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

Abstract

Abstract Reservoir description was a key factor in analyzing the performance of the Golden Spike vertical gravity controlled miscible L.P.G. 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. The originally designed 7% pore volume L.P.G. bank was injected between the gas and oil zones from 1964 through 1975. Extensive monitoring of fluid distribution in this 1,385-acre, 480-foot-thick reef complex indicated that the miscible flood improved the oil recovery until 1979. A geological study, incorporating data from infill wells drilled during the 1970's, shows that the reservoir contains many local barriers to vertical flow. Located in the interior of the reef, these barriers range in size from less than 5 to over 600 acres. The barriers were not recognized prior to infill drilling, because the early development wells were drilled mainly in more homogeneous sections of the reefcomplex. Recent performance analysis indicates the barriers resulted in by-passing of oil and loss of miscible flood benefits after 1978. As a result of the loss of miscibility, the pool is currently under gas drive and the miscible scheme has been terminated. Performance analysis results are confirmed by a cross-section computer model study of the pool. The cross-section model also indicates that the vertical flow barriers combined with the designed bank size results in poor sweep efficiency by solvent. However, future sweep efficiency by gas is predicted to be high due to continued drainage of oil off the barriers. Introduction THE GOLDEN SPIKE D3 "A" Pool, located 18 miles southwest of the City of Edmonton in Central Alberta, was discovered in 1949 (Fig. 1). The pool is located on the western, seaward side of a trend of productive Leduc reefs. This limestone reservoir is a Leduc-age Woodbend reef complex, covering an area of 1,385 acres. Within the crestal area of 1,000 acres, average oil pay thickness is 550 feet. Initial oil-in-place was 319 MMSTB, with no gas cap or water leg present. As indicated in Figure 2, the Leduc Formation at Golden Spike is divisible into three members, designated Lower, Middle and Upper Leduc, Oil occurrence in the Golden Spike D3 ‘A’ Pool is restricted to the Middle and Upper members which, together, make up the reef complex, The underlying Lower Leduc is a shallow-water carbonate bank, more extensive in area than the reef build-up, but limited in reservoir development. This carbonate bank is not in communication with the oil zone. Initial production of this reservoir by fluid expansion resulted in a rapid pressure decline. Pressure maintenance by gas injection commenced 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 (Fig. 3).