Development of Associative Polymer Technology for Acid Diversion in Sandstone and Carbonate Lithology

Abstract

Abstract This paper describes the use of associative polymer technology (APT) to achieve fluid diversion during an acid stimulation treatment. APT involves the use of a very low-viscosity aqueous polymer solution. It reacts immediately with the formation surface to significantly reduce the ability of subsequent aqueous fluids to flow into high-permeability portions of the rock. The first stage containing the APT predominately will enter the most permeable area, diverting following acid stage(s) to less permeable sections of the rock. APT has little or no effect on the flow of subsequent hydrocarbon production. Furthermore, in rock containing significant proportions of sandstone-type lithology, the water permeability of the treated zone is decreased permanently, resulting in post-treatment reduced water production from the treated zone. This paper includes a general description of associating polymers and their properties, as well as a detailed description of the laboratory development of the current system. Laboratory data showing the ability of APT to reduce the ability of aqueous fluids to flow through porous media is presented. Parallel flow studies using water-saturated and oil-saturated cores are presented that show the ability of APT to divert acid in both sandstone and carbonate cores. These tests also show the ability of APT to decrease water permeability in the water-saturated core while the diverted acid increases the permeability of the oil-saturated core. Introduction Successful matrix acidizing depends on acid placement to remove near-wellbore formation damage efficiently. Most intervals are nonhomogeneous, containing sections of varying permeability. Acid treatments tend to predominantly enter the highest permeability portion, thus bypassing the lower permeability, or most damaged layers. In some cases, these high-permeability layers are also predominantly water-bearing, thus the acid also mainly enters those zones because of the relative permeability effect. In other cases, the acid may break into a nearby water-bearing zone. In all of these cases, an acid treatment may result in significant increases in water production after the treatment. In attempts to achieve uniform placement of acid across all layers, various placement techniques have been used. The most reliable method is the use of mechanical isolation devices such as straddle packers that allow injection into individual zones, one by one, until an entire interval has been 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 that have been used include ball sealers, degradable particulates, viscous fluids, and foams. Each of these has been used successfully, but each also has potential drawbacks. In addition, none of these techniques addresses the problem of increased water production that often follows an acid treatment. For this reason, a material that could inherently decrease the formation permeability to water and also provide diversion is desirable. One method of controlling water production has been the use of dilute polymer solutions to decrease the effective permeability to water more than to oil. These types of treatments have been referred to as relative permeability modifiers (RPM), disproportionate permeability modifiers, or simply bullhead treatments. That is, these treatments can be simply bullheaded into the formation without zonal isolation. These systems are thought to perform by adsorption onto the pore walls of the formation flow paths. A large number of such polymer systems have been promoted through the years, and a large volume of literature has been devoted to this topic.1–3 A previous paper from this laboratory described an RPM based on a hydrophobically modified water-soluble polymer.4 Because this polymer reduces water permeability with little damage to oil permeability, it was recognized as a potential acid diverter. This paper describes the laboratory development of this polymer and its ability to decrease water permeability and provide diversion of acidizing fluids in both sandstone and carbonate lithology.