Experimental and theoretical studies of solvent bubble nucleation and liberation processes in different heavy crude oil−solvent systems

Abstract

In this paper, solvent bubble nucleation and liberation processes in the heavy crude oil−CO2 systems, heavy crude oil−CH4 systems and heavy crude oil−C3H8 systems were experimentally studied and theoretically analyzed. First, two respective series of tests were conducted for different heavy crude oil−solvent systems. The first series included eleven conventional isothermal constant-composition-expansion (CCE) tests and the other series consisted of three new isothermal constant-composition-expansion & compression (CCEC) tests. Second, the amount of the evolved gas (i.e., the dispersed gas and free gas) in each pressure reduction step was determined from the measured CCE test data to study the solvent bubble nucleation process in each heavy crude oil−solvent system. A new quantity named the bubble nucleation index (BNI) was introduced and used to represent the solvent bubble nucleation strength. Third, the respective amounts of the dispersed gas and free gas in each pressure reduction step were obtained from the measured CCEC test data to examine the solvent bubble liberation processes in three heavy crude oil−solvent systems. A second new quantity named the bubble liberation index (BLI) was defined and applied to represent the solvent bubble liberation strength. It was found from the CCE and CCEC tests that the measured Pcell−vt data for each heavy crude oil−solvent system had three distinct regions. Region I was the one-phase region. Region II was the foamy-oil region, in which the solvent bubble nucleation started and the solvent bubbles were dispersed in the heavy oil. Region III was the two-phase region, in which the free-gas phase was formed and started to dominate the total compressibility of the heavy crude oil−solvent system. In addition, the solvent supersaturation vs. reduced pressure data indicated that the heavy crude oil−CH4 system had the lowest solvent supersaturation at the same reduced pressure in comparison with the heavy crude oil−CO2 or C3H8 system. Thus CH4 was easier to be nucleated from the heavy oil in comparison with CO2 or C3H8. Moreover, the BNI vs. solvent supersaturation data showed that the BNI of the heavy crude oil−CH4 or C3H8 system was slowly increased at lower solvent supersaturations but quickly increased at higher solvent supersaturations, whereas the BNI of the heavy crude oil−CO2 system was almost linearly increased with solvent supersaturation. Furthermore, the BLI vs. reduced pressure data revealed that in comparison with C3H8/CO2, CH4 was the most difficult solvent to be liberated from the heavy oil once its bubbles were nucleated. A large amount of CH4 bubbles could be trapped in the heavy oil to induce the strongest and most stable foamy oil in comparison with C3H8 and CO2. The above experimental findings help to better understand the foamy-oil strengths and stabilities in different heavy crude oil−solvent systems and determine the most suitable solvent to optimize a solvent-based enhanced oil recovery (EOR) process in a heavy oil reservoir.