Determination of minimum miscibility and near-miscibility pressures for CO2-oil mixtures in shale reservoirs

Accurately determining the miscibility pressure between shale oil and CO2 is crucial for enhancing oil recovery and CO2 storage in shale reservoirs. This study presents a novel method for simultaneously calculating the minimum miscibility pressure and the minimum near-miscibility pressure of CO2-shale oil mixtures. This approach integrates a modified interfacial tension vanishing method with a phase equilibrium module that accounts for nano-confinement effects, introducing innovative criteria for defining miscibility and near-miscibility. Six shale oils with varying maturities from the GL, JM, and Bakken shale formations were analyzed. Results show that the GL and Bakken shale oils achieve miscibility with CO2, whereas JM shale oil only attains near- miscibility. Lower-maturity shale oils require lower interfacial tension values to achieve near-miscibility. Moreover, in nanopores smaller than 100 nm, both the minimum miscibility and near-miscibility pressures decrease significantly with pore size, with more pronounced reductions observed in lower-maturity shale oils. These findings provide a robust technical framework for optimizing CO2 injection strategies and improving recovery efficiency in unconventional reservoirs.