Oil-solvent mixing is essential during solvent injection applications to reduce the viscosity of oil but mass transfer by diffusion becomes slower as the oil becomes heavier. Thus, an interface exists between oil and solvent at certain times being stronger in the beginning of the process. This results in an immiscible displacement controlled by the capillary forces while mixing is in progress. It is of practical and fundamental importance to determine the mechanisms responsible for the displacement of heavy oil and the behavior of solvent (acting as both immiscible and miscible displacement agent) as it could be advantageous to accelerate the dilution of heavy oil in many circumstances, including heterogeneous (fractured, layered, wormholed) systems. This is a complex process consisting of multiple pore phases (oil, solvent, their mixtures, aqueous and vapor phases) at the same time while different mechanisms such as capillary imbibition, miscible interaction (diffusion and convection), and gravity also act simultaneously.

To investigate this complex phenomenon for different oil-solvent systems, a novel experimental method was employed. The underlying mechanisms that dominate the solvent displacement process were comprehensively identified. The movement and evolution of interfaces among different fluid phases in glass capillary tubes was observed and recorded. The oil samples with different viscosities were utilized to examine the effects of oil viscosity on the mass transfer accelerated by imbibition transfer. The effects of temperature, wettability and boundary conditions on the interaction of miscible fluids pairs were also studied. Pentane, heptane and decane were used as the solvent phases. Advanced photographic techniques using UV light and dyed fluids were applied to better track the flow of different phases in the mixing zone.

The experiments demonstrated a slowly smearing interface between solvent and viscous oil. A unique natural convection was induced with the combined effect of gravity, diffusion (mixing) and capillarity all contributing to the recovery of heavy-oil. Based on the saturation method, boundary condition and the Bond number, four different motion modes of mixing zone and interfaces of miscible fluids in the capillary tube were unveiled and categorized to identify the degree of interface development (immiscible flooding). Also, the mixing zone, mass flux, and flow behavior were quantified using dimensionless parameters. The results indicate that priority may be given to a solvent with a high interfacial tension for solvent-based oil recovery technique because of a strong imbibition and further enhancement of the dilution and displacement processes under condition of similar viscosity ratio. The data provided will be useful for the accuracy of modeling studies, especially for complex geologies where oil-solvent interaction is critically difficult to develop in order for mixing to occur.

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