Compressibility of Supercritical Methane in Nanopores: A Molecular Simulation Study

Unmineable coalbeds are a promising
source of natural gas and can
act as a receptacle for CO2 sequestration. This is because
they are composed of extensive nanoporous systems, which allow for
significant amounts of methane or CO2 to be trapped in
the adsorbed state. The amount of the fluid confined in the coal seams
can be determined from seismic wave propagation using the Gassmann
equation. However, to accurately apply the Gassmann theory to coalbed
methane, the effects of confinement on methane in these nanoporous
systems must be taken into account. In this work, we investigate these
effects of confinement on supercritical methane in model carbon nanopores.
Using Monte Carlo and molecular dynamics simulations, we calculated
the isothermal elastic modulus of confined methane. We showed that
the effects of confinement on the elastic modulus of supercritical
methane are similar to the effects on subcritical fluids: (1) the
elastic modulus of the confined fluid is higher than in bulk; (2)
for a given pore size, the modulus monotonically increases with pressure;
and (3) at a given pressure, the modulus monotonically increases with
the reciprocal pore size. However, these effects appeared much more
pronounced than for subcritical fluids, showing up to seven-fold increases
of the modulus in 2 nm pores. Such a significant increase should be
taken into account when predicting wave propagation in methane-saturated
porous media.