Lessons Learned From Using Viscoelastic Surfactants in Well Stimulation

Abstract

Summary 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 4 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 because of 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 multilateral 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 to 275°F). In addition, they were successfully employed to stimulate seawater injectors and disposal wells where the bottomhole temperature was in the range of 100 to 150°F. More than 250 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 polymers, there was no need to flow back water injectors. 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 those treated with other acid systems.