Sedimentological and Structural Controls on Gas Breakthrough, West End Area, Prudhoe Field, Alaska

Abstract

The West End is located on an isolated structural high at the south-western periphery of the Prudhoe Bay field. Although connected to the Main field by a continuous oil column, the West End has a separate gas cap. Pre-development reservoir engineering models indicated that gas injection into this gas cap would improve oil recovery by maintaining reservoir pressure and by causing downward sweep of the gas/oil contact during production. Gas injection has successfully maintained reservoir pressure in portions of the reservoir, but gas breakthrough to producers has based several to be shut in. The causes of this gas breakthrough are both sedimentological and structural. The reservoir interval into which gas is injected is composed of numerous upward-fining fluvial channel fill sequences. The shales capping these sequences form effective vertical permeability barriers, which lower the ratio of vertical to horizontal permeability (Kv/Kh) and inhibit vertical gas movement. The current maximum horizontal stress in the Prudhoe Bay area parallels the major fault trend in the West End, and many faults appear to be capable of conducting gas. Because of the low Kv/Kh and the structural anisotropy of the area, injected gas does not sweep uniformly downward. Rather, gas appears to move laterallymore » to conductive faults, to move downward and laterally, and to break through into producing wells, principally along avenues of higher permeability. An aggressive workover program, now under way, attempts to manage efficiently the reservoir by allocating gas among injectors, and by isolating intervals of gas breakthrough from production.« less