The vapor extraction (VAPEX) process is a promising technology for recovering heavy oil and bitumen resources in an economically viable and environmentally friendly way. Although a number of laboratory experiments have been conducted to study the VAPEX process, the oil recovery mechanisms by gravity drainage in this process are not well understood yet. In this paper, both experimental and theoretical approaches are adopted to study the effects of gravity and the capillary force on gravity drainage in the VAPEX process. First, the interfacial tensions between a heavy oil sample and four different solvents (methane, ethane, propane, and carbon dioxide) are measured at different pressures below their respective vapor pressures by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case. Then, the Bond number, which is defined as the ratio of gravity to the capillary force, is calculated for the VAPEX process in a heavy oil reservoir and in a physical model of sand pack, respectively. It is found that the measured interfacial tension between the heavy oil and a solvent is reduced almost linearly with pressure for the four heavy oil-solvent systems tested. Correspondingly, the Bond number increases with pressure. An increased Bond number indicates relatively large effect of gravity on the VAPEX process and thus enhanced oil recovery.

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