Determining the Minimum Miscibility Pressures of Light Oil−CO2 Systems in Bulk Phase and Nanopores

CO₂ has for a long time been used in the petroleum industry as a solvent for displacement and extraction of the remaining oil in reservoirs. In a specific oil-carbon dioxide system, it is of necessity to determine its minimum miscibility pressures to enhance oil recovery accurately. Various experimental methods, simulations, and theoretical models have been established for determination of the minimum miscibility pressures although vanishing interfacial tension technique was recently developed to offer an alternative to the experimental method. The vanishing interfacial tension method relies on the theory that the oil- carbon dioxide phases’ miscibility is arrived at zero-interfacial tension.

Kaiqiang Zhang, Na Jia, and Fanhua Zeng at University of Regina in Collaboration with Peng Luo at Saskatchewan Research Council in Canada developed a new diminishing interface method based on interfacial thickness for determination of the least miscibility pressures for oil-carbon dioxide system in bulk phase and nanopores. They predicted the minimum miscibility pressures of the three liquid-vapor systems and phase behavior using the new diminishing interface method and modified PR-equation of state in nanopores and bulk phase. They also determined the interfacial thickness that exist between mutually solvable phases through two-way mass transfer. Lastly, the equation of state by Peng−Robinson was adjusted to enable liquid-vapor equilibrium calculation nanopores under consideration of the effect of critical temperature and pressure shift and capillary action shift. Their research work is now published in Journal, Energy & Fuels.

The authors performed at different concentrations of carbon dioxide, the bulk phase pressure-volume-temperature tests at reservoir temperatures. Oil density, saturation pressure, and swelling factor increased with carbon dioxide concentration. The authors’ predicted and measured interfacial tensions for the light oil-pure carbon dioxide system were plotted and indicated a perfect match especially at low pressures. However, the anticipated interfacial tensions were lower than those measured at pressures above 10 MPa. Regarding the method of diminishing interface, Minimum Miscibility Pressures is arrived at through linear regression and extrapolating the interfacial thickness derivative concerning pressure. The resulting minimum miscibility pressures calculated from this method for Pembina light oil-carbon dioxide system agreed with the coreflood tests. The equation of state by Peng−Robinson and the model by parachor were used to predict the interfacial tensions for the Bakken live oil- carbon dioxide at different pressures.

The new method of diminishing interface was successfully used to evaluate the least miscibility pressures for a given light oil-carbon dioxide system for both the nanopores and bulk phase. The modified equation of state was vital to calculating the vapor-liquid equilibrium for nanopores. The carbon dioxide dissolution in bulk phase was a principal process of mass transfer accounting to total change composition of about 90%. Lighter components in nanopores were mostly in vapor phase through temperature increase or pressure decrease. The interfacial tensions in nanopores and bulk phase were predicted by a combination of the parachor model and modified Peng-Robinson equation of state. The diminishing interface method provided meaningful results while determining the minimum miscible pressures which were accurate in bulk phase. For dead/live and pure/impure Pembina oil-carbon dioxide system, the minimum miscible pressures got from diminishing interface method matched those measured from lab tests thus the method in conjunction equation of state by Peng−Robinson is accurate.