Minimum miscibility pressure determination in confined nanopores considering pore size distribution of tight/shale formations

Abstract

This work develops a modified minimum miscibility pressure (MMP) calculation algorithm that couples the effects of pore size distribution, capillarity, and confinement. Also, a volume translated Peng-Robinson equation of state (PR-EOS) is employed in the proposed algorithm to provide a more accurate prediction on phase densities. To calibrate the proposed algorithm, this study uses a real crude oil sample Zhang and Gu (2015) to perform all calculations. The binary interactive parameters (BIPs) of the components in this oil sample are tuned to match the measured oil-CO2 MMP in tight cores by Zhang and Gu (2015). Using the proposed algorithm, the effects of temperature and pore radius on the confined oil-CO2 MMPs are studied in detail. It is found that the oil-CO2 MMP in nanopores decreases with decreasing pore radii. However, the confined MMP becomes almost constant when the pore radius is larger than 10 nm. With an increasing temperature, the confined oil-CO2 MMP first increases to a certain temperature, and then decreases for all the tested pore radii. Hence, there exists a maximum confined MMP for every pore radius, and the maximum confined MMP decreases with a decreasing pore radius. Moreover, when predicting the confined MMP for a real tight reservoir, the pore size distribution is crucial, and the average pore radius is no longer applicable in the calculations because the confined MMP does not change linearly with pore radius. Results show that the confined MMP calculated using the proposed strategy is noticeably lower than the MMP calculated using the average pore radius.