Stability of Moving Combustion Fronts in Porous Media
Authors: Armento Michael E. , Miller Clarence A. 
Published: December 2007

Abstract
The stability of moving combustion fronts during underground combustion processes is analyzed. The behavior of small deformations in front configuration is studied using knew stability theory and relatively simple models of the combustion processes. In this way, the main physical phenomena that influence stability can be physical phenomena that influence stability can be distinguished clearly.
For wet combustion processes, where both air and water are injected, front stability is affected, not only by the ratio of the mobilities of fluids flowing ahead of and behind the front, but also by heat transport near the front and by expansion accompanying vaporization of water at the front. Even more interesting is a "reverse mobility" effect predicted by the analysis. Under some conditions at relatively high air to water injection ratios, fluid flow, and the heat produced by combustion interact in such a way that normally "favorable" mobility ratios become unfavorable. That is, existence of a low mobility in the region of two-phase flow of air and water behind the front actually can promote front instability.
For dry combustion the beat transport and reverse mobility effects on stability do not occur and the combustion fronts are typically neither highly stable nor highly unstable; That is, the effective mobility ratio is near unity.

Introduction
Thermal processes, such as underground combustion and steam drive, are prime candidates for supplementing primary recovery techniques in reservoirs containing highly viscous crude oils. Thermal methods also have been considered for tertiary recovery of lighter oils. Chemical flooding seems currently to be the preferred process, however, for this latter application.
As is true for all recovery schemes, the over-all effectiveness of thermal processes depends on the injected fluids contacting a substantial portion of the reservoir, that is, on achieving good sweep efficiency. Of course, the amount of the reservoir swept depends on various factors, including reservoir heterogeneity and the pattern of wells. But studying the stability of moving displacement fronts in an idealized homogeneous porous medium provides useful information that, at a minimum, provides useful information that, at a minimum, gives a qualitative idea of what to expect in more complex situations. For example, if the front is unstable even in the simple case of a homogeneous medium, sweep efficiency is likely to be poor for any realistic reservoir conditions.
It has been known for some years that a ratio between the mobilities () of displacing and displaced fluids that exceeds unity gives rise to a destabilizing influence on the moving front between them. Only recently has it been shown that other effects on front stability can be equally important during thermal recovery processes. Analysis of a simple situation with steam condensing and displacing water at a moving front indicated that the adverse mobility ratio because of low steam viscosity sometimes could be offset completely by two stabilizing effects. This conclusion is consistent with Baker's laboratory experiments on steam displacement that showed that moving condensation fronts are usually stable.
One stabilizing effect at a condensation front is contraction accompanying condensation of a given mass of swam, a process which extracts mechanical energy from the flow. The other effect is heat transport in the water region, where the temperature varies from the saturation temperature at the front to ambient conditions far ahead of the front. The rate of front advance is controlled by the rate of condensation, high front velocities being associated with low condensation rates. The condensation rate, in turn, is limited by the rate at which the latent heat released at the front can be transported into the water. When the front is deformed as shown in Fig. 1 points such as P are exposed to a large amount of cool liquid so that the local heat flux increases. As a result, the local condensation rate increases at P, and the local front velocity decreases there, producing a stabilizing effect. This qualitative explanation of the transport effect on stability is an improvement over that given in the original paper.
SPEJ
P. 423