SPE 121728

New Insights into the Viscosity of Polymer-Based In-Situ Gelled AcidsA.M. Gomaa and H.A. Nasr-El-Din; SPE, Texas A&M University

Copyright 2009, Society of Petroleum Engineers

This paper was prepared for presentation at the 2009 SPE International Symposium on Oilfield Chemistry held in The Woodlands, Texas, USA, 2022 April 2009.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Diversion techniques must be employed to remove all the damage from the entire producing interval. Diversion in carbonates is more difficult than in sandstones because of the ability of acid to significantly increase the permeability in carbonates as it reacts in the pore spaces and flow channels of matrix. The apparent viscosity of the in situ gelled acid based on polymer playsa key role in diversion because it creates a viscosity differential in treated and untreated zones. An extensive literature survey and field data, there is no agreement on the effectiveness of this acid system. Therefore ,this study was conducted to better understand this acid system and determine factors that impact its visocity build-up. Three commercially available in-situ acidswere examined The effect of salts, iron contamination on the apparent viscosity of these acids was examined. Several new findings were identified, including: Polymer and other additives were separated out of the acid when these acids were prepared in high salinity water. Preparing the in-situ gelled acid with saline water decreased the viscosity of the acid in live and neutralizedconditions. Concentrated HCl solutions produced high concentrations of calcium chloride that reduced the viscosity of the acid system. Therefore, in-situ gelled acids that are based on the polymer should be used at low HCl concentrations (3-5 wt% HCl). Sodium, calcium or ferric chlorides reduced the apparent viscosity of live acids. A brown precipitate was noted during the neutralization of acid systems that contained Fe(III) even in the presence of the recommend concentration of iron control agents.

Introduction Effective diversion of reactive acid to achieve uniform treatment of the entire interval is necessary for the successful matrixacidizing. When injected, the acid tends to follow through the path of least resistance, that is to the higher permeability and/or least damaged zones. Since damage must be removed or bypassed from the entire producing interval, effective diversion techniques must be employed. Diversion in carbonates is generally more difficult than in sandstones because of the ability of the highly reactive HCl to drastically increase permeability in carbonate rock as it reacts in the pore spaces and flow channelsof the matrix. Apart from particulate diverters, a number of methods and techniques which are commonly used for acid diversion in matrix treatments include mechanical (packers, bridge plugs, ball sealers, coiled tubing), chemical methods (gelled acids based on polymer or surfactant, emulsified acids and foams), Chang et al. (2008). The in-situ gelled acid is known as self-diverting acid or viscosity control acid that uses pH value to control a cross-linking reaction. De Rozieres et al. (1994) showed that this viscosity will reduce the diffusion coefficient of the hydrogen ion (H+)and, as a result, the rate of mass transfer of the acid into the rock surface will decrease. The polymer and/or the generated gel forms an external filter cake that can reduce the leak-off rate of the acid. Yeager and Shuchart (1997) showed that in-situ gelled acids that cross-linked by iron formed a gel at a pH value of nearly 2. Conway et al. (1999) mentioned that the in-situ gelled acid was very viscous, especially in the pH range of 2-4. Mohamed et al. (1999) examined matrix acid treatments of a large number of seawater injectors. Field data indicated that this acid can cause damage in some cases. They indicated that thevolume of in-situ gelled acid should not exceed nearly 30 vol% of total acids used in typical matrix acid treatment. Taylor et al. (1999) showed that the type of iron compound depends on the level of hydrogen sulfide present. In sweet wells(no hydrogen sulfide, hydrate iron hydroxides will precipitate at pH values of 1-2. Crowe (1985) and Brezinski (1999) showed that in sour wells, however, iron sulfide species will precipitate at pH value of 1.9. The presence of ferric iron can enhance sludge formation with heavy oils and increase corrosion rate. Saxon et al. (2000) reported positive field results. One of the concerns raised about in-situ gelled acid is the presence of ferric iron in the system. It is well known that iron can precipitate in the formation and caused damage.

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Chang et al. (2001) showed that the in-situ gelled acid combines the capabilities of stimulation and diversion in one process, which significantly reduces the operational complexity. Rheology of the in-situ gelled acid is a preferred mechanism in diversion because it creates a viscosity differential in treated and untreated zones. Lynn and Nasr-El-Din (2001) investigated in-situ gelled acids at high temperature conditions. They noted that polymer residue attached to the walls of the wormholes created by the acid. Also, the iron cross-linker precipitated on the surfaces of the wormholes. Precipitation of iron and polymer residue can reduce the outcome of acid treatments. The in-situ gelled acids enhanced the permeability of the reservoir cores by a factor that depended on acid injection rates. Nasr-El-Din et al. (2002) showed that there is polymer residue on theinjection side of the cores. Flow back will be required to reduce the damage due to polymer residue. They also highlighted various negative interactions of this system in the presence of hydrogen sulfide, even in the presence of iron control agents. One of the noted disadvantages of this system is that the cross-linker may precipitate in the formation in some cases. Taylor and Nasr-El-Din (2003) tested three different in-situ gelled acid systems based on polymer at different temperature.They found the reaction of all three of the in-situ gelled acids was significantly retarded compared to the corresponding HCl solution without additives. The primary cause of this reduction in reaction rate was the polymer present in each of the acid formulae. Coreflood studies showed that the polymer and cross-linker component of in-situ gelled acids irreversibly reduced the permeability of carbonate reservoir rock. Abdel Fatah et al. (2008) found that in situ gelled acids that are based on aluminum formed a gel at pH higher than that noted with iron-based cross-linkers. Also, corrosion inhibitor affected the pH at gelation: it reduced the pH at gelation for the iron cross-linker; whereas it increased the pH at gelation for the aluminum cross liker. Mutual solvent did help in removing polymer residue from the cores. Acid system in the field is sometimes prepared using seawater or other saline water. The objective of the present study is to determine the impact of the salinity of field mixing water on the viscosity of in-situ gelled acids that are based on polymers. A second objective is to assess the effect of iron contamination of the gelation process of these acid systems.

Mechanism of Viscosity Build-up According to Hill (2005), suitable polymers can be any polymer that is stable in an aqueous acid environment and that can be cross-linked in the presence of ferric ions or zirconium ions at a pH of about 2 or greater. The polymer should be containing carboxyl groups, such polymers include acrylamide and acrylamide copolymers. Initial spending of the live acid, during leak-off and worm-holing, produces a rise in pH to a value of above about 2, whichinitiates cross-linking of the polymer (resulting in a rapid increase in viscosity). This increase in viscosity creates the diversion from wormholes, from fissures, and within the matrix. The highly viscous acid will plug off the treated zone, forcing the following stages of fresh live acid to be diverted to the untreated zones. The lower viscosity fresh acid allows penetration intoother areas, until the reaction of the fresh acid increases the pH value and causes cross-linking, thereby diverting the following acid stages to other portions of the reservoir. Acid in the wellbore and from subsequent treatment stages will keep the pH sufficiently low therefore the viscosity will be maintained until the end of the job, at which time the acid is allowed to spendcompletely. Hill (2005) stated that as the in-situ gelled acid system spends further, the pH continues to increase. For polymer that cross-linked by ferric ions, this polymer does not cross-linked by ferrous ions. As the acid spends further and the pH continuesto rise, the reducing agent converts the ferric ions to ferrous ions. The gel structure will collapse and the acid system revertsback to a low viscosity fluid. Further pH increases to values above about 3.5 will reduce the viscosity to that of an uncross-linked polymer thickened fluid at the well temperature. It has been observed that the retention of some viscosity after spendingassists in maintaining the formation fines in suspension and facilitates an improved cleanup (if the reservoir is not too depleted). Hill (2005) examined the effectiveness of various breakers, including sodium erythorbate (a reducing agent, but was found to be too active in live acid; a significant amount of the ferric ion was reduced below a pH of about 2 even at low temperatures. That will lead poor gelling. Therefore, the preferred reducing agents were hydrazine sulfate or hydroxylamine hydrochloride, which dont reduce ferric iron in the live acids. For polymers that are cross-linked by zirconium ions (Boles et al., 1996), these complexes are preferably zirconium or titanium compounds with poly-functional organic acids. Gel breakers used with these fluids are formed from materials capable of complexing with the organo-metallic cross-linkers like fluoride, phosphate, sulfate anions and multi-carboxylated compounds. Boles et al. (1996) stated that fluoride reacts readily when introduced into the cross-linked polymer gel. Therefore, it needed to be coated with a water insoluble wood resin material. This coating controls the release of the fluoridefrom the fluorspar. This resin is essentially a mixture of high molecular weight phenolic compounds, resin acids and neutral materials.

Experimental Studies MaterialsHydrochloric acid (ACS reagent grade) titrated using a sodium hydroxide solution to determine its concentration, and was found to be 36.78 wt%. Calcium carbonate powder and sodium hydroxide (ACS grade) were used to neutralize the live acid. Sodium chloride, calcium chloride dehydrate, iron (III) chloride anhydrous (ACS grade) were used as a source of mono-, di-, and tri-covalent cations. Deionized water obtained from purification water system (BARNSTEAD EASYpure PoDi-model

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D13321) that have resistively of 18.2 M.cm at room temperature. Polymer and other additives were all oilfield chemicals, and were used without further purification.

Measurements Viscosity measurements were made using M3600 viscometer. The viscosity was measured as a function of shear rate in the range of 1020 to 0.1 s-1. Measurements were conduct at atmospheric pressure and temperature range from room temperature. pH values were measured using 950 ROSS FASTQC. All acids were mixed continuously during the experiments to ensure that acid was neutralized to the required pH values.

Procedures Acid formulas that used in this work are listed in Tables 1-3 for acids A, B, and C, respectively. This study will be involved by studying the behavior of gelled acids, and the behavior of in-situ gelled acids. Gelled acids are mainly composed of acid, polymer, and corrosion inhibitor which usually used to minimize the acid leak-off, and stimulated the in deep zones (retardation system). In-situ gelled acids are mainly composed of acid, polymer, corrosion inhibitor, cross-linker, and breakerwhich used as a diversion technique. Acids A and C use iron (III) as a cross-linking agent while Acid B uses a zirconium (IV). To study the behavior of the gelled acids: acids A, B and C were prepared by mixing the acid, corrosion inhibitor, polymer, and different salts in deionized water. Salts used were: sodium chloride (NaCl) with concentrations of 3.5, 6, and 10 wt%; calcium chloride (CaCl2) with concentrations of 3.5, and 6 wt%; and ferric chloride (FeCl3) with concentrations of 1 and 6 wt%. To study the behavior of the in-situ gelled acids: acids A, B and C were prepared with all acid additives as described in Tables 1-3. The prepared acids were examined in live and partially spent conditions when was mixed with salt water (4 wt% NaCl), and in the percent of normal amount of ferric cations (400-500 ppm which equivalent to 0.15 wt% FeCl3). The steps for acid preparation were as follows:

1. Deionized water was mixed with NaCl, CaCl2, or FeCl3 for 5 minutes. 2. HCl acid, polymer, corrosion inhibitor, and other additives were added to the water and mixed according to the

mixing procedures that each company for the three acids. 3. The acids with additives were mixed for 30 minutes using 350 rpm mixer, and after that were centrifuged for 20

minutes at a rotational speed 2,500 rpm to remove air bubbles that formed during mixing. 4. The apparent viscosity was measured at 28C using the M3600 viscometer for live acids. 5. Live acids were neutralized gradually by calcium carbonate powder or sodium hydroxide solid while pH was

monitored. 6. The apparent viscosity was measured as a function of the equilibrium pH value, which was achieved when the pH

value of the partially neutralized acid became constant.

Results and Discussion Compatibility Tests Ferric ions have three positive charges and can cross-link the polymer, and build-up the required viscosity. Acids usually contaminated with ferric iron due to dissolution of rust from dirty tanks, equipment, coiled tubing. Due to surface contamination, the concentration of ferric ions in live acids can reach up to 10,000 ppm (Al-Nakhli et al., 2008). Acids A, B, and C (without additives) were prepared with 1 wt% FeCl3. The results show that there was no phase separation in the case of acid A, while polymers of acids B, and C were separated from the solution, Fig. 1. Polymer A separated out of solution in the presence of 4 wt% NaCl and 1 wt% FeCl3. Acids A, B, and C were prepared in 10 wt% NaCl. The polymers and the corrosion inhibitors separated out from the live acids. Polymers of the three acids systems were incompatible at high ferric ions or salts. It is recommended to minimize the iron contamination in live acids by pickling and cleaning the mixing tank and equipment before preparing the acid. Also, the acid should be prepared using water of low salinity. High salt concentrations reduce the solubility of these polymers in live acids.

Effect of Salts Live Acids - Gelled Acids The viscosity behavior of live in-situ gelled acids was examined at an initial HCl concentration 5 wt%, temperature of 28C, and atmospheric pressure. It is desirable to have a low viscosity in the live in-situ gelled acid to allow pumping of the acid into the formation. This is an important issue when stimulating deep wells or horizontal wells with extended reach. Acid additivesare listed in Table 1 without the breaker or cross-linker for acid A. NaCl, CaCl2, or FeCl3 was added at different concentrations, Table 4. Fig. 2 show the effect of NaCl, CaCl2, and FeCl3, respectively on the apparent viscosity of live acid A (no cross-linker, and no breaker). The apparent viscosity of the live acid decreased significantly, as the salt concentration wasincreased. The viscosity behavior of the live acids at test conditions is summarized in Table 4 using the power-law equation. Viscosity and shear rate relationship for all acid systems examined was described by the power-law model, Eq. 1:

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= k n-1 (1) Where is the fluid viscosity, mPa.s; k is the power-law constant, mPa.sn; is the shear rate, s1; and n is the power-law index, dimensionless. Salts have different effects on the viscosity of live acids. Sodium chloride had less effect on viscosity of live acid thancalcium or ferric chloride. A similar behavior was noted with Acids B and C without their breakers or cross-linkers; and the results are given in Table 5. The results discussed thus far indicate that salts reduced the apparent viscosity of live acids, and this trend increased with salt concentration. Also, the viscosity decreased with the number of positive changes of the cation used. It is important to note that the carboxylate groups of the polymer are not protonated in live acids. Based on these results, it appears that salts affect the configuration of the polymer in such a way that reduces its size, which resulted in this viscosity decrease. Untimely, the polymer separated out of solution indicating that live acids with high salt content become poor solvents to these polymers.

Spent Acids Gelled Acids Acid A (without salts, or additives) was prepared and reacted with increasing amounts of calcium carbonate to simulate spending of the acid with carbonate rocks. The apparent viscosity of the Acid A was measured as a function of shear rate at each equilibrium pH and the data were fitted using the power law model. Table 6 gives the power law parameters at each pH value. The viscosity increased at pH 1.4 and reaches a maximum value of 800 mPa.s at pH of 2. The viscosity remains constant as the pH value was further increased. Acids B, and C exhibited a similar behavior when neutralized by calcium carbonate, Table 7. The increase of viscosity with pH was interesting, especially there was no cross-linker. The carboxylate groups will be protonated at pHs values greater than nearly 2. It appears from these results that the calcium ions with two positive charges interacted with the negatively charged carboxylate groups, and increased the viscosity of the acid. To confirm this explanation, live Acid A was prepared and then neutralized using sodium hydroxide. This is because sodium ion has only one positive charge, which will has minimum interaction with the polymer molecules. Table 6 gives the power law contacts obtained with sodium hydroxide. The viscosity decreased as the pH was increased, Fig. 3. This decrease in viscosity is due to the charge screening effects of sodium ions. These results indicated that calcium ions interacted somewhat with the polymer molecules, which resulted in this viscosity increase. Spent Acids In-situ Gelled acids The effects of salts on the apparent viscosity of live and spent acids, without the cross-linker, highlighted the complexity ofthis system. It is of interest to investigate the combined effects of sodium chloride (added to the system), calcium chloride (produced from acid reaction with calcium carbonate) and ferric chloride (surface contamination) on the viscosity of partially spent acids. Acids A, B, and C were prepared with all additives as described in Tables 1-3. Four acid solutions were prepared: the first acid was prepared without salts, in the second system, the acid contained 4 wt% NaCl, in the third, the acidcontained ferric chloride, and in the last system, the acid contained 4 wt% NaCl and ferric chloride. Fig. 4 shows the effect of 4 wt% NaCl on the apparent viscosity of acid A. The viscosity of acid A prepared in deionized water increased at pH 2.5, which is expected from this system (Taylor and Nasr-El-Din, 2003). However, in the presence of 4 wt% NaCl, the viscosity increased to 1400 mPa.s, then remained constant. The maximum viscosity reached in the presence of sodium chloride was significantly less than that obtained when the acid was prepared in deionized water. Sodium chloride has very detrimental effect on the apparent viscosity of Acid A. A similar effect was noted with Acids B, and C, Figs. 5, 6. It is important to mention that this system contained calcium chloride, from the acid reaction, and ferric chloride or zirconium salts, cross-linker. These salts reduced the viscosity of live acids. Calcium chloride increased the viscosity of partially spent acid without the cross-linker. The results shown in Figs. 4-6, and Tables 8-10 indicate that the polymer did cross-link with ferric, or zirconium ions. Sodium chloride present in the system at pH 0 changed the configuration of the polymer in such a way that reduced the accessibility of the carboxylate groups to ferric, or zirconium ions and, as a result, the viscosity was much less than that noted when the acid was prepared in deionized water. The cross-linker used in Acids A and C is ferric chloride, while it is zirconium in acid B. It is of interest to examine the effect of additional iron on the viscosity behavior of acid B, Table 11. Fig. 7 compares the viscosity obtained with Acid B prepared in deionized water, in 0.15 wt% ferric chloride, and in 4 wt% NaCl + 0.15 wt% ferric chloride. Ferric chloride reduced the maximum viscosity obtained. The viscosity further decreased for the acid system that contained both sodium chloride and ferric chloride. The latter, however, showed an increase in viscosity at much higher pH values, which will not bebeneficial in diversion, and breaking of the produced gel. Effect of the Initial Acid Concentration on the In-situ Gelled Acid Systems Taylor and Nasr-El-Din (2003) noted that there was no increase in viscosity when the in-situ gelled acids were used at high HCl concentrations, but when these acids were tested at low HCl concentrations the viscosity increased significantly when the pH reached a value higher than nearly 2. To investigate this observation further, Acid C was prepared (Table 3) at acid concentrations of 3, 5, and 10 wt% HCl. Fig. 8 shows the viscosity as a function of equilibrium pH. The viscosity at 3 wt% HCl was higher than that noted at higher HCl concentrations. Calcium chloride reduced the viscosity of in-situ gelled acid,

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Fig. 8. Obviously, the concentration of calcium would increase when concentrated acids were used. The produced calcium reduced the viscosity of the in-situ gelled acids, and the higher the initial acid concentration, the lower of the maximum viscosity that can be obtained. Based on these results, it is recommended to use this acid at an initial concentration of 3-5 wt%. Higher acid concentrations will not produce high viscosity that is needed for proper diversion.

Breaking Mechanisms Acids A, and C, are cross-linked by ferric ions, therefore an iron reducing agent was used to break the gel formed by the acids.Acid B is cross-linked by zirconium ions, therefore a resin coated of calcium fluoride was used to break the gel formed by thisacid. Acids B and C were prepared with all additives (Tables 2-3), and were neutralized by calcium carbonate. Theoretically, these systems suppose to break at pH value 4-5, but actually pH was reached to 5.2, and sometimes to 6 with no decrease in the viscosity for either acid. The breakers for these systems were carefully selected to break the gel at high pH values, whenthe acid is almost completely spent. However, at pH 2-3, the concentration of acid is small, less than 0.04 wt% and the viscosity of the gel is very high. The high viscosity of the gel will reduce the diffusion coefficient of H+ to transfer to the surface of the rock and reacts. In addition, the low acid concentration will reduce the driving force of H+ to diffuse into the surface. Both factors will tend to delay the rate of gel breaking, at least under the test conductions. This observation raised a concern with the use of these acids. These gels will not be completely break, and will cause damage inside the formation. This is agreement with Lynn and Nasr-El-Din (2001), where a polymer gel residue was found in cores after coreflood tests using this acid system. One of the disadvantages of the acid system that contain ferric salt is theprecipitation of ferric hydroxide, Fig. 9.

Conclusions Gelled and in-situ gelled acids are used to reduce the leak-off rate during acid fracturing, and to divert the acids during matrixacidizing. The effect of salts, iron contamination on the apparent viscosity of these acids was examined. Based on the results obtained, the following conclusions can be shown:

1. Polymer and other additives were separated out of the acid when those acids were prepared in high salinity water. 2. Preparing the in-situ gelled acid with saline water decreased the viscosity of the acid in live and neutralized

conditions.3. High HCl concentrations produced high concentrations of calcium chloride that decreased the viscosity of the acid

system. Therefore, in-situ gelled acids that are based on the polymer should be used at low HCl concentrations (3-5 wt% HCl).

4. Sodium, calcium or ferric chlorides reduced the apparent viscosity of live acids. 5. A brown precipitate was noted during the neutralization of systems that contain Fe(III), even in the presence of the

recommend concentration of iron control agents

Recommendations In-situ gelled acids can be used at acid concentrations up to 28 wt% HCl. Also, it can be prepared using ether of aquifer, or seawater. However, it is strongly recommended to use polymer based in-situ gelled acids at 3-5 wt% HCl, and the acids should be prepared using fresh water.

Acknowledgments The authors wish to acknowledge the financial support of the Texas A&M U., the Texas Engineering Experiment Station of Texas A&M University and Crisman Institute for Petroleum Research.

References Abdel Fatah, W., Nasr-El-Din, H.A., and Moawad, T., and Elgibaly, A.: Effects of Cross-linker Type and Additives on the

Performance of In-Situ Gelled Acids, paper SPE 112448 presented at the 2008 SPE International Symposium and Exhibition on Formation Damage Control held in Lafayette, LA, 1315 February.

Al-Nakhli, A., Nasr-El-Din, H.A. and Al-Baiyat, A.A.: Interactions of Iron and Viscoelastic Surfactants: A New Formation-Damage Mechanism, paper SPE 112465 presented at the SPE International Symposium and Exhibition on Formation Damage Control held in Lafayette, LA, Feb. 13-15, 2008.

Boles, J.L., Metcalf, A.S., and Dawson, J.C.: Coated Breaker for Cross-linked Acid, United States Patent 5497830, Mar.12, 1996.

Brezinski, M.M.: Chelating Agents in Sour Well Acidizing: Methodology or Mythology, paper SPE 54721 presented at the 1999 SPE European Formation Damage Conference, The Hague, The Netherlands, 31 May 1 June.

Chang, F.F., Nasr-El-Din, H.A., Lindvig, T. and Qiu, X.W.: "Matrix Acidizing of Carbonate Reservoirs Using Organic Acids and Mixture of HCl and Organic Acids," paper SPE 116601 presented at the 2008 SPE Annual Technical Conference and Exhibition held in Denver, CO, Sept. 21-24.

Chang, F., Qu, Q., and Frenier, W.: A Novel Self-Diverting-Acid Developed for Matrix Stimulation of Carbonate Reservoirs, Paper SPE 65033 presented at the 2001 SPE International Symposium on Oilfield Chemistry held in Houston, TX, 1316 February.

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Conway, M.W., Asadi, M., Penny, G.S., Change, F.: A Comparative Study of Straight/Gelled/Emulsified Hydrochloric Acid Diffusivity Coefficient Using Diaphragm Cell and Rotating Disk, paper SPE 56532 presented at the 1999 Annual Technical Conference and Exhibition held in Houston, TX, 3-6 October.

Crowe, C.W.: Evaluation of Agents for Preventing Precipitation of Ferric Hydroxide from Spent Treating Acid, JPT (April1985) 691.

De Rozieres, J., Chang, F.F. and Sullivan, R.B.: Measuring Diffusion Coefficients in Acid Fracturing Fluids and their Application to Gelled and Emulsified Acids, paper SPE 28552 presented at the 1994 SPE Annual Technical Conference & Exhibition held in New Orleans, LA, 25-28 September.

Hill, D.G.: Gelled Acid, United States Patent Application Publication, US2005/0065041 A1, Mar. 24, 2005. Lynn, J.D. and Nasr-El-Din, H.A.: A Core-Based Comparison of the Reaction Characteristics of Emulsified and In-Situ

Gelled Acids in Low Permeability, High Temperature, Gas Bearing Carbonates, paper SPE 65386 presented at the 2001 SPE International Symposium on Oilfield Chemistry, Houston, TX, 13-16 February.

Mohamed, S.K., Nasr-El-Din, H.A., and Al-Furaidan, Y.A.: Acid Stimulation of Power Water Injectors and Saltwater Disposal Wells in a Carbonate Reservoir in Saudi Arabia: Laboratory Testing and Field Results, paper SPE 56533 presented at the 1999 SPE Annual Technical Conference and Exhibition, Houston, 36 October.

Nasr-El-Din, H.A., Taylor, K.C. and Al-Hajji, H.H.: Propagation of Cross-linkers Used in In-Situ Gelled Acids in Carbonate Reservoirs, paper SPE 75257 presented at the 2002 SPE/DOE Symposium on Improved Oil Recovery held in Tulsa, OK, 1317 April.

Saxon, A., Chariag, B., and Abdel Rahman, M.R.: An Effective Matrix Diversion Technique for Carbonate Reservoirs, SPEDC 15(1) (2000) 57-62.

Taylor, K.C. and Nasr-El-Din, H.A.: Coreflood Evaluation of In-Situ Gelled Acid, paper SPE 73707 presented at the 2002 international Symposium and Exhibition on Formation Damage held in Lafayette, LA 2021 February.

Taylor, K.C. and Nasr-El-Din, H.A.: Laboratory Evaluation of In-Situ Gelled Acids for Carbonate Reservoirs, SPEJ 8(4)(2003) 426-434.

Taylor, K.C., Nasr-El-Din, H.A. and Al-Alawi, M.: Field Test Measures Amount and Type of Iron in Spent Acids, paper SPE 50780 presented at the 1999 SPE Oilfield Chemistry, Houston, TX, 16-19 February.

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Table 1: In-situ gelled acid formula A Concentration Component

5 wt% HCl Hydrochloric acid 24 gal/Mgal Acid gelling agent: polymer emulsified in Hydrotreated light petroleum distillates (10-30 wt%);

Alcohols, c11-15-secondary, Ethoxylated (1-5 wt%) 5 gal/Mgal Corrosion inhibitor: Aliphatic amide (10-30 wt%), Methanol (10-30 wt%), Propan-2-ol (10-30

wt%), Quaternary ammonium compounds (10-30 wt%), Aromatic hydrocarbon (5-10 wt%), Prop-2-yn-1-ol (5-10 wt%).

2.5 gal/Mgal Cross-linker: Iron trichloride (30-60 wt%) in water, Specific gravity 1.45.

Table 2: In-situ gelled acid formula B Concentration Component

5 wt% HCl Hydrochloric acid 20 gal/Mgal Acid gelling agent: polymer emulsified in Hydrotreated middle petroleum distillates (10-30

wt%), Nonylphenol ethoxylate (1-5 wt%), Acrylic polymers (30-60 wt%) 4 gal/Mgal Corrosion inhibitor: Methanol (1-5 wt%), Isopropanol (1-5 wt%), Formic acid (30-60 wt%),

Organic sulfur compound (1-5 wt%), Quaternary ammonium compound (1-5 wt%), Haloalkyl heteropolycycle salts (10-30 wt%), Aromatic aldehyde (10-30 wt%), Oxyalkylated fatty acid(10-30 wt%).

10 gal/Mgal Cross-linker: A mixture of zirconium and aluminum salts in water 20 lb/Mgal Breaker: Resin-coated inorganic salt, 85 to 90 wt% calcium fluoride and 10 to 15 wt% resin. 2 gal/Mgal Alkoxylated alcohols: poly (oxy-1,2-ethanediyl)

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Table 3: In-situ gelled acid formula C

Concentration Component

5 wt% HCl Hydrochloric acid 20 gal/Mgal Acid gelling agent: polymer emulsified in Hydrotreated Light petroleum distillates (10-30 wt) 4 gal/Mgal Corrosion inhibitor: Methanol (30-60 wt%), Propargyl alcohol (5-10 wt%) 10 gal/Mgal Cross-linker: Ferric chloride (37-45 wt%) 20 lb/Mgal Breaker: Isoascorbic acid, sodium salt (60 to 100 wt%) 2 gal/Mgal Buffer: Hydroxyacetic acid (30-60 wt%)

Table 4: Power-law parameters of 5 wt% HCl, corrosion inhibitor, and polymer of live Acid A (Table 1 - no cross-liker, no breaker, and no additives) at different salt concentrations (28 C).

Salt, wt % Salt Type K, mPa.sn n R2

0 0 506 0.6008 0.998 3.5 NaCl 319.4 0.6059 0.9973 6 NaCl 124.15 0.5388 0.9725

3.5 CaCl2 151.34 0.5569 0.983 6 CaCl2 78.22 0.5508 0.9795 1 FeCl3 258.21 0.4279 0.9891

3.5 FeCl3 180.14 0.2998 0.9884

Table 5: Power-law parameters of 5 wt% HCl, corrosion inhibitor, and polymer of live Acids B & C (Tables 2 and 3, no cross-liker, no breaker, and no additives) at different salt concentrations (28 C).

Acid Type Salt, wt % Salt Type K, mPa.sn n R2

0 0 997.3 0.5269 0.9932

3.5 NaCl 590.2 0.6094 0.9971 B

10 gal/Mgal Zr Salts 426.73 0.6354 0.9946

0 0 889.54 0.4565 0.9985

3.5 NaCl 595.91 0.4969 0.9991 C

10 gal/Mgal Ferric Salts 426.73 0.6354 0.9946

Table 6: Power-law parameters of 5 wt% HCl, corrosion inhibitor, and polymer of live Acid A (Table 1 - no cross-liker, no breaker, and no additives) at different pH values neutralized by CaCO3, and NaOH (28 C).

Neutralization by pH K, mPa.s

n n R2

live acid 506 0.6008 0.998 0.18 576.52 0.5825 0.9976 1.38 712.57 0.5777 0.9983 2.4 826.45 0.5582 0.9986

CaCO3

4.89 806.97 0.5621 0.998 0.92 393.55 0.6089 0.9909

NaOH4.1 309.39 0.6167 0.9871

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Table 7: Power-law parameters of 5 wt% HCl, corrosion inhibitor, and polymer of live Acids B, and C (Table 2 and 3, no cross-liker, no breaker, and no additives) at different pH values neutralized by CaCO3 (28 C).

TypeEquilibrium

pH K,

mPa.sn n R2

live acid 997.3 0.5269 0.9932 0.018 1572.1 0.4615 0.9968

1.5 1739.4 0.4431 0.9965

2.1 1904.1 0.4277 0.9942

3.9 2055.8 0.4144 0.9913

B

6 706.2 0.5768 0.9942 live acid 748.05 0.4175 0.9812

1.8 860.25 0.469 0.9748 2.5 903.27 0.489 0.99

3.5 910.22 0.4979 0.992

C

5 915.5 0.4889 0.9917

Table 8: Effect of 4 wt% NaCl on the power-law parameters of Acid A (Table 1) at different pH values (28 C).

NaCl, wt% Equilibrium

pH K mPa.sn n R2

Live acid 298.26 0.5996 0.9965 1.7 456.31 0.588 0.9989 02.6 2260.4 0.4382 0.9929

Live acid 234.14 0.6032 0.9945 2.5 457.61 0.6157 0.9955 3.3 1375.2 0.498 0.9981

4

4.7 1463.9 0.4975 0.9982

Table 9: Effect of 4 wt% NaCl on the power-law parameters of Acid B (Table 2) at different pH values (28 C). NaCl, wt%

Equilibrium pH

K, mPa.sn n R2

live acid 426.73 0.6354 0.9946

1.3 533.49 0.6194 0.9951

2.5 873.45 0.5578 0.9968 3.7 6456.1 0.3204 0.9972

4.8 4208.8 0.36 0.9969

0

5.5 3367.3 0.3969 0.9929 live acid 581.52 0.5886 0.9915

1.45 318.22 0.6792 0.991

2.5 762.26 0.5672 0.9936

3.5 3136.6 0.4226 0.997 4.8 2320.1 0.4362 0.9987

4

5.2 2254.8 0.4394 0.9987

SPE 121728 9

Table 10: Effect of 4 wt% NaCl on the power-law parameters of Acid C (Table 3) at different pH values (28 C).

NaCl, wt% Equilibrium

pH K, mPa.sn n R2

live acid 439.22 0.6194 0.9942 1.5 819.1 0.5317 0.9937

3.5 4158.4 0.3511 0.994 0

5.1 4888.8 0.3557 0.9943

live acid 321.86 0.5788 0.9963 1.3 355.11 0.6038 0.9934 4

2.5 567.53 0.5475 0.9967

Table 11: Effect of 4 wt% NaCl, and 0.15 wt% FeCl3 on the power-law parameters of Acid B (Table 2)at different pH values (28 C).

Saltconcentration

Equilibrium pH

K, mPa.sn n R2

live acid 439.22 0.6194 0.9942 1.5 819.1 0.5317 0.9937

3.5 4158.4 0.3511 0.994

0.15 wt% FeCl3 which equivalent to

(400-500)ppm of ferric

5.1 4888.8 0.3557 0.9943 live acid 459.65 0.6282 0.9925

1.4 440.78 0.619 0.9895

2.5 981.17 0.5545 0.9948

3.5 2469.3 0.4661 0.9938

4.75 5778.4 0.3122 0.997

4 wt% NaCl + 0.15 wt%

FeCl3

5.8 4133.2 0.3424 0.9963

10 SPE 121728

Acid A (1 wt% FeCl3 + 4wt% NaCl)

Acid B (1 wt% FeCl3 Acid B (1 wt% FeCl3 ) Acid B (1 wt% FeCl3)Fig. 1: Phase separation of polymer in the live condition due to high salt concentration, 5 wt% HCl, 28 C.

0

50

100

150

200

250

300

350

400

450

500

0 1 2 3 4 5 6 7

Salt, wt%

Vis

cosi

ty, m

Pa.s

NaCl

CaCl

FeCl3

2

Fig. 2: Effect of NaCl, CaCl2, and FeCl3 on the apparent viscosity of live acid: 5 wt% HCl, corrosion inhibitor, and polymer of live Acid A (Table 1 - no cross-liker, no breaker, and no additives), (shear rate = 1 s-1, 28 C).

Phase separation Polymer residue

Phase separation

SPE 121728 11

0

100

200

300

400

500

600

700

800

900

0 1 2 3 4 5 6

Equilibrium pH

Vis

cosi

ty, m

Pa.s

Neutralized By CaCO

Neutralized By NaOH

3

Fig. 3: Apparent viscosity of 5 wt% HCl, corrosion inhibitor, and polymer of live Acid A (Table 1 - no cross-liker, no breaker, and no additives). Acid was neutralized to various pH values using CaCO3, and NaOH (shear rate = 1 s-1, 28 C).

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 1 2 3 4 5

Equilibrium pH

Vis

cosi

ty, m

Pa.s

04

NaCl, wt%

Fig. 4: Viscosity decrease of Acid A (Table 1) when it was mixed with 4 wt% NaCl (shear rate = 1 s-1, 5 wt% HCl, 28 C).

12 SPE 121728

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 1 2 3 4 5 6

Equilibrium pH

Vis

cosi

ty, m

Pa.s

0

4

NaCl, wt%

Fig. 5: Viscosity decrease of Acid B (Table 2) when it was mixed with 4 wt% NaCl (shear rate = 1 s-1, 5 wt% HCl, 28 C).

0

100

200

300

400

500

600

700

0 1 2 3

Equilibrium pH

Vis

cosi

ty, m

Pa.s

0

4

NaCl, wt%

Fig. 6: Viscosity decrease of Acid C (Table 3) when it was mixed with 4 wt% NaCl (shear rate = 1 s-1, 5 wt% HCl, 28 C).

SPE 121728 13

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 1 2 3 4 5 6 7

Equilibrium pH

Vis

cosi

ty, m

Pa.s

No salt

0.15 wt% FeCl

4 wt% NaCl + 0.15 wt% FeCl

3

3

Fig. 7: Viscosity changes when Acid B (Table 2) was mixed with 4 wt% NaCl, and 0.15 wt% FeCl3 (shear rate = 1 s-1, 5 wt% HCl, 28 C).

0

100

200

300

400

500

600

700

800

900

1000

0 0.5 1 1.5 2 2.5 3 3.5 4

pH

Vis

cosi

ty, m

Pa.s

3 wt% HCl5 wt% HCl10 wt% HCl

Fig. 8: Effect of initial acid concentration on Acid C (Table 3) neutralized by CaCO3, (shear rate = 1 s-1, 28 C).

14 SPE 121728

(Acid A) (Acid C) (Acid B)

Fig. 9: Precipitation of ferric hydroxide during neutralization by calcium carbonate of Acid A (Table 1), and C (Table 3), while no precipitation in Acid B (Table 2). (5 wt% HCl, no salt contamination, no iron contamination, 28 C).