Summary

It is well established that treatment of porous rocks with gelled polymer systems can cause the permeability of water at residual oil saturation to be reduced by one to three orders of magnitude more than the permeability of oil at the water saturation that is immobile after treatment. This phenomenon is called disproportionate permeability reduction (DPR) and is of interest because application of gel treatments in production wells has potential to reduce water production.

The mechanisms that cause this phenomenon are not well understood. This paper describes how permeability to oil and water is developed in pore space that is filled with a chromium acetate/ partially hydrolyzed polyacrylamide (HPAM) gel and proposes a mechanism for DPR based on the interpretation of the experimental data. Experimental data for the flow of oil and brine were obtained in unconsolidated sandpacks and in Berea sandstone cores with and without residual oil saturation after a chromium acetate/Alcoflood 935 gelant was injected and gelled in situ.

Interpretation of the experimental data suggests that oil permeability develops as oil penetrates into the gel-filled pore space, dehydrating the gel by displacing brine from the gel structure and creating "new flow channels" within or around the gel. The "new pore space" is a fraction of the original porosity, and the permeability to oil is reduced substantially from its value before placement and in-situ gelation of the gelant. Subsequent brine injection displaces oil from these flow channels but traps some of the oil in the new pore space as a residual saturation. The trapping of residual oil in the new pore space causes the disproportionate reduction in brine permeability because the brine flows primarily in the pore channels created by dehydration of the gel even though the gel has some brine permeability. When gelant is placed in a matrix containing residual oil, dehydration of the gel reconnects some of the trapped oil, and the oil permeability increases. Subsequent brine displacement experiments conducted at the same pressure drop showed that initial brine permeability was reduced by factors of 100 to 1,000 more than the oil permeability, verifying the existence of DPR.

Introduction

Increased water production is a worldwide problem in mature fields produced by natural waterdrive or active waterflood. There are economic and environmental incentives to develop methods that reduce water production without significantly affecting oil production. During the past 15 years, a number of polymer systems have been developed that, when placed in a porous matrix, reduce the permeability to water at residual oil saturation significantly more than the permeability to oil at the saturation at which water is immobile. This phenomenon is termed DPR, and systems that exhibit this behavior are called relative permeability modifiers (RPM).

There are extensive investigations (Liang et al.,1 Dawe and Zhang,2 Liang and Seright,3,4 Thompson and Fogler,5 Nilsson et al.,6 and Al-Sharji et al.7) on the mechanisms that cause DPR. Proposed mechanisms include segregated flow paths on a microscopic level, wall effects caused by a polymer/gel film that covers the pore walls, restriction of pore throats because of adsorption of polymer or precipitation of hydrophilic components of the gel system, changes in wettability, lubrication effects, swelling and shrinking of gels and polymer films, and change in pore morphology. Of the proposed mechanisms, research continues on the microscopic segregated flow path model, wall effect models, restricted pore-throat model, and effects of changes in pore morphology. None of these proposed mechanisms has been unequivocally demonstrated to be the primary cause of DPR.

This paper describes an experimental study of chromium acetate- polyacrylamide gels, which demonstrate DPR when placed in sandpacks and Berea sandstone core material. The research was stimulated by experiments conducted by Dawe and Zhang,2 who used microscale models to observe mechanisms of oil and water flow through a gel placed in a porous medium made by etching pore structure on a glass plate. They observed that oil flowed through the micromodel by fingering through the gel. Water flowed through the gel by diffusing into the gel structure. A subsequent paper by Al-Sharji et al.7 provides additional support for the pore level mechanisms observed by Dawe and Zhang.2

Thompson and Fogler5 studied pore-level mechanisms by altering the permeability of a porous matrix following a gel treatment. In their studies, the interstitial water saturation was gelled in situ by introducing an organic orthosilicate in the hydrocarbon phase after the interstitial water saturation was attained. The orthosilicate reacted with the interstitial water to form a silica gel, effectively immobilizing the initial interstitial water saturation. In subsequent two-phase flow experiments, trapping of a residual hydrocarbon phase and a "new" residual water phase altered the endpoint saturations as well as the endpoint values of both water and oil permeabilities. The presence of the new residual water phase reduced the oil-phase permeability, while the trapping of residual oil in the new pore space reduced the water-phase permeability at residual oil saturation. Disproportionate permeability reduction was observed. However, the permeabilities of both oil and water phases at endpoint saturations were reduced significantly.

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