Insights Into In-Situ Combustion by Analytical and Pore-Network Modeling
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AbstractIn-situ combustion is a subject of continuing interest due to its rich interplay of flow, mass/heat transfer and reactions. The process complexity, however, has precluded its thorough understanding. In this paper, we provide some analytical results and pore-network simulations that explore fundamental aspects of this process.The analytical approach relies on high-activation energy asymptotics. It consists of the assumption that the combustion front is a thin layer, within which reactions occur. We review our recent findings and derive exact results for the front temperature, the front propagation velocity and oxygen consumption. The results are then compared to pore-network simulations, where the effects of pore structure are included in a detailed simulation at the pore-network level. The simulations demonstrate the validity of the thin layer assumption and provide approaches for upscaling.Then, the effect of porous medium heterogeneity, a ubiquitous feature of oil reservoirs, on the front is considered, in particular, in the form of a layered reservoir system. The fronts in two layers can be coupled under certain conditions, depending on the permeability-thickness contrast R between the layers, the extent h of external heat losses and the inlet conditions. We derive the parameter space, where this coupling occurs, and compare the results with the porenetwork simulations. Stability of the derived stationary nonequilibrium states is also analyzed. It is shown that the adiabatic states are always stable; whereas the non-adiabatic states being conditionally stable. This suggests that coherence of the fronts in the layered system is possible under certain conditions and that the effect of heterogeneity is not as detrimental to the process and its sweep efficiency, provided that heterogeneity does not exceed a certain limit, Rc.
