Lessons Learned from Using Viscoelastic Surfactants in Well Stimulation

Abstract

Abstract Viscoelastic surfactant systems are used in the industry for several applications. Initially, the application was focused on low friction and solids suspension (fracturing and CT-cleanout) characteristics of the fluid. In the last three years, the application of viscoelastic surfactants was extended to acid-based systems for carbonate stimulation. These surfactants have the ability to significantly increase the apparent viscosity and elastic properties of the treating fluids. This is due to the ability of surfactant monomers to associate and form rod-shaped micellar structures under certain conditions. Viscoelastic surfactant-based acid systems have been used in Saudi Arabian fields in matrix acid stimulation, and in leakoff control acids during acid fracturing treatments. These surfactants were used to provide diversion during acidizing of vertical, long horizontal and multi-lateral wells. They were used in sour environments where hydrogen sulfide levels reached nearly 10 mol%. They were also utilized in gas wells to reduce acid leakoff, and create deep fractures in dolomitic carbonate reservoirs (250–275°F). In addition, they were successfully employed to stimulate seawater injectors and disposal wells where the temperature was in the range of 100- 120°F. More than 150 wells (oil, gas, water injectors and disposal wells) were treated with viscoelastic surfactant-based acid systems. The acid was placed either by bullheading, by using coiled tubing with or without a tractor. In some cases, these treatments included stages of emulsified or regular acids. All these wells responded positively to the treatment. There were no operational problems encountered during pumping these acids even when low permeability reservoirs were treated. Because these acid systems do not contain polymer, there was no need to flow back water injectors. However, the spent acid in oil and gas wells was lifted from the treated wells in a very short period of time. Finally, wells treated with surfactant-based acid systems showed sustained performance for longer times than wells treated with other acid systems. Introduction Matrix acidizing and fracturing treatments have been used to enhance the performance of oil, gas and water wells for several decades. Water and acid-soluble polymers have been used in these treatments to increase viscosity and hence, enhance diversion during matrix acidizing treatments. High viscosity fluids are needed during acid fracturing treatments to reduce leak-off rate during acid injection into the fracture. Various acid systems were introduced to enhance acid diversion by increasing the viscosity of the injected acid. Depending on the viscosifiying agent, these systems can be divided into two main categories: polymer-based acids and surfactant-based acids. Acid-soluble polymers have been used to increase the viscosity of HCl, and to improve its performance.1,2 As the viscosity of the acid increases, the rate of acid spending decreases and, as a result, deeper acid penetration into the formation can be achieved.3 The addition of uncross-linked polymers to HCl improved acid penetration, however, acid placement did not significantly improve.4 Cross-linked acids were introduced in the mid 70's as was cited by Metcalf et al.5 These acids have much higher viscosity than regular acids or acids containing uncross-linked polymers. Two types of cross-linked acids are available. The first type consists of a polymer, a cross-linker, and other acid additives.6 The acid in this case is cross-linked on the surface and reaches the formation already cross-linked. The second type of cross-linked acid consists of a polymer, a cross-linker, a buffer, a breaker, and other acid additives, e.g., corrosion inhibitors and surfactants. The acid in this case reaches the formation uncross-linked, and the cross-linking reaction occurs in the formation.7 The polymer used in the in-situ acid systems is a copolymer of acrylamide. This type of polymer is soluble in HCl acid over a wide range of acid concentration (1 to 28 wt% HCl). The polymer is cross-linked using multi-valent cations, e.g., Fe(III) and Zr(IV), via the carboxylate groups.3,8 It has been reported that the polymer in this system forms a gel within a narrow pH range.7 As a result of gel formation, the viscosity of the acid increases in-situ and acid diversion can be achieved. This gel will improve acid placement in matrix acidizing, provide more uniform damage removal, and control acid fluid loss in acid fracture treatments.