Performance of a High-Pressure-Gas Injection Project, Swanson River Field, Alaska

Abstract

After 10 years of high-pressure gas injection, performance at the Swanson Riverfield continues to exceed expectations. Contributing factors include the efficiency of the process, the suitability of the reservoir and crude oil, an efficient operating strategy, and the apparent absence of the usually detrimental effects of unstable gravity overriding. Introduction The Swanson River field, the first commercial find in Alaska, contains a highly undersaturated reservoir fluid system that, upon initial production, exhibited a rapid rate of reservoir pressure depletion. Early material-balance analyses indicated the presence of finite aquifers surrounding the various fault blocks that would assist recovery. However, further evaluations indicated that initiation of pressure maintenance would result in much greater ultimate recovery of oil in place. After considerable engineering effort, the decision was made to begin high-pressure gas injection, which represented the most promising economic alternative of the various gas- and promising economic alternative of the various gas- and water-injection schemes evaluated. Numerous reevaluation of project performance during the intervening years since inception of full-scale gas injection have contributed substantially to the understanding of the nature of this recovery process in the Swanson River field. Additionally, operating and engineering requirements for optimizing performance and ultimate recovery have been well defined. Field Description The Swanson River field, located in the Kenai National Moose Range about 50 miles southwest of Anchorage, was discovered by the Richfield Oil Corp. in 1957. Initial development of the field was essentially finished in 1963 with the completion of 55 wells. Undersaturated by 4,500 psi, the reservoir experienced a rapid decline in pressure, psi, the reservoir experienced a rapid decline in pressure, requiring early implementation of pressure maintenance. By mid-1966 facilities for field-wide, high-pressure gas injection were supplying injection gas to crestal wells. The recovery efficiency to date, representing 38 percent of the original oil in place, has exceeded early estimates of performance anticipated for this high-pressure, immiscible, gas drive project. Primary factors contributing to the excellent recovery efficiency from the field are the nature of the recovery process itself and the operating strategy used to obtain maximum volumetric sweep efficiency. Fig. 1 shows the Swanson River field structure mapped on the top of the productive Hemlock zone. Structurally, the Swanson River field is a north-south trending anticlinal flexure about 6 miles long and 1 to 3 miles wide. Closure within the field varies from more than 600 ft in the south to 250 ft in the north. The field is cut by a series of major east-west normal faults. The fault planes dip to the north at about 45 deg. in the southern half of the field to near vertical in the northern half. The displacements on the faults vary from about 100 to 300 ft. In general, the displacement on most faults increases basinward to the west. It is uncertain whether some of the minor faults are presently barriers to fluid migration. Initially, some presently barriers to fluid migration. Initially, some faults were barriers along the western flank of the field and the original oil-water contacts were horizontally offset. These faults, however, die out along the eastern flank of the field where they are not considered barriers. Other major displacement faults offset the original oil-water contacts on both the east and west flanks. JPT P. 99