Member SPE-AIME
The South Welch Unit, in Dawson County, Texas, was selected as a miscible flooding candidate in 1981 based on the recommendation of a feasibility study. The central 300 acres (1,214 Mm) of the unit were designated as the Phase One area, and infill drilling of this area was begun in 1981. CO 9 injection was initiated in February of 1982 with CO, trucked to the location in liquid form. Simulations were made to predict the recovery from the Phase One area under predict the recovery from the Phase One area under several production methods. These methods included:
20% IHCPV slug CO2 injected continuously 20% IHCPV slug CO2 injected using a 2:1 4 cycle WAG25% IHCPV slug CO2 injected using a 2:1 3 cycle WAG
Continued waterflooding on 20 acre line drive patterns
All simulations were made with the current CO2 supply limitations taken into consideration. The 25% slug, 2:1 5 cycle WAG case is the optimum mode of operation based on sensitivity studies.
Introduction The South Welch Unit, located in Dawson County, Texas (Figure 1), is a waterflood producing from the San Andres dolomite. It is part of the Welch field which was discovered in 1936, with development on 40 acre (162 Mm) well spacing completed by 1937. Unitization of all 12,500 acres (50,586 Mm2) of the field was attempted in 1961, but due to problem countered in the unitization proceedings the southern portion of the field was omitted, with the unitized portion of the field was omitted, with the unitized area becoming the West Welch Unit. The southern 3360 acres (13,597 Mm) were later unitized in 1968 as the South Welch Unit with Cities Service as operator. After unitization, injection wells were rilled creating 40 acre (162 Mm2) five spot injection patterns with 20 acre (81 Mm') well spacing.
An engineering study was performed in 1979 which recommended that rock and fluid property data be gathered and analyzed for the purpose of determining the feasibility of CO2 injection in the Unit. This was done, and in 1981, the recommendation was made to start a project. The center 300 acres (1,214 Mm) of the unit were infill drilled to 10 acre (40 Mm2) well spacing, with the injection pattern being changed to an east-west oriented line drive. CO2 injection was initiated in February of 1982, with the design process consisting of a 20% Initial Hydrocarbon Pore Volume (IHCPV) slug of CO2 injected as a 2:1
Water-Alternating-Gas (WAG) ratio in four cycles. It was projected that CO2 injection would take seven years to complete, and, followed by water injection for 21 years, would recover 2.7 MMSTB (0.43 MMm) of oil from the project area. This represents 13.8% of Ale estimated 17.7 MMSTB (2.8 MMm) original oil-in-place in the project area. Due, to lower production response and earlier CO2 breakthrough than production response and earlier CO2 breakthrough than anticipated, it was determined that a re-evaluation of the Phase One area should be performed.
Reservoir Description and Core Data Production in the South Welch Unit is from Permian age San Andres carbonates. The top of the San Andres formation occurs at −1249 feet subsea (−378 m), and the top of the productive interval is at −1790 feet (−546 m) subsea. An oil-water contact at −1890 feet (−576 m) subsea constitutes the base of the productive interval. From the analysis of eight cores, productive interval. From the analysis of eight cores, gamma ray-neutron logs, SEA micrographs, X-ray diffractions, and chin sections, it was determined that porosity and permeability are directly related to porosity and permeability are directly related to depositional facies and diagenesis.
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