Pilot Water Flooding in a Dolomite Reservoir, The McElroy Field

Abstract

Abstract The McElroy field, discovered in 1926, is one of the major oil reservoirs of West Texas. Production is obtained from the Permian Grayburg-San Andres dolomites at an average depth of 3,000 ft. Two gas injection and four waterflood pilots have been conducted in the field. Both of the gas injection projects were unsuccessful because of high free-gas saturations in the reservoir. Three of the pilot water floods have sufficient performance histories to permit thorough evaluations. Pilot I, an inverted seven-spot installed in 1947, and Pilot II, a 20-acre five-spot installed in 1953, have proved the old, better part of the field could be successfully flooded. The best wells in these pilots have produced large volumes of oil at water-oil ratios of approximately one. This performance is thought to be more the result of gravity effects than stratification. Pilot III, two contiguous 40-acre, five-spots, was installed in late 1959 and has proved the floodability of the tight, anhydritic portion of the reservoir. Injectivity problems were encountered but were remedied by increases in pressure. Pilot IV was initiated late in 1959 to determine if the old part of the field could be flooded on wide spacing. It consists of four injection wells closing 160 acres which contain nine producers. There have been some individual well responses; however, interference between injection wells is still one to two years in the future. Introduction and History of Development The McElroy field, one of the major oil reservoirs of West Texas, is located in northeastern Crane County and west-central Upton County (see Fig. 1). It is elongated in a northwest-southeast direction with the center portion located along the Crane-Upton County line, immediately east of the city of Crane. Geologically, the field lies on the eastern edge of the Central basin platform and is situated on a roughly north-south trending asymmetric anticline with the steep limb to the east. The trap is stratigraphic as well as structural. For the most part, the productive limits are controlled by porosity and permeability development. Production is obtained from the Grayburg-San Andres dolomites of Permian age at an average depth of 3,000 ft. During the early development, three portions of the present McElroy field were classified as individual fields. First production was obtained in April, 1926, from the Church and Fields No. 1 University in the Church-Fields area in the northern portion of the field. In June, 1926, the Gulf Oil Corp. completed its J.T. McElroy No. 1 in the Gulf-McElroy area in the south-central portion of the field. In Nov., 1928, production was discovered in the McClintic area in the southern part of the field by the Duffy Investment Co. No. 1 McClintic. As development continued, the merging of these three productive areas led to their consolidation and subsequent classification as the McElroy field on March 5, 1941.Drilling in the northern and southwestern areas of the field has been mainly on 10-acre spacing. The Gulf properties, which are concentrated in the southern half of the field, have been developed mostly on 20-acre spacing. At the present time, the reservoir encompasses some 30,000 acres containing approximately 1,800 wells. JPT P. 1345ˆ