This paper is a field case history on the performance of the 26R Reservoir. This is a gravity drainage reservoir under pressure maintenance by crestal gas injection. The 26R Reservoir is a highly layered Stevens turbidite sandstone. The reservoir is located in the Naval Petroleum Reserve No.1 (NPR-1) in Elk Hills, Kern County, California (Figure 1). The 26R Reservoir is contained within the steeply dipping southwestern limb of the 31S Anticline. The reservoir had an initial oil column of 1,800 feet. Original oil-in-place (OOIP) was estimated at 424 million barrels.1
Pressure maintenance by crestal gas injection was initiated immediately after production began in October 1976. The total volume of gas injected is ahout 586 BCF. This exceeds one reservoir pore volume. Reservoir pressure has declined from 3,155 psia to 2,46l psia at 6,000 feet subsea. This pressure decline is believed to be due to migration of injected gas into the overlaying shale reservoirs. Under the gas injection pressure maintenance strategy, reserves are estimated to be approximately 212 million barrels.
Reservoir studies have concluded that the aquifer at the base of the reservoir has been relatively inactive. Well recompletions, deepenings, and horizontal wells are used to improve oil recovery.
An aggressive program of controlling gas production began in the mid 1980's by the installation of multiple packers and sleeves. As the gas-oil contact (GOC) has dropped, sand intervals have subsequently been isolated behind packers. A cased hole logging program was recently undertaken to identify possible remaining reserves in the gas cap.
Introduction
The Naval Petroleum Reserve (NPR-1) in Elk Hills, is located in the Southern San Joaquin Valley of Central California, about 20 miles WSW of Bakersfield and 10 miles north of Taft (Figure 1). NPR-1 contains three large anticlines, the 3lS, Northwest Stevens and 29R Structures (Figure 2). The 26R Reservoir is located within the steeply dipping southwestern limb of the 3lS Anticline. It is a Stevens turbidite sandstone reservoir in the uppermost portion of the Elk Hills Shale member of the Monterey Formation (Figure 3). The reservoir is contained within productive shales and adjoins the Western 31S and Main Body ‘B’ Reservoirs. The 26R Reservoir had an initial oil column of 1,800 feet and a net prbductive thickness of approximately 1,150 feet.
Pressure maintenance by crestal gas injection was initiated immediately after production began in October 1976. However, due to suspected migration between the 26R Reservoir and the overlying shale reservoirs, the pressure has declined steadily from the initial level of 3,155 psia to the current level of 2,461 psia at 6,000 ft. subsea.
Since initial production, extensive reservoir evaluations have been conducted to establish an optimum operating strategy. Crestal gas injection for pressure maintenance in the 26R Reservoir has been a highly successful strategy for a layered, steeply dipping, gravity drainage dominated sand reservoir. The reservoir and production engineering activities are presented in detail in this paper.
