Mobility Control In Waterflooding Oil Reservoirs With A Bottom-Water Zone

Abstract

Abstract Many oil reservoirs in Alberta and Saskatchewan have a bottom-water zone which leads to very poor areal and vertical sweeps under a water flood. Performance in such reservoirs could be improved greatly if an effective method of partially plugging the bottom-water zone can be implemented. A possible means of accomplishing this would be to precede the water flood with a slug of a mobility control agent. This possibility was investigated in this study using a laboratory flow model, for a series of mobility control agents. In an effort to screen a variety of techniques to water flood an oil reservoir with bottom water, polymer, emulsion, biopolymer, and foam were used as mobility control agents in various slug sizes. Core flood experiments were conducted to study the effect of permeability contrast, relative oil-water layer thickness, oil viscosity, injection rate, slug size and the use of an artificial barrier. Oil of viscosities ranging from 1 to 200 mPa •s were used in 42 displacement tests. A qualitative comparison is made to show the relative merits of different mobility control agents. It is shown that when all the mobility control agents used have a favorable impact on tile water-flood performance, the choice of one over another would depend largely on variables such as permeability contrast, relative layer thickness, oil viscosity, injection interval, etc. Moreover, it is noted that only in certain situations it is better to inject the mobility control agent directly into the bottom-water zone. With polymer, emulsion and foam as mobility control agents, the worse the conventional water flood performance is, the more effective the mobility control agents are. For instance, when the bottom-water zone was as thick and permeable as the oil zone, only about 5% of the oil-in-place was recovered by a water-flood, whereas with polymer, emulsion or foam as mobility control agent, more than 60% of the oil-in-place may be recovered. The effect of slug size is investigated for all mobility control agents, and an optimum slug size is discussed. Introduction Many reservoirs in Alberta and Saskatchewan contain some type of high water saturation zone underlying the oil reservoir. Such reservoirs show rather poor performance under a conventional Water-flood. However, the medium to light gravity of the reservoir oil and the plant facilities dictate that water-flooding is the best suitable technique for these reservoirs. Therefore, there is a need to develop a technique that would improve the performance of a water-flood in such reservoirs. This problem has received attention for more than two decades and several techniques have been proposed in the literature(1–7) to control the mobility of water or gas. However, none of the previous studies reports a systematic study of all these mobility control agents applied to a reservoir with a bottom-water zone. Besides, some of these techniques have not been tested specifically for bottom water. This paper reports a systematic study of all the mobility control agents available in improving oil recovery from a reservoir with a bottom-water zone and discusses the relative merit of each of these under various reservoir characteristics and operating conditions.