The Sloss COFCAW Project-Further Evaluation of Performance During and After Air Injection
Authors: Buxton T.S. , Pollock Charles B. 
Published: December 2007

Air injection in the COFCAW project at Sloss field, Nebraska, was terminated in July 1971, but water injection was continued. The result has been the production of significant volumes of oil. Here is a discussion of field performance, of data obtained from core holes, and of possible modifications for future COFCAW operations.

Introduction
From Feb. 1967 to July 1971 a 960-acre COFCAW (Combination of Forward Combustion and Waterflooding) project was operated in the Sloss field, Nebraska. project was operated in the Sloss field, Nebraska. Well locations for the COFCAW area are shown in Fig. 1 and pertinent reservoir data are given in Table 1. During approximately 4 1/2 years in which air was injected, 646,776 bbl of stock-tank oil was recovered (not including 80,000 bbl captured during pilot operations). In addition, about 34,000 bbl of hydrocarbons vaporized by Rue gas pasting through the reservoir was vented to the atmosphere. Cumulative injections into the COFCAW area to July 1, 1971, were 13,754 MMscf of air and 10,818,000 bbl of water. The over-all average injected-air/produced-oil ratio was 21,266 scf/bbl. When air injection was terminated in July 1971, water injection was continued with encouraging results. To Jan. 1, 1974, an additional 189,000 bbl of stock-tank oil had been recovered: Adding this to that produced during air injection brings the total recovery to 836,000 bbl of oil and reduces the over-all air/oil ratio to 16,452 scf/bbl. (If all hydrocarbons in the vent gas had been recovered, the over-all air/oil ratio would calculate to be about 11,700 scf/bbl). After air injection was terminated in 1971, it was decided that core holes should be drilled in an effort to determine the following:Residual fuel saturation-i.e., how much hydrocarbon was left in those zones through which the combustion zone had moved.Variation in vertical sweep with distance from the injection well.Areal coverage by the burning front.Maximum temperature distribution - i.e., how the maximum temperature to which the rock had been subjected varied both areally and vertically.The effective permeability of the reservoir rock and whether some material had been deposited in the rock and reduced the flow capacity.
Five core holes were drilled in late 1971 and early 1972. The COFCAW process is described in Refs. 1 through 4. Details of a 40-acre pilot test conducted in the Sloss field are given in Ref. 5 and details of the 960-acre project are given in Ref. 6. Our purpose here is to discuss the results of the core hole program and the performance of the COFCAW area under the influence of water injection.

Selection of the Core Hole Locations
The core holes were drilled in the Well 16-Well 17 quadrant of the five-spot pattern in which Well 16 was the center injector. This particular quadrant was selected because Well 16 was one of the better injection wells and because there was evidence that heat breakthrough had occurred at Well 17. Injection rates as a function of time for Well 16 are shown in Fig. 2.
JPT
P. 1439