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.

中文翻译：

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超临界甲烷在纳米孔中的可压缩性：分子模拟研究

不可开采的煤层气是有前途的天然气来源，可作为封存CO ₂的容器。这是因为它们由大量的纳米孔系统组成，这些系统允许大量的甲烷或CO ₂被困在吸附状态。可以使用Gassmann方程根据地震波传播来确定煤层中所含流体的量。但是，为了将Gassmann理论准确地应用于煤层气，必须考虑这些纳米多孔系统中甲烷对甲烷的限制作用。在这项工作中，我们研究了限制条件对模型碳纳米孔中超临界甲烷的影响。使用蒙特卡洛和分子动力学模拟，我们计算了受限甲烷的等温弹性模量。我们发现，封闭对超临界甲烷弹性模量的影响与对亚临界流体的影响相似：（1）封闭流体的弹性模量高于整体流体；（2）对于给定的孔径，模量随压力单调增加；（3）在给定压力下，模量随孔径的倒数而单调增加。但是，这些作用似乎比亚临界流体更为明显，在2 nm的孔中模量增加了多达7倍。当预测在甲烷饱和的多孔介质中的波传播时，应考虑到这种显着增加。