Low-temperature In-situ Combustion of Light Oil

We consider flows possessing a combustion front when a gaseous oxidizer (air) is injected into the porous medium, a rock cylinder thermally insulated preventing lateral heat losses and filled with light or medium viscosity oil. When oxygen reacts with hydrocarbons at low temperatures, a series of reactions occur that will convert a part of hydrocarbons to oxygenated hydrocarbons and gaseous product (water, carbon dioxide etc.). The oxygenated hydrocarbons are compounds like ketones, alcohols, aldehydes and acids. This process is termed low temperature oxydation (LTO). Indeed, LTO only involves some 25 % of the possible sites that can react with oxygen. Therefore also the reaction heat per volume of fuel can at most be 25 % of full hydrocarbon combustion and consequently the temperature in the LTO reaction zone is very low. Upstream of the LTO reaction zone evaporization occurs. We formulate conservation laws for liquid oil, gaseous oil, oxygen and inert gas components (combustion products and nitrogen) that includes the reaction terms. Moreover we give the energy conservation equation. We give an analytical solution to the equations. It turns out that the solution consists, from upstream to downstream, of a thermal wave, an LTO wave that combines oxidation and evaporation and a Buckley-Leverett saturation wave. It is shown that in the solution a major role is played by the existence of a resonance line at which the derivatives of the oxygen and evaporated oil flux versus the oil saturation vanish. It means that the derivative of the oil flux versus saturation is positive upstream of the resonance line and negative downstream of the resonance line. The complete solution is described for typical parameters of LTO oil combustion. Computations show that only a small part of the oil vaporizes or reacts. The oil velocity is close to the LTO speed. Thus the LTO wave represents a mechanism of almost complete oil displacement by means of the temperature increase in the LTO wave, which leads to a decrease of oil viscosity and increase of the gas flux in the wave.