The mutual diffusivity of solvent and bitumen is an important parameter in the design of solvent-based bitumen recovery processes. It is determined from measured concentration profiles in bitumen. These measurements are difficult because bitumen is opaque and current approaches such as X-ray or magnetic resonance tomography are expensive. A new apparatus was designed and commissioned as a lower cost alternative. It measures the mass transfer in liquid solvent/heavy oil systems based on the density profiles established over time in a column of solvent over bitumen. A one-dimensional numerical model based on molecular diffusion was developed to determine the mutual diffusivity from the concentration profiles. The model accounted for the dependence of diffusivity on viscosity through a correlation based on the infinite dilution diffusivities of the solvent and the oil. Mutual diffusivities were determined for Athabasca bitumen with toluene and for maltenes from the same oil with toluene, n-heptane, and n-pentane at ambient conditions and diffusion times from 3 to 15 days. The measured diffusivities were found to increase monotonically with decreasing viscosity (and solvent content) of the mixture but remained below the self-diffusion coefficient of the solvent.

The oil production rate of in situ heavy oil recovery processes involving the injection of gaseous or liquid hydrocarbon solvents depends strongly on the mutual diffusivity of the solvent and bitumen. Data for these diffusivities, particularly above the ambient temperature, are relatively scarce because they are time-consuming to measure. In this study, the diffusion and solubilities of gaseous toluene, n-pentane, and n-heptane in a Western Canadian bitumen and its maltenes were measured at temperatures from 150 to 180 °C using a pressure decay method. The diffusivities were determined as a function of viscosity from a numerical model of the experiments. A previously developed correlation for the infinite dilution viscosity of solvent into bitumen, originally proposed for gas/bitumen systems with low solvent solubility in bitumen, was extended to higher solubility gases and to liquid hydrocarbon/bitumen systems. The correlation was updated using gas/bitumen diffusivities and liquid hydrocarbon/bitumen diffusivities from the literature, and the gas/bitumen data were obtained in this study. Tuned correlation parameters for solvent diffusion in bitumen and in maltenes are provided. The numerical model with the proposed correlations matches the measured cumulative mass transfer from the pressure decay experiments in this study to within ±18% and the measured concentration profiles from liquid solvent/crude oil experiments to within ±4 wt %.