Solvent effects on the measured bubble-point pressures and pseudo bubble-point pressures of different heavy crude oil-solvent systems

In this paper,the bubble-point pressures and pseudo bubble-point pressures of various heavy crude oil-solvent systems were measured and studied by conducting the constant-composition-expansion（CCE） tests,during which the test pressure was depleted in a stepwise manner.A total of fourteen CCE tests were performed for five heavy crude oil-CO2 systems,four heavy crude oil-CH4 systems,and five heavy crude oil-C3H8 systems,respectively.All the CCE tests were conducted by using a pressure-volume-temperature（PVT） system.It was found that for most heavy crude oil-solvent systems with relatively low solvent concentrations,the measured PVT cell pressure vs.molar volume(Pcell-vmix) data in the CCE tests had three distinct regions,which were one-liquid phase region（Regions Ⅰ）,foamy-oil region（Region Ⅱ） and two-phase region（Region Ⅲ）.Accordingly,the PVT cell pressure at the intersection point of Regions Ⅰ and Ⅱ was referred to as the measured bubble-point pressu re,whereas the PVT cell pressure at the intersection point of Regions Ⅱ and Ⅲ was termed as the measured pseudo bubble-point pressure.For some heavy crude oil-solvent systems with high solvent concentrations,however,the measured Pcell-vmix data may have two regions only.In this special case,the PVT cell pressure at the intersection point of the two regions was considered as the measured bubble-point pressure and no pseudo bubble-point pressure could be obtained.It was also found that the heavy crude oil-CH4 system had not only the highest bubble-point pressure and pseudo bubble-point pressure but also the largest difference between the bubble-point pressu re and pseudo bubble-point pressure,in comparison with the heavy crude oil-CO2 system and the heavy crude oil-C3H8 system at the same solvent concentration.These facts indicate that CH4 not only is the most volatile solvent but also can remain as dispersed gas bubbles to induce the most stable foamy oil in the largest pressure range after it is nucleated from a live heavy oil.