In situ organically cross linked polymer gel for hightemperature reservoir conformance control: A review

Transcription

1 Received: 18 May 2018 Revised: 7 August 2018 Accepted: 28 August 2018 DOI: /pat.4455 REVIEW In situ organically cross linked polymer gel for hightemperature reservoir conformance control: A review Zulhelmi Amir 1,2 Ismail Mohd Said 1 Badrul Mohamed Jan 2,3 1 Department of Petroleum Engineering, University Technology Petronas, Bandar Seri Iskandar, Darul Ridzuan, Perak, Malaysia 2 Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia 3 Center for Energy Science, Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia Correspondence Ismail Mohd Said, Department of Petroleum Engineering, University Technology Petronas, Bandar Seri Iskandar, Darul Ridzuan, Perak, Malaysia. ismailsaaid@utp.edu.my Funding information Universiti Teknologi PETRONAS, Grant/ Award Number: YUTP 0153AA H05; Ministry of Education Malaysia; University of Malaya, Grant/Award Number: UMRG RP031B 15AFR; PETRONAS, Grant/Award Number: GR&T 0153CB019 Polymer gel has been established as water blocking agents in oil recovery application. In this practice, a mixture known as gelant is injected into target area and set into a semisolid gel after a certain adequate time. Besides profile modification and water shutoff, the role of the polymer gel in conformance control is to block high permeability regions, before diverting injected water from high permeability to low permeability zones of the reservoir. It is to boost the oil displacement and sweep efficiency. This is the key to improve oil recovery in the heterogeneous oil reservoirs. However, very limited gels are applicable for harsh conditions, especially in high temperature reservoirs. Organically cross linked polymer is 1 of the materials for conformance control at high temperature reservoirs. Many experimental works and field applications have exhibited the potential of this technology. This paper presents a concise review on this polymer gel for conformance control at high temperature wells. Firstly, in situ organically cross linked polymer gel has been introduced, and the reason of the use over other types of polymer gels is summarized. The early studies of organically cross linked gel systems are also discussed, followed by the chemistry and the gelation mechanisms. An extensive review on factors that affect gelation kinetics and field applications is also discussed in some detail. KEYWORDS conformance control, high water cut, organic cross linker, polymer gel 1 INTRODUCTION Numerous high temperature reservoirs have been developed with the exploration of new petroleum fields. Some regions in the world have been known having high temperature reservoirs including in Middle East and South East Asia regions. 1 Besides, thermally oil recovery has been extensively deployed for the development of heavy oil and oil sand reservoirs. It consequently increased reservoir temperature and produced steam channeling. 2,3 An understanding has been founded based on the operational works in dealing with hightemperature reservoirs. Petroleum reservoirs can be categorized as high temperature reservoirs if the temperature is more than 80 C. In fact, the reservoirs can be categorized as extremely high temperature reservoirs if the reservoir temperature is more than 120 C. 2 Besides high temperature, the developed reservoirs have never presented ideal conformance in the course of oil recovery process. Because of these reservoir's characteristics, meticulous polymer gel selection needs to be done especially for conformance control application in high temperature reservoirs. This is due to the pristine polymers especially polyacrylamide and xanthan gum that can deteriorate at temperature higher than 80 C. 4 Similarly, inorganic cross linked polymer gels, as for example with chromium and zirconium cross linker, become ineffective when reservoir temperature is above 80 C. To solve this problem, polymer gels formulated with organic cross linkers, which are polyethylenimine and phenol/formaldehyde, have been broadly deployed in high temperature reservoirs. This selection is contributed by the existence of thermally stable bonds within the organic cross linking polymer gel systems. The gel that is thermally stable is able to maintain a reasonable physical structure for required period of time. Since 2000s, numerous review papers have extensively discussed on the development of several organic polymer gels for conformance control. 5-7 However, we believe that a Polym Adv Technol. 2018;1 27. wileyonlinelibrary.com/journal/pat 2018 John Wiley & Sons, Ltd. 1

2 2 AMIR ET AL. thorough review on the organically cross linked polymer for hightemperature reservoirs, especially on the aspect of gelation kinetics and the factors that affect gelation behaviors, has not been brought up for further discussion. Therefore, the objective of this paper is to provide a review on available cross linked polymer gel systems for high temperature conformance control, highlight from the viewpoint of chemistry and the factors that influence the gelation kinetics, and at the end summarize the field application with this polymer system. This review comes with an intention to increase the understanding on organically cross linked polymer gel for hightemperature conformance control. 2 RESERVOIR CONFORMANCE PROBLEMS Conformance problems are related to problems that occur during water flooding with poor sweep efficiency, which resulted in a pocket of oil uncovered and left behind. 8,9 Heterogeneous or layered oil reservoir formation with high permeability channel, which exists midway vertically within the matrix rock reservoir has been identified as the most prevalent feature for this problem. 10 When injecting water in heterogeneous reservoir, water flows to the wellbore through its easiest own path to the regions with relatively higher permeability. Hence, the oil is bypassed by water in high permeability region. It leaves unsweep hydrocarbons in regions with relatively lower permeability. 11 Excessive water production is also related to conformance problems. 12 It becomes a big challenge since it hinders oil production. In many mature reservoirs which had been water flooded for a long time, water cut can easily rise above 90%. 13,14 High water cut does not only mean less produced oil, but it may also cause other significant challenges such as lead corrosion, scale buildup, and sand and fine production consequences. Additionally, high water production increases operational costs for water treatment, complex water/oil separation, transfer, and discharge or reinjection facilities. 15 In order to keep away from water contact, the producing regions are often aborted even though the intervals still hold large amounts of recoverable hydrocarbons. Therefore, controlling water production as a conformance control has been a most important objective for the oil and gas industry. Conformance control has been defined as a treatment measure of excess water production, and it is associated with sweep efficiency improvement during oil recovery operation. 13,16 Vast majority of conformance control treatments are performed by reducing the permeability of the reservoir with high fluid flow paths, channels, and conduits. 17,18 A proper design of conformance improvement treatment increases the effectiveness and profitability from the particular oil recovery operation. 8 The rate of the recovered reservoir oil increases, and the amount of oil recovery drive fluid (eg, water) that will be coproduced with recovered oil decreases, when the sweep profiles and the degree of conformance are improved. Conformance control treatments normally do not uphold reductions in residual oil saturation. Conformance improvement ought to be performed to reservoirs that have substantial and economically feasible amount of moveable oil to be recovered. 13,18 In order to minimize the water production during the recovery process, 3 common ways have been established in the industry. 14 They are increasing the viscosity of the flooding fluid, reducing the permeability of high permeability zones, and increasing the permeability of low permeability regions. 2 Among those techniques, reducing the permeability of high permeability zones gives better results. This method is basically used to block the least resistance pathways between the injected water source and production wells. 14 This method is also using less amount of material that is economically effective especially within the time of low oil prices. 2,19 Some methods used to block high permeability zones which vary from chemical to mechanical treatments, or combinations of both, have been implemented in the industry. Initial trial to reduce water production involved mechanical isolation, cement squeezing, solid slurry injection, and oil water emulsion. Yet, more positive results have been achieved with chemical methods, such as in situ polymerized solutions, crosslinked polymer gels, and silicate based gel systems. 20,21 Foam and gel systems are broadly deployed for their attributes of inexpensive and modest preparation. However, the effective time of foams is fairly short, and on top of that, the equipment and injection processes are relatively complicated. Yet, gel systems retain many beneficial traits over other methods; for instance, controllable gelation time, adjustable gel strength, the flexibility for pumping without a workover rig, indepth penetration, long effective time, low cost, and easy to remove from the wellbore by water recirculation. 20,22-24 Therefore, gel systems are widely deployed as a chemical water shutoff method for oil wells POLYMER GEL TREATMENT In making a decision on the selection of the chemical agent, it depends on the reservoir lithology, reservoir temperature, brine salinity, ph of the formation and mixing brines, and hydrogen sulfide (H 2 S) content. 25,26 Polymer based gel agent, for example, is the most common technology that is available for conformance control in the past decades. 20,27 Polymer gel has a unique versatility that is able to make hard and soft elastic gels when compared to other chemicals such as sodium silicate. In addition, compared to other methods, it is low cost, easy to pump, and has better penetration into the rock matrix. That is the reason why it is more popular in the 1970s and 1980s even though other materials such as sodium silicate were the first plugging and permeability modification technology that is available in the market. 28 These polymer gels were designed to propagate through fractures and layers with high permeability and high water saturation zones. It helps in creating a barrier to prevent water penetration and consequently to reduce the flow of unwanted water in the reservoir. 29,30 As a result, the injected fluids are able to flow through the formerly unswept low permeability zones resulting in increased oil production. 6,13 In situ gel treatment is 1 of the most popular conformance control methods used in the oil industry. In situ gels were reported in oil industry studies as early as late 1950s. In 1970s, Phillips Co. (now ConocoPhillips) applied the first in situ polymer gels using partially hydrolyzed polyacrylamide (HPAM) and aluminum citrate for conformance control. 31 Since then, research into in situ gel systems has received considerable attention. These gel systems usually start with

3 AMIR ET AL. 3 2 main components, which are high molecular weight polymer and cross linkers. The cross linking agent starts to connect chemically itself to 2 polymer molecules and linking them together by internal or external triggers. This reaction depends on temperature, time, and concentration of each component. The result is a 3 dimensional structure of interconnected polymer molecules that is not behaving like a fluid and eventually established an immobile rigid gel. 8 Additives are added to control gelation time and adjust gel strength and thermostability. Since the 1970s, there have been innumerable in situ gel systems developed for conformance control. 9 Depending on the type of cross linker, the cross linked polymers can be categorized as metallic or organic cross linked systems. Popular inorganic cross linked polymer gel includes polymer gel that use trivalent cations aluminum (Al 3+ ) and chromium (Cr 3+ ) as the crosslinkers Despite the versatility of this type polymer gel, limited penetration depth, low thermal stability, high cost material, and polymer precipitation under harsh reservoir conditions are some of the issues. 28 Inorganic cross linkers are normally produced through ionic bonds of inorganic ions cross linked with the carboxylate groups of polymers. This bond is unstable at high temperatures, causes lower thermal stability. Consequently, metallic cross linker for gelation of some polymers is limited to low temperature around 60 C, which prevents it from being the good agent for conformance control at hightemperature reservoirs. 36,37 It also gives pumping problems because of the short gelation time. In addition, because of a large number of uncross linked amide groups, it degrades the polymer molecules at high temperatures. 38 Furthermore, the overcross link of excess carboxylate groups on the polymer backbone with metallic cations results in syneresis and precipitation of polymer gel. 21 For example, chromium based gel systems have tendency to hydrolyze and precipitate in high ph and high temperature conditions. Finally, the important concern of the application of metallic cross linker polymer based gel is its environmental and safety issues. For instance, chromium(vi) is environmental toxic and carcinogenic material. 9 4 ORGANICALLY CROSS LINKED POLYMER GEL AND ITS MOTIVATION Organically cross linked polymer gel is another type of the crosslinked gel system. For elevated temperature application, this type of polymer gel is preferred over metallic cross linked because of their thermal stability, of which the formed gel is stable in high temperature reservoirs. 5,6,39-42 The covalent bonding formed between amide groups of the polymer and organic cross linker renders stable bonding at high temperatures compared to weaker coordinated ionic bonding in inorganic cross linked gel. 6,43 Generally, the cross linking process relies on the polymer side rather than on the chemistry of the crosslinkers. Polyethylenimine (PEI) and combination of phenol and formaldehyde are the common organic cross linkers used for polymer gel. 44 The cross linking process of synthetic polymer gels with organic cross linkers is activated by the temperature of the well. The crosslinking rate is dependent upon temperature, salinity, ph, and concentration of polymer and cross linker. The organically cross linked polymer system offers the following advantages as has been listed by Vasquez and Eoff. 45 First, the viscosity of organically cross linked polymer gel is very low that it can propagate deeper easily into the matrix of the reservoir. The viscosity of organically cross linked polymer gel is predictable that it can be used to improve diversion over long treatment intervals. At temperatures up to 177 C, the gelant solutions have sufficient pumping times to reach target zones before transformations from liquid to a gel network. This transition time which is the gelation time can be controlled with various factors such as polymer or cross linker concentration, ph, and salinity. It also has thermal stability up to 19 C. Moreover, the formed organically crosslinked gel system is not sensitive to formation fluids, lithology of the rock, or heavy metals. Organically cross linked polymer gel is also capable in reducing water permeability. It has adequate strength to withstand the pressure inside the wellbore and blocking water and gas flow. Based on experimental data, the system provides adequate strength in resisting differential pressures of at least 3600 psi. 5 TYPES OF ORGANICALLY CROSS LINKED POLYMER GEL 5.1 Phenol formaldehyde cross linker A conventional organically cross linked gel is the gelation of acrylamide based polymers with phenol and its derivatives together with formaldehyde as cross linker. The gelation occurred through dehydration condensation reactions that forms C N bonds. 2,46 The reactions are between the amide groups on the polymer chains with phenolformaldehyde reaction products, such as o hydroxymethylphenol or salicyl alcohol. 47 In 1985, Chang et al has introduced a gel system that is reliable for high temperature conditions. The gel system is a synthetic thermosetting material based on the cross linking reaction between resorcinol formaldehyde and acrylamide based polymer. 48,49 It has been patented under the trade name Flowperm 325 by Pfizer company. The viscosity of this gel system is very low, which is advantageous for gel propagation. The gelation time also can be prolonged to 10 days at temperature up to 105 C. However, this gel system is difficult to be deployed at the field due to its sensitivity to salinity, and the gelation hardly appears at ph values below Furthermore, a report shows that the application of the phenol formaldehyde gel system is effective in the fields with reservoir temperatures not higher than 100 C. 50 The activation temperature of phenolformaldehyde cross linked with acrylamide based polymer is range only in 70 C to 80 C. 51 Yet, at temperatures as high as 140 C, the formulation and operational process need to be improved. This can be obtained by the addition of other monomers into the polymer structure, with the function to curb thermal hydrolysis of the acrylamide groups. The combination of phenol formaldehyde with other polymers can make the gelation occur at a ph more than 9, and withstand the temperature up to 149 C. 47,52,53 For example, thermally stable acylamide based polymers dissolved in seawater and cross linked with phenol and formaldehyde produced gel that is very stable and hardly to be syneresis. 52 Gel syneresis is the process of liquid of the gel condensed out from its network structure and shrinks the volume. In 2000, Zhuang et al has developed sulfomethylated resorcinol

4 4 AMIR ET AL. formaldehyde (SMRF) system to improve the traditional phenolformaldehyde cross linked gel system in terms of the tolerance on the salinity and ph. 50 As low as ph value of 5, SMRF system is able to delay its gelation time over wide ph ranges. The phenol formaldehyde resin/hpam gel is known to have advantage and suitable in plugging oil and gas reservoirs containing high H 2 S. 51,54 Nevertheless, the toxicity of phenol and the carcinogenic character of formaldehyde render this organic cross linking system unfeasible. The hazard rating for the both cross linkers has brought environmental concerns, and the safety of workers handling these chemicals requires some additional care. In addition, portioning of phenol into the crude oil inhibits the propagation of the gelants. 44 To overcome these issues, Moradi Araghi in 1994 has found the other cross linkers that can produce stable gels but with lower hazard rating as replacements for phenol and formaldehyde. 55 The alternative for phenol includes o and p aminobenzoic acids, m aminophenol, phenyl acetate, phenyl salicylate, salicylamide, salicylic acid, and furfuryl alcohol. On the other hand, hexamethylenetetramine (HMTA) is the only cross linker that can be used as replacement for phenolformaldehyde. 51, Hexamethylenetetramine cross linker In 1996, Hutchins et al has established a new type of organic crosslinked gel system that is applicable for gel treatments in elevated temperature. 57 They reported that the gel system contains hydrolyzed polyacrylamide (HPAM) and the combination of hydroquinone (HQ) and hexamethylenetetramine (HMTA) cross linkers. The mixture is dissolved in seawater with 2 wt% of sodium bicarbonate. To remove the necessity for the use of expensive thermally stable polymers, the simple polyacrylamide has been used for this gel system. The function of sodium bicarbonate is to control the ph and to soften the water by precipitating the divalent cations existing in seawater. The results show that type cross linked gel is stable for 12 months at 149 C and 5 months at C. In some deployment into the well, the treatments have involved the use of this gel as replacement of cement because of its improved strength, longer lifetime, and lower cost. The treatments were effective in reducing produced water and increasing oil production. For example, the treatments have dealt with water control in the gas wells located in New Mexico and Canada, where the bottom hole temperature is more than 110 C. The job resulted in up to 65% reduction in water production with very significant increment in gas production. In 1997, Dovan et al has developed another HQ HMTA crosslinking system that can be applied at higher temperatures. 58 They described from their laboratory works the gelation of polyacrylamide with a number of combination new cross linkers comprising HMTA, terephthalic acid, terephtalaldehyde, and glutaric acid as primary cross linkers, while hydroquinone, dihydroxynaphthalene, and gallic acid are used as secondary cross linkers. The primary cross linkers are able to produce gels with the polymer by building gelation network themselves, even though the produced gels are unstable. The secondary cross linkers function by strengthening the primary cross linked produced gels at high temperatures. The gelation process in this system occurs at slow rate, thus suitable for treatment at very hot wells. At temperature of C, the terephthalaldehyde cross linked gel system took several days for gelation process. Although experimental data by Dovan et al showed high thermal stability, this gel system has not yet been tested in the field due to the economic issues. There is also recent investigation of 4 kinds of phenolic compounds, specifically, phenol, catechol, resorcinol, and hydroquinone, investigated separately, with HMTA as the organic cross linker system by Zhu et al. 38 They found out that there is limited chance to form network structures on HMTA phenol cross linked gel. Phenol has only 4 hydroxyl groups, which allows only a small number of polymeric chains to cross link with cross linker. The formed gel with HMTA phenol has large grid size, less viscous, and small value of gel strength. However, the big amount of uncross linked polymer that dissolved in aqueous solutions is susceptible to high temperature. They will be unable to form visible bulk gels and will be degraded easily after a few days at high temperatures. The study has also reported that phenol can be replaced by catechol, resorcinol, and hydroquinone. 38 The gelant is able to form visible bulk gels at high temperatures, when the cross linkers are combination of HMTA with catechol or hydroquinone. However, the disadvantage of using these cross linkers is the unpromising gel strength. Another study is the gelation mechanism of HMTA resorcinol cross linked gel system. 38,59 HMTA will thermally decomposed to NH 3 and formaldehyde slowly under high temperature and acidic conditions. NH 3 forms ion complex with the present of heat to prevent NH 3 gas to be absorbed. Then, the formaldehyde will react with resorcinol to form hydroxymethyl resorcinol. Finally, formaldehyde and hydroxymethyl resorcinol will make polycondensation reaction with the accessible amide groups on polyacrylamide backbone to produce 3 dimensional gel structure. The reaction equations are summarized in Figure 1. Resorcinol is having larger number of hydroxyl groups compared with phenol. It leads to more amide groups in the polymer that can be cross linked with those hydroxyl groups. The gel also gives higher value of storage modulus or gel strength, compared to the gel systems with combination of HMTA and other phenolic resins. The laboratory observation also found that the grid size of the formed HMTA resorcinol based polymer gel is small and the dendritic structures are distributed between the pores of grids. The structures make polymer gel able to strongly hold water in the reservoir, even under extreme temperature conditions up to 240 C. 5.3 Polyethylenimine cross linker Morgan et al has initiated a stable gel system at elevated temperature that is established by cross linking between polyacrylamide copolymer t butyl acrylate (PAtBA) with polyethylenimine (PEI). 60 This system is unique because it relies on the polymer to delay the cross linking, rather than on the chemistry of the cross linker. 60 PAtBA has 4.7 mol% of tert butyl acrylate (tba) groups that indicate the degree of esterification of PAtBA. The covalent bonds always encompass the amide groups on the polymer chain. Two mechanisms have been proposed for this cross linking: first, a transamidation reaction at the amide group of PAtBA side and second, a nucleophilic affect by imine nitrogen on the carbonyl carbon of tba part. These mechanisms are shown in Figures 2 and 3 respectively. The proposed gelation

5 AMIR ET AL. 5 FIGURE 3 Cross linking reaction through a nucleophilic affect. 62 Figure from Al Muntasheri et al 29 FIGURE 1 Summary of cross linking reactions between resorcinolhexamethylenetetramine hydrolyzed polyacrylamide. Figure from He et al 59 mechanism by Hardy et al incorporates the formation of covalent bonds by a nucleophilic attack by imine nitrogen from PEI on carbonyl carbon at the ester group of tba. 62 On the other hand, Reddy et al has suggested the second cross linking mechanism between PAtBA and PEI is through a transamidation reaction in which the imine nitrogen in PEI replaces the amide group at the carbonyl carbon of PAtBA. 61 This transamidation reaction is also believed as the gelation mechanism of PAM/PEI cross linked gel. 29,63 PAM has more amide groups than carbonyl groups that (are) available to cross link with PEI, thus increase the cross linking network. In both mechanisms, the hydrolysis of the polymer plays a vital role during the process of forming gel. Under high temperatures and high ph environments, PAtBA and PAM undertake hydrolysis process that produces partially hydrolyzed polyacrylamide (PHPA) which contains ammonia (NH 3 ) and carboxylate groups. Meanwhile, the hydrolysis of the tba part yields tertiary butanol (tert BUOH). Then, tba is decomposed thermally producing isobutene and carbon dioxide (CO 2 ) depending on the temperature. With this kind of partially hydrolyzed form and its negatively charged carboxylate groups, hydrolyzed polyacrylamide (HPAM) becomes accessible point to cross link with imine nitrogen in PEI to form gel. In 1998, Hardy et al discovered advantages which include remarkable propagation and thermal stability that help the development of PAtBA/PEI gel for commercial use. 62 They also reported a method to estimate the pumping times and shut in times based on the gelation time and temperature for several stages of a well treatment. It can travel 8 times farther than chromium based systems under same environment. This is because the cross linking reaction of PAtBA/PEI gel is relied upon a nucleophilic attack of amine nitrogen from the PEI on the carbonyl carbon from the t butyl acrylate. The chemical properties and potential of using simple PAM homopolymers to replace PAtBA have also been investigated. Polyacrylamide might be the most popular synthetic polymer. There is no explainable technical justification behind this trend, except the cost effectiveness of PAM compared with PAtBA. Compared to PAtBA, PAM is readily available with relatively low costs and can provide molecular weights in the millions. Because of cost and performance factors, numerous PAM based polymers were also used to form stable gels cross linked with PEI, for instance, with copolymers FIGURE 2 Cross linking reaction through a transamidation reaction. 61 Figure from Al Muntasheri et al 29

6 6 AMIR ET AL. of acrylamide and acrylamido 2 methylpropane sulfonic acid (AMPSA), 64 and mixture of acrylamide, AMPSA, and N,N dimethyl acrylamide, 65 as well as a simple polyacrylamide polymer. 66 Polyethylenimine is not vulnerable to hydrolysis. Gel system with this cross linker was applied broadly in various fields around of the world. 67 In 2003, Reddy et al has introduced an alternative for PEI, which is chitosan based cross linker. 61 Chitosan is from natural resources and has ability to cross link various types of acrylamidebased polymers. Chitosan cross linked partially hydrolyzed polyacrylamide produced gels at the temperature range between 65 C and 88 C. The system showed short gelation time which is 3 hours at temperature of 88 C. AMPS copolymer acrylamide showed longer gelation time, 48 hours at 121 C with the same formulation. They also studied the cross linking of PAtBA with chitosan, and the results showed 5.5 hours of gelation time at temperature of 88 C. Because of chitosan weakness that precipitates at ph of 6, there are no further developments and the subsequent field testing for this cross linked gel. Allison and Purkaple reported the initial study of the PAM/PEI polymeric gel system. They described that PEI can form aqueous gels with simple polyacrylamide at room temperature. 66 It has been continued by recent study by Al Muntasheri et al that indicates the possibility of cross linking a simple PAM with PEI at high temperatures. 63 From their experimental works, the PAM/PEI gel system produces rigid and thermally stable gel at temperature of 130 C for 8 weeks. The performance of the gel in porous media, specifically in Berea cores, has been evaluated as well. The gel effectively reduced the permeability of the core from 47 to 0 md at 90 C. In addition, strength test in high pressure experiments, the gel is able to withstand pressures of 1000 psi at room temperature. 68 The investigation on this gel system is limited on laboratory works, since field data are not available yet for this system. 6 FACTORS THAT AFFECT GELATION KINETICS The process of forming gel is referred to as gelation and is defined as the transition of molecules from local clustering to an extended cluster covering a macroscopic system which is escorted by singularities in the transport properties. 69 The gelation process can be divided to 3 phases which are induction, reaction acceleration, and completed with equilibrium stage. 70,71 In induction phases, the hydrolysis degree and the mobility of the chain of polymers increase as the temperature and pressure increase. The consistency of gel system changes insignificantly in this regime due to only a small amount of covalent bonds form between polymer and cross linker. Then, in the reaction acceleration phases, the cross linking reaction rate increases sharply. The viscosity of gel system increases substantially, and 3 dimensional network structure is formed as the formation of large amount of covalent bonds. Finally, the equilibrium period shows that the viscosity tends to steady. In this stage, the network structure of gel has been stable, but minor cross linking reaction still continues to occur. It takes place in the intramolecular of polymer to solidify the internal structure of gel but does not contribute in increasing the consistency of gel. Polymer gels comprise of 2 elements, which are a liquid phase and a network of long chain molecules. The liquid phase inhibits the network gel to form a compact mass, while the network of polymer molecules sustains the liquid intact. 72 It is accepted that numerous variables contribute to the influence of polymer gel's viscosity, gelation time, and gel strength. There are 6 main variables derived from the literatures which are the temperature of reservoir, initial ph, fluid salinity, polymer concentration, crosslinker concentration, and the present of additives. 41,63,73 Recognizing these combined effects of factors on gelation kinetics is fundamental in order to develop effective or suitable polymer gel for excessive water control. In this subsection, we will shed light on each factor that affect the rheology of organically cross linked polymer gel system. The previous gelation kinetic studies on this type, in situ polymer gel have also been summarized in Tables 1 and Effect of temperature It is worth highlighting that in relation to most of the chemical gel reactions, the temperature may ultimately control the kinetics that initiates the actual gelation. 41 To formulate in situ cross linked polymer gel, the temperature gradient in the reservoir should be taken into consideration as a controlling factor. This is to ensure that it reaches the targeted treatment zone and able to maintain the gel strength during water flooding operation. 94 The gelation time was predicted to decrease, meanwhile the gel strength to increase with temperature. 95 During the gelation process, the rate of cross linking between polymer and cross linker increases with increasing temperature. 41,88 Higher temperature contributes to molecular motion to be accelerated. This phenomenon refers to the high mobility of gel molecules when heating which induces the gelation process to be faster. For example, the condensation reaction of phenol formaldehyde is accelerated in higher temperature. 89 As the temperature increased, the opportunity for effective collision between amide groups of polymers and resorcinol HMTA molecules increased. 89,91,96 Same results have been obtained for the experiment of viscosity evolution of PAM/PEI system. 63 It can be concluded that higher temperatures can increase the hydrolysis degree of polymer, thus accelerates cross linking. As a result, more cross linkers formed, and at the same time, reaction rate enhanced and gelation time shortened. The relationship between gelation time and its temperature can be related to Arrhenius equation: t gel ¼ Ae E a=rt where E a is the activation energy, R is the gas constant, T is the absolute temperature, and A is the pre exponential factor. From the Arrhenius equation shown above, it can be clearly seen that the gelation time will decrease with rising temperature. Hence, the polymerization rate increases which induces the solidification rate of gel. This is consistent with an endothermic type of reaction. 2,97 Generally, the cross linking rate became higher; thus, the gelation time became shorter in high temperature. Beyond of gelation time, experimental temperature also affects the gel strength, which stronger gel will be formed. 2,75 It was found that the equilibrium elastic modulus increases with the increase of temperature, whether the cross

7 AMIR ET AL. 7 TABLE 1 Summary on the literature review on the gelation kinetic studies of polymer gel that organically cross linked with polyethylenimine (PEI) Authors (Year) References Objectives Polymer Methodology Results Remarks Vasquez To develop and evaluate the et al (2005) 65 different combination polymer systems for high temperature conformance as a function of some parameters PAtBA AMPSA N,N DMA Temperature: up to 190 C The experiments using 3 methods which are ambient pressure viscometer, sealed tube, and using high pressure rheometer to measure gelation time Dynamic sandpack and core flood were used to study the effectiveness of polymer gels in porous media PAtBA/PEI provide long term sealant capabilities up to 176 C PAtBA/d PEI provide gelation time for temperature 121 C to 204 C PAtBA/d PEI shows excellent permeability reduction up to 190 C All gels show good thermal stability and permeability reduction 97% 100% in sandpack The results using core sample need to be confirmed, since the comparison with results of core sample might be unreliable The quality of gel with the effects of parameters such as ph of solution, salinity are crucial to be evaluated simultaneously to design suitable polymer gel Al Muntasheri To investigate the reaction et al (2006) 74 mechanisms of high temperature organic water shutoff gel PAtBA PAM Temperature: 80 C to 120 C C 13 NMR was used to examine the structural changes of PAtBA NMR spectra was applied to confirm the peak assignment of tertiary butanol (tert BUOH) in hydrolysis degree analysis Rheometer to conduct viscosity measurements Temperature reduces degree of esterification of PAtBA Hydrolysis of PAtBA produces tertiary butyl alcohol (tert BUOH) in alkaline and increase when temperature increase Over a range of temperatures, cross linking of HPAM with PEI forms gels that are stable for several days PAtBA gel were the result of transamidation rather than nucleophilic substitution Additional work is required to investigate the gel stability and gelation time as a function of temperature for HPAM/PEI gel Al Muntasheri To study the effect of some et al (2007) 75 parameters on the viscoelastic properties of a high temperature cross linked polymer gel Al Muntasheri To study the gelation kinetics, et al (2008) 63 performance in porous media, and the effect of some variables on gelation time of polymer gel PAtBA Controlled strain rheometer was used to measure the elastic and loss modulus Polymer and cross linker concentration, temperature, gel age and salinity were the variables KCl and NaCl were used to prepare synthetic brines PAM Using C 13 NMR to measure degree of hydrolysis Rheological measurements were conducted at high temperature and pressure rheometer. Coreflood evaluation performed using Berea sandstone cores The elastic modulus showed substantial increase when higher than 100 C, leading to higher cross linking and enhance elasticity Gel strength showed exponential dependence on polymer and cross l inker concentration PEI concentration from 0.3 to 1.2 wt%, elastic modulus increased by a factor of 4.8, and beyond 1.2 wt%, syneresis occurs Gel strength decrease as salinity increase Gel strength decayed at 150 C fast decrease in elasticity Higher temperature, polymer and cross linker concentration, and degree of hydrolysis decreased gelation time High salinity and neutral ph increase gelation time PAM/PEI system stable more than 8 weeks at 130 C PAM/PEI system reduce permeability of Berea core under pressure gradient of 1000 psi at 90 C for 3 weeks At 130 C, concentration of 7 and 5 wt% of PAM with 4 wt% of PEI, syneresis occurred The study is only focus on gel strength and elasticity From this study, gelation time has to be measured using actual conditions that representative the field to avoid serious operational problems (Continues)

8 8 AMIR ET AL. TABLE 1 (Continued) Authors (Year) References Objectives Polymer Methodology Results Remarks Salgaonkar and To evaluate the stability of Das (2012) 76 organically cross linked polymer for water shutoff in high temperature well applications El Karsani To investigate the rheology of et al (2014) 41 a water shutoff gel at high pressure and temperature with different variables effect PAtBA Retarder: carbonate salt Temperature: 150 to 205 C Gelation times were measured using Chandler viscometer Sandpack flooding was carried to study the effectiveness of cross linked polymer gel system, with different retarder concentration Permeability measurements is calculated by flowing the brine until stable pressure drop is observed PAM Concentration ratio of PAM/PEI was fixed 7/0.3 wt% Dynamic testing using rheometer was used to study rheological behavior at different factors and conditions Dynamic shear measurement at different strain and frequency is used to measure gel strength Polymer gel system can be used as effective water shutoff up to 205 C Higher retarder concentration prolonged gelation time in higher temperature Gel was able to withstand a pressure differential of 100 psi at over 205 C for 30 days with 100% permeability reduction Maximum pressure differential is 950 psi PAM/PEI system showed high elastic modulus Increase temperature from 120 C to 130 C, increase elastic modulus 35% Elastic modulus followed exponential relationship on PAM and PEI concentrations Elastic modulus decrease in the presence of salt, which low gel strength Addition of retarders decrease the elastic modulus and increase the gelation time. Thorough discussion on the effect and mechanism of retardation by carbonate salt is needed Further laboratory work needs to be performed to confirm the pressure gradients in porous media. El Karsani To study the retardation et al (2014) 77 mechanisms of NaCl and NH4Cl for PAM/PEI gels PAM Retarder: NaCl NH4Cl Na2CO3 PAM/PEI concentration ratio at 7/0.3 wt% and temperature 120 C Using C 13 NMR to measure degree of hydrolysis by the effect of salt Using rheometer and DSC to study the rheology and thermal behavior respectively NH4Cl increase the hydrolysis more than NaCl As DH increase, strength of gel increase, but gelation time decrease NH4Cl increased gelation time more than NaCl because of a shifting in the onset gelation temperature. NaCl and NH4Cl were more compatible with PAM/PEI gel system than Na2CO3 as no white precipitation observed Addition of salt increase gelation time at high temperature is believed through a charge shielding effect Pu et al (2015) 78 To study the gelation performance of PEI cross linked polymer layered silicate nanocomposite gel system for potential water shutoff use in high temperature reservoirs PAM/MMT nanocomposite Experimental temperature: 120 C PAM MMT nanocomposites (NC) were prepared through in situ intercalative polymerization FTIR and XRD to characterize the prepared PAM MMT nanocomposites Bottle test and viscosity measurement methods are used to investigate nanocomposites/pei gelation performance Gelant was injected to synthetic core to study gelation performance through porous media Gelation performance are greatly affected by TDS, PAM MMT nanocomposite and PEI concentration For thermal stability and gelation performance, PAM MMT/PEI gel system is better than PAM/PEI gel system PAM MMT/PEI gel system that consists of 4.0 wt% PAM MMT and 0.3 wt% or 0.6 wt% PEI can thermally stable at 120 C for 90 days Salinity can delay gelation time of PAM MMT/PEI gel system The presence of solid in nanocomposite improves the gelation time and gel strength of polymer gel The absorption of solid in porous media should be further studied (Continues)

9 AMIR ET AL. 9 TABLE 1 (Continued) Authors (Year) References Objectives Polymer Methodology Results Remarks Bai et al (2015) 79 To study the performance of hydrophobically associating polymer and polyethylenimine gel system for water shutoff treatment in bottle and porous media Hydrophobically associating polymer (HAP) Bottle test and viscometer were used to determine gelation time and gel strength Sandpack core flood was conducted to investigate gel performance in porous media Gelation time is delayed after flowing through porous media, but gel strength does not decrease. Viscosity and gelation time are affected by concentration of HAP and PEI NaCl or CaCl2 resulted in decrease of viscosity and extension of gelation time The effect of CaCl2 is better than NaCl due to higher ionic charge The rock skeleton of the core had a great effect on gelation behavior Viscosity of the HAP/PEI gel system in core is reduced compared to in bottle Gelation time in core was extended 2 times longer than in bottle For both core and bottle tests, increase in temperature decrease the gelation time following the relationship of Arrhenius equation The water residual resistance factor is higher than 55, the shutoff ratio is 98% with the subsequent injected water volume of 5.0 PV After a period of water flooding, the HAP/PEI gel can be broken because of flooding fluid. However, because of the high sticking force between the gel and the rock, a gel film is formed on the wall of pore throats then to reduce permeability further El Karsani To study the performance of et al (2015) 80 PAM/PEI gel system for water shutoff in hightemperature reservoirs Adewunmi To study the gel strength and et al (2015) 81 gelation time of PAM/PEI gels reinforced with coal PAM Retarder: NaCl and NH4Cl PAM Additive: CFA Temperature: 116, 150 C Formulation of PAM/PEI (9/1) wt% was used The thermal stability of the PAM/PEI gel in different water salinity was examined Different salts were examined at to identify the best retarder to achieve low viscosity and long gelation time. Different types of core flooding experiments were used to evaluate the effectiveness PAM/PEI gel at high temperature PAM/PEI gel was evaluated in a core flooding system using Berea sandstone and Indiana limestone core with initial permeability of 45 and 6.5 md respectively Temperature: C Rheology, gel strength, and gelation time experiments were conducted using rheometer Samples in sea water showed better thermal stability compared with distilled and field water NH 4 Cl is more effective as retarder compared to NaCl Lower gel strength is showed with NH4Cl The gel reduced permeability in Berea sandstone and Indiana Limestone by 94% for a period of 2 weeks and 99% for more than 5 months respectively Gelant placement in tight carbonate cores requires long gelation time, thus need higher retarder concentration Rheology of cured gel samples indicated that 1 day of curing in the core is enough to stabilize gel strength Strength of PAM/PEI composite gels filled with CFA is stronger than pure PAM/PEI gel The used of retarder is promising to prolong gelation time. However, the effect on gel strength need to be solved Strong interaction between CFA and PAM/PEI gel matrix has been confirmed. The presence of alumina and silica in high concentrations in CFA induced (Continues)

10 10 AMIR ET AL. TABLE 1 (Continued) Authors (Year) References Objectives Polymer Methodology Results Remarks fly ash for water shutoff treatment SEM revealed the surface morphologies of PAM/PEI gels reinforced with CFA Gelation time of PAM/PEI gel filled with 2 wt% CFA decreased with temperature increase Presence of CFA in PAM/PEI gels make its surface morphology dense without any noticeable micropores strong bond within polymer, which resulted inimproved thermal stability and strength The study on the effect of other variables such as ph and salinity on gelation behavior was not performed Mohamed To study the gelation kinetics et al (2015) 82 of emulsified gel system PAM Surfactant: ethoxylated amides and amine acetates Experimental temperature: 100 C (2 C/min) Differential scanning calorimetry (DSC) was used to study gelation behavior Salinity extend gelation time NH4Cl is more efficient than NaCl in the retardation of the gelation process Emulsified PAM/PEI gel has lower rate of cross linking compared to pure PAM/PEI gel The nature of the surfactant affects the rate of gelation by lowering the rate of reaction of cross linking Qin Yi et al (2016) 70 To investigate the dynamic thickening of gelation process of PAM/PEI system at high temperature and high pressure Adewunmi To study the gelation kinetics et al (2017) 83 and dynamic rheological of fly ash based polymer gels for water reduction PAM Temperature: C, Thickening test was carried out to study the effect of temperature, ph values, concentration of polymer and cross linker, and retarders at pressure 15 MPa and shear rate 150 RPM Viscosity of gel was measured by Brookfield rheometer PAM Additive CFA Experimental temperature: 90 C 2000 ppm NaCl brine was added for all sample Effects of various CFA, PAM, and PEI concentrations on gelation performance of gels were observed through the bottle testing method Viscoelasticity study were performed using the hybrid rheometer at various frequency and strain sweep XRD and SEM analysis were used to study compositions of CFA and morphology of gel respectively High temperature, ph, and polymer and cross linker concentration resulted in higher rate of cross linking, then reduce the gelation time Rigid and stable gel was formed in neutral and alkaline media, while gel could not be formed in acidic condition PEI was limiting reactant when concentrations of PAM and PEI were 0.7 and 0.5 wt% respectively NaCl, NH 4 Cl, CH3COONa, and C6H5Na3O7 were found to be efficient to prolong gelation time. However, CH3COONa and C6H5Na3O7 can form a more rigid gel compare to NH4Cl As concentration of CFA increases, gelation time increases. CFA concentration should be around 1 wt% When concentration of PAM and PEI increase, gelation time of PAM/PEI CFA gel system decrease Gelation time of PAM/PEI CFA gel system in range 3 to 120 hours at different concentration of PAM, PEI, and CFA Gel strength of PAM/PEI CFA system is higher than pure PAM/PEI system across strain and frequency sweep test SEM micrographs of pure PAM/PEI gel and PAM/PEI CFA composite gels filled with CFA proved the consistency of gelation kinetics and rheological data Further study on the compatibility of sodium salts with PAM/PEI gel system is required Gelation time and gel strength are not predominated by the cross linker concentration, which is dependent on polymer concentration. Hence, retardation is efficient on polymer side. Mohamed To explore the use of et al (2018) 84 organoclay to enhance the properties of the polymer gel in harsh PAM Organoclay (Cloisite 15A) Experimental temperature: 180 C Tensiometer was used to measure the liquid liquid As the organoclay concentration increased, stability of the emulsion increased. Organoclay has the potential to be used as cost effective emulsifiers to form emulsified polymeric gels (Continues)

11 AMIR ET AL. 11 TABLE 1 (Continued) Authors (Year) References Objectives Polymer Methodology Results Remarks conditions, especially to those encountered in oilfields for the emulsification applications interfacial tension (IFT) by the pendant drop method Addition of the polymers lowered the stability of emulsion. Stability of emulsion increased when the salinity increased Organoclay particles improve the flow properties and gel strength of emulsified cross linked polymer Higher stability was achieved when organoclay as standalone emulsifier TABLE 2 Summary on the literature review on the gelation kinetics studies of polymer gel that organically crosslinked with phnolic resin and Hexamethylenetetramine (HMTA) Authors, Year and References Objectives Polymer Crosslinker Methodology Results Remarks Jia et al. (2011) 51 To investigate the gelation performance using resorcinol and phenol formaldehyde as primary and secondary crosslinker, respectively To determine the effect of parameters on gelation performance HPAM Primary: Resorcinol Secondary: Phenol formaldehyde Temperature: room temperature, 65, 80, 90 C Investigation based on concentration of primary and secondary crosslinkers, TDS and temperature Gelation rate and gel strength investigation through viscosity measurement and bottle test SEM for gel microstructure observation Gelation performance in porous media Primary crosslinking guarantees the effectiveness of secondary crosslinking Viscosity of gel after resorcinol addition increase within 2 hours at room temperature SEM analysis: typical dendritic shape for primary crosslinked gel, while tight 3D network structure for secondary crosslinked gel TDS increase, gelation time increase but gel strength decrease Gelation time and gel strength of the secondary crosslinked gel increased with increasing temperature Gelation time decrease after sample flow through porous media First crosslinked gel has shear stability to ensure the secondary gelation effectiveness Secondary crosslinked gel provides high thermal stability and erosion resistance because of the tight 3D gel network (Continues)

12 12 AMIR ET AL. TABLE 2 (Continued) Authors, Year and References Objectives Polymer Crosslinker Methodology Results Remarks Sengupta Gelation studies of the polymer et al. (2011) 85 gelant prepared from PAM and HQ/HMTA at different temperatures and ph Al Anazi To evaluate the performance of et al. (2011) 86 organic water shut off gelling system for carbonate formation Yadav and To study in situ gelation of Mahto (2014) 87 organically crosslinked polymer gel system for profile modification jobs He et al. (2015) 88 To develop the formulation of organic/inorganic combined gel combined gel To evaluate the thermal stability and salinity tolerance PAM Hexamethylenetetramine and Hydroquinone PAM Hexamethylenetetramine and Hydroquinone PHPAM Hexamine and Hydroquinone PAM Additive: Silicate Temperature varied from 85 C to 120 C Carried by bottle testing method to measure gelation time Sandpack core flooding experiments for gel performance in porous media Experimental temperature: C Bottle test were used to measure gelation time and thermal stability Carbonate core samples were used for coreflooding test CT scan and ESEM are used to observed plugging Experimental temperature: C Bottle test were used to visually observe gel quality Brine injection into sandpack was used to investigate the permeability of sand pack Various range of temperature is adjusted to meet in situ reservoir conditions of the Indian oil fields Phenol formaldehyde The gelation time measured by bottle test method Thermal stability and salinity tolerance test by aging for 60 days in high TDS (20% NaCl and 2.5% CaCl2) at 130 C Gelation time decreased, but gel strength increased, when temperature increase As polymer and crosslinkers concentration increased, gelation time decreased When salinity increased, gelation time increased When ph increased, gelation time increased (best result at ph 8 9) Permeability of the sandpack reduced from 5.63 to Darcy When temperature increase, gelation time decrease Primary and secondary crosslinkers significantly affected gelation time Acid based activator affects gelation time, but not effective in carbonate rock As polymer and crosslinker increase, post gelation permeability decrease, residual resistance factor increase Gel system is effective in core system with formulation polymer at 7 wt%, primary and secondary crosslinker at 0.5 and 0.6 wt%, respectively Increase in polymer and crosslinker concentration, increase permeability reduction and residual resistance factor With temperature increase, gel system is effective up to 120 C Gel system shows good plugging ability for profile modification jobs in high temperature and pressure oil wells Optimum formulation polymer: wt%, phenol formaldehyde resin: wt% and inorganic silicate: wt% As the polymer, silicate and crosslinker concentration The chemistry and mechanism behind the effect of parameters on polymer and crosslinkers reaction were not thoroughly discussed The work did not investigate the effect of some parameters and compability of gelling system with reservoir fluids The mechanism of gelation inside porous media and permeability reduction by gel were not discussed thoroughly Compared with single polymer gel network structure, another threedimensional network structure formed by silicate (Continues)

13 AMIR ET AL. 13 TABLE 2 (Continued) Authors, Year and References Objectives Polymer Crosslinker Methodology Results Remarks To evaluate the (performance of combined gel in permeability reduction Sandpack flow experiment was used to evaluate the gel performance increased, gelation time decreased As temperature increase from 100 to 130 C, gelation time decreased from 54.0 to 15.5 hours Organic/inorganic combined gel has better thermal stability and salt tolerance Combined gel showed 85% permeability reduction has improved thermal stability and salinity tolerance of the gel by preventing lost water content and providing high gel strength. He et al. (2015) 59 To compare the gelation behaviour and morphology of resorcinol HMTA HPAM gel in bulk, static and dynamic gelation in porous media Zhao et al. (2015) 89 To study gels formed by nonionic polyacrylamide and phenolic resin for in depth profile control HPAM (hydrolysis degree of 24.25%) Nonionic polyacrylamide (NPAM) Resorcinol and hexamethylenetetramine (HMTA) with concentration ratio 1:4 The gelation time measured through viscosity changes 57 C Sand pack flooding was used to study the static and dynamic gelation behaviour in porous media SEM was used to study the morphology of gel in bulk and porous media Phenolic resin Temperature: 70 to 90 C The gelation time was determined using a gel strength code method Breakthrough vacuum method was used to determine gel strength ESEM and SEM were used to determine the microstructure of the bulk gel and flow behaviour of gel in porous media Sand pack flow experiments were used to determine the plugging capacity of the NPAM gel DSC was used to study the thermal stability of the NPAM gel Static gelation time in porous media was much longer than that in bulk Gelation time after dynamic gelation in porous media was much longer than bulk and static gelation Morphology of bulk gelation: compact 3D network Morphology after static gelation low magnification: dense gel membrane structure, high magnification: network and macro pore structure Morphology after dynamic gelation low magnification: dense gel membrane structure, high magnification: incomplete chain structure Gelation time and gel strength can be controlled by adjusting polymer or crosslinker concentration Gel system has a strong salt tolerance, but shorter gelation time. Gelation time decreased but the gel strength increased with increasing temperature. At 147 C, the gel structure was almost destroyed. Uniform spherical protrusions with pore sizes 1 3 μm were distributed over the gel surface Plugging rate and resistance factor are more than 90% and 14%, respectively. Due to some factors, such as adsorption of polymer and shear effect from porous media, resulted slow gelation process in porous media The NPAM gel has good shearing stability and thermal stability which can be crosslinked in high salinity and temperature reservoirs. The COO group in the NPAM polymer reacts with multiple alkylations ( CH 2 OH) of the phenolic ring to form a dense three dimensional gel network structure in the (Continues)

14 14 AMIR ET AL. TABLE 2 (Continued) Authors, Year and References Objectives Polymer Crosslinker Methodology Results Remarks gelation process and porous media. Daoyi Zhu To study the effect of different et al. (2016) 38 phenolic compounds on performance of organically crosslinked terpolymer gel systems at extremely high temperatures Liu et al. (2016) 46 To study the influence factors (temperature, salinity and ph) on gelation time and gel strength Terpolymer (ZP 4) HMTA and phenolic compound (phenol, catechol, resorcinol, hydroquinone PAM Hexamethylenetetramine and hydroquinone Temperature: 150 C The Sydansk Gel Code method was employed to investigate the gelation time Gel strength, viscosity, rheological properties, microstructures and thermal stability of gels were tested and compared using rheometer, ESEM and DSC, respectively Experimental temperature: C Carried by PVS rheometer method to measure gelation time and gel strength DSC was used to study the thermal stability ESEM was used to study the gel microstructure With different type of phenolbased gel systems, gelation time was almost the same; however, the thermal stability varied greatly Catechol and hydroquinonebased gelants were able to form strong bulk gels. However, they could not be maintained for long period, syneresis began after 3 to 12 days. When phenol was replaced by resorcinol, bulk gels with excellent strength and long term thermal stability were able to form at 150 C As polymer or the crosslinker concentration is too high, the gelation time become shorter. As the polymer or crosslinker concentration too low, poor thermal stability of the gel due to large dehydration Applicable concentration ranges for polymer and crosslinker at 110 C are and wt%, respectively As temperature rises, gel strength increases and gelation time decreases As the increase of salt concentration, gelation time becomes longer and gel strength becomes weaker With the addition of acid, gelation time decreased and gel strength increased. Whereas, in alkali conditions, the gelation time increases and the gel strength decreases The gel system can keep stable up to 140 8C Uniformly distributed threedimensional network structure was observed. The gelation mechanism of the crosslinking reaction between the terpolymer and different crosslinker systems consisted of three steps. However, the main focus is the difference in the number of hydroxyl groups in the crosslinked clusters obtained in the first step. The gelation mechanism shows that the field operators can adjust the various parameters to control any of those three steps, so that the gelation time and gel strength can be controlled (Continues)

15 AMIR ET AL. 15 TABLE 2 (Continued) Authors, Year and References Objectives Polymer Crosslinker Methodology Results Remarks Daoyi Zhu et al. (2017) 2 To study an in situ terpolymer gel system formed of resorcinol HMTA Liu et al. (2017) 90 To study the influences of silica nanoparticles on the gelation time and gel strength Lashari et al. (2018) 91 To study thermal resistivity, plugging efficiency, strength of novel composite polymer gel 4 anionic terpolymer PAM Additive: Silica nanoparticles PADC polymer Additive: SiO2 and Petroleum sulphonate HMTA and resorcinol Adjusting terpolymer concentration (0.4% to 1.0%) and crosslinker concentrations (0.5% to 0.8%,) to obtain suitable gelation time and performance Gel system was kept at 150 C for five months Gelation time, viscosity, rheological properties, microstructures and thermal stability of gels were tested using Sydansk code method, rheometer, ESEM and DSC, respectively Hexamethylenetetramine and hydroquinone Experimental temperature: 110 C Gelation time and rheological properties were determined using rheometer DSC was used to study the thermal stability The gel microstructure was studied using ESEM HMTA and resorcinol Experimental temperature: 60 C to 100 C The gel strength was determined by the breakthrough vacuum method The gelation time measured by bottle test method The gel microstructure was studied using SEM Thermal stability was studied using tubes and kept in pre heated oven for 90 days As terpolymer concentration increase, gelation time and gel strength increase The higher resorcinol concentration, shorter gelation time but stronger gel strength With the increase of terpolymer concentration, more dendritic structures took shape and distributed between the pores Very low concentrations of NaCl, KCl and CaCl 2 can delay gelation time. MgCl2 shortens gelation time DSC result shows gel system can keep stable up to 240 C Shorter gelation time with higher silica nanoparticles Gel strength increases with the increase of silica nanoparticles Thermal stability of the gel can be strengthened by the addition of silica nanoparticles It can be observed that silica nanoparticles aggregate together and form varied arrangements in polymer gel SEM results showed that composite gel has spatial network structure, which is because of SiO2 linked together the polymer clusters to form highly stable composite gel Polymer gel have good thermal for long time with optimum concentration of PADC, crosslinker, SiO2 and surfactant SiO2 increase the gel strength The increment in temperature, decreases the gelation time with higher gel strength The composite gel formed under alkaline conditions results better strength The composite gel was studied for 90 days, which results in better thermal stability with negligible degradation and syneresis. Terpolymer synthesized with an additional N vinypyrrolidone (NVP) group has better thermal stability; in turn, was able to form bulk gel at a high temperature. Silica nanoparticles are effective to strengthen polymer gel; to strengthen gel strength and thermal stability Combination of organic/inorganic cross linked polymer gel gave promising performance especially in term of gel strength and thermal stability (Continues)

16 16 AMIR ET AL. TABLE 2 (Continued) Authors, Year and References Objectives Polymer Crosslinker Methodology Results Remarks Gu et al. (2018) 92 To study the gelation mechanism of phenolic resin gel and the impact mechanism of various factors Alhashim et al. (2018) 93 To evaluate the applicability PAM crosslinked by of HMTA and resorcinol of in depth fluid diversion practice at high salinity and high temperature conditions NPAM Phenolic resin (combination of phenol, formaldehyde and urotropine) Experimental temperature: 95 C Rheological properties were measured using rheometer The microstructure of the gel was determined using a Carl Zeiss Jena cryo etching electron microscope PAM HMTA and resorcinol Experimental temperature: 95 C Gelation time and rheological properties were measured using rheometer and bottle test Low field NMR measurements were performed to monitor gelling process Polymer gels with high polymer and crosslinker concentration can be formed quickly with good gel strength Viscosity of the gelant systems varied as shear rate increased. With the increase of reaction time, the crosslinking density increased, and the structure became denser With increase of polymer concentration, the viscosity of the gel increased Viscosity of gels rose as crosslinker concentration increased, while zero shear viscosity of the system with the highest crosslinker concentration As salinity increased and ph decreased, viscosity and viscoelastic moduli of the gel decreased. Longer gelation time as polymer and crosslinker concentrations decreased. But there were lower concentration limits for both the polymer and crosslinker. The gel system had a wide range of tolerance to brine salinity Decreasing ph resulted in faster gelation rate, but gel becomes stronger. In low ph range (<6.0), the gelation time slightly increased with increasing brine salinity. While in neutral ph (6 to 8), gelation time slightly increased with decreasing brine salinity. Rheological measurement and NMR technique, without disruption to the gel structure, can determine the gelation time of a bulk gel better than by bottle test. Gelation time tended to be shorter than bottle test result. Higher polymer concentration contributed to form a stronger membrane structure and a higher crosslinker concentration made the density of the spherical particles higher, both improved the gel strength The studied polymer gel is focusing on the potential for indepth fluid diversion application. Gelation time can reach around four days at 95 C, with sufficient gel strength and therma stability.

17 AMIR ET AL. 17 linker is phenolic resin or PEI. As for example, from the experiment carried out by El Karsani et al, equilibrium elastic modulus of gel increased when the temperature heat up from 120 C to 130 C. 41 Beyond 130 C, the cross linking process stops and the polymer gel strength remains constant. However, the experiment also revealed an interesting phenomenon. From the experiment, after certain temperature limit of 150 C, gel strength showed slight decrease. Possible explanation is during the gelation process, the rate of cross linking increases with increased temperature. However, because of limitation of PEI mass transfer in the presence of highly cross linked networks, further cross linking reactions are unable to be finished, hence affected the gel strength. 41 In short, high temperature promotes short gelation time, strengthen the gel strength, but could possibly reduce gel strength after certain temperature limit. In any case, a longer gelation time of gel is more favorable to ensure that the gel can propagate deeper into the reservoir before it solidified. 63 This indicates that the deployment of the organic polymeric gel system at high temperature reservoirs without gelation retarding agent is a difficult task. Hence, the retarders are essential to elongate the gelation time, slow down the hydrolysis, and strengthen the gel. 41,63, Effect of initial ph The ph of the gelling solution can be affected due to the presence of sour gases, dilution with formation fluids, and contact with the reservoir rocks. It is crucial to investigate the effect of initial ph on the gelation time. In the research paper by El Karsani et al, 3 samples with varied ph value (ph 10.2, 7, and 3) were prepared to investigate the gelation time of gel system. 41 It is found that the gel formation in alkaline and acidic media possesses a slightly higher gel strength but with a slower gelation time if compared to the gel sample formation taken in neutral media. El Karsani et al concluded that the gelation in the alkaline and acidic media is associated with slower gelation times compared with the cross linking in the neutral media. Looking into another article reported by Yi et al, the same pattern was also observed for PAM/PEI gel system neutral media. 70 Gel was unable to be solidified under acid condition (ph 4). While comparing the result of alkaline and neutral condition carried out in 130 C, the gelation process took 15 minutes in ph 9 to achieve equilibrium stage while the gelling system required 30 minutes in ph 7. Hence, it can be concluded that gelation time was shorter in alkaline environments compared to neutral environment. These phenomena are associated to the hydrolysis of PAM in both conditions, which higher degrees of hydrolysis can be reached outside neutral medium, either lower or higher ph values. 98 In alkaline environments, the gelation process was shorter compared with gelant in neural media. The high cross linking rate is due to the fact that the alkalinity accelerates the hydrolysis rate of polymer. The carboxylate groups formed in hydrolysis enhances activity of amides which is adjoining to them (anchimeric effect). Meanwhile, short gelation time in acidic conditions is due to the hydrolysis that leads to the conversion of amide groups to carboxyl groups and the imidization that occurs between amide groups of intramolecular and intermolecular in acid media. 70 On the other hand, the hydrolysis is minimum at an initial ph value of 7 compared to the other cases. Theoretically, low hydrolysis rate minimizes the negative charge on PAM, resulting less polymer network stretching. That gives less accessible cross linking sites on the PAM network. Therefore, the longest gelation time is obtained. The similar observation also has been recorded with the gel studied by Al Muntasheri et al. 63 Comparing the effect of ph on gelation time with another article, the result obtained have slightly difference. Different polymer gels including phenolic resin cross linked gels have different range of ph over which they can maintain their stability. From the experiments carried out in the laboratory by Sengupta et al, it was found that HQ HMTA cross linked polyacrylamide is stable up to ph This is attributed to that the acidic condition promotes the decomposition of HMTA to form formaldehyde. With the acceleration of this decomposition, it leads to higher cross linking and reaction rate. On the other hand, above ph 9.5, proper gelation almost did not take place. As alkali is added into the gelant solution, the gelation time increased but the gel strength decreased. This is due to the inhibition of alkaline environment to the decomposition of HMTA. However, high temperature helps HMTA decompose to form formaldehyde, even though in alkaline environment. This phenomenon makes the effect of alkali on gelation performance of the gel system in high temperature, not noticeable compared with the effect of acid. Gu et al also reported that the strength and stability of the phenolic resin cross linked gel under acidic condition were not promising. 92 The addition of acid caused the negatively charged polymer molecules on polymer backbone to be electrically shielded. H + led to the protonation reaction of some amide groups, which affected the cross linking reaction between amide and hydroxymethyl groups. Furthermore, the reduction of electrostatic repulsion between molecular chains turns polymer molecules into coiled state severely. Moreover, the formed membrane structure is very fragile, and the spherical particles were deteriorated to form a type of filamentous structure. It will affect the gel system to be dehydrated easily. 92 The study of the change in ph of polymer gelant with time also was carried out, which concluded that the ph of the gelant decreased with time. This decrease in ph may be due to the cross linking reaction taking place in solution. It seems like reducing the ph value regards the simplest method to elongate the gelation time and improve the gel strength. However, it is not the best way to control the gelation kinetic as ph value is not consistent in the reservoir. 6.3 Effect of salinity The inorganic salt concentration is another crucial factor that affects the gelation process and performance of polymer gels. As the salt concentration is not accurately evaluated, the gelation time might be negatively affected. Field chemical treatments are prepared in brines containing various salt concentrations. Previous studies have agreed that salinity gives negative impact on gel strength. However, there are some conclusions that have been made on the effect of salinity on gelation. For example, Zhao et al has reported that salinity gives negative impact on gelation time of phenolic cross linked polymer gel. 89 However, after intensive review, we tend to believe that salinity is able to increase gelation time and weaken the gel strength. The proposed explanation for the increase in the gelation time by the addition of salinity is likely caused by the shrinkage in the polymer chains. 77

18 18 AMIR ET AL. The flexibility of the polymer chains makes them fairly responsive to the ionic strength of the higher salinity water. 99 With the increase of the salt concentration, the hydrodynamic radius of the polymer decreases. The decrease of hydrodynamic radius for polyacrylamide molecules under high salinity condition is possibly because of the compression of the electrical repulsion among negatively charged groups on the polymer backbone. 100,101 When the charge number is high, the diffuse layer is compressed by the massive counter ions and the thickness of the diffusion layer is reduced. The polymer chains then adopt a coiled state. In a coiled state, the compacted polymer molecules will not be fully stretched any longer. This phenomenon affects the size of the polymer especially in higher salinity conditions. 99 This shrinkage leads to a decrease in the number of crosslinkable sites and cross linking rate. Moreover, divalent metal ions have greater influence than monovalent metal ions due to the higher charge density. 46 In addition, salt gives shielding effect during hydrolysis process of polymer and prolong the induction period. 41 Hydrolysis process is continued when the time increases and provides a higher density of carboxylate groups on the backbone of polymer. The negatively charged carboxylate groups of polymers attached themselves to the positively charged salts ions through weak ionic bonding. It causes bridging of polymer chains with salt ions. The increase in brine concentration screens the cross linking sites; thus, induction period prolongs due to which gelation takes longer time. Moreover, the gels formed are less elastic in nature. These results revealed the influence of salinity on the electric surface charge at polymer interfaces. Thus, when the salinity of the gelant solution increases, the cross linking rate becomes slower and increases gelation time. This observation was confirmed in previous literature reports. 39,102 This observation also suggests that salts can be used as a retardation agent to slow down the cross linking reaction. This retarder can be used when deploying organically cross linker polymer gel system in hightemperature reservoirs to give longer gelation times. Thus, it is very necessary for engineers to assess the salt concentration of injection water and formation water. 6.4 Effect of polymer concentration Gelation kinetics can be influenced by PAM polymer concentration. It is crucial to design an appropriate and qualitative polymer gel to reinforce the rheological properties of gel and prolong the gelation time during the operation in oil fields. Thus, understanding the effects of various polymer concentrations on gelation kinetics of gelling system is essential. With increasing polymer concentration, we can expect that more cross linkable sites will be accessible for cross linker, and consequently higher gel strength. This expectation is in agreement with the data reported for organically cross linked systems by El Karsani et al and Al Muntasheri et al in which PAM and PAtBA were cross linked with PEI respectively. 40,41 At a higher polymer concentration, more cross linking unit can be achieved and resulting gel can be formed in a short time. 103 Same phenomenon can be observed from the study involving the effect of polymer concentration with phenolic resin or HMTA as cross linker. 2,89 As the polymer concentration increased, the cross linking reaction chances between the amide group and hydroxymethyl group increased, which resulted in a decrease in gelation time. Adewunmi et al has also studied on the effect of various PAM polymer concentrations from 2.87 to 8.4 wt% with the presence of solid particles, which is coal fly ash (CFA). From the result, the gelation time of PAM/PEI CFA system shortens as concentration of PAM increases. 83 From the point of viscoelastic behaviour, the results of study of Adewunmi et al shown that the viscoelastic behavior of PAM/PEI CFA gel system increases with increasing polymer concentration. 83 Moreover, without the inclusion of CFA, increasing PAM concentration can also improve the viscoelastic properties of gel system. From 2.87 to 6 wt% PAM, the viscoelastic moduli of the system increase considerably. However, there was a decrease in viscoelastic moduli at 8.4 wt%. The polymer concentration can also influence the starting time of gel syneresis. Polymer gel of high HPAM concentration is able to postpone the gel syneresis when cross linker is at the lower concentration. 103 It can be concluded that more cross linking sites and the cross linking chance is available in high polymer concentration. Consequently, time to achieve a nonflowing polymer gel will be shortened. Nevertheless, as the polymer concentration increases, more network structures will be formed and spread between the pores of the grids. These network structures will further strengthen the gel to block the water under high temperatures. In fact, high polymer concentration is not favorable in terms of economical cost. Thus, to obtain tolerable high gel strength polymer system, it required high loading of polymer. Hence during the treating of deep reservoir where the gelant viscosity and material cost should be minimized, it is important to consider both the effect of gelation period and gel strength when designing the reasonable amount of polymer loading. 6.5 Effect of cross linker concentration The effect of cross linker concentration has also been examined in some literature. 77 Generally, gelation time decreased but gel strength increased with increasing cross linker concentration. Samples of different PEI concentration were prepared while the concentration of PAM polymer is keep constant. Results show that PEI cross linker with higher concentration possesses a longer gelation period and high strength of gel. This can be explained as the high amount of PEI offers more sites for cross linking. Same trend also has been observed with phenolic resin. 2 Better gelation acceleration was achieved with high concentrations of resorcinol. As the concentration of resorcinol increases, the shorter the gelation time, yet the gel strength slightly increased. Adewunmi et al has studied on the effect of PEI concentration on PAM/PEI CFA gels. PEI concentration was varied from 0.3 to 1.04 wt%. 83 It is evident that for cross linker concentration of 0.3 and 0.67 wt%, viscoelastic moduli of the PAM/PEI CFA system display the similar trend, while for 1.04 wt% PEI concentration of PEI, a lower viscoelastic modulus was observed. From the study from He et al, the effect of phenolic resin concentration on gelation time was determined. 88 Resin concentration is varied from 1.5 to 5.0 wt%, while PAM and inorganic silicate are kept constant at 0.3 and 3.0 wt% respectively. Similar trend as previous studies, the gelation time decreases from 30 to 7 hours under 130 C with increasing resin concentration. Undoubtably, when resin percentage increases, the

19 AMIR ET AL. 19 chances for amide group to cross link with hydroxymethyl group increased, resulting in a shorter gelation time required. According to the proper gelation time and economical cost, the resin concentration should range between 2.5 and 4.0 wt%. Optimum value of cross linker concentration needs to be determined. This is due to there is a possible of overcross linking resulted from excessive concentration of cross linker and cause the gel syneresis consequently. The water expulse out of the gel by osmosis and polymer gel dehydrate. 104 It can be concluded that the higher the cross linker system concentration, the shorter the gelation time and the stronger the gel strength. However, if the cross linker concentration is very low, the reaction rate of cross linking will decrease correspondingly. The lower the cross linker concentration, the worse the degrees of cross linking. The gelling solution with lower concentration of cross linker also demonstrated poor gel strength. The solubility of the polymer gel will increase with low cross linking density. It gave lower chance of cross linking due to more carboxyl groups will be generated that can freely stretch to a high extent, but less alkylation groups can be obtained for cross linking. Hence, there is a need of the guidance for petroleum engineers that allows them to adjust the cross linker system concentration to meet different water management requirements. However, polymer concentration definitely has much more influence over cross linker concentration in determining the rheological properties of gel system. 80 With this observation, there is an understanding in delaying gelation is more efficient by retardation of polymer reaction over cross linker side. 6.6 Effect of additives Improvement of the PAM gel system is of utmost importance in order to be applied in harsh reservoir condition. In organically cross linked gels, the gelation time was found to be shorter at high temperatures. When PAM was cross linked with PEI at a concentration ratio of 7/0.3 wt% PAM/PEI at 120 C, gelation time was less than 1 hour. 68 For field applications, this period of time is not sufficiently adequate to inject the gelant safely into the desired zone, without gelation occurs at the midway. The gel may form inside the tubing before it reaches the target zone, and this may lead to severe plugging problems. High temperature reservoirs demand the use of methods to delay the gelation time. Delaying the gelation time is to prevent premature gelation and to ensure deep penetration of the gelling solution. The feasible gelation time should be longer than 55 minutes based on several field studies. 68 Different options have been implemented to overcome this issue. These options included cooling down with preflush to reduce the temperature of the near wellbore area. This method can lower the temperature by approximately 30 C. 68 However, in some cases, it could be intolerable to place huge amounts of water into oil zones. In addition, the injection of small volumes of water significantly will change the effective permeability to oil in some reservoirs. 60 Looking at its development more closely, it can be discovered that many researchers over the past several years have proposed an improved organic based gel with inorganic or organic compounds with the optimum concentration as additives. 6 For instance, the use of retarder to delay gelation time. The retarder affects the chemistry of the gelation process and prolongs the time it takes to form the gel. On the other hand, the addition of retarders should negatively impact the gel strength of the formed gel. The retarders are available in 2 forms, which first by triggering the activity of PEI side in which modified the chemistry of cross linkers or retardation of polymer hydrolysis and reaction. Several laboratory reports have discussed the modification of the cross linker's chemistry by reducing its activity. Specifically, PEI was modified to derivatized PEI and polyamino acid. 65,105 In derivatized PEI, the amine groups have been converted into amides. Consequently, these groups will require a longer time to be changed into available sites for cross linking. Meanwhile, with a polyamino acid perform the retardation by forming a complex with the cross linker (PEI). As a result, the modification will curb the imine groups of PEI attack on the polymer. This will in turn elongates the time it takes the gel to form. On the retardation of polymer hydrolysis and reaction, sodium carbonate (Na 2 CO 3 ) was reported as a retarder for the PAtBA/PEI system. 68 After doing a sandpack flow test, Eoff et al found that the addition of this salt as a retarder could raise the working temperature without affecting the gel strength and its subsequent ability to stop fluid flow. 67 Although Na 2 CO was found to have an excellent retardation effect for the PAtBA/PEI system, 67,76,77 it has disadvantages and compatibility problems with in high salinity brines. 68 The use of salts as retarder can bring negative impact if the compatibility with the field mixing brines is not carefully assessed. Hence, further research has been continued to find a new retarder that is efficient, cost effective, and compatible with the common mixing brines. NaCl and NH 4 Cl have been studied as alternatives for Na 2 CO 3 to increase gelation time of PAM/PEI system in high temperature applications. 41 In comparison, NaCl delayed the gelation time to about 40 minutes only, whereas NH 4 Cl delayed the gelation time of PAM/PEI gel system to more than 3 hours at same temperature and concentration. 41 From the study by Ren et al, the high amount of ammonium salt concentrations will prolong the gelation time for the phenol formaldehyde based gel systems. 104 However, degradation of gel strength and also dehydration synthesis might happen if too much of NH 4 Cl or NH 4 HCO 3 is added. The retardation mechanism of salts in polymer gel system is because of the charge shielding effect on carboxylate groups of polymer. 106 The potential of compositing solid particles also has been investigated to be used as additives. For instances, Adewunmi et al has studied on the strength and gelation time of PAM/PEI gel reinforced with coal fly ash (CFA). 81 Coal fly ash is a retarder which can strengthen the rheological properties of gel and elongate the gelation time at the same time. From the result shown, the PAM/PEI system that contains CFA was found to be substantially rigid and stronger than that of pure PAM/PEI composite. Comparing the PAM/PEI system filled with and without 2 wt% CFA, gelation time of system with CFA was found to deviate from the gelation time of pristine PAM/PEI. Hence, CFA also has a potential to be a retarder for PAM/PEI gel system. The effect of inorganic silicate concentration on gelation time was studied by preparing different gelant solutions containing polymer, phenolic resin, and the inorganic silicate by He et al. 88 From the experiment, when inorganic silicate concentration increased from 1.0 to 8.0 wt%, the gelation time decreased from 19 to 4 hours. It can be understood that silicate itself can form another 3 dimensional network structure, along with the formation of gel network by polymer. Higher concentration

20 20 AMIR ET AL. of inorganic silicate accelerates the formation of the network structures at high temperature, resulting in decrease gelation time. The combination of network structures by organic silicate and polymer also strengthens the gel. The double network structure also has less free water content, thus has ability to thermally stable in high temperature. Hence, to obtain proper gelation time and economical cost, the reasonable inorganic silicate concentration needs some consideration. There is also a study on the influence of silica nanoparticles on the gelation behavior of HMTA HQ cross linked polymer gel by Liu et al. 90 The result indicates that silica nanoparticles enhance both elasticity and viscosity of the gel significantly. Its presence makes rheological properties of the gel more solid like. Their observation found that silica nanoparticles aggregate themselves and form arrangements with certain pattern during cross linking reaction. The aggregations of nanoparticles occurred typically on polymer chain bunches and meshes of the gel structure. It contributes to the major improvement of the gel strength. The presence of silica nanoparticles also strengthen the thermal stability of the gel. This is because of the water lockup function associated with a large number of hydroxyl groups on the surface of silica nanoparticles. In addition, silica nanoparticles have negative charges in gelling solution. The hydroxyl groups form hydrogen bonds and electrostatic attractions with water molecules. It makes the water molecules become bound water. The higher bound water ratio indicates stronger gel to perform better water holding capacity and thermal stability. A study on the effect of the presence of montmorillonite clay (MMT) in PAM nanocomposite also has been conducted by Pu et al. 78 The result shows that the final strength of PAM/MMT nanocomposite/pei gel was significantly stronger than pristine PAM/PEI gel at the same conditions. It is believed that the presence of high MMT clay content reinforces the gel strength. However, the concentration of MMT has to be considered since higher MMT content resulted in low PAM content, when the total concentration of material needs to remain constant. Lower concentration of polymer means that fewer cross linking chance is available. Furthermore, from the study, the gelation time of the PAM/PEI gel system containing MMT is also significantly prolonged than pure PAM/PEI gel system. The longer gelation was observed even though the PAM concentration of both gel systems was same. They explained that MMT layers might be efficient in shielding amide group groups of PAM, hence making it difficult to PEI cross linker to react with the carbonyl carbon attached to the amide group. The thermal stability of the gel is also improved by the addition of MMT into the polymer. This is because MMT plays a role in insulation and as a barrier during decomposition. The presence of MMT layers in a gel network enhances the rigidity of the gel structure and restrains the volume shrinkage, resulting in a good thermal stability. Another interesting point from Pu et al study is that high cross linker concentration is not required if the gel has sufficient strength and stability. 7 FIELD APPLICATIONS Over 1000 field treatments or applications have been executed with the cross linked polymer gel system around the globe to challenge conformance problems such as water coning, high permeability streaks, gravel packing isolation, fractures, and casing leak. Most of the treatments have been designed for matrix, highly naturally fractured carbonate reservoirs, and high permeability streak shutoff. A review on the published literature shows a broad range of results, from total technical to economic and oil production success. Even at today's low oil prices, cross linked polymer gel technology has been developed to large scale commercial application in certain oil producing regions. Even though not all of these treatments are fruitful in terms of tremendous economic success, the treatment showed field water production reduction. Nearly all of the treated wells exhibited huge improvements in water reduction. Some of the wells underwent 100% reduction in water cut, even though it is producing over 80% water prior to gel treatment. In this subsection, the recent field applications that are related to deployment of organically cross linked polymer for conformance control are highlighted. This subsection does not report in full technical literature reviews, but it attempts to describe some field treatment using organically cross linked polymer gel for excessive water mitigation as has been summarized in Table 3. After a profitable and stable hydrocarbon production from the second recovery especially waterflooding, some heterogonous reservoirs have indicated rapid water breakthrough and sudden increase in the level of water cut as high as 95%. Some factors have been identified in contributing the excessive water cut levels, primarily because of breakthrough of injected water, channeling behind casing, rise of oil water contact, and coning. It is also not uncommon problem that cross flow in a multilayer producing well occurs in highly depleted mature fields. In addition, conventional cement squeeze was ineffective mitigation for this well due to the low reservoir pressure. Ineffective cementing liner causes water channeling behind the casing. This may lead to an extremely high water cut at the very beginning of the productive life of the well. This setback causes the well unable to recover the hydrocarbon reserves in the other layers. To mitigate this problem, field operator planned to pump polymer gel to the layer where the active water cross flow was noticed. Based on the high temperature reservoir condition, the gel solutions were designed to delay the gelation time up to 8 hours. The placement time for this gel can be easily calculated by cross linker concentration and temperature, which have major influence on gelation time. Gelation kinetics also depends on the other operational requirements such as ph adjusting of the system for certain type of organic polymer gel, ie, phenol formaldehyde system. The gel strength of organic cross linked polymer was also considered in high temperature field application. The gel is placed into the target zone through techniques such as bullheading or coiled tubing. 113,114 Mostly coiled tube intervention method was selected instead of workover rigs because of the versatility of its operations. Being a continuous pipe, the coiled tubing allows for a faster operation in comparison to jointed pipe, particularly when multiple runs are necessary to change the bottom hole assembly (BHA). In addition, zonal isolation was necessary before pumping the organic cross linked sealant into the targeted zone. The high H 2 S content can lower the ph of treatment fluid, thus reducing the gelation time and adding the operational challenges for certain field application. To overcome this problem, an ammonium chloride preflush was pumped before the treatment. To improve the poor

21 AMIR ET AL. 21 TABLE 3 Summary on the field applications using organically cross linked polymer Field Descriptions Problems Mitigations Results References The well is located in Meleiha concession in the Western Desert of Egypt It produces initial net of 1900 BOPD and 30% water cut from 2 intervals in Alam El Bueib sandstone early Cretaceous formation; AEB IIIA and AEB IIB The reservoir temperature of the target zone (AEB IIB) is 93 C Saqqara A1 Well is located in the Saqqara Field in the central part of the South Suez Gulf in Egypt The primary reservoir is Nubia and entails of a thick sequence of stacked fluvial/alluvial sand bodies The Nubia formation is consistent within the field and neighboring fields, which are Edfu, Ramadan, and October fields The field has started the production since 2008 The well is located in the Permian Basin in North America Zatchi field is located in the lower Congo basin offshore, at about 5 km of the Congolese Coast, 55 km of northwest of Pointe Noire, with water depth in the range of 70 m Zatchi field consists of 5 hydrocarbon bearing reservoirs named levels A, B, C, D and E Typical well architecture on Zatchi field consists in 3 drilling phases 12¼, 8½, and 6 open hole section, with 9⅝ Casing and 7 Liner The upper interval AEB II B to be depleted to 777 psi Lower interval AEB III A is able to maintain the reservoir pressure of 3300 psi by having a strong aquifer support The reason is 2 reservoirs have different characteristics and driving mechanisms It brings the well unable to recover the hydrocarbon reserves in the other layers, low productivity Hydrocarbon production began to decline with increased water cut as high as 95% Production logging tool (PLT) analysis revealed that the perforation interval between and ft was the primary water production contributor This zone produced 74% of the well fluid with 99% water cut The well was producing a very high water cut Conventional cement squeeze was ineffective due to the low reservoir pressure During the clean up phase, after more than 2 weeks, the well was producing mostly water with a water cut almost constant at 100% It was then assumed that the cross flow was established from B to D level, with water migration behind the 7 Liner, and crossing the C level From the results of log evaluation, it was concluded that poor cementing behind the 7 Liner resulting in almost free pipe condition High strength polymer gel designed to be able delaying the gelation time up to 8 hours Coiled tubing path proposed treatment penetration radius was 3 ft Injectivity test to govern the pumping rates and acid wash to improve the poor injectivity 20 bbl of polymer gel followed with 10 bbl of silica flour added polymer gel as tail in system have been pumped to seal the high permeability zone Hesitation squeeze applied when achieving maximum penetration and sealing efficiency. The well was isolated/shut in for 48 hours Coiled tube intervention method was selected because of its operation versatility Retarder was applied to eliminate the need for a cooldown preflush in high temperatures Ammonium chloride preflush was pumped to overcome high H 2 S problem Zonal isolation was conducted before pumping into the targeted zone using a slurry comprised of the gelant with 50% of silica flour The slurry forms filtrate cake that prevent deep gel penetration and to form formation/wellbore barrier The formed barrier is about 1 deep and withstand up to 2500 psi at BHT of up to 160 C Finally, organic cross linked polymer tailed with sealant/silica flour slurry was pumped to isolate high permeability zones The bbl of organically cross linked gel with polymer concentration of ppm was used The wetted polymer was agitated in the mix tanks for a minimum of minutes before pumping to attain full hydration. The cross linker was added to the tank and mixed thoroughly The gel was spotted as a balanced plug across the perforations and squeezed into the zone below the packer The well was shut in for 3 to 4 days The selected option was combination of mechanical and chemical shut off solutions. The organically cross linked polymer will be pumped in the layer where the active water cross flow was noticed The water shut off is consisted in 2 steps. First, 26 bbl of polymer gelant loaded with calcium carbonate. Then, followed with 8 bbl of polymer gelant without calcium carbonate was pumped. After both stages, shut in period was applied The upper perforated interval was positively pressure tested up to 3500 psi differential pressure The lower perforated interval was reopened after wellbore clean up with initial oil production 2500 BOPD with zero water cut The rigless operation cost was only one fourth of estimated cost using mechanical isolation PLT analysis showed that 100% fluid isolation was successfully achieved for the treated zones Highly successful water reduction from the interval where the polymer gel injection procedure was carried out More field trials are planned in the future After the water shut off intervention. The water cut reduced from 94% to 27%, resulting in an oil production of 264 BOPD The treatment is considered successful as the water flow was dropped at around 25 L/hr Sabaa et al (2017) 107 Beltagy et al (2016) 108 Bhaduri et al (2014) 109 Nziembo et al (2013) 110 (Continues)

22 22 AMIR ET AL. TABLE 3 (Continued) Field Descriptions Problems Mitigations Results References Due to the shallow depth of burial, reservoir contains unconsolidated sands and reinforced sand with interbedded dolomites Water channeling behind the casing leads to extremely high water cut when cementing of the liner is not effective From the injectivity test, it required 1.5 bbl of water to reach 200 psi after the first stage, it only required 0.13 bbl after the second one Finally, straddle packer has been installed to mechanically isolate the perforated zone The Caan field is situated on continental shelf of the Gulf of Mexico, off the coast of Tabasco and Campeche The main reservoir rocks are dolomitic carbonated breccias from the Upper Cretaceous Lower Paleocene (K T) and naturally fractured with secondary vuggy porosity The bottom hole temperature is 143 C. Porosity 6% 14% and permeability md. Hydrocarbon from this field is about 30 API of oil gravity The well is horizontal gas producer with bottom hole temperature of 149 C and a bottom hole pressure of 7000 psi The well was completed with a 7 in liner at a measured depth of ft. The open hole section extends from to ft In 2008, well 1 had a production of 4418 BFPD which are 1902 oil and 2516 water, represents 57% water cut per day This is due to the water oil contact started to reach the perforations, known as water coning The well had 100% level of water cut Water breakthrough in the open hole section at the toe after the well completion After log analysis, mechanical packer was set in the water producing section at ft to isolate the water producing interval The problem was not resolved with mechanical isolation due to the water bypassing the packer through the formation and the packer failure The organic polymer gel together with m OCP as a tail in has been used Tail in system is to prevent overdisplacment of the polymer gel in the near wellbore region caused by the existence of fractures and the low formation pressure The treatment consisted of bullheading 377 bbl of seawater spacer and 201 bbl of the organic polymer gelant for radial penetration of 10 ft. The well was shut in overnight The well was reperforated a few feet up from to ft MD with 43 ft interval The pre flush was injected into the reservoir and the pressure was monitored It was observed that well injectivity was good where the pressure was less than 4000 psi at an injection rate of 2 bbl/min After the injectivity test, 150 bbl of the gelant containing 250 gal/1000 gal polymer were injected. 5 bbl of gelant mixed with silica flour were then injected as a tail in slurry. The tail in slurry was pumped immediately, to avoid any settling of the silica flour Finally, 3 barrels of a viscous guar gel, as a spacer, and 47 barrels of water, displaced the gelling fluids from the CT unit After the successful placement of the gelant into the well, the well was shut in for 3 days After 1 year of production, the well is maintaining less than 1% water cut The water cut was decreased in stages to 58% within a period of 8 days Simultaneously, the gas rate improved from to 2.2 to 17 MMSCFD Hernandez et al (2010) 111 Al Muntasheri et al (2010) 68 The well is situated in the southern part of the GA field in western India The field was discovered in Oil initially in place (OIIP) of the sand at 5.6 million metric tonne and the expected total production is 0.36 million metric tonne The well had experienced production pinnacle of 29 million barrels oil per year in 1994 to 1995 The characterization of the well has extremely low permeability, temperature of 130 C 150 C and highly consolidated, homogeneous, thin sandstone accumulation bearing high gravity oil After second recovery, heterogonous sands, specifically S 5 3, S 8 2, and S 9, have shown rapid breakthrough and fluctuated rise in the level of water cut varying between 15% and 95% The identified factors primarily because of breakthrough of injected water behind casing Phenol and formaldehyde gel system has been employed to solve the high water cut issue Gelation kinetics is depending on the operational requirements and has been designed by adjusting the ph of the system The gel formation of phenol and formaldehyde system is highly dependent on the ph and gelation time can be elongated 4 to 15 hours at temperature of 140 C by controlling the ph Cooling down the bottom of the well by circulating cold water also delay the gelation time After analyzing the reservoir data, approximately 16 m 3 of gel solution has injected. The existing perforation hole was blocked, and 0.5 m reperforation job was carried out Even though the injected volume of gelant solution was not as planned, it has yielded effective water blockage around 4.5 m in radius in the formation The well produced almost 200 barrels oil per day for a few months after treatment. Water cut level dropped to 48% from 100% before treatment The return of investment period was only 5 days and generated an additional profit of roughly USD 0.6 million Banerjee et al (2008) 112 (Continues)

23 AMIR ET AL. 23 TABLE 3 (Continued) Field Descriptions Problems Mitigations Results References With the assistance of a casing collar log (CCL), the packer was accurately placed at 2802 m Without adjusting the well depth and temperature, the injectivity of the well was enhanced to 180 L/ minutes at 1800 psi by the injection of 5 m 3 of acid mud followed by 2 m 3 of 1% NH4Cl Gel injection started at 150 L/minutes and 2000 psig pressure. Then it was stopped at 3500 psig after 3.0 m 3 of gelant injection due to the pressure was close to the equipment limiting pressure. Pumping continued when the pump head pressure dropped to 2000 psi. Five injection cycles were completed The secondary oil recovery by water injection method was introduced in 1990 to maintain the mixed drive mechanism injectivity, acid wash has also been performed. Moreover, in other operations, to deal with high temperature by circulating cold water in order to cool down the bottom of the well could also delay the gelation time. In some treatments, they also combine mechanical using straddle packer and chemical shut off solutions to obtain better results. After pumping a polymer gelant into the target zones, mechanical shut off isolation using straddle packer has been used. Practically, the well was shut in for 3 to 4 days to allow the gelation to completely occur. The posttreatment production data are very promising. Monitoring production returns show that there was highly successful water reduction achieved for the treated zones. After the water shutoff intervention, water cut level reduced until 27%, resulting in higher barrel oil per day after treatment. In terms of financial, the project return period is shorter, because of the cheaper price of used chemicals and treatment method does not take a long time to conduct. That generated an additional profit of almost USD 0.6 million. The rigless operation cost was only one fourth of the estimated workover cost using mechanical isolation. This signified that the treatment has a potential in revitalizing the well. More field trials are planned in the future. 8 CONCLUSIONS Organically cross linked polymer gels are an increasingly popular and effective means of conformance control. The application of organically cross linked polymer gels for conformance control has been the significant subject for research and development for over several decades. This review provides a concise discussion of various organically crosslinked polymer gels reported in recent literatures. Organically crosslinked polymer gels have created very noteworthy impact on water shutoff, profile modification, the sealing of open wellbores, and abandonment of zones or wells. The literature review focused on the early studies, reaction mechanism, characteristics, rheological properties, and field applications. Factors affecting the rheology of the organic gels have been discussed in some detail. System selection primarily depends on gel thermal and rheological stabilities under specific reservoir conditions which are temperature, salinity, pressure, and gel solution properties. It is as worthy as ever to optimally exploit this technology to maximize the efficiency of hydrocarbon recovery, and consequently elongate the life of mature oil and gas fields especially for high temperature wells. The future of polymer gel application as a conformance control method will greatly increase as researchers discover more about organic type polymer gels and gel transport mechanisms through porous media. It is concluded that organically cross linked polymer gels have been demonstrated to be effective in water control applications in areas of petroleum production. Organically cross linked polymer gels proved to be able to increase yield and profits on a large scale and have significant influence on the petroleum industry. ACKNOWLEDGEMENTS The authors appreciate the contributions and financial supports from Universiti Teknologi PETRONAS (YUTP 0153AAH05), PETRONAS (GR&T 0153CB019) and University of Malaya (UMRG RP031B 15AFR), and SLAI Fellowship Scheme from Ministry of Education Malaysia and University of Malaya.

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27 AMIR ET AL. 27 Zulhelmi Amir is a PhD candidate from Universiti Teknologi PETRONAS. He holds BEng and MSc degrees both in chemical engineering, from the University of Yamagata University and University of Malaya, respectively. Ismail Mohd Saaid works as an associate professor with the Department of Petroleum Engineering, Universiti Teknologi PETRONAS. Ismail Mohd Saaid holds BSc in Petroleum Engineering from Missouri University of Science and Technology. He also holds MSc and PhD degrees, both in chemical engineering from University of Manchester and Universiti Sains Malaysia. Ismail Mohd Saaid is actively involved in research in the area of enhanced oil recovery and conformance control, with more than 100 refereed journal and conference publications. Badrul Mohamed Jan, SPE, is a researcher and an associate professor attached to the Department of Chemical Engineering, University of Malaya, Malaysia. He holds BS, MS, and PhD degrees in petroleum engineering from New Mexico Institute of Mining and Technology. Jan's research areas and interest include the development of super lightweight completion fluid for underbalance perforation, ultralow interfacial tension microemulsion for enhanced oil recovery, and conversion of palm oil mill effluent into super clean fuel for diesel replacement. He has published numerous technical conference and journal papers. He is the recipient of the 2016 SPE Distinguished Achievement Award for Petroleum Engineering Faculty for the Northern Asia Pacific Region. How to cite this article: Amir Z, Said IM, Jan BM. In situ organically cross linked polymer gel for high temperature reservoir conformance control: A review. Polym Adv Technol. 2018;