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Volatile oil recovery by means of air injection is studied as a method to improve recovery from low permeable reservoirs. We consider the case in which the oil is directly combusted into small products, for which we use the term medium temperature oil combustion. The two-phase model considers evaporation, condensation and reaction with oxygen. In the absence of thermal, molecular and capillary diffusion, the relevant transport equations can be solved analytically. The solution consists of three waves, i.e., a thermal wave, a medium temperature oxidation (MTO) wave and a saturation wave separated by constant state regions. A striking feature is that evaporation occurs upstream of the combustion reaction in the MTO wave. The purpose of this paper is to show the effect of diffusion mechanisms on the MTO process. We used a finite element package (COMSOL) to obtain a numerical solution; the package uses fifth-order Lagrangian base functions, combined with a central difference scheme. This makes it possible to model situations at realistic diffusion coefficients. The qualitative behavior of the numerical solution is similar to the analytical solution. Molecular diffusion lowers the temperature of the MTO wave, but creates a small peak near the vaporization region. The effect of thermal diffusion smoothes the thermal wave and widens the MTO region. Capillary diffusion increases the temperature in the upstream part of the MTO region and decreases the efficiency of oil recovery. At increasing capillary diffusion the

Combustion can be used to enhance recovery of heavy, medium, or light oil in highly heterogeneous reservoirs. Such broad range of applicability is attained because not only do the high temperatures increase the mobility of viscous oils but also the high thermal diffusion spreads the heat evenly and reduces heterogeneity effects. For the latter reason, combustion is also used for the recovery of light oils. The reaction mechanisms are different for light oils, where vaporization is dominant, whereas for medium nonvolatile oils combustion is dominant. We will only consider combustion of medium and light oils. Therefore we ignore coke formation and coke combustion. It is our goal to investigate the relative importance of vaporization and combustion in a two-component mixture of volatile and nonvolatile oils in a low air injection rate regime. By changing the composition we can continuously change the character of the combustion process. We derive a simplified model for the vaporization/combustion process, and implement it in a finite element package, COMSOL. For light oil mixtures, the solution consists of a thermal wave upstream, a combined vaporization/combustion wave in the middle (with vaporization upstream of combustion) and a saturation wave downstream. For medium mixtures the vaporization/condensation sequence is reversed and vaporization moves ahead of the combustion. Due to its low viscosity, the light oil is displaced by the gases to a region outside the reach of oxygen and therefore less oil remains behind to reach the combustion zone. This leads to a high combustion front velocity. For oil with more nonvolatile components, vaporization occurs downstream of the combustion zone. As more oil stays behind to feed the combustion zone, the velocity of the combustion zone is slower, albeit the temperatures are much higher. The relative importance of vaporization/combustion depends also on the injection rate, pressure, initial temperature, and oil viscosity. Numerical calculations allow to estimate the bifurcation points where the character of the combustion changes from a vaporization-dominated to a combustion-dominated process.

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