Published in Petroleum Transactions, AIME, Volume 204, 1955, pages 49–57. Abstract Pressure has been maintained in the Elk Basin Tensleep reservoir since the initiation of inert gas injection in Sept., 1949. Oil is being produced under conditions favorable for gravity drainage, including high angle of dip, appreciable structural closure, and fairly good permeability. Results being obtained are high per cent recovery of oil-in-place, sustained field productivity, reduced operating problems, and recovery of plant products. The high recovery has been calculated by application of the gravity drainage theory described in an earlier AIME paper. It is also confirmed from field performance by comparing oil recovery to date with gas cap space voided. General Information The Elk Basin field lies in Park County, Wyo., and Carbon County, Mont., approximately 50 miles east of Yellowstone Park. Located at the northern end of the Big Horn Basin, the field is situated on an elongated asymmetrical anticline (Fig. 1). The pressure maintenance project covered by this paper involves the Embar-Tensleep reservoir in this multi-pay field. Discovered in Nov., 1942, it has a proved productive area of over 6,300 acres. It consists of the 210-ft thick Tensleep sandstone of Pennsylvanian Age overlain by 40 ft of Embar dolomite of Permian Age. The two formations apparently are in communication with each other and are produced as a common source of supply. As 98 per cent of the reserves are contained in the Tensleep sandstone, in this paper the Embar-Tensleep reservoir will be called simply the Tensleep. The reservoir is found at an average depth of 4,900 ft, is approximately 7 miles long and 2 miles wide, and has a maximum oil productive closure of 2,330 ft. The strata dip an average of 21° on the west flank and 45° on the east. There are presently 132 Tensleep wells in the field drilled on 40-acre spacing; of these, 17 are shut in because of high gas-oil ratio (GOR) and eight are gas injection wells.
In this paper oil initially in place is calculated by the various methods commonly used for analysis of water drive fields using data available as time progresses. Rate and pressure are predicted by means of the same methods using data available at the end of two years' history. These predictions are then compared with subsequent performance permitting an evaluation of the methods, which are Modified Schilthuis, Simplified Hurst, van Everdingen et al (1952), and the Electric Analyzer. Introduction Several variations of the material balance equation, differing in the term used to evaluate water influx, are in common use for estimating oil in place, predicting rate and volume of water influx, and forecasting producing rates and reservoir pressure in water drive reservoirs. The electric analyzer with an electrical network set up to simulate the oil reservoir and aquifer and their contained fluids may be used for the same purpose. Examples of the application of one or another of these methods have appeared in the literature. The accuracy and reliability of the material balance for calculating oil in place have been covered. However, there have been few articles in which more than one method has been applied to an actual field allowing a comparison. It is felt that a field example, simple in nature, where the Modified Schilthuis, Simplified Hurst, van Everdingen et al (1952), and the Electric Analyzer have been applied would be of benefit in providing an evaluation of these methods as to their comparative usefulness and reliability in calculating initial oil in place and predicting performance.
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