Why coal permeability changes under free swellings: New insights

Numerical simulation of boreholes for gas extraction and effective range of gas extraction in soft coal seams

“…Afterward, on this basis, researchers proposed diverse theories including linear gas diffusion theory, seepage-diffusion theory, nonlinear gas flow, gas flow under the effect of a geophysical field, and gas leakage theory in multiple coal seams. The representative permeability models with a single borehole considering elastic deformation 28,29 include Seidle-Huitt, [30][31][32] Palmer-Mansoori, 33,34 Shi-Durucan, 35,36 Cui-Bustin, 37,38 and Robertson-Christiansen models. Based on the adsorption mechanism, numerous scholars have established different models such as the Langmuir model, 16,17 Brunauer-Emmett-Teller (BET) model, 18 Polanyi adsorption potential, [19][20][21][22] Langmuir-Freundlich adsorption model, 23,24 micropore filling theory, 25,26 and Freundlich Equation.…”

“…At present, the permeability of coal seams is investigated mainly through permeability model with a single borehole for elastic deformation, permeability model with dual boreholes considering elastic deformation of coal blocks, etc. The representative permeability models with a single borehole considering elastic deformation 28,29 include Seidle-Huitt, [30][31][32] Palmer-Mansoori, 33,34 Shi-Durucan, 35,36 Cui-Bustin, 37,38 and Robertson-Christiansen models. [39][40][41] These models only explore the influence of pore pressure on permeability and porosity on the premise that the external stress remains constant.…”

An Elastoplastic Softening Damage Model for Hydraulic Fracturing in Soft Coal Seams

“…Hence, conventional methods cannot effectively drain gas from those seams [1]. e parameters of permeability and porosity play an important role in the study of accumulation and development of coalbed methane [2][3][4]. To increase the permeability of those coal seams, underground hydraulic fracturing technology to transform the seam structure is an effective way to achieve this goal [5][6][7][8][9][10][11].…”

Thermodynamic models for swelling of unconfined coal due to adsorption of mixed gases

“…The limited quality of fit could be due to four possible reasons: (1) insufficient data points lead to significant errors in the values of the fitting parameters C s and K obtained using the method employed; (2) the V 0 values, which we obtained from sorption isotherm and swelling data for similar but nonetheless different Bowen Basin coal, might lead to errors; (3) the experimental swelling data presented by Day et al [14] pertain to rather large samples (30 mm  9 mm  9 mm) that might not have fully equilibrated with respect to changing gas mixture composition, leading to low measured swelling values; (4) interactions between CO 2 and CH 4 in the sorption process might be not negligible, though we assume them to be so in our modeling approach [18,31]. Under point 3 above, we do not wish to imply any shortcomings in the approach adopted by Day et al in their truly excellent work, but note that numerous previous experiments show incomplete equilibrium effects caused by either long equilibration time [14][15][16] or the change of transport path due to swelling effect [31][32][33]. This means that more experimental data using samples of different sizes and longer equilibration times are required to evaluate our models thoroughly.…”