Influence of Wettability on Foamed Gel Mobility Control Performance in Unconsolidated Porous Media

ABSTRACT

Practically all enhanced oil recovery (EOR) processes require mobility control techniques to minimize channeling, gravity override, and viscous fingering of the displacing phase. In oil reservoirs that contain free gas, high gas-oil ratio production is a widespread problem, because gas segregates as a result of its higher mobility. This condition may cause reduced oil production rates, loss of drive energy, loss of recoverable oil, and problems with fluid processing. Use of foam as mobility-control fluid has shown promise in a wide range of EOR techniques including steam flooding, CO 2, light hydrocarbon or N 2 injection, and chemical flooding. The success of any mobility control process, including foams, is determined by the microscopic displacement efficiency of the displacing fluid at pore level. Microscopic displacement efficiency is determined by the interactions of rock pore geometry and interface boundary conditions, which constitute the reservoir wettability. To date, limited information has been published regarding the role of wettability in the performance of foams as a mobility control agent, and accordingly, more evaluation is needed.

The focus of this experimental research was to determine the effect of wettability on the performance of foamed gels in displacing oil and in its efficiency as a mobility control and blocking agent. The experimental observations were made through a series of displacement tests using unconsolidated porous media with an average permeability of 209 D. The wettability of the porous media was modified using an organic coating solution. The oil phase used was Soltro1 ®10 and the foamed gel formulation was based on a partly hydrolyzed polyacrylamide crosslinked with chromium (III) acetate, and an anionic surfactant.

The experimental outcome indicated that mobility control performance of foamed gels is sensitive to the wettability of the porous media. Under oil-wet conditions, foamed gel demonstrated superior efficiency as a mobility control and/or blocking agent. Furthermore, the injection of foamed gels in high permeability porous media demonstrated an efficient fluid diverting capability, which renders important additional oil recovery.

Introduction

Many enhanced oil recovery processes use gas drive to displace trapped oil, such as steam flood and CO 2, floods. The sweep efficiency of these methods is often low because of the unfavourable mobility ratio of gas to oil. Therefore, gas tends to override or finger through oil (1, 2). Improving gas mobility to increase reservoir sweep can add significantly to recoverable reserves and impact the economics of these tertiary recovery processes (3). The use of surfactant-stabilized foams to counteract such problems was suggested decades ago (4). The use of foam is advantageous compared with the use of a single fluid of the same nominal mobility because the foam, which usually has an apparent viscosity greater than the displaced fluids, lowers the gas mobility in the swept or higher permeability parts of the formation. This diverts at least some of the displacing gas into other parts of the formation that were previously unswept or underswept. From these underswept areas, additional oil is recovered.

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