Interaction of Kerogen Combustion and Pyrolysis and Continuous Oil Production during In Situ Combustion for Oil Shale Upgrading

Oil shale is a potential strategic resource with large reserves and widespread application potential, and in situ combustion is an effective method to exploit oil shale. However, the interaction mechanism between pyrolysis and combustion reaction is still not clear; hence, the evolution of components and temperature in the reservoir is hard to describe, limiting the application of in situ combustion in oil shale. In this paper, we established a comprehensive reactive flow model to describe the in situ combustion in oil shale, and the in situ combustion process is analyzed in detail. During the in situ combustion, only pyrolysis reactions occur in the matrix system, while combustion reactions only occur in the fracture system. Under the interaction mechanisms of these two kinds of chemical reactions, in situ combustion upgrading can be carried out continuously. The produced oil and hydrocarbon gas per unit volume of oil shale are 0.06 m3/m3 and 12.5 m3/m3, respectively, but about 83.3 m3/m3 of CO2 is produced that needs to be stored subsequently when air is directly injected into the reservoir to maintain the combustion reactions. Increasing oil content and air injection rate can increase the cumulative oil and gas production, but there are optimal values above which the increase rate slows down. The energy conversion efficiency of in situ combustion is 49.5, and it is much higher than the energy conversion efficiency in other in situ upgrading methods for oil shale.