A Review on the Application of Molecular Dynamics to the Study of Coalbed Methane Geology

Abstract

Over the last three decades, molecular dynamics (MD) has been extensively utilized in the field of coalbed methane geology. These uses include but are not limited to 1) adsorption of gaseous molecules onto coal, 2) diffusion of gaseous molecules into coal, 3) gas adsorption-induced coal matrix swelling and shrinkage, and 4) coal pyrolysis and combustion. With the development of computation power, we are entering a period where MD can be widely used for the above higher level applications. Here, the application of MD for coalbed methane study was reviewed. Combining GCMC (grand canonical Monte Carlo) and MD simulation can provide microscopic understanding of the adsorption of gaseous molecules onto coal. The experimental observations face significant challenges when encountering the nanoscale diffusion process due to coal structure heterogeneity. Today, all types of diffusion coefficients, such as self-, corrected-, and transport-diffusion coefficients can be calculated based on MD and the Peng-Robinson equation. To date, the MD simulation for both pure and multi-components has reached a situation of unprecedented success. Meanwhile, the swelling deformation of coal has been attracting an increasing amount of attention both via experimental and mimetic angles, which can be successfully clarified using MD and a poromechanical model incorporating the geothermal gradient law. With the development of computational power and physical examination level, simulation sophistication and improvements in MD, GCMC, and other numerical models will provide more opportunities to go beyond the current informed approach, gaining researcher confidence in the engagement in the estimation of coal-swelling deformation behaviors. These reactive MD works have clarified the feasibility and capability of the reactive force field ReaxFF to describe initial reactive events for coal pyrolysis and combustion. In future, advancing MD simulation (primarily characterized by the ReaxFF force field) will allow the exploration of the more complex reaction process. The reaction mechanism of pyrolysis and spontaneous combustion should also be a positive trend, as well as the potential of MD for both visualization and microscopic mechanisms for more clean utilization processes of coal. Thus, it is expected that the availability of MD will continue to increase and be added to the extensive list of advanced analytical approaches to explore the multi-scaled behaviors in coalbed methane geology.

Influence of functional groups on the microporous development and CO2-CH4 adsorption: A molecular simulation investigation

“…Coal macromolecule was consisted of aromatic clusters, aliphatic chains, oxygen functional groups, and heteroatom groups (Given et al, 1986; Liu et al, 2019). After entering the 21st century, the research of coal molecular geochemistry has achieved fruitful results combined with the further development of computer-aided molecular design especially the development and progress of 13 C nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy, providing the basis for the simulation of gas behaviors and coal geology (Song et al, 2021). The coal macromolecular representation used here was created via elemental analysis, 13 C NMR, FTIR, and high-resolution transmission electron microscope from Song et al (2019), whose predicted 13 C NMR spectrum was in line with the experimental results.…”

Adsorption and Diffusion Behavior of CH4 and CO2 in Closed and Open Pores from Zhaozhuang Coal

“…Molecular simulations are increasingly popular for exploration in recent years. 17 Based on Newtonian mechanics, MD can supplement the process, which is impossible to measure in experiments. Meng et al 18 found that the affinity between methane and coal molecules reduced in the presence of water molecules, thus leading to less aggregation around the coal molecules.…”

“…Molecular simulations are increasingly popular for exploration in recent years . Based on Newtonian mechanics, MD can supplement the process, which is impossible to measure in experiments.…”

Numerical Optimization of Drilling Parameters for Gas Predrainage and Excavating-Drainage Collaboration on Roadway Head

“…Lin et al [12] established a multiphysics coupled model of diffusion, stress, and seepage fields and introduced the dynamic diffusion coefficient to explore the seepage evolution during gas drainage. In addition, some scholars have studied the desorption and adsorption behavior of coal at the microscopic level [13,14] and the influence of polymer on coal desorption and adsorption [15]. For the borehole spacing, too small spacing will increase the amount of work in the drilling construction and rise the instability risk of drilling such as pierce or collapse holes resulting from the pressure relief damage of coal and rock, while too large spacing will leave blind area of drainage, which not only can hardly eliminate the outbursts but also aggravate the hidden dangers of gas accidents [16].…”