energies Nano-Based Drilling Fluids: A Review Review Zisis Vryzas 1,2 and Vassilios C. Kelessidis 3, *

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1 energies Review Nano-Based Drilling Fluids: A Review Zisis Vryzas 1,2 Vassilios C. Kelessidis 3, * 1 Department Petroleum Engineering, Texas A&M University at Qatar, Doha P.O. Box 23874, Qatar; zisis.vryzas@qatar.tamu.edu 2 Department Chemical Engineering, Aristotle University Thessaloniki, Thessaloniki 54124, Greece 3 Department Petroleum Engineering, Petroleum Institute, Abu Dhabi P.O. Box 2533, UAE * Correspondence: vkelessidis@pi.ac.ae; Tel.: +971-(2) Academic Editor: Dongsheng Wen Received: 13 March 2017; Accepted: 6 April 2017; Published: 15 April 2017 Abstract: Nanomaterials are engineered materials with at least one dimension in range nm. Nanluids nanoscale colloidal suspensions containing various nanomaterials have distinctive properties fer unprecedented potential for various sectors such as energy, cosmetic, aerospace biomedical industries. Due to ir unique physico-chemical properties, nanoparticles are considered as very good cidates for smart fluid formulation, i.e., fluids with tailor-made rheological filtration properties. However, due to great risk adapting new technologies, ir application in oil gas industry is not, to date, fully implemented. Over last few years, several researchers have examined use various nanoparticles, from commercial to custom made particles, to formulate fluids with enhanced properties that can withst extreme downhole environments, particularly at high pressure high temperature (HP/HT) conditions. This article summarizes recent progress made on use nanoparticles as additives in fluids in order to give such fluids optimal rheological filtration characteristics, increase shale stability achieve wellbore strengning. Type, size shape nanoparticles, volumetric concentration, addition different surfactants application an external magnetic field are factors that are critically evaluated are discussed in this article. The results obtained from various studies show that nanoparticles have a great potential to be used as fluid additives in order to overcome stern problems. However, re are still challenges that should be addressed in order to take full advantage capabilities such particles. Finally paper identifies discusses opportunities for future research. Keywords: nanoparticles; fluids; smart fluids; nano-fluid; nanotechnology; formation damage; wellbore strengning; rheology; fluid loss; challenges nanluids 1. Introduction A successful operation depends strongly on effectiveness fluid in use (Figure 1). Drilling for oil gas involves a telescopic hole from surface to reservoir which can be kilometers away from surface. Drilling is accomplished with use a bit connected to a long string drill pipe. Applying weight rotation on bit, bit crushes rock into small fragments, cuttings. Drilling fluid is circulated from surface, through drill pipe to bit face, it lifts generated cuttings brings m to surface, where separation equipment removes cuttings from fluid, which is circulated, with help powerful pumps, back to wellbore. Energies 2017, 10, 540; doi: /en

2 Energies 2017, 10, Energies efficiently 2017, in 10, harsh 540 environments it must be ensured that y do not damage formations 2 34 which are being drilled. Figure 1. Schematic representation process. The wells can be vertical (pictured), Figure 1. Schematic representation process. The wells can be vertical (pictured), inclined inclined even horizontal. even horizontal. Exploration new hydrocarbon fields in complex subsurface environments under high pressure Drilling fluids high temperature perform additional (HP/HT) functions, conditions, mainly, requires y control development subsurface pressures, use exceptional stabilize exposed fluids, rock, which prevent maintain contamination ir rheological subsurface filtration formation properties hydrocarbon even fluids, at such provide hostile buoyancy, environments. cool lubricate bit. Such fluids must be engineered so that y can perform efficiently Nanotechnology in harsh environments has come to forefront it must be research ensured that has y already do not contributed damage significantly formations which to technological are being drilled. advances in various industries, including energy industry. The industry could Exploration not be an exception new hydrocarbon to this norm. fields Nanoparticles in complex subsurface (NP) possess environments enhanced under physico-chemical high pressure properties high temperature compared to (HP/HT) macro conditions, micro-sized requires materials, development which can be attributed use exceptional to ir tiny size fluids, along with which ir maintain extremely ir rheological high surface-to-volume filtration properties ratio. Such even properties at such hostile make environments. NPs most promising Nanotechnology materials for has come design to forefront smart research fluids with has tailor-made already contributed properties significantly that can meet to technological requirements advances in deming various industries, downhole including environments energy [1]. industry. The industry could not be The an exception industry to this can norm. thus Nanoparticles significantly (NP) benefit possess from enhanced nanotechnology. physico-chemical The most promising properties compared prospects is to macro use NPs micro-sized in materials, fluids in order which to can formulate be attributed smart to ir tiny fluids size to along give m with ir optimal extremely properties high under surface-to-volume a wide range ratio. Such operating properties conditions. make NPs Furrmore, most promising potential materials to for manufacture design custom-made smart nanoparticles fluids with tailor-made will play a properties vital role that for can development meet requirements nano-based deming fluids, downhole because environments custom-made [1]. fluids can be developed that can meet needs each operator The to deal industry with different can thus specific significantly conditions. benefit Hence, from nanotechnology. application The nanoparticles most promising to prospects formulate is high performance use NPs in fluids fluids has in order potential to formulate to overcome smart current as well fluids as to future give m technical optimal challenges properties encountered under a by wide range operating industry. conditions. Furrmore, potential to manufacture Over custom-made last few years nanoparticles need for improved will play a vital role fluids for has led development researchers to examine nano-based development fluids, because enhanced custom-made fluids, fluids using can be various developed NPs as that additives. can meet While needs most each reported operator to work deal is with lab work, different re specific are however conditions. two Hence, studies reporting application full-scale nanoparticles field testing to formulate nanoparticlebased high performance fluids fluids [2,3]. has potential to overcome current as well as future technical challenges encountered A brief overview by industry. application nanotechnology to fluid formulation has also been provided Over in last literature few years [4,5]. need Modeling for improved rheology fluids nano-ehanced has led researchers fluids, to examine as a function development shear rate, enhanced nanoparticle volume fluids, fraction using various temperature, NPs as additives. aspects which While are most critical toward reported high-fidelity work is computational lab work, re modelling are however for two design studies reporting planning full-scale cost-effective field testing nanoparticle-based campaigns, has also been reported fluids [2,3]. [6 8]. A This brief paper overview presents a review application for nanotechnology incorporation to various NPs fluid for formulation formulation has also been improved provided in fluids. literature It will [4,5]. focus Modeling on reported results rheology on nano-ehanced use nanoparticles for fluids, as enhancement a function shear rate, fluid nanoparticle properties volume which can fraction be modified temperature, with addition aspects nanoparticles. which are critical Of toward above high-fidelity mentioned computational fluid properties, modelling for ones design readily modifiable planning by cost-effective nanoparticles are campaigns, rheology has fluid also loss been control, reported wellbore [6 8]. shale stabilization, wellbore strengning, magnetic

3 Energies 2017, 10, This paper presents a review for incorporation various NPs for formulation improved fluids. It will focus on reported results on use nanoparticles for enhancement fluid properties which can be modified with addition nanoparticles. Of above mentioned fluid properties, ones readily modifiable by nanoparticles are rheology fluid loss control, wellbore shale stabilization, wellbore strengning, magnetic properties such smart fluids, cuttings suspension rmal properties. It is expected that this review will be a useful guide for both companies researchers for development smart, greener more efficient fluids. 2. Rheology Fluid Loss Control 2.1. Experimental Studies Monitoring controlling rheological properties fluid is an integral part efforts for successful oil gas well. Precise prediction frictional losses is strongly dependent on accurate knowledge fluid rheology. As fluids move through wellbore, ir rheological prile is undergoing significant alterations. The combined effect temperature, pressure, time- shear-history on rheological properties makes characterization forecasting fluids rheological prile a complex task [9]. Accurate determination rheological characteristics complex fluids necessitates deep understing base fluid properties, especially contribution associated microstructure mechanisms on flow properties [10]. Beyond rheology, which is a key property that needs to be optimized for development any stable effective fluid, fluid loss is anor property that drillers should minimize in order to promote safer less expensive activities. Invasion foreign fluids, such as mud filtrate, into newly exposed formations, is one most common causes formation damage, leading to costly stimulation treatments even loss production. This problem has been known for decades as a major contributor to abnormal decline in productivity or injectivity in most reservoirs. During, fluid loss into formation occurs due to normal resultant differential pressure between wellbore pressure reservoir pressure, as in most cases for safety reasons, wells are drilled overbalanced, i.e., with higher wellbore pressures than formation fluid pressures. A filter mud cake is formed on formation face due to build-up mud solids. Satisfactory fluid loss value deposition thin, impermeable filter cake can mitigate problems excessive formation damage [11]. Several researchers attempted to incorporate different NPs into fluids for rheological filtration control instead common polymer additives. Amanullah et al. [1] discussed formulation preliminary test results three nano-based fluids. The initial mud formulation indicated that development a functionally viable, physically stable homogeneous also stable over a long period time nano-based fluid is difficult using water or salt water as fluid phase. The preparation a homogeneous stable nanluid with adequate time stability was very difficult without use highly effective surfactants, chemicals or polymers with high shielding or neutralizing capabilities. The test results indicated also that developed nano-based mud produced suitable high low end rheological properties. Furrmore, y noticed a significant decrease in spurt fluid loss upon addition NP with a deposition a thin compact mudcake, which in turn can lead to major decrease in differential pipe sticking in highly permeable formations. Jung et al. [12] examined rheological properties 5 wt % bentonite fluids (used as base fluid) containing different concentrations (0.5 5 wt %) iron oxide (Fe 2 O 3 ) NP (3 30 nm) as a function temperature ( C) pressure (1 100 atm). The results showed that an increase in concentration Fe 2 O 3 NP in bentonite suspension resulted in increasing yield stress (Table 1), viscosity (Figure 2), strength particle interaction. They attributed this rheological enhancement

4 Energies 2017, 10, to fact that Fe 2 O 3 NP embedded in romly dispersed pore structure on surface clay particle conferred links between bentonite particles, which in turn promoted gelation bentonite particles. Furrmore, y performed stard American Petroleum Institute (API) filtration tests in developed samples (100 pounds per square inch pressure differential at atmospheric temperature, referenced as Low Pressure/Low Temperature LP/LT) y found maximum reduction in fluid loss achieved upon addition 0.5 wt % 30 nm Fe 2 O 3 NP. Higher concentrations NP Energies (5 wt %) 2017, led10, to540 a decreased fluid loss capacity. They suggested that near this critical concentration, 4 33 net repulsive attractive forces were in a ratio such that clay platelets aligned more in platelets face-to-face aligned (FF) than more face-to-edge in face-to-face (FE) (FF) configurations, than face-to-edge thus decreasing (FE) configurations, penetrable thus decreasing surface area penetrable filter cake surface formation. area filter cake formation. Table 1. Change yield stress as a function content Fe Fe2O3 2 O 3 NP in 5 wt % bentonite aqueous suspension (base fluid-bf) at 25 C C atmospheric pressure [12]. Samples Samples Yield Yield Stress Stress (Pa) (Pa) BF 1.27 BF 1.27 BF wt % Fe2O3 NP (30 nm) 1.80 BF wt % Fe 2 O 3 NP (30 nm) 1.80 BF wt % Fe2O3 NP (30 nm) 7.67 BF wt % Fe 2 O 3 NP (30 nm) 7.67 BF wt BF % Fe2O wt NP % (3 Fe nm) 2 O 3 NP (3 nm) BF wt BF % Fe2O wt NP %(3 Fenm) 2 O 3 NP (3 nm) Figure 2. Measured apparent viscosity bentonite fluid samples with without addition NP as a function shear rate at 25 C C atmospheric pressure [12] (with permission from AADE, 2011). Barry et al. [13] investigated fluid filtration rheological properties low solid content bentonite fluids, containing iron oxide (Fe2O3) 2 O 3 ) NP additives two NP intercalated clay hybrids, iron oxide clay hybrid (ICH) aluminosilicateclay clay hybrid (ASCH), under under both both LP/LT LP/LT (25 (25 C, 6.9 C, bar) 6.9 bar) High High Pressure/High Temperature (HP/HT, C, C, bar) conditions. Increasing temperature pressure fluid changed fluid changed rheological rheological properties properties this subsequently this subsequently affected its fluid affected loss performance. its fluid loss performance. They also noticed They also that noticed effect that pressure effect on pressure rheological on rheological properties properties produced suspensions produced suspensions was not aswas significant not as significant as that temperature. as that temperature. The ICH The ICH samples containing samples containing Fe 2 O 3 NP Fe2O3 showed NP showed higher higher stresses stresses at all shear at all rates shear compared rates compared to to base fluid base fluid (5 wt (5 % aqueous wt % aqueous bentonite bentonite suspension), suspension), while ASCH while solutions ASCH solutions showed showed lower shear lower stresses shear at stresses all shear at all rates shear thanrates base than fluid base (Figure fluid 3a). (Figure 3a). Filtration experiments revealed that addition ICH ASCH in bentonite suspensions decreased LP/LT fluid loss, by 37% 47% respectively, compared to control sample (5 wt % bentonite) (Figure 3b). A better effect was observed at HP/HT with a reduction 47%. The authors reported that addition 0.5 wt % 3 nm 0.5 wt % 30 nm Fe2O3 NP unexpectedly increased filtration volume at LP/LT conditions compared to control sample (in 30 min) by 11.5% 2.1%, respectively. However, at HP/HT conditions samples containing 0.5 wt % 3 30 nm Fe2O3 NP reduced fluid loss compared to control sample by 27.6% 23.4%, respectively. The

5 conditions. The restructured mode clay platelet interaction due to a modification in surface charge was revealed by zeta potential measurements SEM images. Finally, authors proposed that improved filtration performance by adding ASCH could be attributed to low permeability filter Energies cakes 2017, 10, which 540 were built due to strong electrostatic repulsion between hybrid particles 5 34 clay platelets which provided good dispersion prevented coagulation flocculation. Figure Figure Shear Shear stress stress versus versus shear shear rate rate various various fluids fluids at 25 at C 25 C 6.9 bar. 6.9 Solid bar. Solid lines indicate lines indicate Herschel-Bulkley Herschel-Bulkley fits; fits; Cumulative Cumulative LP/LT LP/LT fluid filtration fluid filtration volumes volumes as a function as a function squareroot square-root time [13] time (with [13] permission (with permission from Elsevier, from Elsevier, 2015). 2015). Contreras et al. [14] investigated use in-house prepared iron-based calcium-based Filtration experiments revealed that addition ICH ASCH in bentonite suspensions nanoparticles with glide graphite as a conventional lost circulation material (LCM) in oil-based mud decreased LP/LT fluid loss, by 37% 47% respectively, compared to control sample (5 wt % in order to minimize formation damage in porous media. The rheological properties were measured bentonite) (Figure 3b). A better effect was observed at HP/HT with a reduction 47%. The authors at 120 F atmospheric pressure. They noticed that samples containing calcium NP moderately reported that addition 0.5 wt % 3 nm 0.5 wt % 30 nm Fe increased ir plastic viscosity. The samples containing iron NP caused 2 O 3 NP unexpectedly increased a reduction in yield point filtration volume at LP/LT conditions compared to control sample (in 30 min) by 11.5% 2.1%, especially at high graphite concentration. The authors concluded that addition NP LCM did respectively. However, at HP/HT conditions samples containing 0.5 wt % 3 30 nm Fe not significantly affect rheological properties thus NP can be used without requiring 2 O 3 NP reduced fluid loss compared to control sample by 27.6% 23.4%, respectively. The authors additional rheological additives. Both HP/HT filter press at 500 psi 250 F API LP/LT filter suggested that at HP/HT conditions Fe press were used to investigate behavior 2 O 3 NP replaced dissociated Na NP graphite enhanced + cations, deflocculating fluids under solution which yielded a low permeability filter cake. On or h, superior performance different conditions. Ceramic discs 775 md were used as filter media at HP/HT filtration ICH as fluid loss additives, was attributed by authors to a strong cross-linked coagulated platelet experiments. The results indicated that all produced nanluids were capable reducing network, which was less sensitive to pressure temperature this resulted in less permeable fluid loss compared to values given by control sample. More specifically, iron-based NP gave filter cakes, reducing filtration volumes both at LP/LT HP/HT conditions. The restructured mode higher reduction in fluid loss value especially at low concentrations under HP/HT conditions, clay platelet interaction due to a modification in surface charge was revealed by zeta potential while calcium based NP yielded significant reduction at high concentration under HP/HT conditions. measurements SEM images. Finally, authors proposed that improved filtration performance Mahmoud et al. [11] evaluated performance fluids containing commercial Fe2O3 by adding ASCH could be attributed to low permeability filter cakes which were built due to SiO2 NP at various concentrations (up to 2.5 wt %), for minimizing formation damage at HP/HT strong electrostatic repulsion between hybrid particles clay platelets which provided good conditions. A 7 wt % Ca-bentonite suspension was used as base fluid. They reported that adding dispersion prevented coagulation flocculation. Fe2O3 NP changed rheology bentonite-based fluids at temperatures up to 200 F, by Contreras et al. [14] investigated use in-house prepared iron-based calcium-based changing yield point plastic viscosity (Figure 4a). However, addition silica NP decreased nanoparticles with glide graphite as a conventional lost circulation material (LCM) in oil-based mud yield point at higher temperatures (Figure 4a). The Herschel-Bulkley model was found to be in order to minimize formation damage in porous media. The rheological properties were measured best fitted model. The authors performed aging tests at 350 F for 16 h, y observed that at 120 rheology F atmospheric pressure. They noticed that samples containing calcium NP moderately bentonite-based fluid containing iron oxide NP remained stable with minor loss increased ir plastic viscosity. The samples containing iron NP caused a reduction in yield point in gel structure. Addition silica NP showed better rheological stability than Fe2O3 NP when especially at high graphite concentration. The authors concluded that addition NP LCM did not aging under same conditions. significantly affect rheological properties thus NP can be used without requiring additional Filtration experiments were carried out at HP/HT conditions (300 psi differential 250 F) rheological additives. Both HP/HT filter press at 500 psi 250 both at static dynamic conditions (using a filter press cell with F API LP/LT filter press were an agitator). The results showed used to investigate behavior NP graphite enhanced fluids under different conditions. that 0.5 wt % Fe2O3 NP was optimum NP concentration, giving a reduction in filtrate volume Ceramic by discs 42.7%, 775 compared md were to that used as filter base media fluid, at with HP/HT a corresponding filtration experiments. increase in The filter results cake thickness indicated by that 17.32% all (Table produced 2). At nanluids concentration were capable 0.5 wt reducing %, a smoor fluid filter loss cake compared morphology to values given by control sample. More specifically, iron-based NP gave higher reduction in fluid loss value especially at low concentrations under HP/HT conditions, while calcium based NP yielded significant reduction at high concentration under HP/HT conditions.

6 Energies 2017, 10, Energies 2017, 10, Mahmoud et al. [11] evaluated performance fluids containing commercial Fe 2 O 3 with SiO less 2 agglomeration NP at various concentrations was observed from (up to SEM 2.5 images. wt %), for The minimizing filter cakes formation were furr damage examined at HP/HT with Computed-Tomography conditions. A 7 wt % Ca-bentonite (CT) scans suspension SEM analysis was used as base results fluid. They were reported that in anor adding Fe study 2 O 3 [15]. NP changed The authors rheology concluded bentonite-based that addition Fe2O3 NP fluids to at temperatures fluids up improved to 200 F, byfilter changing cake characteristics yield pointunder plastic both static viscosity dynamic (Figure 4a). filtration However, conditions. addition The best silicafilter NP decreased cake characteristics yield were pointobtained at higherat temperatures wt %(Figure Fe2O3 NP. 4a). The Thefilter Herschel-Bulkley cake produced model after addition was found Fe2O3 to be NP best consisted fitted model. two layers, The authors as indicated performed by aging CT scan tests (Figure at 3504b). F for The 16layer h, close y to observed rock surface that was rheology main layer, bentonite-based in which NP played fluid containing a key role in iron building oxide NP a good remained microstructure. stable withmoreover, minor loss at inhigh NP gel concentrations structure. Addition a new layer silicawas NPformed, showedconsisted better rheological mainly stability agglomerated than FeNP, 2 O 3 which NP when adversely aging affected under same filter conditions. cake efficiency. Figure Figure Yield Yield point point values values for for samples samples that that have have wt wt % NP NP compared compared to to that that base base fluid fluid (7 (7 wt wt % Ca-bentonite Ca-bentonite suspension) suspension) at at different different temperatures temperatures [11] [11] (with (with permission permission from from SPE, 2016); SPE, 2016); CT scan CT images scan images filter cakes filter generated cakes generated by by fluids fluids that have that 0.5 have wt 0.5 % ferric wt % oxide ferric oxide (Fe (Fe2O3) nanoparticle under static condition at a differential pressure 300 psi a 2 O 3 ) nanoparticle under static condition at a differential pressure 300 psi a temperature temperature 250 F [15] (with 250 permission F [15] (with frompermission SPE, 2017). from SPE, 2017). Table 2. Filtration characteristics fluids that have different NP types concentrations Filtration experiments were carried out at HP/HT conditions (300 psi differential 250 F) at 300 psi differential pressure 250 F [11]. both at static dynamic conditions (using a filter press cell with an agitator). The results showed that 0.5 wt % Fe 2 O 3 NP was optimum Iron Oxide NP Nanoparticles concentration, giving a reduction in filtrate volume by 42.7%, compared to Percentage Cumulative Filter that Cake base fluid, with a corresponding increase Percentage in Change filter cake Concentration Mode Change Filtrate thickness by 17.32% (Table 2). AtThickness concentration 0.5 wt %, a smoorin filter Filtrate cakevolume morphology In Thickness Volume with less agglomeration was observed from SEM images. The filter cakes were furr examined (wt %) (in.) (%) (cm 3 ) (%) with Computed-Tomography (CT) scans SEM analysis results were reported in anor 0.0 Static study [15]. 0.3 The authors Static concluded that addition 1.25 Fe 2 O 3 NP to10.0 fluids improved filter cake 0.5 characteristics Static under both static dynamic filtration6.9 conditions. The best filter cake characteristics 1.5 were obtained Static at wt % Fe 2 O NP. The filter9.0 cake produced after addition Fe 2 O 3 NP consisted 2.5 two Static layers, as indicated by CT scan (Figure b). The layer 0.83 close to rock surface was 0.5 maindynamic layer, in which NP played a key role in 12.4 building a good microstructure. Moreover, at high NP concentrations a new Silica layer Nanoparticles was formed, consisted mainly agglomerated NP, which adversely 0.5 affected Static filter cake efficiency Static Zakaria [16] developed in-house a new class nanoparticles to be used as loss circulation material to control fluid loss in porous media with very small pore size, such as shale formations. The authors tested two different approaches, in-situ ex-situ, nanoparticle formation when using an oil-based fluid. The authors observed a slight decrease in apparent viscosity at all tested shear rates upon addition both types NP. The rheograms followed a non-linear trend at low shear rates while approaching linearity at high shear rates. They attributed this rheological behavior to fact that NP behavior is governed by NP grain boundary surface area/unit mass. They tested also fluids for ir filtration characteristics y found that under stard API filtration test, more than 70% reduction in fluid loss was achieved compared to 9% reduction in fluid

7 Energies 2017, 10, Table 2. Filtration characteristics fluids that have different NP types concentrations at 300 psi differential pressure 250 F [11]. Iron Oxide Nanoparticles Filter Cake Percentage Change Cumulative Filtrate Percentage Change Concentration Mode Thickness In Thickness Volume In Filtrate Volume (wt %) (in.) (%) (cm 3 ) (%) 0.0 Static Static Static Static Static Dynamic Silica Nanoparticles 0.5 Static Static Zakaria [16] developed in-house a new class nanoparticles to be used as loss circulation material to control fluid loss in porous media with very small pore size, such as shale formations. The authors tested two different approaches, in-situ ex-situ, nanoparticle formation when using an oil-based fluid. The authors observed a slight decrease in apparent viscosity at all tested shear rates upon addition both types NP. The rheograms followed a non-linear trend at low shear rates while approaching linearity at high shear rates. They attributed this rheological behavior to fact that NP behavior is governed by NP grain boundary surface area/unit mass. They tested also fluids for ir filtration characteristics y found that under stard API filtration test, more than 70% reduction in fluid loss was achieved compared to 9% reduction in fluid loss in presence typical lost circulation materials. They reported a thin filter cake which indicates high potential for reducing differential pressure sticking as well as formation damage. Moreover, re was no impact NP addition on viscosity stability fluid for more than 6 weeks. Vryzas et al. [10,17 19] carried out experimental investigations to examine effect addition different concentrations commercial iron oxide (Fe 2 O 3 ) NP as fluid additive in 7 wt % aqueous Na-bentonite suspensions (used as base fluid). They concluded that at HP/HT conditions, iron oxide nanoparticles are more efficient at lower concentrations. Maximum reduction filtration loss was achieved upon addition 0.5 wt %, which reduced filtrate losses by 42.5% compared to base fluid (Figure 5). In contrary, addition silica nanoparticles at different concentrations to bentonite-based fluids affected adversely filtration characteristics at HTHP conditions (Figure 6). The exceptional filtration behavior Fe NP was attributed to thin compact filter cake produced upon addition nanoparticles compared to that base fluid, as it was furr revealed by SEM images. The authors performed rheological measurements at different Fe NP concentrations as well as at temperatures. The results showed that Herschel-Bulkley yield stress increased at higher concentrations Fe 2 O 3 NP (Figure 7). Higher tested temperatures also showed an increase Herschel-Bulkley yield stress at all concentrations nanoparticles. They stated that changes were not excessive, allowing potential use NP without need to use rheological additives. The testing fluid samples containing nanosilica at different concentrations showed small rheological changes with most important being reduction in yield stress for all tested samples at 140 F compared to base fluid. It is worth noting that variation yield stress samples containing nanosilica was small at all tested temperatures compared to case for samples containing iron oxide nanoparticles. This interesting behavior should be explored furr in future studies.

8 showed small rheological changes with most important being reduction in yield stress for all tested samples at 140 F compared to base fluid. It is worth noting that variation yield stress samples containing nanosilica was small at all tested temperatures compared to case for samples containing iron oxide nanoparticles. This interesting behavior should be explored furr in future studies. Energies 2017, 10, Figure min HP/HT cumulative filtrate volume samples containing different concentrations iron oxide (Fe2 O3 ) nanoparticles at 250 F 300 psi differential pressure [10] (with permission iron oxide (Fe2O3) nanoparticles at 250 F 300 psi differential pressure [10] (with permission Energies 2017, 10, from SPE, 2015). from SPE, 2015) EnergiesFigure 2017, 10, 540min HP/HT cumulative filtrate volume samples containing different concentrations 8 33 Figure min-hp/ht filtrate volume base fluid samples containing 0.5 wt % iron oxide wt %min-hp/ht nanosilica atfiltrate 250 Fvolume 300psi pressure [10]containing (with permission SPE, Figure differential basefluid fluid samples 0.5 wt wt % %from iron oxide Figure min-hp/ht filtrate volume base samples containing psi differential pressure [10] (with permission from SPE, 2015). 2015) wt wt%%nanosilica nanosilicaatat 250F F 300 psi differential pressure [10] (with permission from SPE, 2015). Figure7.7.Rheograms Rheogramsfor for samples sampleshave havedifferent different concentrations concentrations iron iron oxide oxide (Fe (Fe22O O33))nanoparticles nanoparticles Figure F [10] (with permission from SPE, 2015). in 7.0 wt % aqueous bentonite suspensions at 78 in 7.0 wt % aqueous bentonite suspensions at 78 F [10] (with permission from SPE, 2015). Figure 7. Rheograms for samples have different concentrations iron oxide (Fe2O3) nanoparticles in 7.0 wt % aqueous bentonite suspensions at 78 F [10] (with permission from SPE, 2015). Vryzas magnetite (Fe(Fe 3O3 4)ONP (Figure 8a) 8a) as Vryzas et etal. al.[20,21] [20,21]investigated investigatednovel novelcustom-made custom-made(cm) (CM) magnetite (Figure 4 ) NP fluid additives, with diameters approximately 8 nm, to improve rheological as fluid additives, with diameters approximately 8 nm, to improve rheological Vryzas et al. [20,21] investigated novel custom-made (CM) magnetite (Fe3O4) NP (Figure 8a) as filtration properties based Full suspensions filtration properties Na-bentonite Na-bentonite basedfluids. fluids. Full characterization characterization Na-bentonite Na-bentonite suspensions fluid additives, with diameters approximately 8 nm, to improve rheological that were used as base fluid in this study was reported in or studies [9,22]. They found that filtration properties Na-bentonite based fluids. Full characterization Na-bentonite suspensions yield stressused as apparent viscosity, all shear rates, became increasingly sensitive temperature. that were base fluid in this at study was reported in or studies [9,22]. Theytound that Yield stress produced nanluids linearly with temperature up to to 60temperature. C (250 F) yield stress apparent viscosity, at allincreased shear rates, became increasingly sensitive (Figure 8b). Apparent viscosity, at all shearincreased rates, also increased at temperature higher temperatures. Yield stress produced nanluids linearly with up to 60Addition C (250 F) CM Fe 3O4 NP at 0.5 wt % showed optimal filtration characteristics with a reduction 40% fluid (Figure 8b). Apparent viscosity, at all shear rates, also increased at higher temperatures. Addition

9 Energies 2017, 10, that were used as base fluid in this study was reported in or studies [9,22]. They found that yield stress apparent viscosity, at all shear rates, became increasingly sensitive to temperature. Yield stress produced nanluids increased linearly with temperature up to 60 C (250 F) (Figure 8b). Apparent viscosity, at all shear rates, also increased at higher temperatures. Addition CM Fe3 O4 NP at 0.5 wt % showed optimal filtration characteristics with a reduction 40% fluid loss compared to base fluid at HP/HT conditions (250 F 300 psi differential pressure). The spurt loss, which is initial fluid loss before starting formation filter cake decreased by 100% upon addition CM Fe3 O4 NP. Filter cake thicknesses increased upon addition NP. Dynamic rmal aging at 350 F for 16 h adversely affected properties base fluid. Energies 2017, 10, Energies 2017, 10, Figure Transmission Transmission Electron Electron Microscope Microscope (TEM) (TEM) image image synsized synsizedfe Fe 3O O4 (magnetite) (magnetite) Figure 44 (magnetite) Figure 8. Transmission Electron from Microscope (TEM) image synsized Fe33O nanoparticles [20] (with permission ASME, 2016).; Yield stress as a function temperature nanoparticles [20] (with permission from ASME, 2016); Yield stress as a function temperature for nanoparticles [20] (with permission from ASME, 2016).; Yield stress as a function temperature for fluid sample containing % CM 4 NP [21] (with permission from SPE, 2016). basebase fluid sample containing wt wt % CM Fe Fe O 3ONP [21] (with permission from SPE, 2016). for base fluid sample containing 0.5 wt % CM 3Fe34O4 NP [21] (with permission from SPE, 2016). However, NF maintained its extraordinary rheological filtration behavior achieving 43% However, NF maintained its extraordinary rheological filtration achieving However, NFloss maintained itsto extraordinary performance filtrationbehavior behavior achieving43% 43% reduction in fluid compared base fluid,rheological similar to NP-enhanced reduction in fluid loss compared to base fluid, similar to to performance performance NP-enhanced NP-enhanced fluid at normal conditions, i.e., no rmal aging. SEM-EDS analysis revealed fluid at normal conditions, i.e., SEM-EDS analysis analysis revealed revealed normal conditions, i.e., no no rmal rmal aging. aging. SEM-EDS microstructure at produced filter cakes. microstructure produced filter filter cakes. cakes. The filter cake produced from base fluid was very smooth without significant anomalies The filter cake produced from base fluid was very smooth without significant anomalies anomalies The filter cake produced (Figure 9a), while filter cakes containing CM Fe3O4 NP showed chain-like structures (Figure 9b). (Figure 9a), while filter cakes containing CM Fe3O O44 NP NPshowed showedchain-like chain-likestructures structures(figure (Figure9b). 9b). The authors concluded that presence se structures increased surface area filter The authors concluded that presence presence se se structures structures increased increased surface surface area area filter filter cake furrmore it enhanced its ability to interact more efficiently finally to attach firmly cake furrmore ititenhanced its ability to interact more efficiently finally to attach firmly on furrmore enhanced on surface filter media. its ability to interact more efficiently finally to attach firmly surface filter media. on surface filter media. Figure 9. SEM images filter cakes formed from after HP/HT filtration test at 24.1 bar (300 psi) Figure SEM SEM images images filter filter cakes cakes formed formed from after after HP/HT HP/HT filtration test psi) Figure filtration test at at bar bar (300 (300 psi) 121 C (250 F) (magnification 5000) for from base fluid; nanluid containing 0.5 wt % 121 C (250 F) (magnification 5000) for base fluid; nanluid containing 0.5 wt (250 (magnification 5000) base fluid; nanluid containing 0.5 wt % % CM 121 Fe3O4CNP [21]F)(with permissionfrom SPE,for 2016). CM Fe Fe3O 4 NP [21] (with permission from SPE, 2016). CM 3 O4 NP [21] (with permission from SPE, 2016). Several researchers have used nanosilica (SiO2) as fluid additive. Mao et al. [23] Several researchers have used nanosilica (SiO2) as fluid additive. Mao et al. [23] developed a hydrophobic associated polymer based nano-silica composite with core-shell structure developed hydrophobic associated polymer based nano-silica composite with core-shell structure (SDFL) via ainverse micro emulsion polymerization sol-gel preparation. The results revealed that (SDFL) via inverse micro emulsion polymerization sol-gel preparation. The results revealed that composite showed excellent rmal stability, lubricity, rheological fluid loss properties. composite showed excellent rmal stability, lubricity, rheological fluid loss properties. More specifically, addition 0.5 wt % SDFL in a fresh water-based fluid decreased More conditions. 0.5 wt % in aconcluded fresh water-based fluidfluid decreased fluid specifically, loss by 69% addition at HP/HT TheSDFL authors that developed has great fluid loss by 69% at HP/HT conditions. The authors concluded that developed fluid has great potential to stabilize borehole protect reservoir. Li et al. [24] formulated a fluid

10 Energies 2017, 10, Several researchers have used nanosilica (SiO 2 ) as fluid additive. Mao et al. [23] developed a hydrophobic associated polymer based nano-silica composite with core-shell structure (SDFL) via inverse micro emulsion polymerization sol-gel preparation. The results revealed that composite showed excellent rmal stability, lubricity, rheological fluid loss properties. More specifically, addition 0.5 wt % SDFL in a fresh water-based fluid decreased fluid loss by 69% at HP/HT conditions. The authors concluded that developed fluid has great potential to stabilize borehole protect reservoir. Li et al. [24] formulated a fluid using common fluid materials such as bentonite, KCL XC-polymer added nanosilica NP. Addition silica nanoparticles improved rheological properties produced fluid, while fluid loss was reduced. Moreover, a thin well textured mud cake was formed. They performed also a cost analysis that showed economic feasibility use such new fluid. Salih et al. [25] stated that use nanosilica NP in range wt % had most significant impact on mud properties than any or concentration y have tested (>0.5 wt %). Furrmore y claimed that smart water-based muds with nanosilica can replace oil-based mud in horizontal, directional, shale operations due to ability newly formed fluid to reduce production problems. They furr reported that nanosilica was very sensitive to ph mud played a significant role in enhancing flocculated mud properties at high ph. Anoop et al. [26] examined rheology mineral oil-sio 2 nanluids (1% 2%) at HP/HT conditions. They noticed that tested nanluids exhibited non-newtonian characteristics at elevated pressures temperatures. They concluded that nanluid viscosity values increased with an increase in particle concentration as well as at higher pressures. Higher than 100 C temperatures caused a decrease in viscosity, while most appropriate rheological model was that a power law for all cases. But for temperatures below 100 C, re was no substantial reduction in viscosity values ir fluids. Changing viscosity values were attributed to chemical alteration nanluids at HP/HT conditions, as observed by infrared spectroscopy analysis. Javeri et al. [27] used nm SiO 2 NP for developing a new fluid examined its impact on mud cake thickness rheological properties. They showed that SiO 2 NP did not affect significantly rheological properties. However, y reduced mud cake thickness by 34%, which is very important for alleviating formation damage issues. Ismail et al. [28] studied applicability multi-walled carbon nanotube (MWCNT) nanosilica as fluid additives for improving rheological filtration characteristics as well as lubricity water-based fluids. The results showed that addition MWCNT nanosilica improved rheological properties such as plastic viscosity yield point compared to that base fluid. Furrmore, y found that maximum fluid loss reduction was achieved upon addition wt % (0.01 ppb) nanosilica MWCNT. Belayneh et al. [29] focused on effect nano-silicon dioxide (SiO 2 ) on polymer (HV-CMC, LV-CMC, xanthan gum) salt (KCl, NaCl) treated bentonite fluid systems. They developed a reference fluid containing 0.2 g low viscosity (LV)-carboxymethyl cellulose (CMC) 0.3 g xanthan gum polymers in 25 g bentonite/500 g water with 2.5 g KCl. They added different concentrations SiO 2 NP in reference fluid (up to 0.4 g) checked rheological filtration properties. The results indicated that addition SiO 2 NP caused an upward shift on rheograms with respect to that reference fluid showed shear thinning behavior. A maximum yield stress 10 Pa was measured upon addition 0.25 g SiO 2 NP compared to 5.5 Pa base fluid system. The API filtration measurements revealed that some reduction, 4.5%, was achieved, compared to that nano-free system, by adding 0.25 g SiO 2 NP. On or h, addition 0.2 g 0.3 g increased fluid loss by 8.7% 13% respectively compared to base fluid. These results are fairly similar to results Vryzas et al. [10] who reported, as mentioned above, deterioration fluid loss performance silica nanluid when compared to base fluid. Agarwal et al. [30] investigated use nanoclay nanosilica, in place polymeric surfactants, to stabilize invert emulsion model fluids for HP/HT application. Poly 1-decene,

11 Energies 2017, 10, an olefin oil, was used as continuous phase for invert emulsions. The dispersed phase was deionized water. The nanoclays were montmorillonite based clays which were modified by various organic cations. Hydrophobic nanosilica particles were used. The authors found that best stability flow properties were obtained when both nanoclay nanosilica are used toger. They also found that nanoclay disperses easily in oil phase shows better gel formation capacity. Finally y reported that when aging at 225 C for 96 h, re was some loss in yield stress but emulsion remained stable. Abdo Haneef [31] investigated significance reducing size distribution particles tested a new clay (ATR) which has a chain like structure fers enormous surface area increased reactivity. The new clay (ATR) consisted mainly montmorillonite. The material was used in different sizes (micro nano) to illustrate tailoring rheological properties fluids without using or additives. Bentonite was also tested with different particle sizes. The nanoparticles tested material ATR were found to be suitable for use in mud due to ir functional Energies 2017, 10, characteristic maintaining low viscosity without compromising density requirement, while also maintaining regular bentonite high gel strength. ATR nanoparticles The authors displayed found that a best combination optimized regular set bentonite properties due ATR to nanoparticles combining displayed characteristics best optimized high density set properties bentonite due to combining low viscosity characteristics high gel strength high density ATR nanoparticles, bentonite which lowis not viscosity possible when high gel any strength two ATR clays nanoparticles, are used alone. which The is nano-modified not possible when any fluids two were clays tested are used in HP/HT alone. The environment nano-modified showed fluids great were rheological tested in stability HP/HTat environment high temperature showed pressure. great rheological stability at high temperature pressure. Abdo Abdo Haneef [32] [32] tested use use clay clay nanoparticles to to stabilize fluid rheology at at HP/HT conditions. In this work, y used palygorskite (Pal, a natural hydrous clay clay mineral with with a fibrous rod-like microstructure), which was was purified, synsized, characterized, functionalized, tested tested in in nano-form nano-form (10 20 (10 20 nm nm diameter) diameter) for its for effectiveness its effectiveness to tailor to tailor rheology rheology fluids. The fluids. authors The authors reported reported that that elongated elongated needle shape needle shape Pal (Figure Pal 10a) (Figure results 10a) in results uniquein colloidal unique properties, colloidal properties, especially especially in terms in terms resistance resistance to high concentrations to high concentrations electrolytes. electrolytes. The added The nanoparticles added nanoparticles were found were to provide found to stable provide stable fluid rheology fluid at HP/HT rheology environment at HP/HT environment (Figure 10b). Montmorillonite (Figure 10b). Montmorillonite alone was not alone stablewas at not tested stable HP/HT at tested conditions, HP/HT but conditions, adding small but concentration adding small concentration Pal nanoparticles Pal solved nanoparticles problem. solved They concluded problem. that They Pal concluded NP can bethat used Pal asnp effective can be rheology used as modifiers effective rheology thusmodifiers eliminate use thus eliminate or expensive use or fluid expensive additives. fluid additives. Figure Figure SEM SEM image image regular regular Pal Pal (needle (needle like like clusters); clusters); Yield Yield point point plastic plastic viscosity viscosity vs. vs. temperature temperature for for nano-modified nano-modified fluids fluids [32] [32] (with (with permission permission from fromelsevier, 2013). 2013). Kosynkin et al. [33] examined use graphene oxide (GO) as a high-performance fluid loss Kosynkin et al. [33] examined use graphene oxide (GO) as a high-performance fluid control additive in water-based fluids. They concluded that GO performed very well as a loss control additive in water-based fluids. They concluded that GO performed very filtration additive in water-based fluids at concentrations 0.2 wt % by carbon content. They well as a filtration additive in water-based fluids at concentrations 0.2 wt % by carbon content. performed stard API filtration tests on aqueous dispersions GO xanthan gum. They They performed stard API filtration tests on aqueous dispersions GO xanthan gum. determined that a combination large flake GO (Figure 11a) powdered GO in a 3:1 ratio They determined that a combination large flake GO (Figure 11a) powdered GO in a 3:1 ratio performed best in filtration tests, with an average fluid loss 6.1 ml (Figure 11b) filter cake thickness 20 µm. They compared se results with stard fluids used by oil industries containing ~12 g/l clays polymers, which gave an average fluid loss 7.2 ml (+18%) a filter cake with ~280 µm thickness. They also observed that GO solutions exhibited shear thinning behavior higher temperature stability compared to clay-based fluid loss additives, thus showing a great potential to be applied for HP/HT wells. They concluded that GO has potential for

12 Energies 2017, 10, performed best in filtration tests, with an average fluid loss 6.1 ml (Figure 11b) filter cake thickness 20 µm. They compared se results with stard fluids used by oil industries containing ~12 g/l clays polymers, which gave an average fluid loss 7.2 ml (+18%) a filter cake with ~280 µm thickness. They also observed that GO solutions exhibited shear thinning behavior higher temperature stability compared to clay-based fluid loss additives, thus showing a great potential to be applied for HP/HT wells. They concluded that GO has potential for industrial scalability through production from abundant graphite sources common reagents can be proved as an effective fluid loss control additive for industry. Energies 2017, 10, Figure Figure SEM SEM image image a single single large large flake flake graphene graphene oxide oxide (LFGO) (LFGO) flake; flake; API API filtration filtration loss loss results results for for LFGO, LFGO, PGO, PGO, a 1:1 1:1 mix mix a 3:1 3:1 mix mix LFGO LFGO PGO PGO suspensions suspensions at at 2 g/l g/l carbon-content carbon-content concentrations concentrations in in g/l g/l (0.75 (0.75 lbm/bbl) lbm/bbl) xanthan xanthan gum gum solution solution [33] [33] (with (with permission permission from from American American Chemical Chemical Society, Society, 2012). 2012). Nasser et al. [34] developed nanluid using nanographite nanosilicon wires as additives. Nasser et al. [34] developed a nanluid using nanographite nanosilicon wires as additives. The authors concluded that nanomud retained all desired rheological properties at higher The authors concluded that nanomud retained all desired rheological properties at higher temperatures (up to 90 C). The viscosity nanomud was higher than this normal mud at temperatures (up to 90 C). The viscosity nanomud was higher than this normal mud all tested temperatures. Finally, y proposed that cost feasibility NP should be assessed in at all tested temperatures. Finally, y proposed that cost feasibility NP should be assessed in future work. future work. Manea [35] focused on low solid content water-based fluids prepared with nano size Manea [35] focused on low solid content water-based fluids prepared with nano size polymers. synsized nano polymer was used as a filtrate reducer additive. The nanoparticles polymers. A synsized nano polymer was used as a filtrate reducer additive. The nanoparticles were were obtained by grinding with a Fritsch Pulverisette planetary mill. The enhanced rheological obtained by grinding with a Fritsch Pulverisette planetary mill. The enhanced rheological properties properties this polymer were due to its capacity forming hydrogels by adsorption free water this polymer were due to its capacity forming hydrogels by adsorption free water from from system. The material was reported to be ph sensitive its swelling capacity increased in system. The material was reported to be ph sensitive its swelling capacity increased in alkaline alkaline media. The author concluded that fluid loss reducer agent keeps cumulative volume media. The author concluded that fluid loss reducer agent keeps cumulative volume filtrate filtrate at low values. at low values. Saboori et al. [36] Fereydouni et al. [37] identified effect carboxymethyl cellulose Saboori et al. [36] Fereydouni et al. [37] identified effect carboxymethyl cellulose (CMC) (CMC) polyanionic cellulose (PAC) polymer nanoparticles on fluid loss mud-cake-thickness. polyanionic cellulose (PAC) polymer nanoparticles on fluid loss mud-cake-thickness. CMC CMC PAC nanoparticles were made in-house. The polymer powders' size distributions before PAC nanoparticles were made in-house. The polymer powders size distributions before entering entering to mill after exiting from mill were measured by Particle size analyzer. The to mill after exiting from mill were measured by Particle size analyzer. The amount amount fluid loss mud cake thickness fluids were measured by stard API fluid loss mud cake thickness fluids were measured by stard API filter press. filter press. The authors found that adding CMC PAC nanoparticles resulted in desirable The authors found that adding CMC PAC nanoparticles resulted in desirable reduction amount reduction amount fluid loss mud cake thickness when compared with conventional fluid loss mud cake thickness when compared with conventional polymers same type. polymers same type. It would be interesting to compare ir results with results obtained It would be interesting to compare ir results with results obtained when regular powder CMC or when regular powder CMC or PAC are used as additives to fluids to denote PAC are used as additives to fluids to denote importance nano particles in improving importance nano particles in improving such performance. such performance. Li et al. [38] investigated addition cellulose nanocrystals (CNC) polyanionic cellulose Li et al. [38] investigated addition cellulose nanocrystals (CNC) polyanionic cellulose (PAC) as additives in bentonite water-based fluids. They showed that presence (PAC) as additives in bentonite water-based fluids. They showed that presence bentonite bentonite CNCs significantly improved rheological properties PAC/CNC/bentonite waterbased fluids, whereas effect PAC was relatively less (Figure 12a,b). Finally, y noticed that API fluid loss PAC/CNCs/bentonite water based fluids remarkably decreased as concentrations bentonite PAC increased, while CNCs had little impact on fluid loss PAC/CNC/bentonite fluid.

13 Energies 2017, 10, CNCs significantly improved rheological properties PAC/CNC/bentonite water-based fluids, whereas effect PAC was relatively less (Figure 12a,b). Finally, y noticed that API fluid loss PAC/CNCs/bentonite water based fluids remarkably decreased as concentrations bentonite PAC increased, while CNCs had little impact on fluid loss Energies PAC/CNC/bentonite 2017, 10, 540 fluid Figure Figure Plots Plots viscosity; viscosity; shear shear stress stress as as a a function function shear shear rate rate for for PAC/CNC/BTWDFs PAC/CNC/BTWDFs at at various various bentonite bentonite concentrations concentrations [38] [38] (with (with permission permission from from American American Chemical Chemical Society, Society, 2015). 2015). Li et al. [39] explored effectiveness cellulose nanoparticles (CNPs), including Li et al. [39] explored effectiveness cellulose nanoparticles (CNPs), including micribrillated micribrillated cellulose (MFC) CNCs in enhancing rheological filtration performance cellulose (MFC) CNCs in enhancing rheological filtration performance bentonite bentonite water-based fluids. They found that addition MFC CNCs increased water-based fluids. They found that addition MFC CNCs increased rheological rheological properties bentonite water based fluids, including viscosity yield point, properties bentonite water based fluids, including viscosity yield point, demonstrating ir capability on improving cuttings transport capacity. Moreover, y suggested demonstrating ir capability on improving cuttings transport capacity. Moreover, y suggested that improved viscosity, core-shell structure as well as formation CNC polymer films that improved viscosity, core-shell structure as well as formation CNC polymer films remarkably reduced fluid loss volume thickness filter cake for CNC/bentonite fluids. remarkably reduced fluid loss volume thickness filter cake for CNC/bentonite fluids. On or h, MFC had little impact on fluid loss yielded thicker filter cakes, which can On or h, MFC had little impact on fluid loss yielded thicker filter cakes, which can cause serious problems such as differential pressure sticking stuck pipe. cause serious problems such as differential pressure sticking stuck pipe. Sadeghalvaad Sabbaghi [40] examined effect TiO2/polyacrylamide (PAM) Sadeghalvaad Sabbaghi [40] examined effect TiO 2 /polyacrylamide (PAM) nanocomposite on water-based fluid properties. They found that nano-enhanced water nanocomposite on water-based fluid properties. They found that nano-enhanced water based fluids (NWBF) increased rheological properties such as plastic viscosity yield based fluids (NWBF) increased rheological properties such as plastic viscosity yield point. Furrmore, shear thinning behavior was increased by increasing concentration point. Furrmore, shear thinning behavior was increased by increasing concentration additive. They performed also SEM analysis pure PAM TiO2/PAM nanocomposite additive. They performed also SEM analysis pure PAM TiO SEM images showed that surface pure PAM sample was smooth. 2 /PAM nanocomposite The comparison SEM images showed that surface pure PAM sample was smooth. The comparison se two images revealed that TiO2 grains appeared on surface inside PAM. se two images revealed that TiO William et al. [41] investigated 2 grains appeared on surface inside PAM. preparation nanluid-enhanced water-based William et al. [41] investigated preparation nanluid-enhanced water-based muds muds (NWBM). They used CuO ZnO nanoparticles (with sizes less than 50 nm) in a based fluid (NWBM). They used CuO ZnO nanoparticles (with sizes less than 50 nm) in a based fluid which which was a 0.4 wt % Xanthan Gum aqueous solution. The nanluids were prepared using was a 0.4 wt % Xanthan Gum aqueous solution. The nanluids were prepared using nanoparticle nanoparticle concentrations 0.1, wt %. An ultrasonication tank was used, sonication concentrations 0.1, wt %. An ultrasonication tank was used, sonication for one hour for one hour was used. The WBM was formulated with 5 cp prehydrated bentonite slurry adding was used. The WBM was formulated with 5 cp prehydrated bentonite slurry adding xanthan gum xanthan gum (XG) as a viscosifier, polyanionic cellulose (PAC-L) as a fluid loss control agent, KCl for (XG) as a viscosifier, polyanionic cellulose (PAC-L) as a fluid loss control agent, KCl for inhibition inhibition KOH to establish a ph range A biocide (formaldehyde) was added to ensure KOH to establish a ph range A biocide (formaldehyde) was added to ensure that that natural polymers do not degrade due to bacterial action. The authors observed that NWBM natural polymers do not degrade due to bacterial action. The authors observed that NWBM showed improved rmal electrical properties by about 35% compared to WBM. Increasing showed improved rmal electrical properties by about 35% compared to WBM. Increasing concentration nanoparticles enhanced electrical rmal properties fluids even concentration nanoparticles enhanced electrical rmal properties fluids more. The NWBM based on CuO nanluids were found to show improved rmal properties even more. The NWBM based on CuO nanluids were found to show improved rmal properties were more resistant to HP/HT conditions than ZnO-based NWBM. High pressure high were more resistant to HP/HT conditions than ZnO-based NWBM. High pressure high temperature rheological studies were conducted on NWBM at varying temperatures (25, 70, C) pressures (0.1 MPa 10 MPa). The effect pressure on rheology NWBM was found to be more significant at higher temperatures. The results showed better rheological stability in case NWBM. The authors reported that most significant role that nanluids play was in stabilizing viscosity at higher temperatures. The Herschel Bulkley model was observed to

14 Energies 2017, 10, temperature rheological studies were conducted on NWBM at varying temperatures (25, 70, C) pressures (0.1 MPa 10 MPa). The effect pressure on rheology NWBM was found to be more significant at higher temperatures. The results showed better rheological stability in case NWBM. The authors reported that most significant role that nanluids play was in stabilizing viscosity at higher temperatures. The Herschel Bulkley model was observed to be best fit-model for describing rheological behavior NWBM. Ponmani et al. [42] tested effect nanluids copper oxide (CuO) zinc oxide (ZnO) at various concentrations (0 0.5 wt %) in various base fluids, such as xanthan gum, polyethylene glycol (PEG-600), polyvinylpyrrolidone (PVP), for development nanluid enhanced mud (NWBM). The results were compared with se obtained from micro fluid enhanced mud (MWBM) in order to assess effect particle size. The results showed that NWBM had better rmal filtration properties than MWBM. The maximum reduction in fluid loss ( 63%) compared to base fluid was achieved by sample containing 0.5 wt % ZnO NP 0.5 wt % ZnO microparticles. Finally, y observed that MWBM had higher filter cake thickness compared to NWBM as well as a decrease in thickness filter cake with an increase in concentration nanoparticles in nanluids. Aftab et al. [43] explored effects zinc oxide NP-acrylamide composite (ZnO-Am) on rheological shale swelling behavior conventional water-based fluid. Results revealed that rheological properties (e.g., AV, PV) were slightly increased upon addition ZnO-Am composite over tested temperature range (up to 150 F). API fluid loss (LP/LT) was reduced by 14%, while HP/HT fluid loss was slightly reduced. Shale swelling was decreased from 16% to 9%. Friedheim et al. [4] discussed use carbon nanotubes (CNTs) as stabilizers for ultra-hpht non-aqueous invert emulsion fluids. Two CNTs were selected for evaluation formulations after screening numerous types CNTs at various concentrations. They found that both CNT materials showed positive results in stabilizing rheological behavior under HP/HT conditions. They also reported that fluid loss control was still an issue with se fluids. Furrmore, y reported effect adding graphene oxide nanoparticles to freshwater slurry bentonite barite on fluid viscosity fluid loss control. The results showed that effect, in terms typical fluid rheological parameters, is quite substantial when only 2 lb/bbl (0.57 wt %) is added. They concluded that graphene oxide nanoparticles affect both rheology fluid loss appear to be relatively effective. Ho et al. [44] carried out experimental investigations in order to examine effect hydrogenated oil-based fluid when dispersed with graphene nano-sheets. Graphene nano-sheets were dispersed via hydrodynamic cavitation dispersion ultrasonic bath for 3 h each. Two rheological models (Bingham model Power Law model) were fitted to predict rheological behavior graphene-oil based fluid (Figure 13). The results indicated that newly developed nano-based fluid exhibited higher viscosity as compared to hydrogenated oil-based fluid over shear rate range s 1. In addition, y noticed that at higher particle loadings, higher viscosity values were obtained. The authors also observed that graphene-oil based fluid behaved like a Bingham fluid but was similar to a Newtonian fluid as it possessed zero shear stress. The viscosity decreased exponentially compared to base fluid s viscosity at increasing shear rate regardless concentration viscosity trend continued to decrease exponentially at increasing shear rates until it reached a base viscosity similar to this base fluid. Finally, y saw that at lower shear rates, experimental data fitted well into both rheological models (Bingham plastic model Power Law model). However, at higher shear rates Power Law model significantly deviated from experimental data.

15 Energies 2017, 10, Energies 2017, 10, Energies 2017, 10, Figure 13. Comparison experimental data rheological models at 40 C [44] (with permission from Elsevier, Figure Figure ). Comparison experimental data data rheological models at at C C [44] [44](with permission from from Elsevier, Elsevier, 2016). 2016). Ismail et al. [45] studied use multi-walled carbon nanotubes (MWCNTs) as an additive to improve Ismail rheological et al. [45] properties studied use water-based multi-walled carbon ester-based nanotubes (MWCNTs) fluids. as They an additive focused to on determining improve optimum rheological concentration properties water-based MWCNTs with ester-based average diameter fluids. 30 They nm, focused to produce on better determining rheological properties optimum at concentration various temperatures. MWCNTs with The with results average average showed diameter diameter that 30in nm, water-based 30 tonm, produce to produce better fluid, rheological better plastic rheological properties viscosity, properties at various yield at various point temperatures. temperatures. gel The strength results The results are showed not showed that much inthat water-based affected in water-based by different fluid, fluid, plastic viscosity, plastic viscosity, yield point yield point gel strength gel arestrength not muchare affected not much by affected different by concentrations different concentrations MWCNTs that were used. However, in ester-based fluid, emulsion stability concentrations MWCNTs that MWCNTs were used. that However, were used. in However, ester-basedin ester-based fluid, emulsion fluid, stability emulsion is stability slightly is slightly increased as MWCNTs concentration increases. It was also found that increase in increased is slightly as increased MWCNTs as concentration MWCNTs concentration increases. Itincreases. was also found It was that also found increase that in temperature increase in temperature led to a decrease in plastic viscosity yield point water-based fluid. On led temperature to a decrease led to in a decrease plastic in viscosity plastic viscosity yield point yield point water-based water-based fluid. fluid. On On contrary, contrary, contrary, ester-based ester-based ester-based fluid fluid showed fluid showed showed an increase an an increase increase rheological in in rheological properties properties with properties an increase with with an in an increase temperature. increase temperature. in temperature. HP/HT HP/HT filtration HP/HT filtration after filtration agingafter indicated after aging aging that indicated that wt % ( ppb) wt wt % MWCNTs %(0.01 (0.01 ppb) ppb) was MWCNTs MWCNTs optimal was was concentration optimal optimal concentration with lowest with with filtration lowest volume filtration (Figure volume 14). (Figure 14). 14). Figure 14. HP/HT fluid loss after aging at different MWCNTs concentration [45] (copyright ANSInet, 2014). Figure Figure 14. HP/HT 14. HP/HT fluid loss fluid after loss aging after at aging different at different MWCNTs MWCNTs concentration concentration [45] (copyright [45] (copyright ANSInet, 2014). ANSInet, 2014). Parizad Shahbazi [46] investigated effects Tin oxide (SnO2) NP on water-based fluid properties. They concluded that adding SnO2 NP enhanced characteristics Parizad Parizad Shahbazi Shahbazi [46] [46] investigated effects Tin oxide (SnO2) fluids such as rheology, rmal electrical conductivities, thixotropy 2 ) NP on water-based filtration characteristics. fluid properties. fluid properties. More specifically, They They y concluded saw a 20% that that reduction adding in SnO2 fluid NP loss enhanced by adding characteristics 2.5 g/l SnO2 NP, however higher fluids fluids such concentrations as such rheology, as rheology, NP rmal rmal could not improve electrical more conductivities, filtration thixotropy characteristics. filtration In characteristics. addition, y stated More specifically, More specifically, that increasing y y saw saw concentration a 20% a 20% reduction reduction SnO2 NP in influid resulted loss in by reduction adding g/l g/l SnO flow SnO2 behavior 2 NP, NP, however however higher index (n) higher concentrations in increase NP could flow consistency not improve index more (K). filtration characteristics. In addition, y stated that increasing concentration SnO2 NP resulted in reduction flow behavior index (n) in increase flow consistency index (K).

16 Energies 2017, 10, concentrations NP could not improve more filtration characteristics. In addition, y stated that increasing concentration SnO 2 NP resulted in reduction flow behavior index (n) in increase flow consistency index (K). Li et al. [47] studied utilization a commercially available soy protein isolate (SPI) as fluid loss additive in bentonite-water based fluids (BT-WDFs). The results indicated that at low SPI concentrations (0.5, 1.0, 1.5 wt %), strong aggregations were formed, resulting in formation thick, high-porosity high-permeability filter cakes giving high fluid loss values. On or h, at higher concentrations SPI (3.0, 4.5, 6.0 wt %), intercalated structures were created that led to formation thin, compact low-porosity low-permeability filter cakes, which had superior filtration characteristics compared to pure BT-WDFs. A critical concentration was determined (3 wt %), above which addition SPI led to significant reduction in fluid loss tested fluids. The authors attributed this behavior to fact that attachment SPI on surface bentonite alteration microstructure bentonite in suspension from house cards to agglomeration or intercalation were responsible for se phenomena. Alizadeh et al. [48] explored rheological behavior a fluid containing alumina/polyacrylamide nanocomposite. The syntic nanocomposite was synsized through solution polymerization method. They noticed that addition 4% nanocomposite increased viscosity fluid up to more than 300 cp for both fresh salt water based mud. Furrmore, y showed that nanocomposite tested was able to decrease thixotropy produced fluid. Amarfio Abdulkadir [49] explored effect Al 2 O 3 NP on rheological properties water-based mud. They showed that Al 2 O 3 NP provided rmal stabilization for fluid under high temperature conditions that Al 2 O 3 NP were able to maintain shear stresses fluid as temperature increases. Afolabi et al. [50] evaluated rheological properties bentonite mud at three different concentrations (6.3 wt %, 13 wt % 15 wt %) containing different concentrations silica nanoparticles (0 wt % 1.5 wt %). They developed a new hyperbolic model to evaluate rheological properties bentonite mud with without silica nanoparticles. In addition y compared its performance against various rheological models: Herschel Bulkley, Sisko, Casson (Figure 15a). They observed that hyperbolic rheological model outperformed or models estimated rheological behavior nano-modified mud with high accuracy. The reliability different models was investigated using Root Mean Square Error (RMSE), residual plot analysis coefficient determination (R 2 ) values. They noticed that range R 2 at all tested concentrations bentonite silica NP was ranging from , 0.999, , for Herschel-Bulkley, Hyperbolic, Casson Sisko model, respectively. The residual plots for Herschel-Bulkley model Hyperbolic model (Figure 15b) indicated that re was a rom variation in plot residuals with fitted data points se two models provided best fit to experimental data. For Casson model, re was a systematic pattern deviation in plot residuals revealing its poor performance.

17 bentonite silica NP was ranging from , 0.999, , for Herschel-Bulkley, Hyperbolic, Casson Sisko model, respectively. The residual plots for Herschel-Bulkley model Hyperbolic model (Figure 15b) indicated that re was a rom variation in plot residuals with fitted data points se two models provided best Energies fit 2017, to 10, experimental 540 data. For Casson model, re was a systematic pattern deviation in plot residuals revealing its poor performance. Figure Figure Predicted Predicted measured measured shear shear stress stress shear shear rate data rate for data 6.3 wt for % 6.3 bentonite wt % bentonite mud containing mud containing 0.5 wt % silica 0.5 nanoparticles wt % silica nanoparticles using different using rheological different models; rheological Residual models; plot for Residual 6.3 wt % plot bentonite for 6.3 wt mud % bentonite containingmud 0.5 wt containing % silica nanoparticles 0.5 wt % silica [50] nanoparticles (copyright Afolabi [50] (copyright et al. 2017). Afolabi et al. 2017). Energies 2017, 10, Field Applications All above studies have been carried out in laboratories. However, it is important to evaluate nanluids in real conditions. There are two studies in literature that highlight successful application novel nanluids in field. Borisov et al. [2] presented results from a field application nanoparticle-based invert emulsion fluids (an oil-based fluid). They emphasize that fluids that combine LCM with nanoparticles can significantly reduce fluid loss create a thinner filter cake, compared to fluids containing LCM alone. Due to ir superior properties, NP have ability to fill gaps between micron-sized particles, which leads to lower permeability decreased filtrate flux (Figure 16). They concluded that ir attempt to scale up NP synsis was successful. Total mud losses were reduced by 22 34% in presence 0.5 wt % calcium NP, which agrees with what was obtained in lab. Figure 16. Aschematic schematicrepresentation representation mud mud losses losses while while in in case case typical typical LCM; LCM; NP [2] NP (with [2] (with permission permission from from Springer, Springer, 2015). 2015). Taha Lee [3] studied application nanluid containing blend proprietary Taha Lee [3] studied application a nanluid containing a blend proprietary surfactants engineered with nano graphene to improve fluid performance. They tested surfactants engineered with nano graphene to improve fluid performance. They tested developed nanluid in field (HP/HT onshore well) y saw a significant improvement in developed nanluid in field (HP/HT onshore well) y saw a significant improvement fluids rmal stability as well as in its lubricity. They also observed a 30% reduction in fluid loss compared to conventional muds. Furrmore, y obtained an improved rate penetration (ROP) by 125%, actual reaming torque reduction 20% more than 75% increase in bit s life span Modeling Rheology

18 Energies 2017, 10, in fluids rmal stability as well as in its lubricity. They also observed a 30% reduction in fluid loss compared to conventional muds. Furrmore, y obtained an improved rate penetration (ROP) by 125%, actual reaming torque reduction 20% more than 75% increase in bit s life span Modeling Rheology There are also several researchers that examined modeling aspects rheology different nanoparticle-enhanced fluids. Such models, can provide credible predictions yield stress viscosity values on basis smart fluid composition formulation, while have potential to be applied to more complex fluid systems. The development first principle models for rheology nano-enhanced fluids, which can characterize fluid behavior as a function shear rate (γ), nanoparticle volume fraction (φ) temperature (T), is critical toward modelling, design planning cost effective campaigns [7]. Reilly et al. [7] proposed a first-principles approach to rheology smart fluids containing Fe 3 O 4 NP which have shown advantages to increasing efficiency in a variety reservoir environments. Their models were based on original experimental data. The model for shear stress was developed based on a force balance between Van der Waals attractions monodispersed Fe 3 O 4 NP spheres. The model Energies 2017, 10, for viscosity was created by considering force required to maintain NP in suspension being equal to drag force as calculated for Stokes flow approximation about a sphere. At first y developed bivariate (shear rate, NP concentration) viscosity shear stress models at a range developed bivariate (shear rate, NP concentration) viscosity shear stress models at a range temperatures (25 C 60 temperatures (25 C) C 60 y concluded that produced results by first principle C) y concluded that produced results by first principle showed good agreement with experimental data for shear stress viscosity. They observed showed good agreement with experimental data for shear stress viscosity. They observed a continuous increase in shear stress apparent viscosity at higher NP concentrations as well as a continuous increase in shear stress apparent viscosity at higher NP concentrations as well increased temperatures reduced degree shear thinning predicted by model leading to as increased temperatures reduced degree shear thinning predicted by model leading to discrepancies in shear stress predicted at high shear rates. They also incorporated parameter T discrepancies in shear stress predicted at high shear rates. They also incorporated parameter in ir bivariate models, this leading to development trivariate viscosity shear stress T in ir bivariate models, this leading to development trivariate viscosity shear stress models (Figure 17). They stated that heating effects low NP concentrations increased stard models (Figure 17). They stated that heating effects low NP concentrations increased stard error concluded that newly developed models described rheological effects shear rate, error concluded that newly developed models described rheological effects shear rate, nanoparticle concentration temperature with high predictive potential with correlation nanoparticle concentration temperature with high predictive potential with correlation coefficients coefficients (R 2 (R > 0.983). 2 > 0.983). Figure 17. Trivariate models plots plots for for shear shear stress stress viscosity viscosity at different at different temperatures temperatures [7] (copyright [7] (copyright Elsevier, 2016). Elsevier, 2016). Gerogiorgis Gerogiorgis et et al. al. [8] [8] started started from from microstructural microstructural arguments arguments force force equilibria equilibria assumptions assumptions developed developed physics-based physics-based (not data-driven) (not data-driven) correlations. correlations. They developed They developed first-principles first-principles rheological rheological models nano-enhanced fluids containing Fe3O4 models nano-enhanced fluids containing Fe NP, which are considered to be 3 O 4 NP, which are considered to be explicit explicit multivariate multivariate functions functions temperature, temperature, NP volume NP volume fraction fraction shear rate. shear They rate. concluded They concluded that all that all composed fluids exhibited a yield stress behavior are sensitive to both NP addition temperature, which induced an upward shift on yield stress values as well as shear stress surfaces. They achieved a very good agreement model consistency, with a slight discrepancy at lowest temperature (Figure 18). They also stated that variation surface inclination as a function temperature is more pronounced for high NP volume fraction, an observation corroborating indication strong microstructural effects (interconnected network

19 Energies 2017, 10, composed fluids exhibited a yield stress behavior are sensitive to both NP addition temperature, which induced an upward shift on yield stress values as well as shear stress surfaces. They achieved a very good agreement model consistency, with a slight discrepancy at lowest temperature (Figure 18). They also stated that variation surface inclination as a function temperature is more pronounced for high NP volume fraction, an observation corroborating indication strong microstructural effects (interconnected network formation leading to gelation). Finally, y examined reliability developed models by calculating comparing R 2 values as well as sum squared errors (ΣQ 2 ) found that trivariate models showed high predictive Energies 2017, potential, 10, 540 with R 2 > 0.97 for all subsets shear stress Figure 18. Trivariate first-principles shear stress versus temperature shear rate at different NP concentrations [8] (with permission from SPE, 2017). 3. Shale Wellbore Stability Several studies were carried out in order to examine use various nanoparticles for reduction shale shale permeability around around wellbore wellbore by plugging by plugging pore throats, pore throats, building building an internal an internal mud cake mud cake reby reby reducing reducing fluid invasion fluid invasion into into shale. shale. Sensoy Sensoy et al. [51] et al. performed [51] performed tests tests using using an apparatus an apparatus called called Shale Shale Membrane Membrane Tester Tester for twor different two different shales (Atoka shales (Atoka Gulf Gulf Mexico Mexico shales). shales). It was concluded It was concluded that a concentration that a concentration least at 10 least wt 10 % wt % 20 nm20 NP nm should NP should be used be used for for successful successful shale shale plugging. plugging. Scanning Scanning Electron Electron Micrographs Micrographs were used were toused visualize to visualize type plugging type plugging that was taking that was place. taking It was place. concluded It was concluded that nanoparticles that nanoparticles plugged primarily plugged primarily pores that fitpores ir that size. fit However ir size. a group However nanoparticles a group nanoparticles could in somecould casesin aggregate some cases toger aggregate to plug toger a bigger to plug pore a throat. bigger Finally, pore four throat. field Finally, muds were four studied field muds withwere without studied with addition without nanoparticles. addition It was nanoparticles. found that addition It was found nanoparticles that addition reduced nanoparticles fluid penetration reduced into Atoka fluid shale penetration by 16 72% into Atoka into shale Gulfby 16 72% Mexico shaleinto by 17 27%. Gulf Mexico shale by 17 27%. Taraghikhah et al. [52] examined nanosilica as an additive in water-based fluid for improving shale stability. They They determined that that optimal optimal concentration nanosilica nanosilica is <1is wt <1 % wt % stated stated that that nano- nano- fluid fluid had an had acceptable an acceptable shale shale recovery recovery in comparison in comparison with ordinary with ordinary shale shale swelling swelling inhibitors. inhibitors. SEM images SEM images collected collected shales after shales performing after performing shale recovery shale recovery test, revealed test, revealed pore plugging pore plugging as a physical as a shale physical inhibition shale inhibition mechanism. mechanism. In additionin toaddition its improved to its improved inhibition inhibition characteristics, characteristics, developed developed nanluid nanluid proved to proved be an efficient to be an lubricant efficient lubricant gave improved gave improved rheological rheological priles with priles minorwith changes minor inchanges fluid loss in characteristics. fluid loss characteristics. Hoelscher et et al. al. [53] [53] studied application application water-based water-based fluids fluids in unconventional in unconventional shale shale formations formations using silica using nanoparticles. silica nanoparticles. They aimed They toaimed minimize to minimize shale permeability shale permeability through physically through physically plugging nanometer-sized pores instead chemical inhibition to impede water flow between wellbore formation, thus eliminating swelling shales reducing formation fractures. The nanoparticles used were nm in size. The nm diameter nanoparticles were found to have lowest amount fluid loss (based on one-third rule filtration ory using 100 nm membranes). After that, samples desired size surface

20 Energies 2017, 10, plugging nanometer-sized pores instead chemical inhibition to impede water flow between wellbore formation, thus eliminating swelling shales reducing formation fractures. The nanoparticles used were nm in size. The nm diameter nanoparticles were found to have lowest amount fluid loss (based on one-third rule filtration ory using 100 nm membranes). After that, samples desired size surface treatment were analyzed with cryo-transmission electron microscopy (c-tem) X-ray photospectrometry (XPS) to assure that re were no major contaminants in samples. The authors used Shale Membrane Tester to better underst plugging mechanism shale pore without taking into consideration any modifications rock itself. The results confirmed that silica nanoparticles can physically plug shale at low loading levels in a water-based fluid while being environmentally friendly cost effective. Sharma et al. [54] developed tested a new family water-based fluids that can be applied to a much border range shales. They used silica nanoparticles with uniform 20 nm diameter. They found that formulated fluids were quite stable at elevated pressures temperatures fered a wide range rheological properties, having also good lubricity. The authors also conducted tests to measure extent invasion water into shales when y are exposed to nanoparticle based fluids. They found that invasion into shale was reduced by times. Tests were also conducted on fractured gas shale samples. They found that nanoparticles alone can effectively plug pores in shales without microcracks. However, a combination properly formulated mud nanoparticles appropriate size concentration is key to prevent water invasion into shale samples with or without microcracks. Srivatsa et al. [55] investigated effectiveness a bio polymer-surfactant fluid blend, containing nanoparticles as fluid loss additives in reducing filtrate losses to formation by forming a thin, non-erodible filter cake. The authors presented results testing rheological properties API filtrate loss compared fluid loss reduction by using nanoparticles as fluid loss additive with an industry stard polymer-based fluid loss additive. The results showed that sized silica nanoparticles can be used instead sized calcium carbonates which are very effective inorganic bridging agents, however, difficult to maintain. They also found that surfactant is not rmally stable as bio-polymer at high temperatures; hence y concluded that bio-polymer nanoparticles might be a good combination for high temperature zones as bio-polymers are generally stable up to 350 F. Increasing concentration nanoparticles was found to reduce fluid loss, however this was limited by aggregation nanoparticles in polymer fluid. Finally, y reported that nanoparticles are most effective for shale applications as y can penetrate pores shale act as bridging material resulting in wellbore strengning. Akhtarmanesh et al. [56] used NP to reduce fluid penetration into Gurpi shale thus promoting wellbore stability. They tested three different fluids with different additives with without NP. It was concluded that for successful shale plugging, a concentration at least 10 wt % 35 nm NP was needed. The fluid with NP reduced fluid penetration into Gurpi shale up to 68% in comparison with control mud. Kang et al. [57] developed evaluated water oil-based fluids containing silica nanoparticles by running tests such as spontaneous imbibition, swelling rate acoustic transit time. Results showed that, for water-based fluids, nanoparticles resulted in higher plastic viscosity (PV) yield point (YP), lower API-filtration. Moreover, because pore throats shale can be plugged by nanoparticles, imbibition amount, swelling rate, Young s-modulus reduction shale reduced significantly. However, for oil-based fluids, nanoparticles did not have such good performance led to some negative effects such as higher filtration larger Young s modulus reduction. The authors attributed this behavior to fact that silica nanoparticles can easily disperse in water-based fluids, effectively prevent filtrate from invading into shale by plugging pore throats. However, it is difficult for NP to disperse in oil-based fluids, thus decreasing ir effectiveness.

21 Energies 2017, 10, Wellbore Strengning One leading causes non-productive time in is wellbore instability. This may lead to lost circulation stuck pipe. It can create significant problems particularly when through depleted formations or in deep water environment where operational windows may be very small. To prevent formation fracturing while, a good practice that has evolved over past years is a preventive technique, usually called wellbore strengning. This involves pumping material downhole, with aim to have material enter or block, eir entrance subsurface fractures (Figure 19a), or enter inside fracture block fracture itself (Figure 19b). In eir way, y are stopping any potential fracture from propagating, achieving this by isolating it from wellbore [58,59]. Several types nanoparticles have been tested as wellbore strengning Energies materials 2017, 10, 540 with good results so far, including field applications Figure Figure 19. A 19. fracture A fracture is is quickly quicklysealed sealedby by wellbore strengning material material isolating isolating it from it from wellbore, wellbore, eir eir at at entrance entrance fracture fracture [58] (with [58] permission (with permission from SPE, from 2015); SPE, or 2015); byor entering by entering fracture fracture [59] (with [59] permission (with permission from SPE, from 2010). SPE, 2010). Nwaoji et al. [60] introduced a new lost circulation material (LCM) fluid blend. They Nwaoji et al. [60] introduced a new lost circulation material (LCM) fluid blend. They aimed at testing ability blend to achieve wellbore strengning by running hydraulic aimed at testing ability blend to achieve wellbore strengning by running hydraulic fracture fracture experiments on Roubidoux sstone impermeable concrete cores. Optimum experiments on Roubidoux sstone impermeable concrete cores. Optimum concentration concentration stard LCM (graphite) in-house prepared nanoparticles (Iron III hydroxide stard LCM (graphite) in-house prepared nanoparticles (Iron III hydroxide calcium calcium carbonate) were established. The authors concluded that optimum blend iron III carbonate) were established. The authors concluded that optimum blend iron III hydroxide hydroxide nanoparticle graphite increased fracture pressure by 1668 psi or by 70% over nanoparticle graphite increased fracture pressure by 1668 psi or by 70% over unblended unblended water based mud. These additives had moderate impact on mud rheology. They also water based mud. These additives had moderate impact on mud rheology. They also found that found that optimal blend by using calcium carbonate nanoparticles graphite increased optimal blend by using calcium carbonate nanoparticles graphite increased fracture pressure fracture pressure by 586 psi or by 36% over unblended invert emulsion (diesel oil) mud with by 586 psi or by 36% over unblended invert emulsion (diesel oil) mud with moderate impact moderate impact on mud rheology. A 25% increase in fracture pressure over unblended mud was on mud rheology. A 25% increase in fracture pressure over unblended mud was achieved in achieved in impermeable concrete core thus confirming applicability designed fluid in shale impermeable concrete core thus confirming applicability designed fluid in shale wellbore wellbore strengning. Finally, four field muds were studied with without addition strengning. Finally, four field muds were studied with without addition nanoparticles. nanoparticles. It was found that addition nanoparticles reduced fluid penetration into It was found that addition nanoparticles reduced fluid penetration into Atoka shale by Atoka shale by 16 72% into Gulf Mexico shale from 17 to 27% % into Gulf Mexico shale from 17 to 27%. Contreras et al. [61,62] applied in-house prepared different NP (NP1 NP2) used m Contreras et al. [61,62] applied in-house prepared different NP (NP1 NP2) used m at low concentrations toger with graphite aiming at wellbore strengning. They have tested at low concentrations toger with graphite aiming at wellbore strengning. They have tested materials in shale in sstone cores. The results indicated that wellbore strengning reached materials in shale in sstone cores. The results indicated that wellbore strengning reached a maximum value 30% when NP2 graphite were used in shales, while maximum fracture a maximum value 30% when NP2 graphite were used in shales, while maximum fracture pressure increased by 20% upon addition NP1 graphite. In sstone cores using NP2 pressure increased by 20% upon addition NP1 graphite. In sstone cores using NP2 graphite resulted in a maximum wellbore strengning value 65% whereas a maximum fracture graphite resulted in a maximum wellbore strengning value 65% whereas a maximum fracture pressure increase 39% was observed upon use NP1 graphite. The differences for ir pressure increase 39% was observed upon use NP1 graphite. The differences for ir performances in both cases were attributed to different capabilities in decreasing filtration performances in both cases were attributed to different capabilities in decreasing filtration between two NP as well as due to viscosity resulting blends. The predominant wellbore between two NP as well as due to viscosity resulting blends. The predominant wellbore strengning mechanism was identified attributed this to tip resistance by development an immobile mass. The authors also noticed that a thin seal was created along fracture plane with a homogeneous NP distribution, while bulk shale formation was not found as being invaded by NP. These are some few attempts aiming at using nanoparticles as wellbore strengning materials, with good results so far. What it remains to be done is to identify best

22 Energies 2017, 10, strengning mechanism was identified attributed this to tip resistance by development an immobile mass. The authors also noticed that a thin seal was created along fracture plane with a homogeneous NP distribution, while bulk shale formation was not found as being invaded by NP. These are some few attempts aiming at using nanoparticles as wellbore strengning materials, with good results so far. What it remains to be done is to identify best nanoparticles which fer effective wellbore strengning under severe downhole conditions. 5. Cutting Lifting Capacity Cuttings Suspension One most important functions mud in operations is to transport drilled cuttings to surface through well bore annulus this called lifting or carrying capacity. There are several factors that affect mud lifting capacity including rheological prile flow rate mud, particles settling velocities, particle size size distribution (geometry, orientation concentration), drill bit penetration, rotary speed, mud density, annulus inclination, drill pipe position in well bore (eccentricity) axially varying flow geometry [63]. Effective cuttings transport remains a major problem especially in vertical inclined wells, where cuttings tend to settle at bottom side borehole due to gravitational force. Samsuri Hamzah [63] investigated using multiwall carbon nanotubes (MWCNTs) as an additive to increase carrying capacity water-based mud. They aimed to study effect different concentrations MWCNTs used, cutting size mud annular velocity on mud lifting capacity. They found that lifting capacity increased as amount MWCNTs increases. They observed that low concentrations MWCNTs ( % volume) had a minimal impact on cuttings recovery. For example, cutting recovery increased about 5 15% when 0.005% volume MWCNTs was added to water based mud, depending on cutting size annular velocity. For 0.01% volume MWCNTs added, cutting recovery increased by 5 21%. They attributed this enhancement to fact that MWCNTs improved stability against base mud, since surface forces balanced gravity force resulting in increase drag force acts to drill cuttings, which led to easily cutting lifting to surface. They also concluded that multiwall carbon nanotubes improved viscosity which significantly increased carrying capacity mud. Many fluids are thixotropic; ability fluids to form a gelled structure over time when not subject to shearing n to liquefy when agitated. This gelling behavior aids suspension cuttings while fluid motion is stopped. A fluid must be able to transport cuttings under dynamic conditions suspend m under static conditions. Gel strength is one most important fluid properties because it reveals ability mud to suspend cuttings weighting materials when circulation is ceased is measured with a viscometer after varying lengths static conditions (generally at 10 s 10 min). Although gel strength is a crucial property for optimal operations, it is ultimately a compromise; it should be carefully monitored since it is directly related to pressure is needed to break gels when fluid circulation is reestablished. Excessive gel strength can also lead to retention at surface, which in turn can cause severe problems such as ineffective solids control, fracturing formation, fluid loss. Low gel strength values indicate that fluid will not efficiently suspend cuttings leading to build-up cuttings bed within bore path resulting in an increased possibility for stuck drill pipe. Several researchers have examined gel strength values various nano-enhnaced fluids in order to evaluate ir capacity to suspend cuttings. Amanullah et al. [1] tested three nano-based fluids against bentonite mud y observed that nano-based fluids exhibited a flat type gel strength prile compared to progressive type gel strength micro-sized bentonite-based mud (Figure 20). They concluded that superior functional behavior nano-based fluids in terms development adequate gel strength will allow homogeneous distributed suspension cuttings within fluid column without causing any accumulation drill cuttings in critical borehole areas. This, in turn, will eliminates

23 Energies 2017, 10, problems such as hole pack-f, pipe sticking, bridging cutting beds formation in horizontal extended reach wells. They also stated that flat type gel strength nano-based fluids will also ensure requirement lower circulation pressure to restart operation thus will aid reduction Equivalent Circulation Density (ECD), induced loss circulation or Energies 2017, 10, problems. Figure 20. Comparison gel strength bentonite-based nano-based fluids [1] (with permission from SPE, 2011). Contreras Vryzas et al. et al. [21] [14] studied investigated gel strength gel strength prile values nano-based oil based-mud fluid containing wt wt %, % custom-made Fe3O4 1 wt % 2.5 wt % NP calcium against bentonite-based iron nanoparticles fluid at before different after concentrations rmal aging glide at 177 graphite C (350 F) as afor conventional 16 h. Their lost results circulation agree with material se (0.5 reported wt % from 2 wt Amanullah %). They noticed et al. [1] that revealing samples flat containing type gel calcium strength NP prile almost doubled nanluids. ir gel Before strength aging, values base compared fluid showed to control a 10 s sample gel strength at 10 svalue 10 min, 5.74 while Pa (12 lbs/100 samples ft 2 ) with10 iron min NP value showed tremendous Pa (31 lbs/100 increases ft 2 ). in After aging gel strength 10 values s 10 min 10 min gel with strength almost values no change were slightly in values decreased obtained to be at s. Pa Finally, (10 lbs/100 y stated ft 2 ) that 11.0 addition Pa (23 lbs/100 graphite ft 2 ), did respectively. not significantly The NF impact before aging gelshowed strengthvalues fluids 3.35 containing Pa (7 lbs/100 ironft NP, 2 ) while 4.31 itpa had(9 albs/100 moderate ft 2 ) impact for in 10 s fluids 10 min containing gel strength, calcium respectively, NP. while after aging 2.87 Pa (6 lbs/100 ft 2 ) 3.83 Pa (8 lbs/100 Vryzas ft 2 et ). The al. [21] authors studied also stated gel strength that prile gel strength nano-based values base fluid fluid containing were significantly 0.5 wt % custom-made higher than its Feyield stress values, while nanluid showed opposite behavior showed 3 O 4 NP against bentonite-based before after rmal aging at 177 C (350 F) for decreased 16 h. Their gel strength results agree values with compared se reported to yield fromstress. Amanullah This complex et al. [1] behavior revealingwas attributed flat type gel to strength ability prile NP to reduce nanluids. progressive Before aging, gel structure, base fluid which showed is mainly a 10caused s gel strength by bentonite value 5.74 particles, Pa (12thus lbs/100 promoting ft 2 ) stability. 10 min value Pa (31 lbs/100 ft 2 ). After aging 10 s 10 min gel strength Abdo values Haneef were[31] slightly studied decreased effectiveness to be 4.79 Pa reducing (10 lbs/100 particle ft 2 ) size 11.0distribution Pa (23 lbs/100 clay ft 2 ), respectively. material (ATR) The NF before its incorporation aging showedas values 3.35 fluid Pa additive. (7 lbs/100they ft 2 ) performed 4.31 Pa (9gel lbs/100 strength ft 2 ) for measurements 10 s in 10 minfluids gel strength, containing respectively, different while concentrations after aging 2.87 ATR Pa nano (6 lbs/100 (2 g, 4 ft g, 2 ) 6 g Pa g) (8 lbs/100 y noticed ft 2 ). Thethat authors newly also stated developed that nanluids gel strength displayed valuesoptimal base gel fluid strength werevalues, significantly which higher is essential thanfor itsavoiding yield stress many values, severe while problems. nanluidat showed low concentrations opposite behavior ATR nano (2 showed g) y decreased showed an gel increase strength between values compared 10 s to10 min yield gel stress. strength This values complex 88%, behavior while was y attributed observed to maximum ability increase NP to reduce gel strength progressive value upon gel structure, addition which 8 g isatr mainly nano caused (+280%). by bentonite particles, Ismail thus et al. promoting [45] examined stability. gel strength values water-based ester-based fluids upon Abdo addition Haneef different [31] studied concentrations effectiveness multi-walled reducingcarbon particle nanotubes size distribution (MWCNTs) at clay different material temperatures. (ATR) The its results incorporation revealed asthat in water-based fluid additive. They fluid, performed gel strength gel strength was measurements not much affected in by fluids different containing concentrations different concentrations MWCNTs. In ester-based ATR nano (2 g, 4 g, fluid, 6 g gel strength 8 g) y was slightly noticed that increased newly as MWCNTs developed concentration nanluids displayed increases. optimal They also gelnoticed strength values, significance which is essential temperature for avoiding at rheological many severe properties problems. produced At low samples. concentrations They found ATR that nano gel (2strength g) y in water-based fluid was decreased with an increase in temperature. However, ester-based fluid showed opposite behavior with increased gel strength values as temperature increases. They concluded that optimal rheological as well as filtration properties were obtained at higher concentrations MWCNTs (0.1 ppb = wt %). 6. Thermal Properties

24 Energies 2017, 10, showed an increase between 10 s 10 min gel strength values 88%, while y observed maximum increase gel strength value upon addition 8 g ATR nano (+280%). Ismail et al. [45] examined gel strength values water-based ester-based fluids upon addition different concentrations multi-walled carbon nanotubes (MWCNTs) at different temperatures. The results revealed that in water-based fluid, gel strength was not much affected by different concentrations MWCNTs. In ester-based fluid, gel strength was slightly increased as MWCNTs concentration increases. They also noticed significance temperature at rheological properties produced samples. They found that gel strength in water-based fluid was decreased with an increase in temperature. However, ester-based fluid showed opposite behavior with increased gel strength values as temperature increases. They concluded that optimal rheological as well as filtration properties were obtained at higher concentrations MWCNTs (0.1 ppb = wt %). 6. Thermal Properties Designing stable fluid systems with high rmal conductivity optimal cooling properties for in deep oil gas reservoirs under extreme downhole conditions (HP/HT) is a major challenge. Drilling fluids with optimal heat transfer properties are highly desirable as operations cause excessive heat due to friction between bit rock surface. Overheating equipment can lead to severe problems with direct impact in cost efficiency operations. Therefore, it is required to formulate muds with excellent heat transfer capabilities. The rmal properties nanluids that can be used in various industrial applications as well as associated mechanisms contributing to enhancement in rmal conductivity, including role Brownian motion, interfacial resistance, morphology suspended nanoparticles aggregating behavior, have been well reported in or studies [64,65] will not repeated here. Here we will review efforts made by several researchers over last years related to investigation rmal properties various newly formulated fluid systems containing different nanoparticles. William et al. [41] examined effect addition CuO ZnO nanoparticles on rmal electrical properties water-based fluids. The nano-enhanced fluids were prepared at various NP concentrations (0.1, wt %) in a xanthan gum aqueous solution (0.4 wt %) as base fluid. The authors observed that nano-enhanced water-based mud (NWBM) showed improved rmal electrical properties by about 35% compared to WBM. Increasing concentration nanoparticles enhanced electrical rmal properties fluids even more. The NWBM based on CuO nanluids were found to show improved rmal properties were more resistant to HP/HT conditions than ZnO-based NWBM. The increase in rmal conductivity ZnO nanluids was found to be from 12% to 23%, while CuO nanluids showed an enhancement in rmal conductivity by 28% to 53%. The authors also stated that increased rmal conductivity nano-based fluids indicates ability mud to cool faster as it moves up to surface, which is really significant when dealing with HP/HT environments. Ponmani et al. [42] developed nano-enhanced containing CuO ZnO nanoparticles at various concentrations (0 0.5 wt %) in various base fluids, such as xanthan gum, polyethylene glycol (PEG-600), polyvinylpyrrolidone (PVP). The results were compared to micrluid-enhanced mud in order to reveal effect particle size (Figure 21). The authors observed that enhanced rmal conductivity properties were achieved when nanoparticles were added compared to micron-sized materials that higher concentrations nanoparticles promoted better rmal conductivity properties. The authors also noticed that system contained PEG-600 showed low rmal conductivity compared to or materials this was attributed to fact that PEG-600 is highly viscous in nature, nanoparticles may get entrapped in its microstructure network-forming aggregates.

25 enhanced rmal conductivity properties were achieved when nanoparticles were added compared to micron-sized materials that higher concentrations nanoparticles promoted better rmal conductivity properties. The authors also noticed that system contained PEG-600 showed low rmal conductivity compared to or materials this was attributed to fact that PEG- 600 is highly viscous in nature, nanoparticles may get entrapped in its microstructure networkforming Energies 2017, 10, aggregates. Figure 21. Variation Variation rmal conductivity nano-enhanced water-based mud mud (NWBM) micrluid-enhanced water-based mud mud (MWBM) for for CuO CuO particles; for for ZnO ZnO particles. Unfilled symbols: NWBM, filled symbols: MWBM [42] (with permission from SPE, 2016). Sabbaghi et al. [66] synsized TiO 2 nanoparticles with sol-gel method incorporated m into a bentonite base fluid in order to enhance its heat transfer properties. The TiO 2 NP were characterized with a particle size analyzer, X-ray diffraction, scanning electron microscopy fourier transform infrared spectroscopy showed an average size 20 mn. The stabilization nano-enhanced fluid was optimized using CTAB surfactant by changing ph visually examined with sedimentation tests. These tests revealed that titania nanluid was stable after one month. The authors also observed that addition NP increased rmal conductivity by about 150% compared to base fluid. They also indicated that at increasing concentrations nanoparticles ( wt %), rmal conductivity was moderately increased. Sedaghatzadeh et al. [67] investigated impact MWCNTs volume fraction, ball milling time, functionalization, temperature dispersion quality (by SEM) on rmal properties water-based mud. The rmal conductivities nano-based fluid were measured with a transient hot wire method. They observed that rmal conductivity MWCNT-based mud increased nonlinearly by increasing volume fraction MWCNTs. They obtained a maximum rmal conductivity enhancement by 23.2% in presence 1 vol % functionalized MWCNTs at room temperature. This was attributed to fact that surface MWCNTs was functionalized with hydrophilic functional groups, this causes nanotubes to disperse more efficiently in water-based mud. The authors also stated that increasing temperatures enhanced rmal conductivity. Finally, y examined rmal conductivity all samples as a function time noticed that rmal conductivity decreased initially n, due to gel strength water-based fluid, levels f. However, this reduction varied for different dispersion methods. Pure ball milled MWCNTs showed highest reduction in rmal conductivity due to agglomeration as explained by authors. Halali et al. [68] studied role CNTs in improving rmal stability polymeric fluids. They stated that optimum formulation sample was achieved by using CNTs, surfactant polymers all toger. They observed that with increasing temperature, rmal conductivity increased that combination CNTs polymethacrylic acid methyl ester (PMMA) exposed highest rmal conductivity. Fazelabdolabadi Khodadadi [69] developed nano-based fluids using functionalized CNTs. CNTs functionalization was performed by applying hydrophilic functional groups onto surface nanotubes via acid treatment. The time evolution rmal conductivity was examined. The rmal conductivity significantly enhanced by 23.2% (1 vol % functionalized CNT) in CNT-water-based fluid at ambient temperature, with improved even furr by 31.8% at 50 C. In

26 Energies 2017, 10, case oil-based fluids rmal conductivity was improved by 40.3% (unfunctionalized) 43.1% (functionalized) 1% volume fraction CNT. Ho et al. [70] investigated incorporation carbon NP at different concentrations (up to 1 wt %) as fluid additives. They used an ultrasonic bath for dispersion NP into base fluid. They concluded that carbon NP enhanced viscosity base fluid as well as its rmal conductivity. Thermal conductivity nanluid increased nonlinearly with increasing mass fraction nanoparticles. They also focused on impact size nanoparticles on nanluid s rmal conductivity. They stated that ball milled nanoparticles had sizes 4 µm averagely. However, authors explained that large particles do not possess Brownian motion anymore as particles approach micrometre size, thus leading to lower rmal conductivity enhancements. In addition, results showed that 0.2 wt % se particles showed higher rmal conductivity than 0.4 wt % 0.6 wt % respectively due to increase distance between particles. Finally, higher nanoparticle volume fractions gave higher rmal conductivity improvement but induced higher settlement nanoparticle cluster sizes in end. Li et al. [71] developed self-assembled silver nanoparticles with an average diameter 5 nm y incorporated m in kerosene-based fluids. They carried out rmal conductivity measurements at three different temperatures (25, C). They concluded that rmal conductivity each silver nanluid was higher than that its base fluid increased nonlinearly with increasing concentration nanoparticles. In addition, enhancement rmal conductivity was greater at higher temperatures which was explained by fact that an increase in temperature led to enhanced Brownian motion particles, which improved rate heat transfer. They suggested that capping surfaces nanoparticles with oleic acid significantly impacted rmal conductivity as oleic acid layer capped on silver cores tended to change so that bare part surfaces was enlarged at higher temperatures. 7. Effect Magnetic Field Engineering a fluid tailored to meet specific downhole environmental dems with tunable rheological properties can revolutionize industry. Such fluids, containing magnetic nanoparticles, could potentially fer in-situ control fluid viscosity yield stress, under application an external magnetic field. They can thus provide a great potential for drillers to formulate fluid systems with instantaneous responses to continuously changing environment, leading to enhanced well control contributing to decreased non-productive time costs. Lee et al. [72] investigated performance fluids containing magnetic iron oxide (Fe 2 O 3 ) nanoparticles that can fer possibility for in-situ control viscosity under application a magnetic field. They tested two different fluids, one based on hybrid particles where NP are embedded in interlayer space bentonite particles or based on a simple mixture nanoparticles bentonite particles. The results indicated that produced fluids have capability to increase viscosity by one order magnitude upon application 0.7 T magnetic field. Vryzas et al. [73] examined novel fluids containing magnetic custom-made (CM) iron oxide (Fe 3 O 4 ) NP at two concentrations (0.5 wt % 1 wt %) for ir potential to be used for in-situ rheological control under application an external magnetic field. They did it under application different magnetic field strengths ranging between T. They concluded that all tested fluids exhibited a typical monotonic increase shear stress apparent viscosity with increasing magnetic field strength. The authors attributed this behavior to strong chain-like structures created between CM Fe 3 O 4 NP that were formed at high magnetic flux densities. The results showed maximum yield stress values upon application 0.7 T with increases up to +386% +609% for 0.5 wt % 1 wt % NP, respectively (Figure 22). Finally, authors reported that developed magnetic nanluids had ability to recover ir original state upon removal magnetic field, reflecting disintegration particles chains because rom movements due to Brownian forces.

27 structures created between CM Fe3O4 NP that were formed at high magnetic flux densities. The results showed maximum yield stress values upon application 0.7 T with increases up to +386% +609% for 0.5 wt % 1 wt % NP, respectively (Figure 22). Finally, authors reported that developed magnetic nanluids had ability to recover ir original state upon removal magnetic field, reflecting disintegration particles chains because rom Energies 2017, 10, movements due to Brownian forces. Figure 22. Yield stress at different magnetic flux densities for fluids that contain wt % CM iron oxide (Fe3O4) 3 O 4 ) NP [73] (with permission from SPE, 2017). 8. Challenges Nanluids Applied research in nanluids is growing at a very fast pace is expected to play a vital role in near future leading to development well performing fluids which can sustain harsh conditions. This review allowed us to report many advantages use nanoparticles as fluid additives for rheological fluid loss control, for enhancing shale stability for wellbore strengning. There are some challenges that researchers should address before y can be fully implemented in applications. Firstly, stability NP dispersions remains a technical challenge is one basic requirements to apply such fluids in field. Furrmore, process used to disperse nanoparticles in a liquid, is a critical factor for an effective dispersion. Researchers have reported various pieces equipment that can be used to disperse solid, dry nanoparticles such as ultrasonic baths, magnetic stirrers, high-shear mixers homogenizers. However even after high shearing, where nanoparticles are broken to ir primary size y tend to re-agglomerate due to strong van der Waals attractive forces, which limits ir advantages stemming from ir high surface area. Electrostatic repulsion or steric hindrance are necessary to overcome such attractive forces form stable dispersions [74]. This can be achieved by adding certain surfactants which are able to create steric barriers between nanoparticles. Lack surfactant can have a negative effect on stability nanluids, as can be seen in Figure 23. It can be observed that aqueous nanluids containing Al 2 O 3 nanoparticles (20 nm) at 0.5 wt % without any surfactant, completely separated after 5 h [74].The primary factors that affect stability such nanluids are particle surface properties, size morphology nanoparticles [74]. Sidik et al. [75] presented a review on challenges nanluids stated that it is impossible to achieve stable nanluids without addition surfactants or without surface modification suspended particles. Choi et al. [76] stated that addition surfactants should be done with extreme care as excessive quantities may adversely affect viscosity chemical stability nanluids.

28 primary factors that affect stability such nanluids are particle surface properties, size morphology nanoparticles [74]. Sidik et al. [75] presented a review on challenges nanluids stated that it is impossible to achieve stable nanluids without addition surfactants or without surface modification suspended particles. Choi et al. [76] stated that addition surfactants should be done with extreme care as excessive quantities may adversely affect Energies 2017, 10, viscosity chemical stability nanluids. Figure 23. Samples α-al2o3 2 O 3 nanluids (without any stabilizer) stability change with time [74] (with permission from Elsevier, 2009). It is well established that NP affect rheological properties fluids at relatively low It is well established that NP affect rheological properties fluids at relatively low concentrations (<0.5 wt %). It is thus critical to find out optimal nanoparticle mass fraction, which concentrations (<0.5 wt %). It is thus critical to find out optimal nanoparticle mass fraction, which will give optimal rheological filtration properties leading to less expensive more efficient will give optimal rheological filtration properties leading to less expensive more efficient operations. The low concentrations NP may eliminate use potentially harmful operations. The low concentrations NP may eliminate use potentially harmful chemicals, currently used in fluids, thus enhancing environmental footprint, chemicals, currently used in fluids, thus enhancing environmental footprint, from use improved nanluids. from use improved nanluids. Anor challenge is field scale applications appropriately developed nano- Anor challenge is field scale applications appropriately developed nano- fluids, with use identified in literature nanoparticles. This can reveal ir full fluids, with use identified in literature nanoparticles. This can reveal ir full advantages advantages also identify challenges in real conditions making possible for developers to focus also identify challenges in real conditions making possible for developers to focus on specific on specific properties problems such fluids. Furrmore, cost some nanoparticles can properties problems such fluids. Furrmore, cost some nanoparticles can be an obstacle be an obstacle that may hinder application such fluids in specific operations in oil gas that may hinder application such fluids in specific operations in oil gas industry. However, industry. However, many types nanoparticles are already commercially available at affordable many types nanoparticles are already commercially available at affordable prices (e.g., iron oxide, prices (e.g., iron oxide, which is abundant in nature in various forms) that can be used instead which is abundant in nature in various forms) that can be used instead many chemicals that are currently used by many oil gas companies. There are numerous research groups that are currently working on scaling up synsis various types nanoparticles in order to render this process economically viable [77,78]. Preparation measurement protocols that were followed by researchers when dealing with formulation testing nano-based fluids are critical challenges as well. Such information is obscure in literature. The American Petroleum Institute (API) procedures specifications [79 81] were developed in order to establish common procedures but sometimes se specifications do not deal with newer additives or newer requirements that are used or needed in different mud formulations that perform difficult tasks under varying conditions [82]. So, it is very difficult to compare results from different researchers laboratories because mixing preparation protocols are almost never same [22]. Factors such as pre-shearing time, hydration additives, raw materials, mixing time order adding different additives are crucial that can significantly affect reported results. Caution should thus be taken when comparing results fluid samples among different, but also even within same, laboratories, because preparation measurement procedures are vital for producing consistent results this is anor challenge facing researchers oil companies in order to take full advantage such superior fluids [22]. 9. Recommendations for Future Work Researchers so far have mainly focused on fluids containing only one nanoparticle type while few studies have been carried out using complete fluid formulations. Hence, furr studies should be attempted focusing on use different nanoparticles in combination with commonly used polymers (for e.g., CMC or PAC). Furrmore, quantification side effects by using nano-based fluids should be fully carried out, e.g., any issues with filter cake removal.

29 Energies 2017, 10, Measurement integration methodology development for full assessment formation damage by any fluids should be carried out as formation damage can cause well integrity problems which may lead to enormous costs. Future studies should focus on interfacial phenomena taking place modes interaction between nanoparticles or fluid particles especially bentonite particles aided by macroscopic measurements, so that we can better underst causes behind good performance nano-enhanced fluids, particularly at HP/HT applications. In-depth characterization produced filter cakes using sophisticated quantitative techniques such as Nuclear Magnetic Resonance (NMR) Magnetic Resonance Imaging (MRI), which can assess formation damage minimization potential any novel fluids along with comparisons against conventional experimental filtration data at HP/HT conditions needs to be developed in order to formulate nano-based fluids with tailor-made properties that can minimize formation damage risks leading to costless more efficient activities. Drilling fluids containing custom-made magnetic nanoparticles, tailored to meet specific downhole environmental dems with tunable rheological properties that could potentially fer in-situ control fluid viscosity yield stress can be investigated by researchers in future. Such fluids have unique ability to rapidly increase viscosity yield stress in presence an external magnetic field thus, fer potential for drillers to formulate use fluid systems with instantaneous responses to continuously changing environment, leading to enhanced well control contributing to decreased non-productive time costs. Several works reported in this review use what we may call, minimal exposure testing developed nanluids, i.e., reporting only high shear rate rheological measurements (PV YP) not full rheogram, as well as only LP/LT API filtration tests. The main aim to develop incorporate appropriate nanoparticles into fluid formulations is to combat harsh conditions high temperatures high pressures, hence fluid loss tests should only be carried out for HP/HT conditions. Furrmore, full rheograms should be tested, because extreme danger with respect to pressure loss evaluation is in annulus region where fluids encounter low shear rates, where PV YP have little meaning. Furrmore, true yield stress fluids should be determined which gives good information regarding cuttings carrying capacity fluids. 10. Conclusions This review has highlighted recent advancements on development fluids using different nanoparticles. Challenges directions for future research are also presented. Based on this critical review following conclusions can be drawn: Nanoparticle shape, size concentration have been identified as driving factors affecting performance nano-based fluids. The major effect use nanoparticles in fluids is significant enhancement fluid loss particularly at HP/HT conditions. This can lead industry to great cost savings. Optimal concentrations reported range at lower than 1 wt %, typically range around 0.5 wt %. Nanoparticles affect rheological properties various water or oil base fluids at different temperatures (up to 300 F) at relatively low concentrations (<0.5 wt %). The reported effects are not detrimental for use such nanoparticles as fluid additives. Nano-enhanced fluids exhibited flat type gel strength prile while maintaining optimal yield stress values, which reveals ir great potential for better cuttings suspension properties as well as improved cuttings lifting capacity fluids. Promising attempts were reported to model modification rheological behavior fluids upon addition nanoparticles at different temperatures, confirming ir potential for modeling complex fluid systems toward commercial application.

30 Energies 2017, 10, Nanoparticles have capability to reduce shale permeability by efficiently plugging pores thus ir use is going to play a vital role for future shale explorations exploitations. Wellbore strengning is possible with use different nanoparticles because reported results proved that nanoparticle-based fluids can lead to increased fracture pressures fering thus more efficient safer activities. Researchers attempted to quantify enhancement fluid rmal properties with nanoparticles for utilization in heat transfer studies flow se fluids in wellbore found that nanoparticles can significantly improve ir rmal conductivity, especially at high temperatures. The incorporation magnetic nanoparticles as fluid additives shows great potential for development smart fluids with in-situ rheological controllability upon application an external magnetic field. Stability cost nanluids should be properly addressed in order for nanoparticles to make substantial impact on fluid industry. Future directions should focus on interfacial phenomena taking place modes interaction between nanoparticles or fluid additives aided by macroscopic measurements, so that researchers can better underst reasons behind such a good performance in order to optimize ir effect. The ability to synsize custom-made nanoparticles by changing ir surface properties or by optimizing ir terminal units in order to accomplish different functional tasks promises to substantially influence lscape fluid industry by developing smarter greener fluids that can aid significantly industry. Conflicts Interest: The authors declare no conflict interest. References 1. Amanullah, M.; AlArfaj, M.K.; Al-abdullatif, Z.A. Preliminary test results nano-based fluids for oil gas field application. In Proceedings SPE/IADC Conference Exhibition, Amsterdam, The Nerls, 1 3 March Borisov, A.S.; Husein, M.; Harel, G. A field application nanoparticle-based invert emulsion fluids. J. Nanopart. Res. 2015, 17, 340. [CrossRef] 3. Taha, N.M.; Lee, S. Nano graphene application improving fluids performance. In Proceedings International Petroleum Technology Conference (IPTC 18539), Doha, Qatar, 6 9 December Friedheim, J.; Young, S.; De Stefano, G.; Lee, J.; Guo, Q. Nanotechnology for oilfield applications Hype or reality? In Proceedings SPE International Nanotechnology Conference, Noordwjik, The Nerls, June Hoelscher, K.P.; Young, S.; Friedheim, J.; De Stefano, G. Nanotechnology application in fluids. In Proceedings 11th Offshore Mediterranean Conference Exhibition, Ravenna, Italy, March Gerogiorgis, D.I.; Clark, C.; Vryzas, Z.; Kelessidis, V.C. Development parameter estimation for an enhanced multivariate herschel-bulkley rheological model a nanoparticle-based smart fluid. Comput. Aided Chem. Eng. 2015, 37, Reilly, S.I.; Vryzas, Z.; Kelessidis, V.C.; Gerogiorgis, D.I. First-principles rheological modelling parameter estimation for nanoparticle-based smart fluids. Comput. Aided Chem. Eng. 2016, 38, Gerogiorgis, D.I.; Reilly, S.I.; Vryzas, Z.; Kelessidis, V.C. Experimentally validated first-principles multivariate modelling for rheological study design complex nanluid systems. In Proceedings SPE/IADC Drilling Conference Exhibition (SPE/IADC), The Hague, The Nerls, March Vryzas, Z.; Wubulikasimu, Y.; Gerogiorgis, D.; Kelessidis, V.C. Understing temperature effect on rheology water-bentonite suspensions. In Proceedings Nordic Polymer Days Nordic Rheological Conference, Helsinki, Finl, 30 May 1 June 2016.

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