Simulation of Adsorption–Desorption Behavior in Coal Seam Gas Reservoirs at the Molecular Level: A Comprehensive Review

Abstract

This work reviews the processes of methane (CH4) and carbon dioxide (CO2) adsorption and desorption, as well as the displacement of CH4 by CO2, at the molecular level. For the adsorbate, laboratory experiments and numerical simulations have been carried out to examine the adsorption and desorption behavior of CH4 and CO2. For the adsorbent, graphite, graphene, carbon nanotube, and heteroatom-containing carbon-based models have been developed for adsorption and desorption processes. In recent decades, simulations of carbon dioxide enhanced coalbed methane (CO2-ECBM) have achieved significant progress. This success has generated significant interest among the scientific communities, as coalbed methane is one of the cleanest forms of energy. Furthermore, an improved understanding of this process can help to sequester CO2 in deep unminable coal seam gas reservoirs to mitigate CO2 emission. This work reviews the advantages and weaknesses of three simulation methods, namely, molecular dynamics (MD), Monte Carlo (MC), and the density functional theory (DFT), with respect to their applications in CO2-ECBM and CO2 sequestration in CBM. Based on such simulations, dynamic and thermodynamic properties have been analyzed to evaluate the effectiveness of CO2-ECBM and CO2 sequestration. In recent studies, MD simulation has been commonly used to optimize the configuration of the adsorbate and adsorbent, as well as to define the self-diffusion coefficient and mean squared distance. However, this approach only works for a small atomic system or a short time simulation process. MC, on the other hand, is used to predict the adsorption behavior of pure component and binary and ternary mixtures of adsorbates. Nevertheless, the simulation cannot obtain information on the dynamic processes. Meanwhile, DFT is commonly used to predict the adsorption site, orientation, and adsorption energy. Dynamic and thermodynamic properties are discussed based on the three simulation methods. Certain aspects facing the future are also addressed.

Insights into the Molecular Competitive Adsorption Mechanism of CH4/CO2 in a Kerogen Matrix in the Presence of Moisture, Salinity, and Ethane

“…A particle–particle and particle–mesh (PPPM) solver is taken, whose precision is 1 × 10 –4 with a 12 Å cutoff radius to compute the electrostatic interactions among particles. The integrator of the Verlet method is used to assess the velocity and position of particles by Newton’s equation of motion …”

“…The integrator of the Verlet method is used to assess the velocity and position of particles by Newton's equation of motion. 5 To establish the absorbent, relaxing the single kerogen unit is the first step. Following that, eight macromolecules are put into one cubic box using the replicate command, and the initial box size is 100 × 100 × 100 Å 3 with a 0.033 g/cm 3 density at the beginning.…”

“…However, the recovery of the unconventional resources is at low efficiency because of the insufficient understanding of the comprehensive reservoir properties and the complex gas competitive adsorption behavior. In coalbed and shale reservoirs, methane is the primary gas component associated with other alkanes, such as C 2 H 6 , C 3 H 8 , other heavy hydrocarbons, CO 2 , and sometimes SO 2 and N 2 . ,, There are usually three forms of CH 4 in shale reservoirs: the free phase in pores and fractures, the adsorbed phase in organic matter and minerals, and the dissolved phase in water . In contrast, the coalbed gas-bearing layers usually hold 20–85% CH 4 as an adsorption phase at a given reservoir condition, and the left part remains a free phase and dissolved phase. , Thus, high CH 4 recovery techniques have become a hot topic and draw attention worldwide.…”

“…In coalbed and shale reservoirs, methane is the primary gas component associated with other alkanes, such as C 2 H 6 , C 3 H 8 , other heavy hydrocarbons, CO 2 , and sometimes SO 2 and N 2 . 2,5,6 There are usually three forms of CH 4 in shale reservoirs: the free phase in pores and fractures, the adsorbed phase in organic matter and minerals, and the dissolved phase in water. 6 In contrast, the coalbed gas-bearing layers usually hold 20−85% CH 4 as an adsorption phase at a given reservoir condition, and the left part remains a free phase and dissolved phase.…”

Experimental Study on Desorption Characteristics of Coalbed Methane under Variable Loading and Temperature in Deep and High Geothermal Mine

“…An adsorption-desorption experimental device was developed and used to study the energy variation rules in the process of gas adsorption and desorption in coal [6]. e processes of methane (CH 4 ) and carbon dioxide (CO 2 ) adsorption and desorption were reviewed, as well as the displacement of CH 4 by CO 2 , at the molecular level, and the adsorption and desorption behavior of CH 4 and CO 2 was examined [7].…”