Molecular Dynamics Simulation of Phase Behavior of Fluid in Confined Nanopores

The reservoir proportion with deep and low permeability, where oil and gas exist widely in nanopores, has been discovered increasingly in China. Affected by the nano-confinement effect, the phase behavior of fluid in nanopores varies with nanopore size rather than being constant. In this paper, the density, viscosity, and saturation pressure of pure and mixed fluids in nanopores are studied by molecular dynamics (MD) simulation combined with statistical physics. The feasibility of using the MD method to investigate fluid behavior in nanopores was verified with laboratory data. It was found that the fluids’ phase behavior parameters in nanopores are lower than those in the bulk phase due to the confinement effect. The boundary of confinement effect (BCE) is defined as a size range of nanopores that separates the pore scale into the confinement scale and bulk scale. Each fluid has a specific BCE influenced by the molecular size of fluid. The phase behavior of crude oil in shale and tight oil reservoirs is most affected by the molecular adsorption and interaction energy between the fluid molecule and pore wall. Clarifying a specific BCE in shale reservoirs can significantly enhance the understanding of reservoirs and guide reservoir development strategies.