Naturally occurring clays, metallic minerals and additives (catalytic agents) change morphology and surface properties of the reservoir matrix. In in-situ combustion processes these pore-scale modifications lead to variations in combustion front performance. It has been previously reported that catalytic agents have a dual effect on combustion: they modify the kinetics of oxidation reactions inside the front, and they increase the specific surface area of sand grains ahead of the front, promoting hydrocarbon deposition. The emphasis of this paper is the use of analytical approach to investigate the front performance in the presence of catalytic agents under reservoir conditions. The model describes front propagation in a homogeneous porous medium. The front involves coherent propagation of lowtemperature (fuel-generating) and high-temperature (fuel-burning) reaction regions under the influence of reservoir heat losses. The catalytic agents are implicitly introduced to the model in terms of their dual effects. It is found that a strong catalytic effect exists due to changes in the specific surface area of hydrocarbons reacting with the injected oxygen. Variations in the activation energies of the oxidation reactions, on the other hand, are compensated by the reaction frequency (pre-exponential) factors, and they do not influence the combustion performance significantly. The catalytic effect is more pronounced at low air injection rates where heat losses are dominant. Changes in fuel deposition improve the combustion process, in particular at high air injection rates. Then, the front behavior is analyzed in a space of parameters that includes the catalytic agents. For analysis purposes, the space is divided into two sub-domains: one for the fuel generation efficiency and another for the dual effects. The results show existence of optimum conditions in the presence of catalytic agents. Such optimal reservoir conditions lead to frontal coherence and have the potential to significantly enhance combustion performance.
Air injection and in-situ combustion processes have long been used as a thermal oil recovery method. During injection, the development of a self-sustaining combustion front and its propagation throughout the reservoir are necessary for improved recoveries. Front propagation is dictated by the availability of reactants, i.e., oxygen and fuels, and their reaction kinetics under the influence of reservoir heat losses.
Unlike steam-based recovery methods, an in-situ combustion process produces significant amounts of heat in the reservoir. Because of the high temperature gradients to the surrounding formations, the process is subject to acute heat loss rates, however. Consequently, temperatures may be reduced considerably, eventually leading to a deteriorated performance and debilitated field operations [1–3]. In this paper, the goal is to determine under which conditions combustion temperature can be maintained at sufficient levels for the combustion front to be controlled adequately and the air injection process to become optimized.
Previous laboratory investigations have addressed this issue using kinetic and combustion tube experiments. In the absence of external heat losses, it has been repeatedly shown that naturally occurring clays, metallic minerals and some water soluble metallic additives in the oil/sand mixtures improve self-sustainability of the combustion front.
