Simulation of In-Situ Combustion in a Two-Dimensional System

Abstract

Abstract A two-dimensional mathematical model of the forward combustion process was developed and used for studying the effects of fuel content and air injection rate on the sweep efficiency attained in in-situ combustion. The model incorporates the effects of single-phase gas flow, heat generation by a combustion front, change of permeability in the burned zone, heat transfer by conduction and convection within the reservoir, and heat loss by conduction to the adjacent formations. A numerical scheme was developed to solve the system of partial differential equations constituting the model. The model was used to investigate combustion in a quadrant of a five-spot pattern. It was found that as the fuel content of the reservoir increases (0.5 to 2.0 lb/cu ft), the sweep efficiency at combustion front breakthrough increases (68 to 84 percent) and tends to reach a plateau. The advantage of a high sweep may be offset, however, by a corresponding increase in heat loss and total air requirement. Sweep efficiency did not show a consistent trend with variation in the air injection rate for a given fuel content. For a constant air injection rate, the percent heat loss from the reservoir increases percent heat loss from the reservoir increases linearly with the fuel content. Also, for a constant fuel content, the heat loss decreases with increasing injection rate. Feedback of heat from the adjacent formations into the reservoir was observed in the vicinity of the injection wellbore. Based upon the cumulative heat loss, however, this effect was negligible. Breakthrough time for the combustion front increases rapidly as the fuel content increases. Accordingly, the cumulative air requirement for a project also increases with increasing fuel content. For a constant injection. rate the peak temperature along the combustion front increases as the fuel content is increased. The temperature at any given point on the front varies in response to the existing fuel content, the magnitude of air flux, and the heat loss rate at that point. For a constant fuel content, the air flux is the factor which determines whether portions of the combustion front will be extinguished, resulting in poor reservoir sweep, high air-oil ratios, and economic failure. Introduction In-situ combustion is a highly promising oil recovery technique that has been tested under a wide variety of conditions. Up to now, however, a complete mathematical model of this complex process is lacking.