Determination of the propagation state of the combustion zone during in-situ combustion by dimensionless numbers

Ensuring the sustainable propagation of the combustion zone is a prerequisite for in-situ combustion (ISC). The combustion zone propagation processes with different gas injection parameters and reservoir properties were simulated with CMG STARS and a method of determining the propagation state was proposed based on the theoretical analysis of the simulation results. The simulation results of 67 cases with different gas injection parameters and reservoir properties show that there are three states of combustion zone propagation: the sustainable state, the declined state and the extinguished state. The temperature of the combustion zone shows a good correlation with the state of the combustion zone propagation. With increases in the gas injection rate, oxygen concentration, and oil saturation and decrease in the rock permeability, the combustion zone temperature increases and the propagation state changes from the extinguished or declined state to the sustainable state. Analysis of the nondimensionalized energy conservation equation shows that the combustion zone temperature is determined by the two dimensionless numbers HD and CW with the determined core property and domain. A diagram for determining the time-averaged temperature of the combustion zone using the HD and CW numbers was established, with relative deviations within 7%. The combustion zone propagation state under the combination conditions of the gas injection rate, oxygen concentration, oil saturation and reservoir permeability can be judged according to the predicted combustion zone temperature. The proposed method helps optimize the design of gas injection parameters in combustion tube tests and ISC oilfield applications.