Phase Behavior and Critical Properties of Long-Chain Alkane Confined in Shale Nanopores: A Gauge-Gibbs Monte Carlo Simulations Study

Shale oil has gained widespread attention as an important unconventional energy source due to the depletion of conventional resources. <i>n</i>-alkanes with a carbon number more than eight are the main components of shale oil. However, their phase behavior confined to shale nanopores has not been reported. Our study first investigates <i>n</i>-octane, <i>n</i>-dodecane, <i>n</i>-pentadecane, and <i>n</i>-eicosane in type II-C kerogen slit pores of 2-10 nm. We used Gaussian-Gibbs Monte Carlo simulations to obtain vapor-liquid equilibrium properties. Then, we applied the Maxwell equal-area rule, rectilinear diameter law, and Clausius-Clapeyron equation to determine critical properties. The results show that under the same confinement, vapor phase density deviation increases from 240,974% to 5,675,987% as carbon number increases from 8 to 20. We also found that critical parameter deviations have a nonlinear relationship with the carbon chain length. Therefore, we established empirical correlation equations between pore size and critical parameters. Extrapolation reveals that the critical pressure is most sensitive to confinement effects. Alkanes with longer chain lengths adapt better to confined spaces due to their stronger intermolecular interaction. This study establishes empirical correlations for critical parameter shifts of different alkanes, which provides theoretical support for the phase behavior prediction and development optimization of shale oil reservoirs.