In Situ Combustion of Heavy, Medium, and Light Crude Oils: Low-Temperature Oxidation in Terms of a Chain Reaction Approach

Oil oxidation reactions have attracted considerable interest in terms of mechanism comprehension for thermally enhanced oil recovery applications. Many hypotheses regarding oil oxidation mechanisms appear to be disputable even now. The aim of our work was to broaden current knowledge on the crude oil oxidation chain reaction mechanism including the formation behavior of free radicals and hydroperoxides. In this context, we attempted to shed light on the main differences in the oxidation reactions between heavy and light oils. We have found a way to solve both analytically and numerically a set of differential equations for concentrations corresponding to the reaction scheme. Taken together, our findings allowed us to obtain hydroperoxide concentration dependence on time for the initial stages of oxidation. Two main time dependencies were observed, one for low-temperature oxidation (LTO) and the other for high-temperature oxidation (HTO). Both dependencies were revealed in the oxidation experiments of different types of oils and were taken for the matching procedure, which is also presented in this work. The φ-factor of branched-chain reactions, obtained as a combination of reaction rates, determines the efficiency of LTO and transition from LTO to HTO. By matching the experimental data, we were able to find that the success of self-ignition may be achieved only if the concentration ratio of saturated hydrocarbons to inhibitors in crude oil is equal to 2 or more and the temperature is more than 415 K. Under these conditions, the ignition time for heavy oil was 5–7 days, and that for light oil was 15–30 min in oxidation experiments, which were well matched by the presented chain reaction model.