Effects of Gelant Composition and Pressure Gradients of Water and Oil on Disproportionate Permeability Reduction of Sandpacks Treated with Polyacrylamide-Chromium Acetate Gels

Summary

For some polymer gels applied in reservoirs to control water flow, a favorable disproportionate permeability reduction (DPR) occurs in which permeability to water is reduced to a much greater extent than it is to oil. Permeability reduction in sandpacks by partially hydrolyzed polyacrylamide-chromium acetate gels was studied as functions of gel composition and the pressure gradients imposed on the gels. For the range of parameters studied, increased gel composition increased the factors by which the permeabilities to water and oil were reduced. Increased gel composition also increased selectivity, a measure of the water-permeability reduction with respect to oil-permeability reduction. Applied pressure gradients during steady-state flows had little effect on oil permeability and a moderate effect on water permeability. Material balances on phases and components in the sandpacks provided insights into mechanisms responsible for the development of flow channels through gelled sandpacks and mechanisms contributing to favorable DPR. Increased pressure gradient during channel development decreased the selectivity of the treatment.

Introduction

High water production is a major concern in mature hydrocarbon reservoirs. Costs of handling and disposing of water produced from oil reservoirs often shorten the life of a production well. Disposal of the water is also an environmental concern. In order to reduce water production, polymer gels have been used to modify the mobility of water and oil in petroleum reservoirs.

When some gels are placed in a petroleum reservoir, permeability reduction occurs to a much greater extent for water than for oil. This phenomenon is known as favorable DPR. Reduced permeability to water can lead to decreased production of water, and sometimes increased oil production, thereby prolonging the useful life of the reservoir. Results reported in the literature have shown that the application of several polymer gel systems can result in DPR. Mechanisms for DPR have been debated, and the magnitude of the effect has been unpredictable from one application to another. Mechanisms for DPR that have been proposed and studied by several researchers are shown in Table 1. The usual method to study DPR is to saturate a porous medium with gelant, allow time for gelation to occur, and then inject oil and water to steady-state conditions and determine permeabilities at 100% fractional flow of each fluid. One aspect of this procedure that most of these experimental works do not describe or examine is the process that occurs when oil or water is first injected into the gel-treated porous media. It is our experience that the medium has very little permeability at the start of injection and that considerable time is required for the injected fluid to develop channels or flow paths through the system before a steady state is approached.