Abstract

The Prudhoe Bay field, located on the north coast of Alaska, is the largest oil field in North America. Discovered in 1968, the Prudhoe Bay field is a massive sandstone reservoir complex covering over 200 square miles.

Eileen West End (EWE) is a western extension of Prudhoe Bay field, connected only through the aquifer. It consists of two tilted fault blocks, each with a gas cap and an oil leg having a combined OOIP over 750 MMSTB. Production began at EWE in June, 1988, approximately 11 years after Prudhoe field startup.

Initially, EWE was considered an analogue to the gravity drainage area of Prudhoe Bay. The reservoir management strategy employed at EWE was gas cap expansion and gravity drainage of oil with injection of Prudhoe Bay produced gas into the EWE gas caps. A review of surveillance data, material balance studies, fluid mapping, simulation and other analogue data, changed the perception that gravity drainage was an efficient recovery process at EWE. The work also showed that aquifer influx was stronger than anticipated. As a result, gas injection was stopped in 2001 and water injection into the gas caps was started for pressure maintenance and to prevent oil from resaturating the gas cap.

A pattern water alternating miscible gas (WAG) flood was initiated in 2003. The initial WAG patterns targeted areas with large gas cap expansion and gas underruns to capture oil trapped in these gas invaded intervals. The WAG flooding is being expanded to areas of low gas cap expansion and peripheral regions.

This paper describes the technical process used to select the major design parameters of the pattern WAG flood to optimize EWE recovery. The key parameters evaluated were slug size, injection rate, WAG ratio, and WAG sequencing. A fine grid fully compositional numerical simulator was used to determine the optimal design parameters. The modeling indicated that optimal design consists of multiple WAG cycles with high voidage replacement ratio (VRR) during the gas cycle with an overall WAG ratio of 1. Timing of the first gas slug within the first year after start of waterflood in addition to realignment of the injection and production wells also optimizes recovery. Surveillance of the secondary and tertiary recovery through well performance monitoring and surface and down hole data acquisition combined with classical reservoir engineering is ongoing to ensure that the EWE enhanced oil recovery (EOR) project remains optimized.

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