The inherently high viscosity of bitumen makes SAGD the ideal technology for in-situ recovery of these unconventional hydrocarbon deposits. Its conceptual simplicity and proven performance in the field have directed research to explore new opportunities to further improve its recovery efficiency and minimise its environmental footprint. Solvent addition to steam in a SAGD process makes the thermal and solvent dilution effects on oil viscosity act together in improving the bitumen recovery. Various experimental, numerical and pilot studies have shown the incremental benefit of solvent co-injection in accelerating the oil recovery at reduced steam-oil ratio compared to steam alone injection. The objective of this work was to evaluate the effectiveness of different single-component and multi-component additives for improving the SAGD performance.

Among the various process variables, the optimal solvent design and the injection pressure play influential roles in determining the process efficiency, as they directly affect the phase behavior of the system. This paper uses numerical simulation to examine the solvent-SAGD mechanisms and presents a comparative analysis of single and multi-component solvent injection at low and high operating pressures. A lab scale physical model saturated with Athabasca bitumen, used in an earlier experimental study, was adapted to a simulation model for this purpose. After tuning the numerical model by history matching the physical model experiments, it was used to compare the benefits of co-injecting different solvents with steam at two different operating pressures. A range of single component solvents (C4, C5 and C6) and a multi - component gas condensate were aevaluated for low and high-pressure injection scenarios.

The results show that all tested solvents reduce the steam oil ratio and enhance the rate of oil production. The relative performance of different solvents is affected by the innate bitumen viscosity and the operating pressure. With lower bitumen viscosity, C4-steam co-injection outperformed C5, C6 and gas-condensate addition for both low- and high-pressure ES-SAGD cases. However, the performance of heavier solvents was better at higher operating pressures due to their relative ease of condensation, which enhances their interactions with bitumen.

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