Abstract
Unlike other thermal recovery methods, air injection and in-situ combustion produces significant amounts of heat in the reservoir. However, the process is subject to acute external heat loss rates due to high temperature gradients in place. Consequently, reservoir temperatures may be reduced considerably, leading to a deteriorated combustion performance and debilitated field operations. 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 water-soluble metallic additives improve the self-sustainability limit of combustion front in oil and sand mixtures. In general, this has been attributed to dual-role of the additives on the combustion performance: (1) kinetics of heterogeneous oxidation reactions inside the combustion front are modified, namely the catalytic effect; (2) specific surface area of the porous medium ahead of the combustion front is increased, namely the fuel deposition effect. 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. Determining their role on in-situ combustion performance requires that the mechanisms of combustion are well understood. 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 (HTO/LTO) combustion front propagation model, based on the large activation energy asymptotics, and introducing systematically the reaction kinetics and fuel deposition effects of the additives to the model. Coherent propagation of the reaction regions are then investigated in terms of their temperatures, propagation velocity and the oxygen consumption efficiency. It is found that an improved combustion performance can be observed only if both the kinetics and fuel deposition effects develop under the reservoir conditions.
