Implementation of a Gravity-Stable Miscible CO2 Flood in the 8000 Foot Sand, Bay St. Elaine Field

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Summary This paper reviews implementation of a gravity-stable, miscible CO2 solvent flood in a reservoir located in the coastal marshes of south Louisiana. Key reservoir properties are presented, and field tests to determine residual oil saturation and to define reservoir limits and continuity are described. Unique equipment used to transport and inject the CO2 solvent is discussed along with the special instrumentation used for quality control. Also itemized are the regulatory agencies contacted and the permits acquired before the project could be implemented. Introduction Texaco U.S.A. initiated a gravity-stable, miscible CO2 solvent flood on Jan. 20, 1981, in the Bay St. Elaine field, Terrebonne Parish, LA (Fig. 1). The flood in the 8000 Foot Reservoir E Sand Unit (RESU) is being conducted to prove the effectiveness of gravity-stable, miscible CO2 solvent flooding in a steeply dipping, depleted sand. The results of this project will help to determine whether fieldwide CO2 flooding will be economical in similar south Louisiana reservoirs. The project consists of three wells-one injector and two producers. A structure map of the 8000 Foot RESU is shown in Fig. 2. Approximately 84.4 Mg/D [84.4 metric tons/d] CO2 solvent, consisting of CO2, methane, and normal butane, was injected into updip Well 22–26 over a 9-month period. The CO2 solvent slug occupied one-third of the reservoir PV. The CO2 solvent slug size and composition was designed by Texaco's Bellaire (TX) Research Laboratories. Nitrogen, used as a drive gas, is being injected into Well 22–26 to displace the CO2 solvent slug downdip in the reservoir. The CO2 solvent, as it moves downdip, will become miscible with the in-place residual oil. Production of tertiary oil will be from downdip Wells 22–5 and 22–31. An estimated 11 924.0 stock-tank m3 [75,000 STB] tertiary oil is expected to be produced from this depleted water-drive reservoir. The CO2 and n-butane were trucked in liquid states and off-loaded into storage tanks at Cocodrie, LA. These fluids were later transferred to similar storage tanks mounted on barges. The barges then were transported 11.3 km [7 miles] to the injection facility in the Bay St. Elaine field. The methane was obtained from the gas-lift system already in the field. The three components were mixed and injected into Well 22–26. Reservoir Description. The Bay St. Elaine field overlies a salt dome. The recoverable oil is found in numerous sands separated by shale layers. Fig. 3 is a typical electric log of the 8000 Foot Sand from Well 22–5. These sands dip steeply as a result of the upward movement of the salt dome. The 8000 Foot Sand is separated into segments by faults radiating from the salt dome. The 8000 Foot RESU varies in net sand thickness from about 9.1 m [30 ft] near the updip unconformity to about 30.5 m [100 ft] in the downdip portion of the project area. This Miocene sand exhibits high permeability and porosity. The reservoir oil has an 840-kg/m3 [36API] gravity and was produced at a normal GOR. The reservoir has a strong edgewater drive. Key reservoir parameters are presented in Table 1. The 8000 Foot RESU was determined a suitable candidate for a gravity-stable, miscible CO2 solvent flood for the following reasons. Well-Defined Reservoir. Reservoir E and Segment 790 are isolated from the rest of the 8000 Foot Sand by well-established faults. Well Availability. All wells in the two segments except Well 22–3 are completed in the 800 Foot Sand with no remaining primary or secondary potential. Well-Delineated Sand. The 12.7-cm [5-in.] log of Well 22–5 (Fig. 3) indicates significant shale breaks between the sands. The 8000 Foot RESU is not in communication with any other sand within the project area. Fairly Homogeneous Sand. The few shale streaks within the 8000 Foot Sand are not continuous, and the sand is uniform. Expansion Potential. Large reservoirs with millions of barrels of residual oil are adjacent to Segment 790 in both the 8000 Foot Sand and the 8050 Foot Sand (directly beneath the 8000 Foot Sand). The reservoir characteristics are quite similar and the same CO2 solvent flooding process can be used in these reservoirs. JPT P. 101^

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