Diversion and Cleanup Studies of Viscoelastic Surfactant-Based Self-Diverting Acid

Abstract

Abstract A self-diverting-acid based on viscoelastic surfactant (SDVA) has been used recently on stimulation treatments of carbonate formations. The new system has been proven successfull in more than 250 field applications. The decrease of acid concentration during the spending process viscosifies the fluid by the transformation from spherical micelles to an entangled wormlike micellar structure while penetrating the carbonate rock. The highly viscous fluid acts as a temporary barrier and diverts the fluid into the remaining lower-permeability treating zones. After treatment, the SDVA barrier breaks when contacted either by formation hydrocarbons or pre-and post-flush fluids. Quantifying diversion, fluid efficiency, and cleanup are important factors for successful candidate selection and job design. Laboratory tests defining these key factors are presented in this paper. This paper demonstrates the diverting ability of the acid as a function of permeability, characterized by introducing the concept of maximum pressure ratio (dPmax/dPo) supported by core-flow and acid conductivity tests using limestone and dolomite cores. Results demonstrate high dPmax/dPo in high-permeability cores and low dPmax/dPo in low-permeability cores. Retained permeability measurements are presented that assesses the level of cleanup. Flow initiation experiments of spent acid systems with gas and brine were performed to illustrate the cleanup behavior of SDVA in comparison to gelled acid systems under conditions encountered in gas and oil wells. The results indicate that SDVA systems clean up easily and that SDVA provides higher regained permeability than conventional gelled acid systems. Background The purpose of matrix stimulation in limestone and dolomite reservoirs is the formation of wormholes, which can bypass the damaged areas and increase the effective wellbore area. When acid enters the formation with the highest injectivity it creates highly conductive flow channels, called wormholes, by dissolving the carbonate-containing minerals. Consequently, the injectivity will be further increased. The other zones are left untreated by the acid. To overcome this problem, a diverting agent is used. Mechanical diverters such as ball sealers, degradable ball sealers, rock salt, and benzoic acid flakes are used alone or in conjunction with chemical diverters based on foams or polymeric gels.1,2 These materials can work effectively only in a narrow permeability contrast and may result in residual damage.3 These characteristics are highly undesirable, particularly in low-pressure gas wells, and in long vertical and horizontal sections. Polymer based systems such as in-situ crosslinked gelled acids (XLGA) have been used in the field as self-diverting fluids. These systems rely on a pH-triggered increase of viscosity during the acid spending process. Essentially, the pH change activates a metallic reagent that crosslinks the polymer chains, and the resulting viscosity increase causes a higher flow resistance.4,5 Further increase of the pH deactivates the metallic crosslinker and breaks the fluid down to the original linear gel with dissociated polymer chains. However, because of the nature of the long polymer chains, potential damage of the formation may occur.3 Recently, a new polymer-free self-diverting acid system was developed with a fluid stability in temperatures greater than300°F.6,7 The fluid system has been applied successfully in both matrix8 and acid fracturing9,10 treatments. It causes rapid viscosity development throughout the spending process. The reduction in acid concentration, together with the simultaneous release of ions in solution, promotes the transformation from spherical micelles into worm-like micelles, resulting in increased viscosity of the fluid. The highly viscous fluid subsequently diverts the remaining acid treatment fluid into zones of lower injectivity by reducing the acid loss into wormholes, resulting in an improved zonal coverage of the treatment interval. Diversion tests using multiple parallel cores with varying permeabilities showed effective stimulation in all cores.6,7 This paper presents new data providing further insight into the understanding of the unique properties of this self-diverting acid (SDVA) based on laboratory studies. Specifically described are the chemical and physical properties of the SDVA fluid, including cleanup efficiency that is relevant to low-pressure reservoirs.