Results of secondary and tertiary displacements of Maljamar crude oil by N2 and CO2 are compared to examine the effects of solubility and extraction on local displacement efficiency: The flow visualization experiments were performed in pore networks etched in glass plates. In those experiments, the much higher solubility of CO2 in the oil caused only marginal improvement in displacement efficiency over that observed for N2, which was much less soluble. At pressures high enough for CO2 to extract hydrocarbons efficiently, however, displacements were much more efficient.
Secondary and tertiary displacements in a more heterogeneous glass model arte also compared. In a secondary displacement with high solubility but low extraction, adverse capillary and viscous effects limited the area swept to preferential flow paths. High CO2 solubility did not appear to have significant effects. When water was present in tertiary displacements with efficient extraction and high solubility, combined effects of viscous instability, capillary forces, and heterogeneity sharply reduced sweep efficiency below that observed in secondary displacements in the same model.
CO2 flooding is a process that is influenced by a variety of mechanisms operating simultaneously. , Depending on the pressure and temperature, CO2 can dissolve in the oil and extract hydrocarbons from it. When CO2 displaces oil, the flow is likely to be unstable, given the low viscosity of CO2 and CO2 rich phases. The flow and mixing of CO2 and oil-rich phases are influenced by the heterogeneity of the pore space and the saturation of water. Given the number of factors that may influence a particular displacement, it can be difficult to determine which factor(s) had the greatest effect on displacement performance. In this paper, we report results of flow visualization experiments that provide additional evidence concerning the roles of the various process mechanisms.