Experimental And Numerical Simulation Studies Of the Wet Combustion Recovery Process

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Abstract
Laboratory combustion tubes can yield much information about applying the in-situ combustion process to petroleum reservoirs. However, because of practical limitations on combustion tube length, end effects may sometimes distort the experimental results. These obscuring phenomena may be especially troublesome when water is injected while burning (wet combustion). In order to bolster the reliability of predictions of reservoir performance of the latter process, a numerical model has been developed which accounts for conductive and convective heat transfer, fluid flow, and vaporization and condensation of the various components. The kinetics of the oxidation reaction which has been studied independently are also incorporated into the model. A comparison of computed and experimental results shows reasonable agreement. Computed results for adiabatic reservoir-length systems indicate that combustion behaviour may differ significantly from that observed in a relatively short combustion tube. Introduction: FORWARD COMBUSTION, sometimes referred to as in-situ combustion, fireflooding, etc., has been studied extensively by the petroleum industry for a number of years. The technical literature contains numerous reports on laboratory investigations and field experience with the process. These studies have shown that during dry forward combustion, much of the heat released by the burning oil remains behind the combustion front. Thus, the dry in-situ combustion process is inefficient in terms of heat reaching the displacement front. A technique for improving the heat transport efficiency of forward combustion has been discussed in recent year The procedure, after achieving ignition of the oil, is to inject water either simultaneously or alternately with air. The water, having a specific heat approximately four times that of air, absorbs heat, evaporates with the absorption of more heat and, as steam, carries the heat downstream beyond the combustion zone. Another potential effect of water injection is that the air requirement, a major expense, may be diminished by partially quenching the combustion reaction. By reducing the length of the high-temperature region upstream from the steam zone as well as moving it ahead more rapidly by convection, the time available for vigorous oxidation to occur may not be sufficient to consume all of the available fuel. As indicated qualitatively in Figure 1 and as discussed by Dietz and Weijdema,(2) increasing the water/air injection ratio (,WAR) can eventually reduce the temperature level in the moving heat zone to that of saturated steam. At higher water/air ratios, injected water can flow through the zone without being converted to steam. For convenience, this type of recovery process will be referred to here as "wet combustion" regardless of the water/air ratio. Even though the major physical and chemical phenomena involved in forward combustion are recognized, investigators have not reported methods to predict with certainty the performance of the process in a particular reservoir. The interplay of three-phase relative permeability, vaporization and distillation, oxidation kinetics, thermal cracking of the oil, etc., is complex. Consequently, the major approach has been experimentation. This generally has involved both laboratory research and field tests. In this paper, we have described laboratory experiments carried out in a high-pressure combustion tube.

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