During a solvent-based heavy oil recovery process, significant viscosity reduction is achieved through sufficient solvent dissolution and possible asphaltene precipitation. In the second viscosity reduction mechanism, the heavy oil is in-situ upgraded as the asphaltene content in the produced heavy oil is greatly lowered. In this paper, the respective and synergistic effects of asphaltene content and solvent concentration on heavy oil viscosity are studied by measuring viscosities of heavy oil samples with different asphaltene contents and/or solvent concentrations. More specifically, a deasphalted heavy oil sample is obtained by using the standard ASTM method to extract asphaltenes from a crude heavy oil. Then reconstituted heavy oil sample is prepared by adding the extracted asphaltenes into the deasphalted heavy oil at a different asphaltene content each time and its corresponding viscosity is measured at the atmospheric pressure. It is found that the viscosity of such prepared heavy oil is sensitive to the asphaltene content. In particular, the viscosity of the deasphalted heavy oil is reduced by 13.7 times in comparison with that of the original heavy oil at T=23.9°C. This indicates that high asphaltene content results in high viscosity of the heavy oil. On the other hand, the viscosities of propane-saturated heavy oil samples with different asphaltene contents are measured at high pressures. As the equilibrium propane concentration reaches certain value, the viscosity of the crude heavy oil-propane system is reduced by almost two orders. This result shows that solvent dissolution plays a dominant role in heavy oil viscosity reduction. When the equilibrium propane concentration is high enough, all the three reconstituted heavy oil-propane systems have extremely low viscosities. In this case, effect of asphaltene content on heavy oil viscosity becomes negligible.

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