Evaluation of COFCAW as a Tertiary Recovery Method, Sloss Field, Nebraska
Authors: Parrish David R. , Pollock Charles B. , Craig F.F. 
Published: December 2007

In this 960-acre project, more than a million barrels of oil was displaced from a previously waterflooded reservoir. Producing rates were as high as 550 BOPD and injected air/water ratios were low. Although COFCAW poses many problems, most of them can be handled, and its great advantage is that the injecting media are the cheapest to be found: air and water.

Introduction

Purpose Purpose Several years ago Amoco Production Co. began field testing a new oil recovery process. This process involves Combinations Of Forward Combustion And Waterflooding. For brevity we call it COFCAW.Pilot COFCAW tests were conducted in several reservoirs of several types. This program of pilot tests is discussed in a separate paper. One of these pilot tests, which is discussed in another separate paper, was conducted in a previously waterflooded portion of the Sloss field, Nebraska. The results of this tertiary recovery pilot were encouraging. It became apparent, however, that the economic potential of COFCAW as a tertiary recovery method could not be determined by a pilot test alone. The pilot therefore was terminated before completion.A full-scale tertiary COFCAW project was undertaken at Sloss. Begun with six 80-acre five-spots and later expanded to 960 acres, this became the largest test of any tertiary recovery method yet reported.The purpose here is to discuss the full-scale COFCAW project at Sloss. Descriptions of the COFCAW process' and the Sloss field are given elsewhere.

Discussion

As shown in Figs. 1 and 2, the expanded or full-scale project at its final maximum size involved about 960 project at its final maximum size involved about 960 acres. Pertinent reservoir data for this area are given in Table 1.Previous engineering studies indicated that better economics could be obtained by operating COFCAW in phases or stages than by operating in the entire reservoir at once. Therefore, the plan was to begin with six 80-acre five-spots. When these had been burned out, the same equipment was to be used for a second similar phase, and so on. Events did not work out exactly that way.As planned, the project began in Feb. 1967 with six 80-acre five-spots called the Phase I area (see Fig. 2). Incidentally, note in Fig. 2 that one of the five-spots was highly asymetrical; this was because offcenter Well 105 was used for injection instead of center Well 18, a COFCAW pilot producer. Note also that the northeast half of the pilot area was arbitrarily included in Phase I, even though Well 13 was never produced during the expanded project.For various reasons, additional wells were added to the project from time to time. Because intermittent testing indicated the presence of combustion gas, two producers in the poorer-quality, lower-productivity producers in the poorer-quality, lower-productivity pay near the northwest edge of the reservoir were pay near the northwest edge of the reservoir were added in March 1968. More important, because the available air-compressor and water-pump capacity could not be fully used in the Phase I injection wells, other wells were added in attempts to use all of the available fluids.