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.

Fluids confined in nanoporous materials exhibit thermodynamic properties that differ from the same fluid in bulk. Recent experiments and molecular simulations suggested that the isothermal compressibility is among these properties. The compressibility determines the elastic response of a fluid to mechanical impact, and in particular, the speed of acoustic wave propagation through it. Knowledge of the compressibility of fluids confined in nanopores is needed for understanding the elastic wave propagation in fluid-saturated nanoporous media, such as hydrocarbon-bearing shales. Molecular simulations allow for the prediction of the elastic properties of a confined fluid but require computationally expensive calculations for each system and pore size. Therefore, there is interest for a more straightforward model that can predict the elastic properties of a confined fluid as a function of the external pressure and confining pore size. Such models can be based on an equation of state (EOS) for a confined system. Here, we explore a possibility for a generalized van der Waals EOS for confined fluids to predict the compressibility. We also calculate the elastic properties of argon confined in silica nanopores from grand canonical Monte Carlo simulations. We obtain comparable adsorption isotherm predictions of the EOS and simulations at various pore sizes and temperatures without changing any other parameters. We then see how the predictions of the elastic properties from simulations compare to the EOS and find reasonable agreement. Additionally, we vary the solid–fluid interaction parameters in both the EOS and molecular simulations to represent solids other than silica and see how the elastic moduli depend on the other properties of confining pores related to the interaction strength. This work is a step toward a quantitative description of wave propagation in fluid-saturated nanoporous media.

The phenomenon of adsorption-induced deformation of nanoporous materials has recently attracted a lot of attention in chemical, materials, and geoscience communities. Various theoretical and molecular simulation approaches have been suggested to predict the stress and strain induced by single component gas adsorption. Here, we develop a thermodynamic method based on the notion of the adsorption stress to predict the deformation effects upon multicomponent adsorption. As a practically important example, the process of the displacement of methane by carbon dioxide from microporous carbons is considered. This process is the foundation of secondary gas recovery from shales and coalbeds associated with carbon dioxide sequestration. Theoretical predictions are correlated with the original experimental data on CO 2 and CH 4 individual and binary adsorption on coal samples coupled with in situ strain measurements. With the model parametrized and verified against the experimental data at ambient temperature, the projections are made for the adsorption deformation at geological conditions of elevated pressure and temperature, which increase with the depth of the reservoir. The proposed approach may have multifaceted applications in modeling the behavior of hydrocarbon mixtures in nanoporous geomaterials, gas separations, and energy storage on flexible adsorbents.

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