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

Abstract

Summary A self-diverting-acid based on viscoelastic surfactant (SDVA) has been successfully used recently on numerous stimulation treatments of carbonate formations in various fields. The decrease of acid concentration during the spending process viscosifies the fluid through 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 postflush 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/dP0) supported by core-flow and acid conductivity tests using limestone and dolomite cores. Results demonstrate high dPmax/dP0 in high-permeability cores and low dPmax/dP0 in low-permeability cores. Retained permeability measurements are presented that assess 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 (Williams et al. 1979; Economides and Nolte 1989). These materials can work effectively only in a narrow permeability contrast and may result in residual damage (Lynn and Nasr-El-Din 2001). 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 (Mukherjee and Gudney 1993; Saxon et al. 1997). 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 (Lynn and Nasr-El-Din 2001). Recently, a new polymer-free self-diverting acid system was developed with a fluid stability in temperatures greater than 300°F (Taylor et al. 2003; Chang et al. 2001). The fluid system has been applied successfully in both matrix (Al-Mutawa et al. 2001) and acid-fracturing (Al-Muhareb et al. 2003; Artola et al. 2004) 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 (Taylor et al. 2003; Chang et al. 2001). This paper presents new data providing further insight into the understanding of the unique properties of this 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.