Gravity drainage of liquid toluene over bitumen in a packed Hele-Shaw cell: Effect of permeability, irreducible water saturation, and dispersion

Condensing solvent bitumen recovery processes are a promising alternative to steam-assisted processes in terms of greenhouse gas emissions and energy consumption reduction. However, the details of the underlying physical mechanisms of these processes (gravity drainage and mass transfer) are not fully understood, and predictive models are lacking. Three key parameters in these mechanisms are permeability, irreducible water saturation, and mechanical dispersion. In particular, mechanical dispersion can significantly impact mass transfer rates but has not been yet been measured for mass transfer of bitumen into a draining liquid solvent layer. The objectives of this study are to measure and model the impact of permeability and irreducible water saturation on a lab-scale representation of a condensing solvent process, and to assess mechanical dispersion. Mass transfer rates were measured with toluene injected over a bitumen layer in a Hele-Shaw cell filled with silica sands and glass beads with and without irreducible water saturation. With this apparatus, key variables in the gravity drainage process such as the drainage layer flow rate, inclination, and composition are measured. Hence, there is sufficient information to identify and model the process mechanisms. Toluene volumetric flow rates between 0.5 and 15 cm3/min and permeabilities within the range of 47 to 254 D were considered. The high dilution (high flow rate) data were predicted with an analytical model. The model was previously derived from Fick’s second law of diffusion for the effective orthogonal mass transfer of bitumen into a drainage layer flowing according to Darcy’s law. The full range of data was matched with a numerical model. This model was based on convective mass transfer of bitumen into the drainage layer using a correlated convective mass transfer coefficient derived from effective molecular diffusivity and Darcy flow. Neither model required mechanical dispersion even in presence of irreducible water. A square root dependence of mass flux versus permeability was observed, consistent with molecular diffusion with no mechanical dispersion. The numerical method is a step towards predicting mass fluxes at reservoir permeabilities and condensing solvent processes at the field scale.