Interactions and phase behaviors between oleic phase and CO2 from swelling to miscibility in CO2-based enhanced oil recovery (EOR) process: A comprehensive visualization study

In this work, a comprehensive study of interactions and phase behaviors between CO2 and oleic phase, including CO2 dissolution, oil swelling (So), hydrocarbon extraction, and miscible process, was conducted using a high-pressure-temperature visualization cell. The experimental data indicated that the magnitude of CO2 solubility decreased with temperature and oil density; while at a given temperature, this parameter increased steadily with pressure until the oleic phase was CO2-saturated. The value of So remained increasing up to the first threshold extraction pressure, Pext,1, from which the hydrocarbons began to be extracted into the CO2 phase. Furthermore, the extracted portion became heavier (C10-C19) with the rise in pressure, which thus caused the oil volume to shrink and led to the value of So below 1.0 from the second threshold extraction pressure, Pext,2. Further increasing pressure resulted in more heavy hydrocarbons (C20-C29) participating in CO2-oleic phase interchanges until the miscibility was finally achieved. At the miscible stage, the composition of the gas phase was found to be fairly close to the oleic phase except some heavy fraction (i.e. asphaltene). Based on the results, the dominant oil recovery mechanisms contributing to CO2 cyclic injection EOR were qualitatively defined.