Slaughter Estate Unit CO2 Flood: Comparison Between Pilot and Field-Scale Performance

Abstract

Summary This paper describes the performance of a pilot and a unitwide CO2 flood inthe Slaughter Estate Unit, Slaughter field, Hockley County, TX. The performanceand design of both projects are compared to yield insight into the process, theimpact of flood design variables, and the effects of project scale. Introduction Amoco Production Co. drilled an "oil-in-the-tank" pilot in the Slaughter Estate Unit to improve its understanding of the miscible-gas process. Oil-in-the-tank refers to a pattern in which the pilot producing wells areconfined by EOR injection wells. The pilot producing wells are not influencedby any offset continuous-water-injection wells. The pilot was waterfloodedduring 1972-76, Miscible-gas injection was initiated in the pilot in Aug. 1976. The initial gas stream contained an acid gas (72 mol% CO2 and 28 mol% H2S). Theacid gas was eventually replaces with a chase gas (various concentrations ofresidue gas and nitrogen). Water was injected alternately with gas. The pilottest was completed in July 1984. A reservoir description of the pilot andsurrounding area was obtained by starting with geological andpressure-transient data and then modifying the least-known parameters to matchprimary and waterflood performance. Reservoir description was important forquantitative interpretation of the tertiary performance. Pilot design, 2performance 3 through 1981, and mechanical design considerations are discussedin other technical papers. The favorable tertiary oil response from the pilot provided comfort with the CO2 flood process, and a unit-wide CO2 flood was initiated in Dec. 1984. Previous feasibility studies were used to design a fieldwide CO2 flood. Reservoir descriptions for various segments of the Slaughter Estate Unit hadbeen developed by history matching primary and secondary (waterflood)performance. Once history matched, CO2 predictions were made for a variety ofconditions to determine the most profitable manner to inject CO2. Ongoing CO2flood modeling has been part of the reservoir monitoring process. With changingoil prices, additional CO2 flood predictions were made, resulting in changes inthe slug size and grading predictions were made, resulting in changes in theslug size and grading of the gas/water injection ratio to improve verticalsweep. This paper updates pilot performance through termination of the pilot, describes how the tertiary performance was matched, and compares pilot and unitperformance. The procedure used for fieldwide design is also discussed.performance. The procedure used for fieldwide design is also discussed. Slaughter Estate Unit Miscible-Gas Pilot The Slaughter Estate Unit tertiary pilot was one of several miscible-gas EORpilots in the Permian Basin in west Texas. The pilot was located in the Slaughter field in Hockley County, TX (Fig. 1). The pilot, a 12-acre, doublefive-spot oil-in-the-tank pilot, was drilled in 1972 in an area that hadundergone only volumetric depletion. The pilot was then waterflooded from Nov.1972 to Aug. 1976. The volume of oil drained from the pilot predicted beforewaterflooding was 60.7 MSTB, or 9.4% original oil in place (OOIP). The pilotoil recovery during waterflood operations was 191.9 MSTB, or 29.9% OOIP. Themodel predicted additional oil recovery for continued waterflood operationspredicted additional oil recovery for continued waterflood operations throughpilot termination to be 72.0 MSTB (11.2% OOIP). From the predicted recoveriesplus the actual waterflood oil recovery, the predicted recoveries plus theactual waterflood oil recovery, the ultimate primary plus secondary oilrecovery for the pilot is 324.6 MSTB, or 50.5% OOIP. An alternate acid-gas (72% CO2 and 28% H2S)/water-injection project wasinitiated in Aug. 1976. Acid gas and water were injected over 10 cycles at agas/water injection ratio (GWR) of 1:1 (1 res bbl/res bbl). A 25 % HCPV slug ofacid gas was injected. The pilot oil production rate increased from 37 to apeak of 152 STB/D in Feb. 1979 (Fig. 2). Chase-gas injection began in Oct.1979. The chase gas was composed of residue gas or nitrogen, depending on theavailable supply. The chase gas was immiscible with the Slaughter Estate oilbut was first-contact miscible with the acid gas. A 40% HCPV slug of chase gaswas injected over 11 cycles. After five chase-gas cycles, the GWR was reducedto 0.7:1.0 to improve vertical sweep. Chase-gas injection was completed in June1982. Chase-water injection continued until July 1985. The incremental tertiaryoil recovery from the pilot was 125.9 MSTB (19.6% OOIP). JPT p. 1026