A Case History of the Pegasus Ellenburger Reservoir

Abstract

The performance of the 13,000-ft-deep Pegasus Ellenburger in West Texas has shown that, even in an extremely heterogeneous reservoir, gravity segregation drive can be an efficient producing mechanism. Introduction The Pegasus field is a large multizone field in Midland and Upton Counties, Tex. Oil productive intervals have been found in the San Andres, Spraberry, Pennsylvanian, Devonian, Fusselman and Ellenburger zones. The Pegasus Ellenburger is the deepest reservoir at an average depth of 13,000 ft and is also the most prolific oil producer. Since discovery in 1949, over 60 million STB of oil have been produced from this reservoir. The 4.5 million STB produced in 1967 was the highest annual oil production during the life of the reservoir. As production during the life of the reservoir. As of April, 1968, the reservoir pressure was more than 1,100 psi below the bubble point and the producing GOR had not significantly increased. producing GOR had not significantly increased. Development and Unitization Magnolia Petroleum Co. (now Mobil Oil Corp.) drilled and completed TXL "A" Well No. 1 as the discovery well in the Pegasus Ellenburger field. This well, which is now the Pegasus Ellenburger Unit Well No. 15-1, was drilled in March, 1949, to a total depth of 13,295 ft, with a potential of 1,167 BOPD flowing with over 1,000 psig tubing pressure. From 1950 to mid-1954 the field was pressure. From 1950 to mid-1954 the field was developed at an average rate of 24 wells/year. In all, 119 wells have been drilled. The Pegasus Ellenburger unit became effective June 1, 1954. Fig. 1 shows the location of wells and the unit boundary. Production and Injection History Production and Injection History Fig. 2 shows the oil and water production history, water and gas injection history, and GOR performance. The sharp decrease in oil production in 1951 resulted when the allowable was reduced from 395 to 200 B/D/well to allow time to initiate pressure maintenance operations before the bubble-point pressure was reached. Pilot water injection was begun in 1955 and expanded in 1959 to a full-scale peripheral injection program. In 1960 the allowable was increased to 435 B/D/well (the 1947 yardstick value for 80-acre spacing). Water injection performance did not prove efficient and, in performance did not prove efficient and, in 1963, injection rates were curtained so that the effects of continued pressure decline could be observed. Gas injection was begun in 1967 and expanded to a full-scale crestal gas injection program in early 1968. Geology Structurally, the Pegasus Ellenburger reservoir is a three-domed anticline with a general northeast-south-west axial trend. (See Fig. 3.) The production area encloses almost 10,000 acres. There are major faults along the east and west sides of the field, and it is believed that numerous other faults (having smaller displacements) also exist throughout the field. JPT P. 1330