Unmineable coalbeds can be useful as a source of natural gas and have the potential to act as a receptacle for CO2 sequestration. This is due to their extensive nanoporous systems, which enables methane or CO2 to be trapped in the adsorbed state. The amount of the fluid confined in the coal seams can be estimated from seismic wave propagation data combined with the Gassmann equation [1,2]. However, to accurately apply the Gassmann theory to coalbed methane, the confinement effects on methane in the nanopores must be considered. In this work, we explore these effects of confinement on supercritical methane in model carbon nanopores. Grand canonical Monte Carlo and canonical ensemble molecular dynamics simulations were used to calculate the isothermal elastic modulus of confined methane. We find that the effects of confinement on the elastic modulus of supercritical methane are significant, similarly to the effects on subcritical fluids [3,4] in three major ways: 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 increases monotonically with the reciprocal pore size. Furthermore, these effects appeared much more pronounced than for subcritical fluids, showing up to seven-fold increases of the modulus in smaller pores of 2 nm in diameter [5]. Such a large increase should be accounted for when predicting wave propagation in methane-saturated porous media.
References:
[1] Gassmann, F. Uber die Elastizitat poroser Medien. Viertel. Naturforsch. Ges. Zurich 96 (1951), 1-23.
[2] Gor, G. Y., and Gurevich, B. Gassmann Theory Applies to Nanoporous Media. Geophys. Res. Lett. 45, 1 (2018), 146-155.
[3] Gor, G. Y., Siderius, D. W., Rasmussen, C. J., Krekelberg, W. P., Shen, V. K., and Bernstein, N. Relation between pore size and the compressibility of a confined fluid. J. Chem. Phys. 143 (2015), 194506.
[4] Dobrzanski, C. D., Maximov, M. A., and Gor, G. Y. Effect of pore geometry on the compressibility of a confined simple fluid. J. Chem. Phys. 148, 5 (2018), 054503.
[5] Corrente, N. J., Dobrzanski, C. D., and Gor, G. Y. Compressibility of supercritical methane in nanopores: a molecular simulations study. Energy & Fuels 34, 2 (2020), 1506-1513.
