Theoretical model and numerical solution of gas desorption and flow mechanism in coal matrix based on free gas density gradient

Liu, Wei; Xu, Hao; Qin, Yueping; Chu, Xiangyu; Jia, Lei; Wu, Deyan; Zhao, Wei

doi:10.1016/j.jngse.2021.103932

Cited by 19 publications

(15 citation statements)

References 45 publications

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“…In this work, the so-called particle method , is used to evaluate gas desorption and diffusion kinetics in a coal matrix. The following assumptions are made prior to developing a numerical model for gas flow in coal particles: (i) a coal particle is spherical and homogeneous, while a sphere containing coal particles is infinite; (ii) pore deformation caused by pressure fluctuations is ignored; (iii) surface diffusion on a surface of a coal particle is not involved; and (iv) gas desorption and diffusion flow in a coal particle are driven by the FGDG. , In this way, mass gas flow is proportional to the FGDG, and then, we have where J m is the mass flux of gas, g/cm 2 s; D fg is the effective diffusion coefficient of free gas, cm 2 /s; and ρ fg is the free gas density, g/cm 3 .…”

Section: Theoretical Formulationsmentioning

confidence: 99%

“…The total gas concentration can be expressed by the Langmuir equation , where X is the total gas concentration, cm 3 /g; a and b are the Langmuir adsorption constants, cm 3 /g, 1/MPa; ρ a is the apparent density of the coal, g/cm 3 ; p is the gas pressure, MPa; ϕ is the porosity; T 0 is the temperature at the standard condition, 273.15 K; p 0 is the pressure at the standard condition, 0.101325 MPa; and T is the actual temperature, K.…”

Section: Theoretical Formulationsmentioning

confidence: 99%

“…Eq contains nonlinear expressions that require Gaussian iteration for their solution. Our previously developed program codes ,, have been modified for solving the developed model, and the solution workflow can be briefly illustrated in Figure .…”

Section: Numerical Solutionsmentioning

confidence: 99%

“…Accordingly, we previously demonstrated that the gas diffusion flow in a coal matrix is driven by the free gas density gradient (FGDG) and then developed a FGDG model (FGDGM), assuming that free gas is the main contributor to the gas flow and that the diffusive mass flux is proportional to the FGDG. In addition to validating the FGDGM by performing isothermal gas adsorption/desorption experiments with different coal ranks, gas pressures, and gas types at a constant temperature, − experimental data from the literature have been used for such a purpose …”

Section: Introductionmentioning

confidence: 99%

“…Based on our previous works, , in this paper, the theoretical FGDGM has been further modified and formulated as a function of temperature to quantify gas desorption and diffusion kinetics in a coal seam. Once such theoretical models have been validated with the experimentally measured methane desorption data at different temperatures, comparisons and discussions are made to evaluate the accuracy of the proposed FGDGM and traditional UDM with justifications.…”

Section: Introductionmentioning

confidence: 99%

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Modeling of Gas Desorption and Diffusion Kinetics in a Coal Seam Combined with a Free Gas Density Gradient Concept

Qin

Yang

et al. 2022

Energy Fuels

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Gas diffusion in a coal seam plays an essential role in the efficient and effective exploitation of underground coalbed methane (CBM). Based on the concept of free gas density gradient-driven flow, in this work, we formulated and validated a theoretical model to quantify gas desorption and diffusion kinetics in a coal seam at different temperatures. Once the governing equations are formulated and non-dimensionalized, they are numerically solved with the finite difference method and then validated with the experimental measurements on gas desorption at different temperatures. The mechanism of temperature affecting gas diffusion kinetics was elaborately explored. Also, the advantages of the proposed free gas density gradient model (FGDGM) compared to the traditional unipore diffusion model (UDM) were well evaluated and discussed. It is found that the FGDGM simulations are basically consistent with the experimental data, but the UDM prediction deviates significantly from the experimental measurements at different temperatures. By introducing a new dimensionless temperature variable and performing sensitivity analysis, the dimensionless gas desorption is found to increase with temperature, while the incremental gas desorption varies across the entire temperature range. With the temperature increasing from 293 to 313 K, both the gas desorption increment and gas yield enhancement are found to be the largest. In addition, there exists a linear relationship between diffusivity and temperature because a high temperature increases the gas molecules’ activity and accelerates the pore expansion. It is clearly found that temperature has a considerable contribution to the gas diffusion kinetics in a coal seam, and appropriately boosting the coal temperature to enhance CBM recovery is feasible and valuable.

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