A Tertiary COFCAW Pilot Test In the Sloss Field, Nebraska

Abstract

The operability of COFCAW in previously waterflooded reservoirs was demonstrated by the production of 80,000 barrels of oil from a 40-acre five-spot. This thermal recovery pilot was unusual in several respects. The pay was thin and deep, the oil was light, and the viscosity was low. Introduction The purpose of this paper is to describe a pilot test of COFCAW in a previously waterflooded portion of the Sloss Field, Nebraska.The Sloss pilot was one of a series of COFCAW field tests conducted in several reservoirs of differing types. This field test program is the subject of a separate paper.The COFCAW pilot at Sloss was not carried to completion but was replaced by a full-scale project. The expanded COFCAW project is the subject of another separate paper.The decision to expand COFCAW operations at Sloss was based partly on the encouraging results of the pilot and largely on an urgent need for a full-scale evaluation of this tertiary oil recovery method. The Sloss Field The Sloss field, with about 38 million bbl of oil originally in place is one of the larger fields in the Denver basin. It is near the city of Kimball, in Kimball County, Nebr.The field was discovered in Nov. 1954. By mid-1958, when primary operations ended, there were 89 wells in the field, most of them on 40-acre spacing. Additional wells completed later brought the total number to slightly more than 100.The field produces a light (38.8 deg. API) paraffinic oil from a stratigraphic trap in well consolidated Muddy "J" sandstone. There are two main reservoirs -- the J1 at a depth of about 6,200 ft and the slightly deeper J2. In some areas, the two reservoirs are separated by only a few feet of shale. Much of our discussion will be confined to the J1 reservoir (Fig. 1), since only that reservoir was involved in the COFCAW operations. Production History Production History Early performance was typical of volumetric reservoirs. Peak production of 9,600 BOPD occurred in mid-1957, about 21/2 years after discovery. By 1958 the reservoir pressure had declined from the original pressure of 1,328 psia to less than 400 psia, well pressure of 1,328 psia to less than 400 psia, well below the bubble-point pressure of 689 psia. About 11 percent of the original oil in place was recovered percent of the original oil in place was recovered during pressure depletion operations.The field was unitized with Amoco Production Co. as operator and a waterflood was started in mid 1958. Response to the waterflood was good, and a maximum incremental oil producing rate of about 3,000 BOPD over the estimated primary rate was obtained. By the start of the COFCAW pilot project in mid-1963, most of the reservoir had been waterflooded. (For a description of the COFCAW process, see Ref. 1 and its references.) JPT P. 667