Impacts of Kinetics Scheme Used To Simulate Toe-to-Heel Air Injection (THAI) in Situ Combustion Method for Heavy Oil Upgrading and Production

摘要

While simulating toe-to-heel air injection (THAI), which is a variant of conventional in situ combustion that uses a horizontal producer well to recover mobilized partially upgraded heavy oil, the chemical kinetics is one of the main sources of uncertainty because the hydrocarbon must be represented by the use of oil pseudo-components. There is, however, no study comparing the predictive capability of the different kinetics schemes used to simulate the THAI process. From the literature, it was determined that the thermal cracking kinetics schemes can be broadly divided into two: split and direct conversion schemes. Unlike the former, the latter does not depend on the selected stoichiometric coefficients of the products. It is concluded that by using a direct conversion scheme, the extent of uncertainty imposed by the kinetics is reduced as the stoichiometric coefficients of the products are known with certainty. Three models, P, G, and B, each with their own different kinetics schemes, were successfully validated against a three-dimensional combustion cell experiment. In models P and G, which do not take low-temperature oxidation (LTO) into account, the effect of oil pseudo-component combustion reactions is insignificant. For model B, which included LTO reactions, LTO was also found to be insignificant because only a small fraction of oxygen bypassed the combustion front and the combustion zone was maintained at temperatures of over 600°C. Therefore, in all the models, it is observed that coke deposition was due to the thermal cracking taking place ahead of the combustion zone. During the first phase of the combustion, peak temperature curves of models P, G, and B closely matched the experimental curve, albeit with some deviations by up to 100°C between 90 and 120 min. After the increase in the air injection flux, only the model P curve overlapped the experimental curve. The model P cumulative oil production curve deviated from the experimental one by only a relative error of 4.0% compared to deviations in models G and B by relative errors of 6.0 and 8.3%, respectively. Consequently, it follows that model P provided better predictions of the peak temperature and cumulative oil production. The same conclusion can be drawn with regard to the produced oxygen concentration and combustion front velocity. With regard to American Petroleum Institute (API) gravity, it is found that all the three models predicted very similar trends to the experiment, just like in the case of the oil production rate curves, and therefore, no model, in these two cases, can be singled out as the best. Also, all the models’ predictions of the produced COX concentration prior to the increase in the air flux closely match the experimental curve. There are, however, serious differences, especially by model P, from the reported experimental curve by up to 15% after the increase in the air flux.