Molecular simulation of CH₄ adsorption behavior in slit nanopores: Verification of simulation methods and models

Abstract

The aim of this study is to select an appropriate method for CH 4 adsorption in organic nanopores for shale-gas development. Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations were performed. Three comparison studies were included: (i) comparison of the adsorption behavior in kerogen nanopores using different schemes for dispersion correction, (ii) comparison of the adsorption behavior in graphite nanopores using MD and GCMC simulations, and (iii) comparison of the adsorption behavior in kerogen and graphite nanopores using MD simulations. The result was reliable when using a particle-mesh Ewald scheme or a cutoff ≥1.5 nm without dispersion correction. The simulation results were essentially identical for the MD and GCMC simulations. The free-gas CH 4 density inside the nanopores started to deviate from the bulk density at~2 nm for the graphite model and at~7-10 nm for the kerogen model, whereas the total CH 4 density deviates from the bulk density at~20 nm.

A new approach for modeling adsorption kinetics and transport of methane and carbon dioxide in shale

“…The behavior of adsorption 2–6 and transport 16–19 of a gas in pores depend on the pore size. For a media with wide PSD, the adsorption and transport behavior of a gas in small pores differ significantly from that in large pores.…”

“…Shale gas (CH 4 ) can be extracted as high‐efficiency energy. Since the adsorbed shale gas takes up approximately 20%–85% of the total shale gas reservoir, 1–4 the CH 4 recovery depends greatly on the CH 4 desorption. The adsorbed CH 4 in a shale may be desorbed by the injected CO 2 because shale has a stronger affinity for CO 2 than CH 4 .…”

“…Shale usually has a wide range of pore size distribution (PSD) from nanometers to micrometers, 3–11 which determines that the adsorption kinetics and transport mechanism of gas flow in the shale are very complicated. Most of the adsorbed gas is stored in the nanopores of shale due to their large specific surface areas and strong adsorption potential 2–9 . The adsorption rate in small pores is different from that in large pores.…”

“…Gas flow in shale undergoes transition of Darcy regimes to other regimes 16–18 . The flow dynamics of both CO 2 and CH 4 in shale are dominated by diffusion in the pores, and Darcy flow in fractures 3,10 . The adsorption affects molecular collisions with the pore walls, thereby affecting significantly gas transport.…”

O2 Physisorption in Coal Pore: Effects of Pore Size, Pressure, Temperature and Function Group

“…The numerous slit-type and round-type pore structures below 10 nm diameter were found on the surface of coal (lignite from the Xilinhot region of Inner Mongolia, China) by scanning electron microscopy (SEM) techniques, as shown in Figure 1 a-c. When studying the coal pore structure on gas adsorption characteristics, the graphite slit model was often used [13,31,35,37,51], as shown in Figure 1 d. As in Figure 1(c), the default distance between graphite slit models in the molecular simulation software was H, but the distance between the pore walls of coal in the actual case was H`, the conversion equation between H and H` was as follows.…”