Thermal-Solvent Assisted Gravity Drainage are recovery processes in which the stimulation mechanism for bitumen viscosity reduction is by heating and/or dilution. Different gravity drainage recovery processes can be included in this category depending on the range of the injection temperature and the solvent concentration in the injection stream. Examples are SAGD, SA-SAGD, VAPEX and heated VAPEX. The performance behavior of these processes is significantly driven by the complex thermodynamic interaction of steam and solvent, heat transfer, multiphase fluid equilibrium and flow in the porous medium.

In this study, we develop a general analytical model for gravity drainage processes by incorporating mass transfer mechanisms (including diffusion and dispersion) and heat transfer mechanism by conduction. In particular, we incorporate the dependency of diffusion and dispersion coefficients on concentration, temperature and drainage velocities, respectively. We utilize a novel approach to analytically solve the second order non-linear partial differential equation which governs mass transfer within the mass boundary layer. The resulted closed-form analytical model provides oil drainage rate due to gravity and heat and dilution effects as a function of reservoir and fluid properties. The developed model in this work provides a new perspective into the mass transfer mechanisms and their relative importance within the mass transfer boundary layer at different operating conditions.

The consistent application of the new model to the gravity drainage processes ranging from SAGD to VAPEX demonstrated using laboratory data from literature. It is shown that the predicted concentration distribution profile by the model with the concentration-dependent diffusion coefficient is profoundly different than the predicted profiles with a constant diffusion coefficient. The modeling results demonstrate that the dispersion can be several orders of magnitude greater than diffusion for solvent assisted gravity drainage process at the elevated temperatures. In addition, the contribution of the mass transfer boundary layer to oil production rate can be significantly greater than the heat transfer boundary layer despite being considerably narrower than the heat transfer boundary layer at the elevated temperatures. These findings confirms that the performance of solvent assisted gravity drainage process can be more favorable at the elevated temperatures.

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