Abstract
Abstract The uppermost 200 ft of the main pay interval at Prudhoe Bay is a moderate permeability, high net-to-gross fluvial sandstone which contains a number of thin, discontinuous shales. Most of this interval is being produced by waterflood and miscible gas WAG, but the updip third of Zone 4 has been produced by gravity drainage since 1977 and is very mature. Fullfield models and material balance estimates indicated good recovery in the updip area. One Zone 4 core was taken in the gravity drainage area and one was taken in the waterflood/WAG area. Both cores were history matched with a reservoir simulator to determine the ultimate recovery potential of the process currently being employed. Detailed simulations indicated that the gravity drainage process was leaving high oil saturations behind the gas front which could not be produced economically because of excessive gas-oil ratios (GORs). Both water injection and miscible gas injection mobilized large volumes of oil and dramatically reduced the GOR. The Updip Zone 4 Injection Project (UZI) is designed to mobilize and capture remaining oil trapped above the current gas-oil contact. In the updip area, both water and miscible injectant (MI) have been injected in Zone 4 while maintaining gravity drainage in the lower intervals (Zones 2 and 3). UZI patterns employ MI in a unique, highly efficient, single-slug oil swelling process rather than as a tertiary WAG. UZI injection has mobilized large volumes of oil and dramatically reduced GORs. The 14-01A pilot has produced roughly 2,000 BOPD of additional oil for the last 5 years from MI injection into a previously gassed-out producer. Total incremental oil from the MI pilot is approximately 3.5 MMBO. The 1-11 water injection pilot is another success, with five of the eight offset wells producing an incremental 960 BOPD and 35 MMscf/D less gas. This major reduction in gas production has allowed increased production of about 700 BOPD from other Prudhoe Bay wells. A tracer program in the waterflood pilot has verified water containment within Zone 4. Based on this success, the Prudhoe Bay Unit has been expanding the UZI Project. Ten UZI patterns are already developed in the Eileen West End area. In the mainfield area, four patterns are currently on-line with another four patterns approved. This paper briefly reviews the core analysis data and the reservoir simulation history match of the two Zone 4 cores and the process comparison studies. Production data from the updip waterflood and miscible gas flood pilot tests is presented, as is the performance of the early UZI patterns.
Published Version
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