A New Method for Acid Stimulation Without Increasing Water Production
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 103771, "A New Method for Acid Stimulation Without Increasing Water Production: Case Studies From Offshore Mexico," by G.H. Reza, Pemex, and E. Soriano, SPE, L. Eoff, SPE, and D. Dalrymple, SPE, Halliburton, prepared for the 2006 SPE International Oil Conference and Exhibition, Cancun, Mexico, 31 August-2 September.
Successful acid stimulation requires a method to distribute the acid between multiple hydrocarbon zones. Because almost all producing wells contain sections of varying permeability, this can be a problem. Because acid is an aqueous fluid, it tends to enter the zones with the highest water saturation. These water zones also are often the highest-permeability zones, so acid stimulation often will result in large increases in water production. The full-length paper describes use of a new low-viscosity system that reduces formation permeability to water with little effect on hydrocarbon permeability and also diverts acid from high-permeability zones to lower-permeability zones.

Introduction
In matrix-acidizing treatments, the acid tends to enter the highest-permeability layers and bypass the most-damaged (lower-permeability) layers. Various placement techniques have been used in attempts to achieve uniform placement of acid across all layers. The most reliable method uses mechanical isolation devices (such as straddle packers) that allow injection into individual zones until the entire interval is treated. However, this technique often is not practical, cost-effective, or feasible. Without a packer, some type of diverting agent must be used. Typical diverting agents include ball sealers, degradable particulates, viscous fluids, and foams. Although these agents have been used successfully, all have potential disadvantages, and none address the problem of increased water production that often follows acid treatments.
One method of controlling water production uses dilute polymer solutions to decrease the effective permeability to water more than to oil. These treatments are referred to as relative permeability modifiers (RPMs), disproportionate-permeability modifiers, or bullhead treatments. RPM systems are thought to perform by adsorption onto the pore walls of the formation flow paths.
Previous papers have described the development of and laboratory studies of an RPM based on a hydrophobically modified, water-soluble polymer, referred to as an associative polymer (AP). This group of polymers was selected for study because their properties can be altered in ways that render them valuable for oilfield applications. Another paper has described a laboratory study of this polymer for use as an acid diverter.

AP Properties
The solution properties of both ionic and nonionic, water-soluble polymers are uniquely modified when hydrophobic groups are introduced into the polymer chains. The primary factor responsible for the property modification is the associative tendency between the hydrophobic groups when placed in an aqueous medium. Previous testing has shown a unique shear-thickening phenomenon for the AP used in the current work. However, the solutions used in diversion operations show very low viscosity (less than 2 cp) at surface conditions.