Mass transfer mechanisms in VAPEX and estimation of diffusion and dispersion coefficients of solvents into bitumenare popular subjects in heavy oil and bitumen research. In previous work, it was shown that low-field NMR could be used as a useful tool in determining diffusion coefficients of solvent into bitumen. The present paper extends this previous work into new areas as follows: First, the effect of the presence of sand on diffusion coefficient measurements is studied. This is done by mixing sand and Cold Lake bitumen and exposing it to a liquid solvent. As the solvent comes into contact with the bitumen-sand matrix, it gradually diffuses into the liquid-filled pores and dilutes the bitumen. NMR spectra are continuously acquired for the duration of each diffusion test. The changes in solvent and bitumen concentrations are detected through changes in the NMR relaxation characteristics of the sample. Diffusion coefficients are generated through different mass transfer models and are compared to sand-free data that were presented previously.
In solvent-based recovery processes for heavy oil andbitumen, mass transfer phenomena compete with viscous forces, gravity and capillary forces as the predominant means for oil recovery or trapping. In order to understand the relative merit of such forces one must be able to measure mass transfer at reservoir conditions and potentially verify such measurements in the field.
There is considerable literature on the measurement of recovery mechanisms and recovery efficiency through solvent based processes for heavy oil and bitumen. The bulk of the work presented in the recent literature focuses around the VAPEX process but the fundamental work presented can easily apply to any solvent displacement process.
Reduction in the viscosity of the heavy oil is achieved by the diffusion process mechanism and interplay of gravity and capillary forces in draining the solvent enriched oil (1).
VAPEX involves primarily the interaction of gravity forces, capillary forces and mass transfer aspects associated with gas absorption on oil films in a porous medium with flow caused by the action of gravity towards a horizontal production well. However, near the VAPEX/bitumen interface, the viscosity and diffusion coefficient values in heavy oil recovery using VAPEX can assume values that vary by several orders of magnitude, depending on the concentration of extraction vapor absorbed( (1) (. This fact implies the importance of mass transfer phenomena in VAPEX in comparison to the gravity and viscous forces.
Das and Butler( (2) (suggested that capillary driven countercurrent flow of solvent vapor and bitumen in the mixing zone is one of the main factors in enhanced mass transfer in porous media. They proposed several mechanisms as the potentialcontributors to the enhanced mass transfer. These processes include: physical dispersion, improved interfacial contact, enhanced surface renewal by capillary imbibitions and development of transient mass transfer across the interface, increased stability due to solvent vapor condensation in fine capillaries and enhancement during the rising of the solvent chamber.
If we were to measure mass transfer phenomena in the field, it appears that a possible logging tool with significant potential is magnetic resonance.
