A Multipilot Evaluation of the COFCAW Process

Abstract

To evaluate COFCAW as an improved oil recovery method in reservoirs with widely varying conditions, single-injection-well pilot tests were conducted in seven different fields. Much valuable experience was gained in the practical application of the method. This paper summarizes the information practical application of the method. This paper summarizes the information obtained in that test program. Introduction Both forward combustion and waterflooding are recognized oil recovery processes. Forward combustion involves the injection of air to sustain a combustion zone that moves through the reservoir. This combustion zone displaces most of the reservoir oil, but consumes as fuel some of the heavier oil components that are frequently referred to as "coke." Behind the combustion zone the reservoir rock is clean, dry, and hot. Waterflooding displaces only a portion of the reservoir oil, but its sweep efficiency is good.A combination of these two processes has been termed COFCAW, an acronym standing for "combination of forward combustion and waterflooding." COFCAW involves the injection of both air and water either simultaneously or alternately at air/water ratios of less than about 3,000 scf of air per barrel of water (3,000 scf/bbl).During forward combustion, much of the heat generated from the expensive compressed air is wasted by being left behind the combustion zone. When water is injected with the air, some of the water (and air) remains in the burned-out region behind the combustion zone. The remaining water is converted into steam, which flows through and ahead of the combustion zone and creates a steam zone that displaces much of the oil. The water thus picks up heat from a region where it is virtually useless and transports the heat into the oil-containing region where it can serve a useful purpose. Connate water cannot accomplish this since it is boiled away ahead of the combustion zone. During COFCAW only enough air (heat) must be supplied to vaporize the water flowing past the combustion zone. The sensible heat is taken care of by a heat-wave type of action. But, since the latent heat or heat of vaporization of water depends upon pressure, the air/water ratio (AWR) must be modified pressure, the air/water ratio (AWR) must be modified according to the pressure. Generally, for higher pressures, lower AWR's can be used. A good description pressures, lower AWR's can be used. A good description of what happens as more and more water is injected during a combustion operation has been given by Dietz and Weijdema. COFCAW has a major advantage over waterflooding: it displaces more oil. It also consumes less fuel and thus displaces more oil than forward combustion. Its greatest advantage, however, is the reduced cost of injected fluids. By substituting cheaper water for some of the expensive compressed air, COFCAW may bring the cost of injected fluids down into the realm of economic possibility for tertiary oil recovery.A series of laboratory COFCAW experiments has been previously reported. The experiments provided a quantitative understanding of the mechanism and permitted estimation of the potential of this process permitted estimation of the potential of this process as a field oil recovery method. The next step in a planned development of a new and improved oil recovery planned development of a new and improved oil recovery process normally is field pilot evaluation. Pilot process normally is field pilot evaluation. Pilot testing yields experience with the actual operation of the COFCAW process and provides an evaluation of its performance in natural heterogeneous reservoirs. JPT P. 659