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

Abstract

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 (P_cell−ν_mix) 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 pressure, 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 P_cell−ν_mix 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 pressure 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.

Keywords

Heavy crude oil−solvent systems / Phase behaviours / PVT studies / Constant-composition-expansion (CCE) tests / Bubble-point pressure / Pseudo bubble-point pressure

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