Reservoir Performance Field Studies - Case History of Reservoir Performance of a Highly Volatile Type Oil Reservoir

The clue history of performance of a highly volatile type oil reservoir which is now greater than 80 per cent depleted is presented. The reservoir is at a depth of approximately 8,200 ft and includes an area greater than 7,500 acres. The reservoir has been exploited by 10 wells which to date have yielded about 10,880,000 bbl of stock tank oil by pressure depletion. Reservoir pressure has declined from an original of 5,000 psi to approximately 1,450 psi. Produced gas/oil ratio has increased from 3,200 to 23,000 cu ft/bbl with a corresponriing increase in APl gravity of the stock tank oil from 440 to 620 . Pressure and fluid data are given for different stages of depletion. INTRODUCTION In papers by Sloan,' and Cook, Spencer and Bo-browski,' properties of highly volatile type reservoir oil were discussed in detail and methods were presented for predicting performance of reservoirs containing this type of fluid. The purpose of this paper is to present performance history of a highly volatile type oil reservoir designated as Reservoir A which has been produced by pressure depletion and field separation without processing of gas GEOLOGY The reterence field which includes Reservoir A is a structural feature north of a trend of major folding along a northeast-southwest axis. In the area of development, the structure has the appearance of a dome with formation dip of three to four degrees, but structural relations between the reference field and the axis of major folding to the south have not been determined. No faulting within the field is evident. Oil and gas productive sands have been found at depths of 7,000 ft to 10,000 ft in a columnar section of Mio-Oligocene age. The sand bodies in the section are lenticular and usually continuity of sand is limited to a small area. Sand A which forms Reservoir A at a depth of 8,200 ft, however, has exceptional continuous development in the reference field. Fig. 1 shows an isopachous map of net oil sand for Reservoir A. Net thickness of oil sand at producing wells ranges from 14 to 34 ft with an average of 22 ft. Pinch-out of sand has been shown by drilling in the southwest part of the field, but extent of the reservoir on the northwest and northeast parts of the field is indicated only by thinning of the sand. The vertical oil column between the highest and lowest producing well in the reservoir is 339 ft. No water-oil contact has been found within the reservoir. As interpreted from the isopachous map, the reservoir includes about 8,900 acres with an average net sand thickness of 12 ft. Because of limited data on extent of the reservoir, the isopachous map permits only an approximation of sand volume in the reservoir. DEVELOPMENT AND PRODUCTION Reservoir A was discovered during 1938 and was developed with eight wells to the end of 1942. One well was completed during 1946 and the tenth well in the reservoir was completed during 1951. Three of the original wells and the last completion have produced from Sand A only. Six wells have produced from Sand A through the casing side of dual completions. Initial producing rates of the original wells ranged from 800 to 1,000 BOPD. Production history for the reservoir is shown in Fig. 2. Production of the reservoir was begun during 1940 and in May, 1952, peak production from seven wells was 5,300 BOPD. At the end of March, 1954. cumulative production was 10.880,000