Mechanistic simulation study of expanding-solvent steam-assisted gravity drainage under reservoir heterogeneity

Abstract

Expanding-solvent steam-assisted gravity drainage (ES-SAGD) is a potential method to reduce steam-oil ratio (SOR) of SAGD, which is a critical concern especially for highly-heterogeneous reservoirs. The main objective of this research is to investigate the flow characteristics of heterogeneous reservoirs in which solvent is more likely to lower SOR of SAGD.SAGD and ES-SAGD with normal hexane are simulated for fifty geostatistical realizations consisting of clean sand and shale, qualitatively representative of the middle member of the McMurray formation. Thermodynamic models are calibrated with experimental phase behavior data for reliable comparison between SAGD and ES-SAGD, including the water solubility in oil at elevated temperatures.Results show that the SOR reduction by steam-solvent coinjection is positively correlated with the increase in SAGD's SOR due to heterogeneity. Enhancement of bitumen flow by dilution is more important for lowering SOR for those reservoirs in which the permeability variation makes slow-flow regions during SAGD.Simulation results show that a larger amount of bitumen tends to be diluted by solvent in those reservoirs for which SAGD exhibits slow production of bitumen. Then, the observed results are analyzed by use of SAGD analytical equations that clarify several influential factors for bitumen flow beyond the edge of a steam chamber. It is shown that dilution of bitumen by solvent in steam-solvent coinjection becomes more significant where flow barriers limit the local bitumen flow under SAGD even at high temperatures. In such slow-flow regions, the bitumen flow rate can be substantially increased by accumulation of solvent in ES-SAGD, which reduces the oleic-phase viscosity and increases the oleic-phase saturation and, therefore, relative permeability. Solvent accumulation within a steam chamber can also reduce thermal losses because of lower operating-chamber temperatures.