Compressibility of Supercritical Methane in Nanopores: A Molecular Simulation Study

Abstract

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.

Elastic properties of confined fluids from molecular modeling to ultrasonic experiments on porous solids

“…Showing that multiple differing methods of molecular modeling (i.e., MD, GCMC, and DFT) and use of various thermodynamic ensembles are able to predict the same values for the elastic properties of the confined fluid. [44,57,60,162] Note that these theoretical results have practical implications, in particular they suggest the pore-size dependent correction for parameters for the Gassmann equation often used by practitioners. The dependence of fluid compressibility on the pore surface properties could be important for processes such as enhanced oil recovery, or carbon dioxide sequestration, which cause the surface modifications of the geological porous media [172,173].…”

“…Future experiments should focus on other nanoporous solids that have different pore sizes, pore shapes, surface properties, etc. In particular, a series of measurements on similar samples with different pore sizes could help verifying the pore size dependence of the elastic modulus predicted by molecular simulations [30,44,62]. Furthermore, ultrasonic experiments with a broader family of liquids are desired in order to explore how molecular properties, such as polarity, molecule size, and shape, affect the compressibility of confined fluids.…”

“…Specifically, experiments are needed for fluids which have practical importance for geophysics, i.e., water, hydrocarbons, and carbon dioxide. The two latter compounds are of special interest at supercritical conditions, at which, according to molecular modeling, the compressibility is more sensitive to effects of confinement [44].…”

“…Corrente et al utilized Eq. 11 for calculating compressibility of methane confined in carbon nanopores, which was to model the natural gas found in coal and shale systems [44]. They performed simulations on slit pores of widths ranging from 2 to 9 nm using GCMC and molecular dynamics (MD) simulations in N V T ensemble.…”

Description of the thickness of the adsorbed layer, identification of the instability characteristics of the liquid–vapour interface and assessment of criticality in elliptical pores through the Broekhoff de Boer theory

“…The understanding of adsorption results from experiments [4], molecular simulation studies [5][6][7][8][9] and sophisticated approaches such as density functional theory (DFT) [10][11][12][13][14]. Molecular simulation allows the treatment of complex pores with interconnectivity and surface defects and it successfully predicts hysteresis in cylindrical pores [7,15].…”