Expanding-solvent steam-assisted gravity drainage (ES-SAGD) is a widely-investigated alternative to SAGD, considering its potential to reduce thermal losses while enhancing bitumen recovery. However, most prior studies on ES-SAGD were limited to homogeneous reservoirs. This research presents a mechanistic analysis of ES-SAGD in heterogeneous reservoirs in terms of cumulative steam-oil ratio (SOR) as a function of steam-chamber size.
Simulation case studies for SAGD and ES-SAGD with normal hexane are conducted for geostatistical realizations of two types of heterogeneous Athabasca-bitumen reservoirs. For the first type, shale barriers are oriented horizontally relative to the top and basal planes of the reservoir. For the second type, they are inclined and more representative of the middle McMurray member. The solubility of water in the oleic phase at elevated temperatures is properly modeled to ensure reliable comparison between SAGD and ES-SAGD.
Results show that ES-SAGD is less sensitive to heterogeneity than SAGD in terms of cumulative SOR for simulations at 35 bars and 2 mol% solvent-injection concentration for a reservoir thickness of 20 m. On average, the reduction in SOR due to steam-solvent coinjection is simulated to be greater under heterogeneity. The margin of SOR reduction is greater in reservoirs with inclined shale barriers than in those with horizontal shale barriers.
Analysis of simulation results indicates that the injected solvent tends to accumulate more significantly under heterogeneity, which enhances the mechanisms of ES-SAGD, such as dilution of bitumen by solvent and reduced thermal losses to the overburden. Tortuous hydraulic paths and slower gravity drainage under heterogeneity enhance the mixing between solvent and bitumen in the transverse direction along the edge of a steam chamber. Then, a larger amount of the accumulated solvent tends to facilitate lower temperatures near the chamber edge. Lower chamber-edge temperatures combined with restricted access to the overburden under heterogeneity alter the chamber geometry such that the contact area for overburden heat losses is further reduced.