In-situ combustion is a potential method for the recovery of heavy oil. The effect of reservoir heterogeneity, a ubiquitous feature of oil reservoirs, on in-situ combustion has not been systematically addressed in prior studies, however. In this paper, we present analytical models for filtration combustion, namely the combustion of a stationary solid fuel, in the specific case where the reservoir consists of two layers of different permeability and thickness, separated by nearly impermeable shales. We investigate the conditions for the propagation of steady combustion fronts as a function of some key parameters, including the permeability-thickness contrast R between the layers, the thickness ratio η, and the external heat loss coefficient h.
We find that heterogeneity acts in two distinct ways: It reduces the temperature of the leading front in the high-permeability layer in all cases, and uncouples the propagation of the fronts in the two layers if R is smaller than a critical value Rc. The first effect may lead to low-temperature oxidation conditions, and therefore to the effective extinction of the front in the high-permeability layer. The second leads to a reduced sweep efficiency (and early breakthrough). However, if R exceeds the critical value, the fronts in the two layers travel coherently (with the same speed). This coherence is identified for the first time. The resulting thermal coupling greatly retards the front in the more permeable layer, and accelerates only slightly that in the less permeable one, until the two fronts reach a common velocity.
We study the effects of R, the heat loss rate, and the ratio of thickness η. The coupling is aided by moderate heat losses (small h), and smaller η, which affect the critical value Rc.Asinthe homogeneous case, at sufficiently high heat loss rates, steady front propagation cannot be sustained and the combustion process becomes extinct.
The work is useful for the understanding of the viability of in-situ combustion process in heterogeneous layered reservoirs and the effect of a number of injection, combustion, and reservoir parameters.