The classical concept of in situ combustion kinetics assumes the high-temperature combustion of a coke-like phase which has been deposited by thermal cracking reactions occurring ahead of the high-temperature front. Although this model appears adequate to describe the behaviour of low-pressure, high-air flux tests on most oils under dry and normal-wet conditions, its limitations are apparent when evaluating the performance of most high-pressure experiments, especially those carried out at high oxygen concentrations and low oxygen fluxes. A comprehensive program on low- and high-temperature oxidation kinetics was therefore undertaken with the aim of understanding the mechanisms associated with in situ combustion.
Accordingly, the highlights of findings from a ramped-temperature oxidation study on Athabasca Oil Sands is presented in this article. The results have significantly expanded the knowledge of in situ combustion kinetics, but perhaps the most important observation arising from the work is the recognition of the significance of the low-reaction rate region existing at the temperatures intermediate between those associated with low-temperature oxidation and high-temperature combustion reactions. The kinetics in this temperature range are characterized by decreasing oxygen uptake and energy generation rates with increasing temperature. This behaviour is analogous to the negative temperature gradient region described in conventional combustion literature. The ability of an oil to transcend the negative temperature gradient region appears to dictate its ultimate in situ combustion behaviour, and it appears that many field projects and tests utilizing high pressures and oxygen-enriched air operate in the low-temperature oxidation mode because of this phenomenon.