Enhanced Oil Recovery By Gas Injection: Proposed Screening Criteria

Abstract As production from oil-bearing reservoirs matures, the need for enhanced oil recovery becomes increasingly important. In many of these reservoirs, waterfloods have been implemented and are presently approaching their economic limit in terms of producing water-oil ratio. One of the most important processes for revitalization of mature reservoirs is EOR by gas injection. Over the last 20 years, the authors have seen significant change in the approach to gas injection design and thus in the 1990's the oil industry has a greater understanding of gas EOR than ever before. This knowledge notwithstanding, many times operating companies jail to understand some of the complexities associated with appropriate gas injection design and consequently target reservoirs for gas EOR which may have characteristics which would preclude the reservoir from such EOR applications. This paper summarizes approximately two years work performed wherein effects of interfacial tension, viscosity ratio, gravity and wettability, all in the context of the microscale pore size distribution, have been evaluated. In particular, four reservoirs, wherein specific laboratory measurements including coreflood testing have been performed, are analyzed. Some counterintuitive responses are then described on the basis of these parameters. Finally, screening criteria for gas EOR projects has been proposed Three reservoirs which have been evaluated on a field scale are then ranked according to the screening criteria proposed herein. The approach used appears to confirm the response observed in the field thus lending credence to the gas EOR screening criteria developed in this paper. The objective of the screening criteria developed from this work is to allow an operating company to rank candidate gas injection EOR reservoirs according to that which has the most promise to that which has the least promise. In this manner, limited resources can be focused on that reservoir or reservoirs which are most likely to result in the best performance, thus stabilizing the reserves base for the corporation and allowing for more optimal corporate performance in the future. Displacement Paradigms In analyzing corefloods over the past fifteen years, the simple paradigm which has exhibited repeated efficacy is that which is described n the literature. Dullien1 described the features of importance in multiphase flow in porous media whereas many practitioners of reservoir engineering have tended to employ global observations to infer small-scale response, Chatsis2, Diaz3, Kwiecien et al4, Kantzas et al 5 and Ionnadis et al6,7,8 have focused on pore level to describe macro-scale performance. A host of other researchers has also contributed to the body of insightful literature which helps us to comprehend why porous media systems behave as they do. The domain of flow in porous media inheres of such challenging subjects that recently the pundits of the fractal domain have directed much attention to the treatment of the problems which were reserved, in previous decades, for the more pragmatic reservoir engineers. Feder, Mandlebrot and a host of others have begun to see flow in porous media as a pons asinorum to which fractal mathematics may hold an insightful eye.