Summary The reservoir engineering aspects of the design of a major west Texas CO2 flood are presented. The design included a detailed fieldwide geologic study, a CO2 injectivity test, laboratory work, and reservoir simulation. CO2 floodingis predicted to recover an additional 8% of the original oil in place(OOIP). Introduction The North Ward Estes (NWE) field, located in Ward and Winkler Counties, TX(Fig. 1), was discovered in 1929. Cumulative oil produced is more than 320million bbl (25% OOIP). The field has produced is more than 320 million bbl(25% OOIP). The field has been waterflooded since 1955. Geologically, the NWEfield resides on the western flank of the Central Basin Platform. The Yates, the dominant producing formation, includes up to seven major reservoirs and iscomposed of very-fine-grained sandstones to siltstones separated by densedolomite beds. Within the 3,840-acre Sect area, average depth is 2,600 ft. Porosity and permeability average 16% PV and 37 md, respectively. Reservoirtemperature is 83 degrees F. The flood patterns are 20-acre five-spots andlinedrives. patterns are 20-acre five-spots and linedrives. CO2 flooding wasimplemented in early 1989 in a 6-section project area located in the betterpart of the field in terms of project area located in the better part of thefield in terms of cumulative oil production and reservoir-rock quality. Thispaper reviews the field history and reservoir geology and focuses on thereservoir engineering aspects of the design of this CO2 flood-laboratory work, CO2 injectivity test, and CO2 flood performance predictions. performancepredictions. Field History and Development The NWE field was discovered in 1929. Except for the most productive parts, which were drilled on 10-acre spacing, the field was initially parts, whichwere drilled on 10-acre spacing, the field was initially developed on 20-acrespacing. Until the early 1950's, a typical completion consisted of drilling tothe top of the Yates, drilling ahead and checking for gas caps, setting casingthrough the gas sands, drilling to total depth, shooting the producing sectionwith nitroglycerine, cleaning out the hole, and hanging a perforated liner fromthe casing. Practices changed in the early 1950's to eased-hole completions, hydraulic fracturing, and acidizing. About half the current injectors are shot, open-hole completions. Vertical sweep has been adversely affected because ofthe inability to measure and control the injection profiles. Fig. 2 shows theproduction and injection history of the project area. Primary production peakedin 1944 and was approaching the economic limit in the mid-1950's. A 960-acrepilot waterflood began in 1954. Oil production responded quickly, and the floodwas expanded to the rest of the project area during the next 2 years. Theprevailing flood patterns were 4O-acre five-spots. Oil production increasedsteadily after 1954, reached a peak in 1960, and then declined at 11 %/yr until1979, when it began to stabilize as a result of drilling infill and replacementwells, injection profile modifications by means of polymer treatments, andpattern profile modifications by means of polymer treatments, and patterntightening and realignment (Sections 3 and 6 through 8 were converted to20-acre five-spot patterns and Sections 9 and 10 to 20acre linedrive patterns). By the end of 1988, the 6 sections had produced 29% of the OOIP. Waterfloodingthe Yates has been very successful, as evidenced by the 2.3 ratio of ultimatesecondary to ultimate primary production from wells existing at the beginningof waterflooding. The production from wells existing at the beginning ofwaterflooding. The favorable mobility ratio in these reservoirs indicates goodarea sweep efficiency. Because of the high Dykstra-Parsons coefficient (0.85)and permeability contrast among the major sands, the vertical conformance hasbeen poor. Even after injection of 2.6 waterflood-movable PV, only 50% of theoil recoverable by waterflooding has been produced. Reservoir Geology and Properties A comprehensive geologic study and reservoir characterization was conductedto characterize the individual reservoirs of the Yates, which consist ofvery-fine-grained sandstones to siltstones separated by dense dolomite beds. Indescending order, these sands are Sands BC, D, E, Strays, J1 and J2 (Fig. 3). The general depositional environment was a tidal-flat-to-lagoona settingsituated to the east of and behind the shelf margin. The reservoirs weredeposited as sand and silt in the subtidal-to-beach environment and silt toclay in the supratidal environment. Depositional strike was parallel to theshelf margin, which is parallel to the present northwest/southeast sectionlines. Sand BC is a siltstone to fine-grained sandstone with detrital clay. Thedepositional environment was that of a shallow-water tidal flat with anabundant amount of windblown sediments. A zone of low porosity and permeabilitytrends northwest/southeast through the porosity and permeability trendsnorthwest/southeast through the middle of the project area. Most of Sand BC wasin the original gas cap. Sands D and E are similar to Sand BC, but theirporosities and permeabilities are more variable. The Strays sand is composed ofthin-bedded, lenticular, intertidal to subtidal siltstones and fine-grainedsandstones with the highest clay content of any Yates interval. Because ofthis, permeability and reservoir continuity suffer while porosity remains high. Sands J1 and J2 are composed of coarser sands with much less clay content andtherefore higher effective porosities and permeabilities. The depositionalenvironment was a beach to near-shore marine where turbulence winnowed finersilts and clays out of the strike-oriented sand deposits. Table 1 lists averagereservoir properties for the Yates. Laboratory Work Extensive laboratory work was conducted to support the evaluation of CO2 flooding in the NWE field.Black-oil PVT and oil/CO2 phase-behavior studiesof recombined separator oil and gas samples (Table 2) determined oil swelling, viscosity reduction, and phase transition pressures vs. mole percent CO2 (Fig.4). The PVT data show the typical complex phase behavior exhibited by CO2/light-crude-oil systems at low reservoir temperature.Slim-tubeexperiments determined minimum miscibility pressure (MMP). Fig. 5 shows theresults of the displacement of pressure (MMP). Fig. 5 shows the results of thedisplacement of reconstituted reservoir fluid by pure CO2 in a packed column atdifferent pressures. Additional displacement tests were conducted with fivedifferent CO2/hydrocarbon-gas mixtures. The MMP ranged from 1,010 to 1,350 psiavs. 937 psia for pure CO2. No significant changes in ultimate slim-tube oilrecovery were observed. These tests verified that a published correlationadequately estimates the MMP for NWE oil and impure CO2.CO2 flooding ofrestored-state composite cores determined the mobilization and recovery of thewaterflood residual oil saturation, S orw. The core assembly (Fig. 6) wasconstructed from 1-in.-diameter plugs drilled from NWE cores epoxied intoconfining stainless-steel sleeves. SPERE P. 11