Steady-State Propagation of In-situ Combustion Fronts with Sequential Reactions

Abstract

Summary The sustained propagation of a combustion front is necessary for the improved recovery of oil during an in situ combustion process. In situ combustion involves the added complexity of chemical reactions. In this paper, combustion will involve two sequential oxidation reactions. High-temperature oxidation represents proper combustion, its fuel generated by the preceding low-temperature oxidation. The interaction between the two reactions in the presence of reservoir heat losses and their overall influence on front propagation are investigated using a perturbation, analytical approach, based on the assumption of large activation energies. The places where the reactions occur are treated as spatial discontinuities for heat and mass fluxes, across which appropriate jump conditions are developed. Under certain conditions, the two reaction regions are coupled and travel coherently with the same velocity. The corresponding parameter space is delineated. The resulting common velocity is investigated as a function of the various parameters, including heat losses. The work finds application to our understanding of in-situ combustion processes and their application to oil and bitumen recovery. Introduction The 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 oil to catalyst regeneration, coal gasification, smoldering, waste incineration, ore calcination and high-temperature synthesis of powdered materials (SHS). 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 processes preceding the combustion region, such as pyrolysis and low-temperature oxidation reactions. The dynamics of filtration combustion are influenced by the flow of injected and produced gases, the heat and mass transfer in the pore space and the solid matrix and the reaction rates. An analytical treatment of these fronts can be done assuming a sharp exothermic oxidation front and using large activation energy asymptotics, a technique widely considered for laminar flames, namely gaseous phase combustion in the absence of a porous medium1,2. Akkutlu and Yortsos3 used such an approach to model forward in-situ combustion in which a single reaction occurs. References 4 and 5, by the same authors, delved into the delineation of the effects of heat losses and their impact on sustained front propagation and extinction. These investigations were based on a single, high-temperature oxidation reaction. In this paper, we will consider instead two oxidation reactions, a low-temperature oxidation (LTO) that precedes the main combustion front and generates fuel, and a high-temperature oxidation (HTO), that follows. The two fronts where oxidation occurs may become coupled, in which case they propagate coherently, by maintaining a finite distance from each other, or they are uncoupled, with the LTO front moving at a faster velocity ahead. Our objective will be the understanding of this coupling, as a function of the parameters and its overall effect on combustion front propagation, hence, the oil recovery at a larger scale. The paper is organized as follows: First, the typical oxidation behavior of crude oil/sand mixtures is briefly introduced, to clarify our motivation for the present work. The associated chemical reactions and their influence on in-situ combustion are in fact more complex and possibly ambiguous. Then, the single-reaction in-situ combustion analysis presented in Reference 4 is outlined, as it constitutes the basis of the present extension to two reactions. Following is the analysis of the conditions under which coupling of the two fronts occurs. Under conditions of coherent propagation, the effect of the various parameters on the properties of the combustion front is analyzed. Effects of heat losses are particularly addressed.