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• Thin-interbed composite reservoir has high permeability characteristics. • Cross-layer seepage effect reflects the advantage of thin-interbed permeability. • The coupling effect of fracture and pore changes the seepage mode of the fluid. • The sedimentation-tectonics control permeability prediction model is constructed. The successful development of coal measure gas (CMG) in the Surat Basin in Australia shows that the geological conditions of thin-interbed CMG development may be more favorable, but the above understanding is still in the geological law summary level. This study combines numerical simulation and physical simulation experiments to study the core geological problems of CMG in the geological control of the permeability of a multi-thin coal seam composite reservoir (MTCCR). The results show that in the initial stage of stress loading, the permeability advantage of the coal-mudstone/sandstone composite reservoir is reflected in the bedding permeability. Based on this phenomenon, the concept of "cross-layer seepage effect of fluid in MTCCR" is proposed for the first time, which is analyzed as the result of the permeation effect of adjacent rock fluids in the composite reservoir under the action of pressure difference. The numerical simulation shows that the permeability increase of the MTCCR depends on the increase in the fracture, and the coupling effect of the fracture and pore changes the fluid pressure field and seepage mode around the fracture. From the experimental perspective, the above understanding confirms the results of previous conclusions on the geological law of high permeability of MTCCR, and explains the geological phenomenon of high permeability of MTCCR from the seepage mechanism. Based on numerical and physical simulation experiment, a sedimentation-tectonics controlled permeability prediction model was constructed. The seepage mechanism and the permeability prediction model established of MTCCR in this study can provide theoretical support and quantitative basis for the selection of high permeability zones and evaluation of high permeability sections in the exploration and development process of thin interbed CMG. [ABSTRACT FROM AUTHOR]

• The micropores of fusain are extremely developed, especially the pores with D = 0.33–0.38 nm. • Aromatic interlamellar pores are important components of the fusain micropore structure. • The Langmuir volume (V L) under dry conditions is much larger than that under wet conditions. • The micropore volume and surface area of fusain did not positively correlate with the V L. • The blocking effect of water molecules resulted in the decrease of methane adsorption. Fusain is widely distributed in coal seams and studying its porosity and adsorption capabilities helps clarify the physical properties of coal reservoirs and gas adsorption mechanisms. The microporous structure characteristics and gas adsorption mechanism of fusain in lignite were studied based on methane and CO 2 adsorption experiments, combined with nuclear magnetic resonance (13NMR) and high-resolution transmission electron microscopy (HRTEM) measurements. Numerous micropores were found in the sample, and the pore volume and specific surface area were relatively large, which is similar to the results of the micropore test for meta-anthracite. The adsorption capacity of fusain was higher under dry base conditions but significantly lower under air-dry base and equilibrium water conditions. The chemical structure analysis results indicate that the micropores of fusain are mainly the interlamellar pores of the aromatic layers and the pore channels formed by them. Under dry conditions, the water in the microporous structure was removed, which provides an effective space for gas adsorption, resulting in more micropores for CO 2 adsorption and a higher methane adsorption capacity. Under wet conditions, the water occupies the adsorption site of methane, which hinders the gas entering the microporous channel of the fusain, resulting in a low adsorption capacity. On comparing the pore structure characteristics of fusain and meta-anthracite, it was found that despite possessing similar chemical structure characteristics of high polycondensation and similar micropore structure, there were clear differences in pore size, and their adsorption properties for methane were different. [ABSTRACT FROM AUTHOR]

• A stress distribution model for hydraulic fracturing in coal is developed. • The deformation–failure linkage law of coal fracturing is revealed. • The entire fracture process of the columnar coal sample is recovered. The continuous injection of fracturing fluid results in a redistribution of stress in the area affected by fracturing, which promotes the deformation and failure of coal. To reveal the distribution law of water pressure in coal and the deformation properties of the coal matrix induced by fracturing. Based on triaxial compression experiments and hydraulic fracturing physics simulations, we have developed a fracture stress distribution model for cylindrical coal samples by combining the principles of elastic mechanics. The concept of the strain ratio (S) is proposed, and its physical significance is explained. Combining the strain, crack propagation and acoustic emission (AE) energy of the coal sample during fracturing, we have recovered the entire deformation and failure process of columnar coal samples. It was shown that S has a negative logarithmic dependence on the confining pressure and an exponential dependence on the stress difference (Δ σ) and fracturing rate (v w). The deformation and failure process of hydraulic fracturing is dominated by injection pressure. At high Δ σ or high v w , coal samples deform and fracture along the direction of the minimum principal stress as the water pressure increases in stage III. During the fracture, the injection pressure is linearly and positively correlated with the radial stress but negatively correlated with the axial stress. The stress concentration is more pronounced near the borehole, which is the starting point for damage to coal samples. Hydraulic fracturing increases the complexity and connectivity of fractures, which in turn tend to simplify under the control of the maximum principal stress as v w increases. The AE energy of hydraulic fracturing has a linear and positive correlation with S max. After Δ σ and v w are determined, the deformation and failure process of the cylindrical coal sample during fracture is characterized by the combination of strain, fracture propagation and AE energy. [ABSTRACT FROM AUTHOR]

• The difference in pore characteristics between vitrain and durain initially increases and then decreases with the increase in pore size. • Physical compaction considerably reduces the pore volume of the vitrain during the early evolution stage of high-volatile bituminous coal but has no effect on the durain. • The pore volume and specific surface area of vitrain and durain in the high-volatile bituminous coal reach their maximum near the first coalification jump point. The coal type of high-volatile bituminous coal (HBC) is an important component of coalbed methane (CBM) reservoirs in China. Moreover, the accurate characterization of pore structures of coal is crucial to the exploration and development of CBM. In this work, the pore structure of vitrain and durain in high-volatile bituminous coals was detected using a scanning electron microscope, mercury intrusion porosimetry, low-temperature N 2 adsorption, and low-temperature CO 2 adsorption across multiple length scales and the effect of coalification and maceral on pore differential development law of HBC was analyzed. The findings reveal that at different pore size scales and the pore characteristics of different coal lithotypes under the influence of coalification are clearly different. In the same sample, the specific surface area (SSA) and total pore volume (TPV) of supermicropores in vitrain are higher than those of durain but the SSA and TPV of micropores and transition pores in vitrain are lower. The fractal results reveal that with the increase in pore size, the complexity of pore structure of coal increases. The relationship between the vitrain and durain pore fractal dimensions in different scale sections will change. At different pore size scales, the difference in pore structure in the vitrain and durain exhibits a trend of initially increasing and then decreasing with pore size. The inertinite content is positively correlated with the adsorption pore volume and SSA of vitrain and durain, and the vitrinite content is negatively correlated. Physical compaction considerably reduces the pore volume of vitrain during the early evolution stage of HBC but has no effect on durain. The pore parameters of vitrain and durain initially increase and subsequently decrease with the increase in vitrinite reflectances. The maximum value is obtained near the first coalification jump point. The evolution trend of the pore volume of vitrain and durain is consistent at different pore size scales; however, the total variation range of durain is higher than that of vitrain. The adsorption pore structure is mainly affected by asphaltification and polycondensation, whereas physical compaction has a greater impact on the seepage pore structure. [ABSTRACT FROM AUTHOR]

• The water saturation of medium rank coal is lower than that of high rank coal. • A calibration method of saturable pore diameter (d *) is established. • The coal rank has a power relationship with the d *. • When the d * is between 1 and 10 nm, the S w typically exceeds 60%. The fluidity of medium-high rank coal reservoirs is different at the microscopic scale, which can lead to differences in coalbed methane (CBM) production. Taking the medium-high rank coal reservoirs in eastern Yunnan and western Guizhou as examples, the characteristics of atmospheric-variational pressure-saturated water based on nuclear magnetic resonance (NMR) technology were studied. The results show that under atmospheric pressure and vacuum pressure, the water saturation (S w) of medium rank coal is lower than that of high rank coal; the former is less than 50%, and the latter is more than 60%, which is attributed to the higher developed micropores and hydrophilicity of high rank coal. With an increase in the saturation pressure, the S w of medium-high rank coal is not evident. A conversion method of the T 2 spectrum and the low-temperature liquid nitrogen average pore size are proposed. Thus, the calibration method of the minimum pore size that water can enter under atmospheric-variational pressure conditions is established, and the saturable pore diameter (d *) under atmospheric-variational pressure conditions is obtained. Under natural atmospheric pressure, water can enter the micropores and transition pores (pore diameter d < 100 nm). The higher the coal rank is, the smaller the d * is. Most of the high rank coal can enter the micropores (pore diameter d < 10 nm). As the saturation pressure increases, the d * of the medium-high rank coal gradually decreases. Under different saturation conditions, the coal rank has a power relationship with the d *. As the d * decreases, the S w increases. [ABSTRACT FROM AUTHOR]