Numerical simulation of the impact of geological heterogeneity on performance and safety of THAI heavy oil production process

Abstract

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.