Carbon Dioxide Test at the Mead-Strawn Field

Abstract

Core data and production histories from the CO2 test flood area, compared with similar data obtained from areas that had been water flooded, confirmed the results of laboratory experiments, which had shown that a CO2 flood recovers 50 to 100 percent more oil than a conventional water flood. Introduction A field pilot project has been conducted to test the effectiveness of carbon dioxide as an oil recovery agent in a primary-depleted reservoir. Our experimental oil recovery process consisted of injecting a small slug of CO2 (4 percent PV) followed by a slug of carbonated water (12 percent PV) and then brine. Prior to CO2 injection, water was injected to raise the reservoir pressure in the test area from about 115 to 850 psi; the objective was to maintain the average reservoir pressure at a minimum of 850 psig throughout the test to ensure maximum effectiveness of the CO2. The test was run in a small area of the Upper Strawn sand in the Mead field located in Jones County, near Abilene, Tex. Oil is produced from two zones in the Strawn sands; these zones are separated by about 400 ft of shale and lime. Approximately 2,000 acre-ft to the south and west of the CO2 pilot in the Upper zone, and all 2,000 acre-ft of the Lower zone, were waterflooded conventionally at the same time the CO2 flood was being conducted. Geological Description of the Sands The Upper and Lower Strawn sands are Pennsylvanian in age and are at depths of 4,500 and 4,900 ft, respectively. Both Strawn sands occur as northeast-southwest trending bars or strand lines in which irregular elongate, lenticular porosity traps were developed. The traps are controlled by pinchouts of porosities and permeability in all directions. The sands usually are poorly developed, and are both limey and shaly, with porosities and permeabilities uniformly low. They contain abundant, thinly interbedded shales and subordinate, thin limestone stringers. There is no known oil-water contact in the Mead-Strawn sands. Structural attitude is monoclinal, with a dip of 80 to 100 ft/mile in a northwesterly direction. The hydrocarbon traps occur independently of structure. A petrographic analysis showed the rock to be a sandstone of fine to very fine grain. The average grain size is 0.1 to 0.18 mm, and the grains are mainly subangular and fairly well sorted. The quartz grains have important amounts of secondary growth, which reduces porosity and permeability. The sandstone is very clean, with little matrix material. Field Information Figs. 1 and 2 are isopachous and structural maps of the Upper Strawn sand. The Mead field was discovered in March, 1951, with the completion of the R. A. McCollum No. 1, Sec. 15, Block 17, T and PRR survey. Solution gas furnished the primary drive mechanism. At the start of the waterflood and CO2 test flood, oil production from the areas to be flooded averaged less production from the areas to be flooded averaged less than 40 B/D, and reservoir pressure was about 115 psig. psig. The following are the properties of the entire Upper Strawn sand reservoir in the Mead field: JPT p. 431