Oil recovery mechanisms and asphaltene precipitation phenomenon in immiscible and miscible CO2 flooding processes

In this paper, oil recovery mechanisms and asphaltene precipitation phenomenon of immiscible and miscible CO2 flooding processes in the tight sandstone reservoir core plugs are experimentally studied. First, the vanishing interfacial tension (VIT) technique is applied to determine the minimum miscibility pressure (MMP) between the original light crude oil and CO2. Second, a total of five coreflood tests are performed at the actual reservoir temperature. It is found that the oil recovery factor (RF) increases monotonically as the injection pressure increases during the immiscible CO2 flooding. The increased oil RF is attributed to the increased CO2 solubility in oil, reduced oil viscosity, increased CO2 viscosity, and reduced equilibrium interfacial tension (IFT) of the light crude oil-CO2 system. Once the injection pressure exceeds the MMP, the oil RF increases slightly and will reach an almost constant maximum value. In this case, it is the multi-contact miscibility (MCM) and the extremely low equilibrium IFT that jointly make the ultimate oil RF high and unchanged. Moreover, the oil RF after CO2 breakthrough (BT) increases slightly in the immiscible case but substantially in the miscible case. This is due to the strong light-hydrocarbons extraction by CO2 and the miscible displacement in the second case. On the other hand, the measured average asphaltene content of the produced oil and the measured oil effective permeability reduction are found to be higher in the immiscible flooding process. They both reach some lower values in the miscible case. Finally, the experimental results obtained in this study at the actual reservoir temperature of T-res = 53.0 degrees C are compared with those attained previously at the laboratory temperature of T-lab = 27.0 degrees C to examine the temperature effect on the CO2-based oil recovery process. (C) 2013 Elsevier Ltd. All rights reserved.