A Novel Pressure Maintenance Operation in a Large Stratigraphic Trap

Abstract

Abstract This paper presents a discussion of a program of pressure maintenance for the Adena sand field, a shallow dipping stratigraphic trap having a large gas cap. It is concluded that a reservoir of this type be successfully water flooded by injection into a row of wells along the gas-oil contact. Reservoir and laboratory studies showed that water channeling in gas saturated streaks existing in the oil zone would not occur, and that an effective water barrier could be formed between the large gas cap and producing area. Studies indicated gas-oil contact water injection far superior to the other programs investigated. One year of field performance indicates that the pressure maintenance program is functioning satisfactorily. History of Development and Unitization In May, 1953, Falcon Seaboard Drilling Co. drilled the Snodgrass No. 1 in Sec. 20, T 1 N R 57 W, as a rank wildcat approximately 14 miles south of Fort Morgan. Colo. (Fig. 1). This well encountered gas in the J sand of the Dakota series and was completed for a good gas well in what is now recognized to be the gas cap of the Adena field, the largest oil field in the Denver-Julesburg Basin. This well created only mild interest in this area of the basin and the next few wells drilled were considered to have but slightly more than wildcat prospects. The oil discovery came in Nov., 1953, when an independent operator completed the Cochran No. 1 in Sec. 12, T 1 N, R 58 W, flowing 900 bbl of 42 gravity oil from the J sand. The Adena J sand field is approximately seven miles long and 3.5 miles wide. The oil zone has an areal extent of approximately 8,425 acres and the gas cap covers approximately 4,650 acres. Basic reservoir data is shown in Table 1. Production is primarily from the Dakota J sand of Cretaceous age, encountered at an average depth of 5,650 ft. The formation dips gently to the west at a rate of only 48 ft per mile, with minor nosing and flattening within the field proper (Fig. 2). Basically, the field is a stratigraphic trap, with local structure playing a very minor role as a trapping component. Oil accumulation is controlled by permeability barriers on the east, north and south sides of the field, and by a shale seal at the base of the first bench. The formation is a light gray, fine-to-medium grained grindstone. To date 170 oil wells and 15 gas wells have been drilled. The gas wells have been shut-in since completion to conserve reservoir energy. Wells were completed by drilling the entire sand section, running 5.5-in. casing and perforating the pay interval. It took only two weeks to drill and complete a well. Approximately 85 per cent of the wells in the field were both cored and logged.