Bubble/dew point and hysteresis of hydrocarbons in nanopores from molecular perspective

The effect of nano-confinement on the thermodynamic behavior and saturation properties of petroleum fluids in nanoporous media are important parameters for shale gas/oil production. Confined petroleum fluids can have hysteresis and bubble/dew point behaviors can be very different depending on the thermodynamic route. In this work, we use grand canonical Monte Carlo (GCMC) simulations and engineering density functional theory (DFT) to study the effect of pressure, temperature, and nanopore sizes on the bubble/dew point and hysteresis of hydrocarbons in nanopores. By comparing to GCMC simulations, engineering DFT reliably predicts the vapor-liquid equilibrium of confined hydrocarbon fluids. We also find that dew point of pure confined fluids approaches bulk saturation point as pore size increases, but bubble point can be very different from bulk even for very large pores. As a result, the hysteresis between bubble and dew points increases with pore size. For single-component, the hysteresis decreases as pressure approaches critical pressure. For binary mixtures, the dependence of hysteresis on temperature is dependent on the pore sizes. The cricondentherm point of confined mixtures is shifted toward higher temperatures comparing to the bulk.