Dual Role of Catalytic Agents on In-Situ Combustion Performance
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SummaryUnlike other thermal recovery methods, air injection and in-situ combustion generates significant amounts of heat in the reservoir. However, the process is subject to acute heat loss rates from the reaction zones because of high temperature gradients; consequently, the reaction temperatures may be reduced considerably, leading to a deteriorated combustion performance and debilitated field operations. The goal of this paper is to determine under which reservoir conditions the combustion temperatures could be maintained at sufficient levels. Previous investigators have partially addressed this issue using kinetic and combustion tube experiments. In the absence of heat losses, it has been repeatedly shown that catalytic agents (naturally occurring clays, metal oxides, and some water-soluble metallic additives) improve the self-sustainability limit of combustion front in crude oil and sand mixtures. In general, this has been attributed to the dual role of these agents on the combustion performance, namely the catalytic and fuel deposition effects. It is currently a common belief that appropriate introduction of such materials in a reservoir environment could enhance the performance of combustion process and, hence, improve the recoveries. An investigation of their dual role on combustion requires that the mechanisms of combustion are well understood in their presence. Complex physical and chemical nature of the problem at the pore-scale has prevented detailed investigations using physical and numerical models, however. Here, we approach the problem analytically using a sequential-reaction [high-temperature oxidation/low-temperature oxidation (HTO/LTO)] combustion front propagation model, based on large activation energy asymptotics, and introducing the reaction kinetics and fuel deposition effects to the model systematically by varying the related variables and parameters. Coherent propagation of the reaction regions are then investigated using reaction region temperatures, propagation velocity, and the oxygen consumption efficiency. General characteristics of an ideal catalytic agent are discussed in terms of its potential to improve in-situ combustion. It is found that the front propagation can be improved under the reservoir conditions only if both the catalytic and fuel deposition effects of the agents are present. The work is important for our understanding of in-situ combustion processes and can be used for development of screening criteria to identify high-performance catalytic agents in the laboratory using conventional apparatus.
