Toe-to-Heel Air Injection (THAI) is a variant of conventional In-Situ Combustion (ISC) that uses a horizontal production well to recover mobilised partially upgraded heavy oil. It has a number of advantages over other heavy oil recovery techniques such as high recovery potential. However, existing models are unable to predict the effect of the most important operational parameters, such as fuel availability and produced oxygen concentration, which will give rise to unsafe designs. Therefore, we have developed a new model that accurately predicts dynamic conditions in the reservoir and also is easily scalable to investigate different field scenarios. The model used a three component direct conversion cracking kinetics scheme, which does not depend on the stoichiometry of the products and, thus, reduces the extent of uncertainty in the simulation results as the number of unknowns is reduced.The oil production rate and cumulative oil produced were well predicted, with the latter deviating from the experimental value by only 4%. The improved ability of the model to emulate real process dynamics meant it also accurately predicted when the oxygen was first produced, thereby enabling a more accurate assessment to be made of when it would be safe to shut-in the process, prior to oxygen breakthrough occurring. The increasing trend in produced oxygen concentration following a step change in the injected oxygen rate by 33 % was closely replicated by the model. The new simulations have now elucidated the mechanism of oxygen production during the later stages of the experiment.The model has allowed limits to be placed on the air injection rates that ensure stability of operation.Unlike previous models, the new simulations have provided better quantitative prediction of fuel laydown, which is a key phenomenon that determines whether, or not, successful operation of the THAI process can be achieved. The new model has also shown that, for completely stable operation, 2 2 the combustion zone must be restricted to the upper portion of the sand pack, which can be achieved by using higher producer back pressure.

The Toe-to-Heel Air Injection (THAI) in-situ combustion process is an efficient way to extract heavy oil and bitumen. However, such reservoirs are often geologically heterogeneous. This work studied the impact of a range of different geological heterogeneities, often found in bitumen deposits, on the performance and safety of THAI. These heterogeneities included random heterogeneity, layered reservoirs, shaly reservoirs, and semi-permeable cap-rocks. A further aim was to also develop potential remedial measures, such as selective well placement. It was found that the degree of symmetry assumed for the reservoir model had a substantial impact on the predicted level of oil production. This is of particular relevance to otherwise apparently symmetrical well placement designs such as staggered line drive. While the presence of impermeable zones resulted in the decrease in the overall oxygen utilisation for shaly reservoirs, compared to simply low permeability reservoirs, there was no evidence of oxygen breakthrough due to preferential channelling into the production well. In layered reservoirs, the development of a rich oil bank during THAI operation depended upon the distribution of permeability around the horizontal producer (HP), and did not occur when there was high permeability just above the HP. It has been shown that the proper representation of the cap-rock in reservoir models for the simulation of THAI is essential in order to accurately mimic the full pattern of heat distribution into the oil zone of the reservoir, and, thence, fuel lay-down. While THAI can operate stably with a permeable cap-rock, vertical permeabilities above ~1-3 mD led to significant loss of combustion gases from the reservoir.

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 CO X 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.

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Some of the bitumen/tar sand/heavy oil reservoirs are underlain by bottom water (BW) layer, which often severely affects the performance of thermal EOR (enhanced oil recovery) processes. The effect of bottom water on the performance of the toe-to-heel air injection (THAI) in-situ combustion (ISC) process is investigated through reservoir simulation using CMG STARS simulator. The current study has shown that there is a limit to BW thickness above which the performance of the THAI process is affected even though the combustion front propagated stably. It is found that the thickness of the ''basal gas layer'' (BGL) depends on how further down into the BW zone the horizontal producer (HP) well is located. From this study, it is found that the critical BW thickness, when the THAI process is implemented in any heavy oil BW reservoir with the wells arranged in an SLD (staggered line drive) pattern, should lie in the range of 50% OL (oil layer) < BW (bottom water) < 100% OL (oil layer). A comparative study between the active and non-active aquifers models shows that the same cumulative volume of water is produced and that over the 715 days of the process, only negligible amount of oil is produced from BWN (i.e. static aquifer model). It is found that in neither of the models does oxygen bypass the combustion front and as in the previous studies, both fronts are restricted to the upper part of the reservoir, within the oil zone. Therefore, it follows that even in the presence of active aquifer (i.e. BWA model), the THAI process still operates stably in terms of combustion front propagation and sustenance. For the combustion initiated at the oil-water (O-W) interface, it is found that controlled gravity override resulted in a high rate of advancement of combustion front at the top of the reservoir. The combustion is observed to not propagate along the BGL, rather, it propagates as though it is initiated at the top of the reservoir. It is shown that the BGL is only formed during the early period of air injection as the combustion gases could not reach the HP well without displacing the water to create initial gas flow pathway into the HP well. It is also observed that initiating the combustion at the oil-water interface results in a massively improved oil recovery rates, most especially when implemented in the DLD (direct line drive) pattern.