Slaughter Estate Unit Tertiary Miscible Gas Pilot Reservoir Description

Abstract

Summary A reservoir description for the Slaughter Estate Unit tertiary pilot and surrounding area and the procedure that we used to obtain it are discussed in this paper. The procedure is based on matching waterflood performance procedure is based on matching waterflood performance prior to pilot miscible gas injection with a black oil prior to pilot miscible gas injection with a black oil reservoir simulator. An initial estimate of the reservoir description is obtained from petrophysical data and single-well pressure transient tests. The initial estimate is then modified by a trial and error procedure until a good match between the actual and calculated waterflood performance is obtained. performance is obtained. It was determined that the Slaughter Estate Unit tertiary pilot had an original oil in place (OOIP) of 642,400 STB [102 133 stock-tank m3]. A waterflood prediction derived from the reservoir description in this paper indicates that a primary-plus-secondary recovery through Sept, 30, 1983, of 49.6% OOIP would have been obtained from the pilot if waterflood operations had been continued. On the basis of this prediction, it was established that the tertiary oil recovery resulting from the miscible gas process was 18.5% OOIP as of Sept. 30, 1983. Introduction The Slaughter Estate Unit tertiary pilot is one of several miscible gas EOR projects operated by Amoco Production Co. in the Permian Basin of west Texas. The pilot Production Co. in the Permian Basin of west Texas. The pilot is located in the Slaughter Estate Unit of the Slaughter field, in Hockley County. The proven productive geological zones in the Slaughter Estate Unit are designated as San Andres Zones IV, V, and VI. The unit is not influenced by a gas cap or water drive. A type log taken from Well 279 is illustrated in Fig. 1. Log-produce-log testing shows that the pilot processes San Andres Zones IV and V. The pilot configuration is a 12.3-acre, [49.8-km2] double five-spot pattern (Fig. 2). Pilot wells were drilled in mid-1972 in an area of the unit that was still under volumetric depletion. Native state cores were collected from the pilot production Well 279. A pilot waterflood program was initiated on Nov. 30, 1972. Gas collapse occurred in the pilot within 3 months after the start of water injection. A peak pilot oil-production rate of 407 STB/D [64.7 stock-tank m3/d] was measured in June 1973. By Aug. 1976 a secondary decline was established with a pilot oil-production rate of 37 STB/D [5.9 stock-tank m3/d] and a water cut of 89.5%. Acid gas (72% CO2 and 28% H2S) and water were alternately injected in the plot beginning on Aug. 23, 1976. The maximum pilot oil-production rate measured during the acid-gas injection process was 152 STB/D [24.2 stock-tank m3/d] in Feb. 1979. Chase-gas injection (residue gas or nitrogen, depending on the available supply) replaced the acid-gas injection on Oct. 16, 1979. Chase-gas injection was completed in July 1982 and the pilot has been under waterflood operations since then. In Sept. 1983 the pilot oil-production rate declined to 28 STB/D [4.5 stock-tank m3/d]. At this time, the pilot water cut (calculated for liquids only) was 96.5% and the producing GOR was 2.8 Mscf/STB [498.7 std m3/stock-tank m3]. The objectives of the pilot are to provide reservoir and performance data that are necessary to determine performance data that are necessary to determine primary-plus-secondary oil recovery, incremental primary-plus-secondary oil recovery, incremental tertiary oil recovery, and to develop a quantitative understanding of the acid-gas displacement process. This procedure will help establish the technology for reliable procedure will help establish the technology for reliable predictions of fieldwide performance for miscible gas predictions of fieldwide performance for miscible gas injection. To achieve these objectives, a good reservoir description of the pilot and the surrounding area is necessary. A reservoir description will allow for the separation of the effects of reservoir heterogeneities and miscible gas injection process variables on the tertiary pilot performance. pilot performance. Physical representation of the waterflood process in the black oil simulator is assumed to be realistic. If representative fluid and rock properties (excluding phi h, kh, and h) can be determined in the laboratory, then the only unknown quantity required to match waterflood performance is the reservoir description. History performance is the reservoir description. History matching waterflood performance by use of sound engineering principles is therefore considered a satisfactory method principles is therefore considered a satisfactory method to determine an accurate reservoir description. History matching a miscible gas flood to determine the reservoir description would not be a valid procedure because not all the physics of the miscible gas process are completely understood at this time. The first purpose of this paper is to present the reservoir description of the pilot and the procedure we used to develop it. This includes discussions of the model study area, well development history, and fluid and rock properties. A petrophysical study was combined with properties. A petrophysical study was combined with single-well pressure transient tests to provide an initial estimate of the reservoir stratification. JPT P. 837