An Update of the Polymer-Augmented Alkaline Flood at the Isenhour Unit, Sublette County, Wyoming

Abstract

Summary. An Almy sand polymer-augmented alkaline flood at the Isenhour Unit, Sublette County, WY, is reviewed. This paper updates process technology, including the use of clay stabilization, sweep improvement, soda ash alkaline agent [to reduce interfacial tension (IFT) and mobilize residual oil], and anionic-polymer-blend mobility buffer. Oil production has been increasing at 20%/yr since the process start. Introduction The Almy sands in Sublette County, WY, have some 15 years of volumetric-sweep-improvement history. In an effort to reduce residual oil saturation in the swept area, the first fieldwide alkaline flood was initiated in Sept. 1980 at the Isenhour Unit. On the basis of core testing, crude oil alkaline-agent screening determined that soda ash (Na2CO3) and anionic polymer would be most beneficial in reducing IFT and mobilizing residual oil. Coreflooding indicted that alkaline agent could be injected into the high-clay-content Almy sand. A significant decrease in oil saturation on a percent PV basis was predicted. This paper details flood performance up to Jan. 1, 1985. Injection is now 48% of the total PV. A cationic and anionic polyacrylamide spearhead preceded the addition of the alkaline polyacrylamide spearhead preceded the addition of the alkaline agent. Currently, the program continues to use a tripolyphosphate/anionic-polymer blend for long-term wettability control and alkaline slug stability. Recovery to date is 27% of the original oil in place (OOIP). Oil production has been inclining at a rate of 20%/yr since the start of the polymer-augmented alkaline flood. polymer-augmented alkaline flood. Formation Data The M-42 reservoir of the Almy sand series, a tertiary sand deposition in a river bed, was subjected to eight stages of diagenesis before hydrocarbon emplacement. Feldspar overgrowths, clay and calcite cement, and emplacement of abundant chlorite and kaolinite clays and calcite reduced primary intergranular porosity. porosity. The average depth of this stratigraphic trap is 3,700 ft [1130 m], with net pay thickness 15.1 ft [4.6 m], average porosity 15.5%, and average permeability 21 md at reservoir temperature, 97 degrees F [36.1 degrees C]. Oil gravity is 43.1 degrees API [0.81 g/cm3], with oil viscosity at bottomhole temperature 2.8 cp [2.8 mPa.s]. History The Isenhour field was discovered in Aug. 1970. Primary development continued and eight producers were completed by 1980. Core studies of the M-42 reservoir of the Almy sand series indicated abundant chlorite and kaolinite content. Migrating-fines problems associated with these clays were noted. problems associated with these clays were noted. Alkaline-agent screening test showed that soda ash, along with sweep improvement techniques proven in 70,000 acre-ft [86.3 × 10(6) m3] of other Almy sand reservoirs already under flood, would be effective. With soda ash mines 100 miles [161 km] away, alkaline-agent costs were low. Dry solids handling, wetting, mixing, and proportioning equipment was provided when the Isenhour injection plant was built. Wells 3, 9, and 10 were completed as injectors and presoaked with KCl followed by cationic polymer. At the time, this treatment was standard practice for clay control in the Almy. In addition, tracer chemicals were used in Wells 9 and 10 to determine subsurface fluid movement. Cumulative production through Sept. 1980, when the engineered oil recovery program started, was 437,202 STB [69 510 stock-tank m3] oil. Isenhour Unit Performance Fig. 1 presents the M-42 reservoir net-pay isopach and well-location plat for the Isenhour Unit as of Jan. 1, 1985. Note the southern plat for the Isenhour Unit as of Jan. 1, 1985. Note the southern and western extension of the reservoir with drilling in late 1983 to mid-1984. Two of the original injection wellbore shut in. One producer was converted to injection and then shut in. Three producer was converted to injection and then shut in. Three producers have been shut in and one shut-in wellbore was converted producers have been shut in and one shut-in wellbore was converted to injection status. As of Jan. 1, 1985, the Isenhour Unit consisted of two injectors, five producers, and five shut-in wellbores. Fig. 2 is total field production performance as of Jan. 1, 1985. Since initiation of the polymer-augmented alkaline flood, 264,843 STB [42 107 stock-tank m3] oil has been recovered for a total recovery to date of 702,045 STB [111 616 stock-tank m3] oil. Production response was noted right after the start of the chemical oil recovery program. Oil rate inclined at about 20%/yr. Quarterly reservoir management meetings provided for the shut-in of various wells and the drilling program in 1983 and 1984. This close reservoir surveillance prevented many problems associated with other alkaline floods. Individual well performance will be detailed in this paper. Fig. 3 is total field WOR vs. cumulative oil recovery. Early breakthrough in southeastern Well 51 (which was shut in before the start of the chemical program) caused a 0.7 WOR fieldwide. Well 51 was converted to injection; however, less than 5,600 bbl [890 m3] was injected, and the well was shut in. As noted in Fig. 3, Wells 1, 2, and 6 were shut in during late 1983. These wells were marginal producers with low fluid recovery. The production performance of these wells confirms the complex geology affecting this portion of the M-42 reservoir. Fig. 4 provided secondary recovery analysis model projections of WOR vs. oil recovery for base-case waterflood, polymer-only flood, and polymer-augmented alkaline flood. In addition, actual field performance is included. Note that after the shut-in of the watered-out producer, Well 51, actual performance was much better than original projections. At this update, the incremental oil estimates remain unchanged. The drilling of the west and southwest wells and the complex geology in the center of the reservoir may have changed volumetrics. No attempt to revise the original volumetrics is made in this paper. Note that corefloods indicated about 3% PV mobilization of residual oil saturation. Chemical Program Status Table 1 summarizes the polymer-augmented alkaline flood as of Jan. 1, 1985. Table 2 details the chemical injection status for the Isenhour Unit. SPERE P. 604

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