Acidizing Geothermal Applications EOST Workshop 06. Nov 2014 Kutzenhausen J. Scheiber, GEIE
Acidizing in Geothermal Outline General Target of Acidizing What are Damages? Reservoir Characteristics History of Acidizing Chemicals in Use Basics of Water-Rock/Mineral Reactions during Acidizing Challenges: Non-Welcome Acid-Mineral Reactions Acidizing: How Does it Work in Practice? Laboratory Tests of Reservoir Materials Examples: Best Case and Worst Case Scenario References 1
Acidizing in Geothermal Stimulation applications in geothermal and oil and gas: Hydraulic Fracturing Acidizing Perforation To increase or to keep up well productivity or injectivity Wellbore cleaning or improving rock permeability removing damages inside the wellbore by chemical cleaning of fractures and pores or by creation of new flow channels 1
General Target of Acidizing Acidizing Techniques Wellbore cleaning Removing drilling-mud cake or scales in the wellbore No or only minimized fluid flow Matrix acidizing Injection of acid into the formation below hydraulic fracturing pressure Create new flow channels or remove damages Acid Fracturing Injection of acid into the formation above hydraulic fracturing pressure Create new flow channels Penetration of the acid into the formation 3
What are damages? Skin (term widely used in the oil and gas industry): The skin factor is a dimensionless pressure drop caused by a flow restriction in the near-wellbore region (http://petrowiki.org/formation_damage) Zero skin: Positive skin: Negative skin: Well productivity without damages Damages are present Establishing new flow channels How to reach a negative or zero skin? Conservative theory of hydrocarbon industry: Negative skin: only in carbonate reservoirs by wormhole creation Zero skin: best result for Sandstone reservoirs Host rocks like granite, basalt, metamorphic rocks are not even considered 3
Das Bild kann zurzeit nicht angezeigt werden. What are damages? Damages in the reservoir or in the near-wellbore region can be induced or natural Induced or natural damages in the near wellbore region Induced Damages Drilling-mud cake Cuttings Cement Injection of suspended matter (scale particles) Natural Damages Fines (Suspended matter) Already existing fracture fillings Secondary mineral precipitation Swelling of clays Bacteria water block (oil and gas) organics (very heavy hydrocarbons, oil and gas) Scalings in the injection well GPK-1 in 80 m depth 4
Matrix Acidizing: General Remarks Target To improve or to keep up well productivity or injectivity without damaging the host rock by Enhancing rock permeability What does that mean? Dissolution of minerals Etching of mineral surfaces Mobilization of particles by decomposition of the rock structure Inhibition of secondary or tertiary reaction products Control of reactivity of mineral surfaces (Clay swelling) 5
Matrix Acidizing: General Remarks GEOTHERMAL Wellbore cleaning after drilling: drilling mud removal Same Objectives: Wellbore HYDROCARBON Application is considered for wellbore only Mud loss zones: possible loss zones for chemicals Different Objectives: Formation Increasing permeability by increasing skin Removing pre-existing fracture fillings Formation Inhomogeneous distribution of fracture fillings High fracture density in fracture zones, partly high permeability Restoring permeability by restoring skin Removing damages: scales or fines < 1m around the wellbore Homogeneous distribution of damages around the wellbore Low fracture density, low permeability (hydrocarbon production tries to avoid open fracture systems in sandstones due to unwelcome water breakthrough) 9
Reservoir Characteristics of Deep Geothermal Heat Exchangers Reservoir Characteristics Thermophysical Parameters temperature, pressure, heat flow density, thermal conductivity, heat capacity, rock density, Hydraulics permeability (pores, fractures, fissures), production/injectivity index, hydrostatic pressure, transmissibility, Physico-Chemical Fluid Properties TDS, ph, Eh, conductivity, gas, density, viscosity, Geology and Mineralogy petrography, mineralogy of the host rock, mineralogy of secondary precipitations in fractures and fissures green: relevant for acidizing procedures 10
Reservoir Types: Sandstone Sandstone characteristics Sedimentary rock Mineralogy Silica grains (quartz, feldspar, phyllosilicates) Grains are cemented by quartz, clay minerals or carbonates Pore fillings: clay minerals, oxides Minor amounts of oxides and clay minerals Permeability Porouse matrix Total porosity vs. open pore space Fractures and fissures Fracture fillings Target of sandstone acidizing Dissolve cementing minerals Increase open pore space Clean fractures from secondary minerals 6
Reservoir Types: Carbonates Carbonate characteristics Sedimentary rock Mineralogy Limestones (mainly calcite and dolomite) Minor amounts of clay minerals and oxides Permeability Compact matrix Low porosity Flow channels Target of carbonate acidizing Creating new flow channels Etching new pits in the host rock 7
Reservoir Types: Granites Granite characteristics: Igneous rock (intrusive) Mineralogy Quartz, feldspar, mica Secondary minerals: calcite, quartz, clay minerals (illite, chlorite, kaolinite, smectite), barite and oxides Permeability Compact matrix Low porosity Fractures and fissures Pores in alterated granites Target of granite acidizing Dissolving and mobilization of fines and secondary minerals in fractures 8
Reservoir Types: Basalt and Andesite Basalt characteristics: Igneous rock (extrusive) Mineralogy Plagioclase, pyroxen (augit), possibly: olivine, hornblende, magnetite secondary minerals: calcite, silica, epidot Permeability Compact matrix Low porosity Fractures and fissures Target of basalt acidizing Dissolving and mobilization of fines and secondary minerals in fractures 9
History of Acidizing in the Oil and Gas Industry 1896 Patent: Limestone acidizing with HCl by Herman Frasch, Standard Oil Co s Solar Refinery, Ohio 1928 Successful use of HCl for cleaning pipes and other equipment from calciferous scales by Gypsy Oil Co., Oklahoma 1932 John Grebe, Dow Chemical Co., used arsenic acid as corrosion inhibitor for HCl acid treatment 1933 Patent: Sandstone acidizing with HF by Jesse Russel Wilson, Standard Oil Co., Indiana 1933 Acidizing of a well with a HCl/HF mixture by McPherson, Halliburton. Result of the treatment: Creation of huge amount of sand 1940 Commercial use of Mud Acid (12% HCl and 3% HF) by Dowell 1984 McLeods guidelines for mud acid ratios as a function of type and concentration of silicate minerals present in the reservoir 2
Chemicals in Use for Matrix Acidizing Acids Mineral dissolution (surface or the whole grain) Mobilization of mineral grains by decomposition of the rock structure Chelating agents Precipitation inhibitor Complexing ions in the fluid Reducing reactivity of specific mineral surfaces (ion exchange processes, adsorption) Additives Corrosion inhibitor Iron control Clay controler Keep in mind! Chemical compatibility reservoir fluid host rock minerals of the fracture fillings temperature pressure 11
Chemicals in Use for Acidizing: Acids ACIDS Inorganic Organic Mixtures Hydrochloric acid (HCl) Acetic acid (CH 3 COOH) Organic Clay Acid: Organic Acids/HBF 4 Hydrofluoric acid (HF) Formic acid (HCOOH) Rock Mud Acid (HCl/HF) Citric Acid (C 6 H 8 O 7 ) Tetrafluoroboric acid (HBF 4 ) Biodegradable acids 12
Chemicals in Use for Acidizing: Acids Which mineral dissolves in which acid? Nearly all acids HF, HCl/HF, HBF 4 Low solubility in acids Carbonates Silicates Sulfates/Sulfides Calcite (CaCO 3 ) Quartz (SiO 2 ) Gypsum (CaSO 4 x 2H 2 O) Dolomite (CaMg(CO 3 ) 2 ) Feldspars (Plagioclase, Orthoclase, ) Anhydrite (CaSO 4 ) Micas (Biotite, Muscovite) Barite (BaSO 4 ) Clay minerals (Kaolinite, Smectite, Illite, Chlorite) Galena (PbS) Pyrit (FeS 2 ) 12
Chemicals in Use for Acidizing: HCl Hydrochloric acid (HCl) Inorganic, strong acid Inexpensive Very good dissolver of carbonates: Calcite CaCO 3, Dolomite CaMg(CO 3 ) 2 Stable up to high temperatures and pressure Very fast reaction rate Reaction rate increases with increasing temperature Reaction products are highly soluble in water Etching wormholes in limestone (after Crowe et al., 1992) Dissolution of calcite Dissolution of dolomite CaCO 3 + 2HCl CaMg(CO 3 ) 2 + 4HCl CaCl 2 + CO 2 + H 2 O CaCl 2 + MgCl 2 + 2CO 2 + 2H O Reaction products CaCl 2 and MgCl 2 are highly soluble in water 13
Chemicals in Use for Acidizing: HF basic reactions Hydrofluoric acid (HF) Inorganic, strong acid Inexpensive Dissolution of silicates and carbonates Stable up to high temperatures and pressure Reaction rate increases with increasing temperature Secondary reaction products can precipitate Primary reaction Dissolution of Quartz SiO 2 + 4HF SiF4 + H2O SiF 4 + 2F - SiF 2-6 Dissolution of Feldspar (Anorthite) 18HF + CaAl 2 Si 2 O 8 + 2 HCl CaCl 2 + 2AlF 3 + 2H 2 SiF 6 + 8H 2 O Secondary reaction Formation of aluminium fluorides 3H 2 SiF 6 + H 2 O 3H 4 SiO 4 + 12HF + 3F - and 3F - + mineral(si, Al) + H + AlF3 Tertiary reaction Formation of aluminium flourides with decreasing F/Al ratio until all HCl is spend 14
Chemicals in Use for Acidizing: Mud acid Mud acid: Mixture of HCl and HF How does it work? Dissolving silicate and carbonates HCl supports secondary and tertiary reactions Solubility of Bentonite in HF/HCl and in HCl Solubility of Quartz in HF/HCl and in HCl Different solubilities of silicate minerals in HF HF/HCl HCl HF/HCl HCl (after Bradley, 1989) 15
Chemicals in Use for Acidizing: Mud acid Selection of the ideal HF/HCl ratio depends on the mineralogy of the formation Efficiency is a function of Amount of silicate minerals in the reservoir Type of silicate minerals (clay minerals are higher soluble in HF than quartz) Carbonate concentration Acids varies between HF: 0.5 3 % and HCl 4 15 % First guideline for HF/HCl ratios as a function of the mineralogy: McLeod 1984 Guidelines were extended until today to very specific mixtures Selection of the proper mixture requires detailed knowledge of the mineralogy Example for HF/HCl mixtures as a function of mineralogy and permeability for BTH >200 F (93 C) Mineralogy >100 md 20-200 md < 20 md High quartz (> 80%), low clay (< 10%) 10% HCl. 2% HF 6% HCl. 1.5% HF 6% HCl. 1% HF High clay (> 10%), low silt (< 10%) 6% HCl, 1% HF 4% HCl, 0.5% HF 4% HCl, 0.5% HF High clay (> 10%), high silt (> 10%) 8% HCl, 1% HF 6% HCl, 0.5% HF 6% HCl, 0.5% HF Low clay (< 10%), high silt (> 10%) 10% HCl, 1% HF 8% HCl, 0.5% HF 8% HCl, 0.5% HF (after Crowe et al., 1992) If a little is good then more is better, right? Not necessarily! 16
Non-welcome Acid Mineral reactions Dissolving minerals means It influences the electrolyte equilibrium of the reservoir fluid (increasing concentration of specific electrolytes) In the worst case: Formation of insoluble products Dissolution of calcite: CaCl 2 is highly soluble in water CaCO 3 + 2HCl CaCl 2 + H 2 O CaCl 2 + CO 2 + H 2 O Ca 2+ + 2Cl - + H 2 O 2HF + Ca 2+ + 2Cl - CaF 2 + HCl + H 2 O Non-welcome secondary precipitation products calcium fluorite CaF 2 amorphous silica SiO 2(am) sodium hexafluosilicate Na 2 SiF 6 sodium hexafluoaluminate Na 3 AlF 6 potassium hexafluosilicate K 2 SiF 6 In sandstone reservoirs mud acid should not be used at carbonate concentrations >20% (McLeod, 1984) calcium hexafluosilicate CaSiF 6 and others 22
Chemicals in Use for Acidizing: Retarded Acids Benefits of Reaction Rate Retardation Acid placement in the reservoir: deeper penetration of the chemicals Two ways to retard the reaction rate of acid dissolution processes 1) use of weak or diluted acids 2) slow decomposition of acids Weak Acids Organic Acids: Acetic acid (CH 3 COOH), Formic acid (HCOOH), Citric Acid (C 6 H 8 O 7 ) Slow reaction rate Reactive from the near-field to the far-field region Carbonate minerals 2HOrg + CaCO 3 CaOrg 2 + H 2 O + CO 2 Retarded Acid: Tetrafluoroboric acid HBF 4 Hydrolysis in water Formation of hydroflouric acid (HF) and hydroxoflouboric acid (HBF 3 OH) HBF 4 + H 2 O HBF 3 OH + HF HBF 4 forms borosilicates at clay mineral surfaces which act as a coating 17
Chemicals in Use for Acidizing: Chelating agents Chelating Agents (Examples) Ethylenediaminetetraacetic acid (EDTA ) C 10 H 16 N 2 O 8 Nitrilotriacetic acid (NTA) C 6 H 9 NO 6 1 2 Complexing specific cations in form of soluble complexes Inhibition of nucleation ph Coordination number of the complex Temperature Co-ions (Ca 2+, Mg 2+, Sr 2+, Pb 2+, Cu 2+, ) Adsorption on growth-active surfaces Retardation of the crystal growth: different morphology example: EDTA M...metallic ion 1 2 3 4 5 6 favorized not favorized favorable energetic state (after Kleber, 1990) 18
Chemicals in Use for Acidizing: Additives Corrosion inhibitors Retardation the reaction rate between acid (HCl) and metal Protection of casing, pumps, valves or in short: every metal equipment in contact with acid No interruption of the reaction between acid and minerals like carbonates and silicates Time of effectiveness is a function of the temperature and type of the acid Concentration in % Temperature in F Protection Time in h 0.6 175 (79.4 C) 24 1.0 250 (121.1 C) 6 2.0 300 (148.9 C) 6 2.0* *with inhibitor aid 350 (176.7 C) 4 Table after Bradley, 1989 Iron control Dissolving of corrosion products in the casing or tubing Dissolution of iron minerals in the reservoir Chelating agents: citric acid, acetic acid, EDTA, NTA Reducing agents: erythorbic acid Reducing agents do not prevent sulfide precipitation Sandstone acidizing 19
Chemicals in Use for Acidizing: Additives Clay Controler Control of ion exchange processes on clay mineral surfaces Reducing ion exchange capacity of clay minerals by controlled adsorption or ion exchange of a specific chemical on the mineral surface Chemicals in use: NH 4 Cl, KCl, polyquarterny amines (Very!) Simplified sketch of clay swelling Impact of the Clay controler Many more additives are available for matrix acidizing (Antiscalings, diverting agents, surfactants, ) 20
Basics of Rock/Mineral Fluid Interactions during Acidizing Mineral Fluid Interactions during Acidizing Dissolution (acid mineral surface) Ion exchange (acid, chelating agent, additive mineral surface) Adsorption (acid, chelating agent, additive mineral surface) Formation of water soluble complexes Formation of insoluble complexes Localized disturbance of the mineral saturation state in the reservoir fluid Fluid selection for acidizing (Type and concentration of acid, chelating agent, additive) Reservoir mineralogy Mineralogy of fracture and fissure fillings Physical properties of the host rock (permeability, flow pathes) Type of damage (carbonates, silicates or both) Compatibility issues Compatibility with the reservoir fluid (electrolyte concentration, dissolved gases, ) Temperature and pressure Diffusion processes 21
Acidizing procedure from the technical point of view The technical organization of an acid treatment is challenging until today Well condition Well preparation (e.g. pickling of production pipe) Injection of clean solutions Best injection practice for the specific well: Wellhead or coiled tubing Injection volume and injection rate Injection intervalls Shut down times Acidizing Soultz 2006. Photo GEIE Acidizing Soultz 2007. Photo GEIE Background knowledge for designing an acidizing operation Well history Laboratory test data Modelling And in the best case: Experience of acidizing the specific reservoir 23
Laboratory Tests of Reservoir Materials Testing the compatibility of acids and additives on reservoir material (cores, cuttings, fluid and if possible of the damaging material) Characterization of the reservoir mineralogy before and after treatment Laboratory analyses: XRD, XRF, SEM, DTA, Thin section analysis Cores, cuttings Acid response curves Change of permeability by injection of acid and additives into a core (p/t conditions of the reservoir) Compatibility issues Reservoir fluid analyses Treating fluid Reservoir fluid mixtures Dissolution experiments for investigation possible secondary and tertiary reactions Before After Stimulation of a core sample from Soultz (GPK4) with Mud Acid 12% HCl and 3% HF 25
Acidizing procedure from the technical point of view Acidizing of sandstone and igneous reservoirs: silica rich environments How does it work Preflush Preparation of the formation Dissolution of carbonates Transport of Ca 2+ to a far-field region of the borehole Acid applied: HCl up to high concentrations Presence of swellable clays: NH 4 Cl injection as an anti-clay swelling agent Cooling of the formation Main Flush Damage removal Dissolving minerals in fractures (igneous rocks) and of cementing minerals and pore fillings (sandstone) Acid applied: HF/HCl mixture or HBF 4 /HCl for retarded dissolution reactions Overflush Cleaning the treated formation Transport of non-reacted mud acid to a far-field region Transport of reaction products to a far-field region Much bigger volume required than the volume used for the main flush (permeability and anisotropy of the reservoir) several stages of the overflush: acids (HCl or organic acids) for the first stage 24
Best Case and Worst Case Scenarios: GPK 2, GPK 3 and GPK 4 Soultz Soultz sous Forêts, France: production well GPK 4 Acidizing operations in 2005, 2006 and 2007 GPK-4 Best case scenario: GPK 4 Soultz 13 3/8" casing with DV Tool and packer @ 530 m 20" casing shoe 576 m Drill 24" 13 3/8" casing cut-off @ 473 m to install a 13 3/8" pumping chamber later Productivity of GPK 4 increased from 0.2 to 0.5 Ls -1 bar -1 after acidizing 13 3/8" casing shoe 1446 m Top of Cement # 4000 m MD 9 5/8" Casing Shoe 4489 m TVD 4756 m MD Drill 17 1/2" Drill 12 1/4" Kick off Point 2135 m 2 10" Inflatable CuNi Packers Drill 8 1/2" Worst case scenario: GPK 4 Soultz NTA injection decreases productivity: Formation of a plug in the downhole section During OCA stimulation fracturing pressure was exceeded Casing of the down hole section of GPK 4 was damaged in 4100 m depth 4982 m TVD 5260 m MD 26
Acidizing of GPK 4 between 2005 and 2007 GPK 4: 4 well acidizing operations were conducted after hydraulic stimulation (Extensive evalutions of the acidizing operations by Nami et al., 2008 a&b, Portier et al., 2009 and Schindler, 2007) 2005 February 4500 m 3 of 0.2% HCl, Q~27l/s Productivity: 0.2-0.3 Ls -1 bar - 1 2006 May Preflush: 25 m 3 15% HCl, Q~22 l/s Main Flush: 200 m 3 RMA (12% HCl, 3% HF), Q~22 l/s Productivity: 0.5 Ls -1 bar -1 2006 October 200 m 3 Na 3 NTA 19% - NaOH (ph 12), Q~22 l/s Formation of a plug 2007 March 200 m 3 OCA (5-10% citric acid (C 6 H 8 O 7 ), 0.1 1% HF, 0.5-1.5% HBF4 and 1-5% NH4Cl), Q~55 l/s Productivity: 0.4-0.5 Ls -1 bar - 1 Acidizing operation in Soultz sous Forêts by Schlumberger, 2006 27
References Bradley, H.B.: Chapter 54, Petroleum Engineering Handbook, SPE (1989) Crowe, C., Masmonteil, J., Thomas, R., 1992. Trends in matrix acidizing. Oilfield Review 4, 24 40. McLeod, H.O., 1984. Matrix acidizing. Journal of Petroleum Technology 36, 2055 2069. Nami, P., Schellschmidt, R., Schindler, M., Tischner, R., 2008a. Chemical stimulation operations for reservoir development of the deep crystalline HDR/EGS system at Soultz-sous-Forêts (France). In: Proceedings of the 32ndWorkshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, USA. Nami, P., Schellschmidt, R., Schindler, M., Tischner, R., 2008b. Chemical stimulation results at Soultz and overview of the flow performance of the wells. In: Proceedings of the EHDRA Scientific Conference 24-25 September 2008, Soultz-sous-Forêts (France). Portier S., Vuataz F.-D., Nami P., Sanjuan B., Gérard A., 2009. Chemical stimulation techniques for geothermal wells: experiments on the three-well EGS system at Soultz-sous-Forêts, France. Geothermics, 38, 349-359. Schindler, M., 2007. Chronology of holes and hints in GPK 4. Internal Report, GEIE. Walsh, M.P., Lake, L.W., Schechter, R.S., 1982. A description of chemical precipitation mechanisms and their role in formation damage during stimulation by hydrofluoric acid. Journal of Petroleum Technology 34, 2097 2112. 31
Merci pour votre attention Thank you for your attention France, Germany, Switzerland French & German Industry