Relative-Permeability Modifiers and Their Use in Acid Stimulation in HP/HT Low-Permeability Carbonate Formations: Offshore Mexico Cases

Abstract
Successful acid stimulation requires a method for diverting an acid across the entire hydrocarbon-producing zone. Because most producing wells are not homogeneous and contain sections of varying permeability, being able to completely acidize the interval is a major problem. This paper describes the use of a new low-viscosity system that uses a relative permeability modifier (RPM) that diverts acid from high-permeability zones to lower-permeability zones and inherently reduces formation permeability to water with little effect on hydrocarbon permeability. This system has been used effectively offshore Mexico with success for more than two years. The cases presented in this paper show the first application in a low-permeability carbonate formation where oil production was increased significantly compared to previous traditional acid treatments using conventional diverters.

The other important feature of this work is that the downhole conditions were high-pressure/high-temperature (HPHT). Details from the jobs using this new RPM acid-diversion system will be presented showing pre- and post-job production results.

Introduction
Matrix acidizing enhances well productivity by reducing the skin factor. The skin factor can be reduced if near-wellbore damage is removed or if a highly conductive structure is superimposed onto the formation. In either case, the result is a net increase in the productivity index, which can be used either to increase the production rate or to decrease the drawdown pressure differential. Although the benefits of an increased production rate are evident, the benefits of reduced drawdown are often overlooked. Decreased drawdown can help prevent formation collapse in weak formations, reduce water or gas coning, minimize both organic and mineral scaling, and/or shift the phase equilibrium in the near-wellbore zone toward smaller fractions of condensate or solution gas. A reduced drawdown pressure can also help ensure that a greater percentage of the completed interval contributes to production.1

In attempts to achieve uniform placement of acid across all layers, various placement techniques have been used.2 The most reliable method uses mechanical isolation devices (such as straddle packers) that allow injection into individual zones one at a time until the entire interval is treated. However, this technique is often 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. Therefore, it would be a major advantage to have a material that could inherently decrease the formation permeability to water while also providing diversion.

One method of controlling water production uses dilute polymer solutions to decrease the effective permeability to water more than to oil. These treatments may be referred to as relative permeability modifiers (RPM), disproportionate-permeability modifiers, or simply, bullhead treatments. The latter name is so called because these treatments can be bullheaded into the formation without the need for zonal isolation. RPM systems are thought to perform by adsorption onto the pore walls of the formation flow paths.3-5