Thermodynamic Properties of Methane Considering Molecule Size and Confinement Effect in Nanopores

Accurately determining the viscosity and density of methane holds great significance for numerical simulations of tight gas reservoirs. Density and viscosity of methane are studied in nanopores in this paper. First, using potential theory, we develop a modified P-R equation of state while taking confinement effect into account. Modified PR EOS can accurately predict the critical temperature of methane than the previous models. Second, Modified PR EOS can be used to calculate the thermodynamic phase behavior. This method can precisely predict the P - V diagrams of methane existing in nanoscale pore. Calculations using the modified PR EOS indicate that the P - V diagrams of methane in nanoscale pore are shifted upward in relation to the corresponding diagrams of the bulk phase. Finally, we can find density and viscosity of methane decreases with the decrease of pore radius owing to confinement effect in free space. When the pressure is less than 10MPa, the nano-confinement effect has a limited impact on the density and viscosity of methane. Under high pressure, we can find obviously that the density and viscosity of methane are lower in nanopores than those in bulk. When pore radius exceeds 20nm, the nano-confinement effect has no effect on density and viscosity. When the pressure is equal to 30MPa and the temperature exceeds 400K, it can be found that the nano-confinement effect has no effect on viscosity. Therefore, the confinement effect improves methane gas flow ability and increases tight gas production.