Improved Oil Recovery for Oil Reservoirs with Gas Cap

Abstract

Current oil recovery method for a gas cap oil reservoir mostly involves gas reinjection. Gas reinjection is performed to supplement the reservoir energy, and to maintain oil production by sweeping oil towards the production well. The gas injection process also gives advantages of component exchange that occurs in the gas oil contact as the injected gas steam sweeps the oil in the reservoir. However, employing the strategy of gas reinjection does not necessarily guarantees high oil recovery efficiency, as the total amount of oil recovered depends on a wide range of reservoir characteristics as well as the production strategy. In this work, a compositional reservoir simulator was used to investigate the improved recovery capabilities of a light oil reservoir with gas cap by concept of gas reinjection into the reservoir. The study comprised of changing operational parameters such as oil flow rate, type of production well, time to initiate/stop gas injection and changing well positions to see how each parameter affects the oil recovery efficiency. The results showed that producing the reservoir at different flow rates for example at 500 stb/d compared to 2000 stb/d, the lower flow rate gave slightly higher oil recovery efficiency compared to the higher flow rate. With gas reinjection into the reservoir, the recovery efficiency was more than double the recovery efficiency obtained from natural depletion, in addition, the horizontal wells under gas reinjection schedule gave much higher oil recovery than vertical wells under similar conditions of oil production rate and gas reinjection specifications. Initiating the gas reinjection process from the start of production resulted in higher oil recovery than starting the gas reinjection process later on during production. Also, performing the gas reinjection process from start of production till end of production gave higher oil recovery than terminating the gas reinjection process at some point during production. Finally, placing the production well a longer distance away from the injection well gave much higher oil recovery than placing the injection and production well closer apart. Gas reinjection helped in sweeping the oil and reducing the oil saturation in the swept area by vaporizing the oil into produced gas.