Improved Oil Recovery in Under-Saturated Reservoirs Using the US-WAG Process

Abstract This paper describes the improved oil recovery mechanism in under-saturated reservoirs using an immiscible water-alternating-gas (IWAG) process. When lean gas is injected into an under-saturated reservoir, incremental oil recovery is achieved through oil viscosity reduction, oil swelling, and improved sweep efficiency. Oil viscosity reduction is found to be the dominant mechanism in improving oil recovery from severely under-saturated reservoirs. The Kuparuk River formation at Milne Point field on the North Slope of Alaska contains highly under-saturated oil with a bubble point pressure of 1875 psi and an initial reservoir pressure of 3500 psi. When lean gas is injected into the reservoir, oil viscosity can be reduced by as much as 43% and oil formation volume factor increased by 4%. These changes in oil properties lead to a calculated incremental oil recovery of 6–9% of original oil-in-place (OOIP). Results from compositional reservoir simulation as well as laboratory measurements are presented in this paper. The theoretical study is supported by actual field production data from the Milne Point field. Reservoir management strategy has changed significantly based on this technical study and detailed economic analysis. The once on-going MWAG process has been replaced with a US-WAG (Under-Saturated Water-Alternating-Gas) process, which substantially improves the economics of the field. The US-WAG process is also applicable in any severely under-saturated reservoirs where miscible injection might be hard to achieve, such as the heavy oil reservoirs on the North Slope of Alaska. Significant improvements in oil recovery can be achieved through reduction in oil viscosity by injection of separator gas into the reservoir.