Abstract
Rigorous validation of a simulation model of the THAI –Toe-to-Heel Air ijection process has been performed using results obtained from a 3D combustion cell experiment on Athabasca Oil Sands. The numerical model includes a new kinetics formulation, based on the Athabasca thermal cracking kinetics scheme proposed by Phillips (1985). The kinetic model excludes low temperature oxidation, since THAI operates in a high temperature oxidation mode. Excellent agreement was obtained between the predicted and experimental oil production rate, and there was generally a good match obtained for other dynamic variables, including the residual coke profile, produced oxygen, and peak combustion temperature. The numerical model provides a fundamental platform for upscaling to field scale, that will enable fine scale details of the process to be captured.
Simulations were performed under dry in situ combustion conditions at the high air injection fluxes used in the experiment. Under these conditions, vertical-plane temperature profiles in the sandpack confirm that the combustion front is quasi-vertical and forward leaning in the direction of the heel of the horizontal producer well. The shape of the combustion front was predicted more accurately from horizontal-plane profiles, which showed that there was no oxygen in regions ahead of the combustion front. Oil displacement occurs mainly by gravity drainage, but pressure draw down in to the horizontal producer well below the Mobile Oil Zone (MOZ) is also significant. Various zones that develop during the in-situ combustion process include a ‘Steam Zone’, and this is located upstream of the Mobile Oil Zone (MOZ). All of the mobilized oil is produced from the MOZ, which comprises two regions. The first part contains oil produced by thermal cracking of the heavy residue and vaporized ‘lighter oil’. The main bulk of the oil produced in THAI comes from the second region of the MOZ, containing ‘banked’ original oil. The oil which is eventually produced is partially upgraded oil, due to the thermally upgraded and ‘lighter oil’ fractions mixing with the original oil when they enter the horizontal producer well.
