E.T. O'Daniel Project A Successful Spraberry Flood

Abstract

Abstract The Upper Spraberry zone on Mobil Oil Corp's 2,160-acre E. T. O'Daniel lease has been under waterflood since Aug. 7, 1959. Rows of injection wells are spaced about 1/2 mile apart, and are oriented in line with the major fracture trend. Dump-flooding, consisting of gravity injection of water from the Santa Rosa water sand at 1,000 ft into the Upper Spraberry zone at 7,200 ft in the same wellbore, has been the injection mechanism from project start to 1966. At the time waterflooding was initiated the leave was at its economic limit after having produced some 1 million bbl of oil by primary depletion. This primary ultimate recovery of 463 bbl/acre is, representative of those leases in the Spraberry Trend area field on which only the Upper Spraberry zone is productive. Since start of waterflood operations the lease has produced an additional 834,000 bbl of oil and is currently producing at a rate of 130,000 bbl year. Thus, the project has already recovered waterflood reserves equivalent to eight-tenths of its primary ultimate and continues to produce at an average rate of 23 BOPD/well. This case history of a successful recovery effort in this field presents performance data which may be useful for comparison in establishing flood development plans for future secondary recovery efforts in other parts of the field. Introduction The Spraberry Trend area field proved to be unique among oil-producing reservoirs almost from the day of its discovery. This vast reservoir has challenged the industry's ingenuity under both primary and secondary recovery operations. Many large investments have been made in recent years to develop waterflood operations in the Spraberry Trend in an effort to improve low primary recoveries. The tight matrix porosity coupled with the extensive fracture system make the waterflood performance of the reservoir particularly difficult to predict. Geology The Spraberry Trend area field is located in the Midland basin, a feature of the greater Permian Basin. Structure is a north-south trending monocline with beds dipping 30 to 50 ft to the west. Estimated productive area of the entire reservoir is approximately 500,000 acres. The Spraberry formation is of Permian Leonard age and is characterized by a complex stratigraphy composed of sandstone, shale, siltstone or limestone interbedding. Gross formation thickness varies from 800 to 1,200 ft with two distinct zones of developed porosity. The upper zone, found at an average depth of 7,200 ft, is productive essentially throughout the trend area. The lower zone, found at an average depth of 8,000 ft, has proven productive only in certain areas of the field. The O'Daniel lease is commercially productive from the upper zone only. Development The discovery well in the Spraberry Trend area field was completed in 1949 with much of the initial development on 40-acre spacing. Over-all well density now ranges from 40 to 160 acres. Throughout the trend area, the majority of the wells were initially completed in the Upper Spraberry zone only. In 1956 and 1957 an extensive program to drill deeper to the Lower Spraberry zone (in the areas in which the lower zone is productive) resulted in a substantial oil production increase. During 1956 and 1957 the lower zone on the O'Daniel lease was tested in Wells 17, 18 and 21 and found to be noncommercial. Initial development of the O'Daniel lease in 1951 was accomplished with a spacing of approximately 100 acres/ well. At the start of secondary recovery operations in Aug., 1959, 21 wells were on the 2,160-acre lease. Six producing wells were drilled after start of the flood to complete the flood development pattern. The wells were completed open hole within a completion interval ranging from 7,000 to 7,350 ft Rock and Fluid Properties The Spraberry matrix rock is a well cemented sand and siltstone which is porous but of very low permeability. Permeability of the producing formation is primarily governed by an extensive vertical fracture system that is oriented on a northeast-southwest trend along an azimuth of N 45 to 50 degrees E. The presence and particularly the orientation of this fracture system is of primary importance in selecting a flood development pattern if optimum recovery is to be achieved. The reservoir permeability is many times greater in the direction of the major fracture trend. The ratio of permeability parallel to the fracture trend as compared with that perpendicular to the trend has been reported in excess of 100:1. JPT P. 1137ˆ