Although heavy oil reserves are abundant, recovering them efficiently and economically remains a crucial technical challenge as a result of their high viscosity. Solvent based non-thermal recovery processes are designed to reduce the heavy oil viscosity through mixing and dilution with solvent. Solvent and heavy oil mixing occurs over a narrow zone, so localized viscous fingering can have a significant impact on the effectiveness of the solvent. In this study, direct pore scale modeling was used to simulate viscous fingering phenomenon during unfavorable mobility ratio miscible displacement of heavy oil in a three dimensional heterogeneous porous medium pattern.

In direct pore-level modeling, Navier-Stokes, Diffusion-Convection and Continuity equations, as the governing equations of dispersion, are directly applied and solved on the 3-D porous medium without any simplification in medium geometry.

To study the impact of unfavorable mobility ratio on the miscible displacement at the sub pore scale level, simulations have been run to model miscible displacement at five different unfavorable mobility ratios on the same porous medium pattern. Additional simulations were run to investigate the effect of pore pattern and different injection rates on the patterns, which were generated based on the process/object based reconstruction method. Base line simulations also have been done to model miscible displacement on the same medium when the mobility ratio is equal to one.

Heterogeneity of the pattern and lower viscosity of the solvent leads to appearance of some fingers just after starting solvent injection. The results show that growth rate of the fingers become smaller by decreasing mobility ratio. Finger transitions are the same for different mobility ratios but the fingers size and growth rate of the fingers are different for different mobility ratios. Generated fingers accelerate concentration spreading, so the solvent is mixed faster than that predicted by Convection-Dispersion equation. As the mobility ratio decrease toward one, growth of mixing zone length tends to 0.5, which is the growth rate caused by dispersion alone. By increasing the mobility ratio, fingers causes the mixing zone length growth tends to 1, so, for large mobility ratio, mixing zone grows because of two mechanisms: Dispersion and Fingering.

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