A Note on COFCAW
Authors: Harmsen G.J. 
Published: December 2007

Those who studied both the paper by Dietz and Weijdema on wet and partially quenched combustion and that by Parrish and Craig on the COFCAW process may have begun to wonder about the mechanism of the process. Although the two papers deal with the same subject - namely, the injection of water and air into a burning oil sand at such high WOR's that water enters the combustion zone - the phenomena described are not completely in agreement. According to Dietz and Weijdema only part of the water evaporates, so that the temperature in the combustion zone is limited to that of saturated steam. Combustion is not extinguished, but the velocity of the combustion zone is now imposed by the cooling front created by the injected water, instead of by the concentration of the available fuel. Consequently, some unburned fuel is left ("partially quenched combustion"). Parrish and Craig also notice that the "fire is not put out" and that some "coke" is left. Nevertheless, put out" and that some "coke" is left. Nevertheless, they observed combustion temperatures exceeding those of saturated steam. In a forthcoming paper by Beckers and Harmsen the effect of water injection on sustained combustion is analyzed in detail for linear flow and under conditions of perfect insulation. This was also simulated in the experiments by Parrish and Craig. These experiments are of specific interest because several of them seem to support the theory in a range not covered in the experiments by Dietz and Weijdema.

Partially Quenched Combustion Partially Quenched Combustion Dietz and Weijdema show that in their form of the partially quenched combustion process the steam partially quenched combustion process the steam content of the gas that leaves the combustion zone is independent of the water/air injection ratio. The remainder of the water flows through the combustion zone unevaporated. This implies that this process can take place only if the amount of water entering the combustion zone at least equals the amount of steam leaving it. Consequently, there is a minimum water/ air injection ratio for which this process is possible. Beckers and Harmsen show that this minimum ratio is higher than the maximum water/air injection ratio for which the process is, as Dietz and Weijdema call it "normal wet". In other words, there is a range of water/air injection ratios for which the injected water overtakes the combustion zone without causing "super wet" combustion as described by Dietz and Weijdema. In this range the water completely evaporates in the upstream part of the combustion zone while in the downstream part the temperature rises to above that of saturated steam. Yet, because of the low temperature at the upstream end, combustion may be incomplete, i.e. partially quenched. The situation is illustrated in Fig. 1, taken from Ref. 3. According to Beckers and Harmsen's theory, under laboratory conditions the type of process described by Dietz and Weijdema can only occur at water/air ratios at the combustion zone higher than 1.67 lb/lb (air/ water ratios less than 2,600 scf/B), so that only three of the 11 experiments reported by Parrish and Craig were in the "super wet" range. For small fuel availabilities (in the order of 1 lb of fuel per cubic foot of formation) the range wherein the process is neither "normal" nor "super wet" is very small.

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