NMR-Observable Prediction of CO2- n -Decane Minimum Miscibility Pressure under Nanoconfinement: Molecular Mechanisms in Calcite Slits

Minimum miscibility pressure (MMP) governs CO2-enhanced oil recovery (EOR) and concurrent CO2 storage. However, nanoconfinement within shale-relevant mineral pores can reshape the miscibility pathway and bias bulk-derived criteria. In this study, molecular dynamics simulations coupled with NMR relaxation theory reveal the microscopic mechanisms of miscibility under nanoscale confinement and introduce a method for predicting the MMP based on the proton longitudinal relaxation time (T1). The results indicate that enhanced mass transport in the calcite adsorption region induces a curved meniscus, while a persistent CO2-n-decane interface stabilized by interfacial free-energy barriers and orientational disorder restricts molecular diffusion; increasing pressure progressively weakens these barriers until the interface vanishes. At ultra-high pressures, dense CO2 combined with wall interactions over-constrains C10 and suppresses diffusion, indicating an optimal injection-pressure window rather than a monotonic benefit. Importantly, the T1 computed from trajectories correlates strongly with interaction energy, enabling an NMR-accessible MMP criterion. T1 predicts a bulk MMP of 11.77 MPa (consistent with experimentally measured value) and a reduced MMP of 8.08 MPa in the 5 nm slit. Spatially resolved T1 further characterizes wall-induced confinement, providing insight into the anomalous increase of MMP observed in slits narrower than 5 nm. This MD-NMR workflow provides a translatable route for NMR-guided MMP screening and CO2 injection optimization in nanoporous reservoirs.