Comparison of three different interface mass transfer models used in the experimental measurement of solvent diffusivity in heavy oil

Design and modeling of solvent-based heavy oil recovery processes require the knowledge of solvent diffusivity in heavy oil. The solvent diffusivity in heavy oil at constant temperature can be measured by using the so-called pressure decay method. In this method, the solvent diffusivity is determined by monitoring the decaying pressure of solvent phase while it is in contact with heavy oil in a closed high-pressure cell. This paper examines the effects of three different boundary conditions at the solvent-heavy oil interface on the determined diffusion coefficient. The so-called equilibrium, quasi-equilibrium and non-equilibrium boundary conditions are applied to represent three rather different interface mass transfer models. An analytical solution to the bulk diffusion equation subjected to each boundary condition is presented to determine the concentration distribution of solvent in the heavy oil. With the determined solvent concentration distribution in the heavy oil, pressure in the solvent phase is then predicted by utilizing the solvent mass conservation and the equation of state for a real gas. Therefore, an average solvent diffusivity, i.e., independent of solvent concentration, is determined by matching the numerically predicted pressures with the experimentally measured data. Experimental data for two different systems, CO2-heavy oil and CH4-heavy oil, are analyzed and the diffusion coefficients are compared with the previously published values. Based on the minimum discrepancy between the predicted and measured pressure data, it is found that the non-equilibrium BC is more applicable to CO2-heavy oil system and that the equilibrium BC is most suitable for CH4-heavy oil system. Thus it is concluded that the obtained solvent diffusivity is sensitive to the boundary condition applied at the solvent-heavy oil interface. Hence, a proper boundary condition for each solvent-heavy oil system should be used to determine the solvent diffusivity in heavy oil. (C) 2004 Elsevier B.V. All rights reserved.