Influence of in situ fuel deposition on air injection and combustion processes
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AbstractSustained 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.IntroductionPropagation 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.
