Pool Description and Performance Analysis Leads to Understanding Golden Spike's Miscible Flood

Abstract

Reservoir description was a key factor in analyzing the performance of the Golden Spike vertical-gravity-controlled, miscible, LPG bank scheme. Understanding the performance of the pool was enhanced by including a detailed reservoir description in a computer-model study of the miscible scheme. Introduction The Golden Spike D3'A' pool, located 18 miles south-west of Edmonton in central Alberta, was discovered in 1949 (Fig. 1). The pool is located on the seaward side of a trend of productive reefs. This limestone reservoir is a Leduc-age Woodbend reef complex, covering an area of 1,385 acres with an average thickness of 480 ft. At discovery, the reservoir contained 319 MMSTB of oil with no gas cap or water leg present. The base of the pool is sealed by an impermeable carbonate bank. Initial production of this reservoir by fluid expansion resulted in a rapid pressure decline. Pressure maintenance by gas injection began in 1953. Analysis of the available well data and gas flood performance up to 1963 indicated a relatively homogeneous reservoir. The apparent good quality of the reservoir and the availability of a suitable, inexpensive hydrocarbon solvent (LPG) prompted installation of a miscible flood in 1964. prompted installation of a miscible flood in 1964. The solvent, containing 60-mol-percent C2-C4, was obtained from the reservoir crude by producing at rates in excess of government-allocated allowables, stripping out the light ends, and reinjecting the surplus dead oil. The wells used in this cycling scheme are shown in Fig. 2. Studies indicated that 95 percent of the original oil in place would be recoverable percent of the original oil in place would be recoverable ultimately by injecting a 7-percent hydrocarbon-volume solvent bank between the secondary gas cap and the oil zone. When the miscible flood was installed, four producers were capable of producing far more than the plant capacity of 45,000 BOPD. Near the end of the bank placement in 1972, well productivities declined to the point where rising allowables could not be met by -the existing wells (Fig. 3). Extensive infill drilling was begun at this time to meet the allowables and offset the declining productivity. By the end of 1975, the pool had been developed extensively, with 52 wells drilled (Fig. 4) and 44 percent of the original oil produced. This provided the basis for a performance analysis that indicated that the miscible flood performance analysis that indicated that the miscible flood was not performing as expected. Performance Analysis Performance Analysis The major part of the performance analysis involved identification of fluid distribution in the reservoir. The upper surface of the oil leg in Golden Spike has been monitored extensively since 1957, mainly through the use of standard neutron logging. Through 1970, results indicated uniform displacement of the oil zone with little difference in oil-zone surface elevation between wells. Distortion of the oil surface was first noted in 1971, when 77-ft differences in elevation were observed across the new. By 1973, the oil-surface distortion had increased to 130 A. Result of this survey, based on bottom-hole samples from eight wells and logs from 30 wells, showed an irregular fluid distribution associated with the presence of an impermeable zone, referred to in this paper as presence of an impermeable zone, referred to in this paper as the "main barrier." JPT P. 867