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

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA 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 highpermeability 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.

An Experimentally Validated Wormhole Model for Self-Diverting and Conventional Acids in Carbonate Rocks under Radial Flow Conditions

“…The idea is that, by increasing the viscosity, injection pressure will rise in the wellbore resulting in better coverage along the wellbore. In particular, self-diverting acids 13 have been developed such that, as acid spends and wormholes propagate in a higher injectivity region of the reservoir, the acid temporarily thickens significantly, resulting in partial diversion of the acid into zones of initially lower injectivity.…”

Development of a New Multifunctional Cationic Surfactant System with Corrosion Inhibiting Ability

“…The viscoelasticity of solution does not develop until the acid reacts with carbonate in the formation (in this article, we use sodium salicylate to produce gel). The increases in Ca 2+ ions and pH due to the HCl-carbonate reaction caused in situ gelling of the acid [22]. The high viscosity temporarily blocks the wormholes formed in the rock matrix, allowing the acid to cover the unacidized area.…”

Pore-Network Modeling of Carbonate Acidization

“…Lungwitz et al (2007) studied a viscoelastic acid system in core floods that increases in viscosity as acid is spent. While self-diverting fluids are typically thought to be most useful in acid-fracturing applications, Majdi et al (2003) found that visco-elastic self-diverting acid was highly effective for matrix acidizing in a field study involving multi-layered carbonate structures (3md to 400md).…”