Molecular Dynamics Simulation of Diffusion Behavior of CH4, CO2, and N2 in Mid-Rank Coal Vitrinite

Abstract

The diffusion characteristics of CH4, CO2, and N2 in coal are important for the study of CO2-enhanced coalbed methane (CO2-ECBM) recovery, which has become the most potential method for carbon sequestration and natural gas recovery. However, quantitative research on the diffusion characteristics of CH4 and the invasive gases (CO2 and N2) in coal, especially those in micropores, still faces enormous challenges. In this paper, the self-, Maxwell’s, and transport diffusions of CO2, CH4, and N2 in mid-rank coal vitrinite (MRCV) macromolecules were simulated based on the molecular dynamics method. The effects of the gas concentration, temperature, and pressure on the diffusion coefficients were examined via the comparison of various ranks. The results indicated that the diffusion coefficients have the order of D(N2) > D(CO2) > D(CH4) in their saturated adsorption states. However, when MRCV adsorbed the same amounts of CH4, CO2, and N2, the self- and transport diffusion coefficients followed the order of DS(N2) > DS(CO2) > DS(CH4) and Dt(CO2) > Dt(N2) > Dt(CH4), respectively. Independent of the gas species, all these diffusion coefficients decreased with increasing gas concentration and increased with increasing temperature. In the saturated adsorption state, the diffusion activation energies of CH4, CO2, and N2 were ordered as CH4 (27.388 kJ/mol) > CO2 (11.832 kJ/mol) > N2 (10.396 kJ/mol), indicating that the diffusion processes of CO2 and N2 occur more easily than CH4. The increase of temperature was more conducive to the swelling equilibrium of coal. For the pressure dependence, the diffusion coefficients first increased until the peak pressure (3 MPa) and then decreased with increasing pressure. In contrast, the diffusion activation energy first decreased and then increased with increasing pressure, in which the peak pressure was also 3 MPa. The swelling rate changed more obviously in high-pressure conditions.

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

“…Values first increased and then decreased. 48 The MSD data can be used to describe displacement vs time. It is important to simulate sufficiently long to relax to the equilibrium state.…”

“…A 500 ps NVT ensemble and a 500 ps NVE ensemble were used to ensure the equilibrium state. 23,48 2.4. Calculation of Diffusion Coefficients.…”

“…The dynamics were via an NVT ensemble to construct a linear dependence between mean square displacement (MSD) and time. A 500 ps NVT ensemble and a 500 ps NVE ensemble were used to ensure the equilibrium state. , …”

Simulation of methane adsorption in diverse organic pores in shale reservoirs with multi-period geological evolution

“…The sorption module in Materials Studio (Accelrys) software was utilized to research methane adsorption behavior with the macro-regular Monte Carlo method (GCMC), and a Compass force field was adopted as the force field type (Mosher et al 2013;Sun 1998;Liu et al 2018Liu et al , 2019Hu et al 2010;Zhang et al 2019). The interactions in the simulation include van der Waals force interaction and Coulomb force interaction.…”

“…The interactions in the simulation include van der Waals force interaction and Coulomb force interaction. For the specific expression of van der Waals force interaction and Coulomb force interaction, refer to Zhang et al (2019) and Liu et al (2019).…”