Coupling effect of fluid molecular structure and nanoporous structure on the confined phase behavior of butane isomers in shale nanopores

Phase behavior of light hydrocarbon isomers in nanoscale pores of shale reservoirs differs from that in the bulk state, and its microscopic characteristics and molecular mechanisms remain to be clarified. This study employs the grand canonical Monte Carlo method to simulate the phase behavior of butane isomers in shale nanopores, and discusses the impacts of nanopore size and shale components. By coupling the molecular structure of butane with the structural characteristics of shale nanopores, the shift in the critical properties of butane isomers is elucidated, and the microscopic mechanisms by which nanoconfinement effects influence the phase behavior of butane is revealed. The results show that the confinement effect can reduce the critical temperature of butane by 17 % and the critical pressure by 82 %. When the slit size reaches 16 nm and 22 nm, respectively, the critical temperature and pressure of n-butane are essentially the same as in bulk. Compared to kerogen, the potential energy and physical density of the n-butane adsorption layer on the quartz wall are stronger, leading to a greater shift in the critical properties of the confined n-butane. Compared to n-butane, isobutane is more accessible to the internal and surface micropores of the kerogen matrix due to its smaller molecular size, which results in a greater confinement effect of isobutane by the kerogen. The results of this work improve the understanding of the phase transition behavior of confined fluids in nanoporous media.