Effects of the Imbibitional Flow of Steam on Oil Recovery from Carbonate Reservoirs

Abstract

Carbonate reservoirs contain over 90% of oil within an extremely tight matrix and are usually oil-wet, which makes them unfavorable for oil production. During water injection, water does not imbibe into the matrix, but flows preferentially through the fractures, resulting in very low oil recoveries. We propose to inject steam or hot water, inducing a wettability change and rendering the matrix water-wet. Then, water can move into the matrix by capillary imbibition. From this perspective, it is crucial to analyze the contributions of conductive and convective heat transfer during steam injection. We account for the mechanism of wettability changes when steam is injected into carbonate reservoirs. The effect of strong capillary pressure in a tight matrix on the oil recovery is also examined. The purpose of this study is to evaluate the rate of conduction and convection and to determine optimum conditions that maximize heat transfer and wettability alteration in the tight matrix. An analytical solution and numerical results were compared for heat conduction within the matrix during steam injection. Investigations were also performed to determine the optimum injection time, steam injection pressure, and steam quality required to make the carbonate matrix water-wet and maximize oil recovery. The results from this study showed that convectional flow might be the most important heat-transfer mechanism to shorten the matrix wettability converting time. Based on analysis of the thermal process, an optimum steam injection strategy can be designed for a given matrix permeability and fracture spacing.