Sustained propagation of a combustion front is necessary for improved recovery of oil during an in-situ combustion process. The front is a sharp moving boundary layer and involves the complexities of combustion reactions. In this work the combustion front is represented by two oxidation reactions: (high-temperature) fuel-burning reaction and (lowtemperature) fuel-generating reaction. Due to distinct reaction kinetics and stoichiometry, the fuel-generating and -burning reactions occur in sequential regions within a finite separation distance in the reservoir. Strongly nonlinear interaction of these regions and its overall influence on the combustion front propagation are investigated using an analytical approach based on large activation energies of the reactions. Reservoir conditions under which the regions could travel in the formation with a common propagation speed is identified and the limits of their coherence in the presence of external heat losses are investigated. Consequently, we have found and formulated a new intricate relationship between the reservoir heat loss rate and separation distance of the reaction regions: the regions propagate closely spaced, thus minimizing the influence of deleterious external heat losses and maximizing the process performance. This two-reaction self-sustainability mechanism keeps the combustion front propagating steadily even though under the same conditions front extinction has been predicted for the equivalent single-reaction problem. The work is essential for improved oil and in-situ bitumen recovery using air/oxygen and emphasizes the importance of local chemical processes during the injection.

Sustained propagation of a combustion front is necessary for improved oil recovery during an air injection and in situ combustion process. The front is a sharp diffusive layer and involves the added complexities of reactions. In this work, the combustion front is represented by two oxidation reactions: a high-temperature fuel burning reaction and a low-temperature fuel generating reaction. Due to distinct reaction kinetics and stoichiometry, the reactions occur in sequential regions within a finite separation distance in the reservoir. Interaction of these regions and its overall influence on the front propagation are investigated locally using an analytical approach based on large activation energies of the reactions. Reservoir conditions under which the regions could travel with a common propagation speed is identified and their limits of coherence are investigated in the presence of external heat losses. Consequently, a new intricate relationship between the reservoir heat loss rate and separation distance of the reaction regions is found and formulated. The regions propagate closely spaced, thus minimizing the influence of deleterious heat losses and improving the combustion process performance. This two-reaction self-sustainability mechanism keeps the combustion front propagating steadily, even though under the same conditions front extinction has been predicted for the equivalent single-reaction problem. The work emphasizes the importance of local nonlinear chemical processes during air injection. Introduction Propagation of combustion fronts in porous media has been studied extensively in the filtration combustion literature. It is a subject of interest to a variety of applications, ranging from in-situ combustion for the recovery of heavy and light oils to catalyst regeneration, coal gasification, smoldering, waste incineration, ore calcinations or the high-temperature synthesis of powdered materials(1). The fuel may pre-exist as part of the solid matrix or, as in the case of in-situ combustion, it may be created by local processes such as pyrolysis and low-temperature oxidation reactions. Dynamics of filtration combustion is influenced by the flow of injected and produced gases, the heat and mass transfer in the porous medium and the rates of reactions. An analytical treatment can be accomplished assuming a sharp exothermic oxidation front using large activation energy asymptotics; a technique widely considered to investigate laminar flames in the absence of a porous material(2,3). Akkutlu and Yortsos(4,5) considered the application of the technique for modeling dry forward in situ combustion fronts in porous media. They investigated the effects of reservoir heat losses(4) and the impact of reservoir heterogeneity(5) on ignition, sustained front propagation and extinction. Their investigations were based on a main (high-temperature) oxidation reaction only, however. In this paper, the presence of an additional oxidation reaction occurring at lower temperatures is considered. The latter precedes the main combustion region, takes place at lower temperatures and generates the fuel necessary for sustained propagation of the former. Under certain conditions, the two reaction regions are thermally coupled, in which case they may propagate coherently, albeit at a finite distance from each other. Otherwise, they become uncoupled, with the region of low-temperature oxidation traveling far ahead at higher velocities.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSeveral experimental investigations have previously considered naturally occurring clays, metallic minerals and additives as catalytic agents to improve oil recovery during air injection and in-situ combustion processes. It is reported that these materials could change the morphology and surface properties of the porous medium ahead of the combustion front; thus, (1) increase hydrocarbon deposition ahead of the front, and (2) modify the kinetics of oxidation reactions inside the front. However, their on the combustion front dynamics has not previously been investigated under varying reservoir conditions. In this paper, we approach the problem using an analytical method based on large activation energies of the oxidation reactions. The model describes combustion front as a moving boundary layer in a uniform porous medium. It involves coherent propagation of low-temperature (fuelgenerating) and high-temperature (fuel-burning) reaction regions under the influence of reservoir heat losses; thus, it is suitable for investigating the role of catalytic agents on the front propagation. In the absence of these materials, it has been previously found that the reaction regions propagate closely spaced, thus reducing the influence of deleterious heat losses on the combustion front [6,7]. At low air injection rates, however, the peak temperature and propagation velocity of the front noticeably decrease; namely, the front approaches its extinction limit. Here, we propose that any possible improvement (whether catalytic or not) on combustion performance should preclude the observed temperature drop with the low injection rates. The presence of catalytic agents is consequently introduced to the model in terms of systematic variations in the reaction kinetics parameters, and deposited hydrocarbon/generated fuel amounts. It is found that, although kinetics influences the system dynamics, it may not improve the combustion performance due to a compensation effect. The improvement is rather due to an implicit role of the increased specific sand grain surface area on the hydrocarbon deposition ahead. The surface area promotes fuel deposition; hence, significantly increase total heat content of the combustion process in the reservoir. The work is important for developing guidelines to screen catalytic agents that could be used during the air injection processes.

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