Solvent injection has emerged as a recovery technique in low pressure and shallow heavy oil reservoirs in cold systems or in combination with steam in steam-based thermal processes. The performance of solvent-based processes strongly depends on the rate of mass transfer in the boundary layer on the edge of solvent and/or steam chamber. Therefore, understanding of the mass transfer phenomena is required for realistic prediction of the performance of solvent-based or solvent-assisted processes. This study was designed to investigate the capillarity and aqueous phase saturation effects on the rate of mass transfer occurring in the boundary layer on the edge of the solvent chamber. In this study, a new experimental approach was designed and developed to eliminate any disturbance in the boundary layer to experimentally simulate the gravity drainage process in vapor extraction process at two different drainage heights. The porous media and oil-solvent characteristics required for history matching study were determined by measuring the capillary pressure and pore size distribution of sand packs, as well as conducting a complete phase behavior study to determine the properties of the oil-solvent system in a wide range of solvent concentrations. Total number of 12 experiments was conducted in three different sandpacks with different pore size distributions, but similar range of permeabilities (i.e., 5.1-6.5 D) to investigate the effect of capillary forces on the mass transfer phenomena.
Conducted analytical and numerical modeling showed that the effective diffusion coefficient was in the range of 4.91×10-8-1.10×10-5 cm2/s. In absence of immobile water saturation, effective diffusion coefficient was in the range of 4.91×10-8-7.89×10-6 cm2/s. In presence of immobile of aqueous phase saturation, the effective diffusion coefficient varied between 2.96×10-6 and 1.10×10-5 cm2/s. The comparison between the calculated effective diffusion coefficients and reported molecular diffusion in literature by different researchers confirmed that the velocity-dependent term in convective dispersion does not play a major role at higher capillarities in heavy oil systems, i.e., lower permeability range compared to other studies (i.e., >100 D). This study highlights the need for selection of a realistic mass transfer coefficient for the simulation of the performance of the solvent injection processes.