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The preparation of suitable specimens is important for obtaining credible mechanical and methane migration parameters for tectonic coal, which help to guide methane extraction and disaster prevention. In this study, a dual-deformation mechanism for porous media was introduced along with two powder compression models, and the issues that should be considered in the preparation of coal specimens were analyzed. By compression tests, the relationship between bed relative density and the applied stress in the compression of coal particles was obtained. The method of coal specimen preparation was introduced in detail. The results indicated that the Kawakita model is suitable for describing the compressive process of tectonic coal powders and guiding the preparation of tectonic coal specimens. The key parameters a and b in the Kawakita model are 0.411 and 0.108, respectively. The bed relative density shows a slight increasing trend followed by an obvious rising tendency with an increase in the applied stress. A compressive stress of 150 MPa was determined to be suitable for preparation of the tested coal specimens.

The diffusion coefficient of methane in coal is a key parameter that influences the methane extraction effect. Former studies on the methane diffusion coefficient mainly paid attention to the intact coal. The diffusion coefficient of tectonic coal is different from that of intact coal, which may be caused by the effect of tectonism on the pore characteristics. In this study, the pore characteristics of the intact coal and tectonic coal were tested using mercury intrusion porosimetry (MIP) and the physisorption method, while the counterdiffusion experiments were adopted to measure the methane diffusion coefficients of both coal samples. The results indicate that the methane diffusion coefficient of the tectonic coal is higher than that of the intact coal, and the diffusion coefficients decrease with the increasing methane pressure. The tectonism increases the volumes of mesopores and macropores and the porosity of coal, which is the main reason for the higher methane diffusion coefficient of the tectonic coal. The pore structure of coal becomes more winding or complicated by tectonism, thus showing a higher tortuosity of the tectonic coal. The tectonism makes it easier to extract methane, and coal mining in the tectonic region should receive more attention.

The establishment of the equivalent structural model (ESM) is the foundation to simplify the structures of the coal matrix and fractures for the study of methane migration. The ESM of the intact coal has been proposed and widely used by many scholars, but the ESM of the tectonic coal has not been found in the literature. The application of the ESM of the intact coal to the tectonic coal is not reasonable for the study of the methane migration properties, so the establishment of the ESM for the tectonic coal is necessary and meaningful. In this study, a tectonic coal specimen was remodeled from collected coal powders, and methane permeability tests were conducted. Then the ESM of the tectonic coal was established by analyzing the fracture structure. The results show that the tectonic coal can be idealized as a model containing a cubic matrix with square section fractures located at the twelve edges of the matrix. Because of the variation of the ESM, the permeability and the fracture porosity has a square relation for the tectonic coal, rather than the cubic relation for the intact coal. The matrix shape factor of the tectonic coal has also been proposed, which has a value of 480bf/7a3m and is much smaller than that of the intact coal.

Coal spallation is a visible product of outburst, and it represents a unique failure type for coal. A theoretical analysis of coal spallation was constructed to study its formation mechanism. Then, physical experiments and numerical simulations were performed to determine the physical features of coal spallation and evaluate the theoretical analysis. In addition, the effect of gas, stress and coal strength on coal spallation was discussed. The results show that coal spallation is a comprehensive product of multiple physical factors, including stress, gas and the coal strength. The formation process is related to stress transfer and mechanical behavior changes, gas migration and the evolution of the coal strength. Coal is eventually subjected to tensile failure, and the mechanical condition for coal spallation requires that the gas pressure difference ( p i − p a ) near the exposed surface is enough to overcome the tensile strength ( σ t ). The gases in coal provide the power necessary for coal spallation; thus, coal gas promotes plastic failure and is the leading factor for tensile failure. Stress is the leading factor for plastic failure and has a great influence on the extent of coal spallation. Meanwhile, with an increase in the coal strength, which represents the resistance to coal spallation, the acreage of the plastic area decreases, and thus, a greater pressure difference is required for tensile failure.

Multiple coal seams widely develop in the deep Chinese coal-bearing strata. Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe. When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams. The safety and economics of exploited first-mined coal seams are tremendously restricted. Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult. Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane (CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs. Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction. In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam. During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods (combination of underground and ground CBM extraction methods). A typical integrated reservoir reconstruction technology, “One well for triple use”, was detailed introduced and successfully applied in the Luling coal mine. The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction. Keywords: Reservoir reconstruction, Coalbed methane, Multiple seam, Surface well, Gas drainage

Researches on the failure process of coal during coal and gas outbursts are of great importance in underground mining. Based on the theories of coal spherical shell failure, coal spallation and coal powder pneumatic conveying, the flow state and transport mechanism of coal and gas outbursts were studied. This paper concludes that in the outbursts' development stage, with the front of the outburst being gradually exposed to air, the gas gradient behind the outburst acts as a reciprocating air knife of high pressure that cuts the ejected coal powders, thus forming a plug flow with a high solid-gas ratio and a low speed. By referring to the laboratory results from other researchers and data recorded from field outbursts, an appropriate explanation was obtained for the wave-like coal powder distribution and the sound explosion using the dense-phase plug flow pneumatic conveying theory, which conversely confirms the possible existence of the plug flow.

A coal and gas outburst can lead to a catastrophic failure in a coal mine. These outbursts usually occur where the distribution of coal seam gas is abnormal, commonly in tectonic belts. To study the effects of the abnormal distribution of this gas on outbursts, an experimental apparatus to collect data on simulated coal seam outbursts was constructed. Experiments on specimens containing discrete gas-enriched areas were run to induce artificial gas outbursts and further study of these outbursts using data from the experiment was conducted. The results suggest that more gas and outburst energy are contained in gas-enriched areas and this permits these areas to cause an outburst easily, even though the gas pressure in them is lower. During mining, the disappearance of the sealing effect of a coal pillar establishes the occurrence conditions for an outburst. When the enriched gas and outburst energy in the gas-enriched area is released suddenly, a reverse unloading wave and a high gas pressure gradient are formed, which have tension effects on the coal. Under these effects, the fragmentation degree of the coal intensifies and the intensity of the outburst increases. Because a high gas pressure gradient is maintained near the exposed surface and the enriched energy release reduces the coal strength, the existence of a gas-enriched area in coal leads to a faster outburst and the average thickness of the spall is smaller than where is no gas-enriched area.

The method of explosion venting is widely used in industrial explosion-proof design due to its simple operation, economical and practical features. A dump vessel vented platform was built. By changing the vacuum level and the gas in the dump vessels and the structural size of linked vessels, the pressure in the explosion vessel and the dump vessel was compared, and the influencing factors of explosion venting investigated. The main conclusions are as follows: In the explosion venting process, the higher the vacuum in the dump vessel, the smaller the pressure peak of the explosion vessel and the dump vessel, and the faster the explosion pressure is lowered. When the dump vessel is under the same vacuum level and the gas in the dump vessel is CO2, the maximum pressure of the explosion vessel and the dump vessel is less than the maximum pressure when the containment medium is air. Under the same vacuum condition, the larger the volume ratio of the dump vessel and the explosion vessel, the smaller the pressure peak of the explosion vessel, the faster the explosion pressure drops, and the volume of the dump vessel reaches or exceeds the explosion vessel. Increasing the volume ratio of the containment vessel to the explosion vessel facilitates protection of the explosion vessel and the containment vessel. Under the same vacuum condition, when the gas explosion in 113 L vessel vents into 22 L vessel, the longer the length of the pipe, the greater the maximum pressure in the spherical vessel. When the gas explosion in 22 L vessel vents into 113 L dump vessel, as the pipeline grows, the maximum pressure in the two vessels decreases, but the reduction is not significant. In practical application, it is recommended to use a vacuum of 0.08Mpa or more for the dump vessel vented, and the containment medium is CO2.In terms of the structural size of the container, it is recommended that the ratio of the receiving container to the explosion container be as large as possible, and the pipe length be as long.

The mining of protective coal seams can cause changes in geostress, leading to changes in the permeability of coal rock and creating favorable conditions for gas extraction from coal seams. At the Haishiwan Coal Mine, field tests using remote protective coal seam mining were performed in the protected layer, which is rich in CO2 gas. In remote protective longwall mining, the permeability and composition of extracted stress-relief gas can vary. Under the conditions of remote protective longwall mining, the permeability of a protected coal seam can be generally described by the Liu model. During protective layer mining, the permeability of the protective layer increases rapidly with the release of stress, then decreases gradually with the recovery of the geostress. However, matrix shrinkage and decreased pore pressure caused by CO2 desorption from coal seams also cannot be ignored when considering the factors that affect the permeability. Thus, it is necessary to appropriately configure the cross-measure boreholes in advance to drain the stress-relief gas during remote protective layer mining. Stressrelief CO2 gas extraction presents multiple consecutive peaks. The No. 2 coal seam has different trap pressure systems as CO2 migrates into the coal seam. The protected seam experiences different effective stresses during protective layer mining, and the permeabilities appear to periodically increase due to differences in the original permeability. The various permeability and diffusion coefficients for CO2 and CH4 in coal induce CO2 and CH4 fractionation in the roof and floor of the No. 2 coal seam.

The permeability of coal is an important parameter in mine methane control and coal bed methane exploitation because it determines the practicability of methane extraction. We developed a new coal permeability model under tri-axial stress conditions. In our model, the coal matrix is compressible and Biot’s coefficient, which is considered to be 1 in existing models, varies between 0 and 1. Only a portion of the matrix deformation, which is represented by the effective coal matrix deformation factor \(f_\mathrm{m}\), contributes to fracture deformation. The factor \(f_\mathrm{m}\) is a parameter of the coal structure and is a constant between 0 and 1 for a specific coal. Laboratory tests indicate that the Sulcis coal sample has an \(f_\mathrm{m}\) value of 0.1794 for \(\hbox {N}_{2}\) and \(\hbox {CO}_{2}\). The proposed permeability model was evaluated using published data for the Sulcis coal sample and is compared to three popular permeability models. The proposed model agrees well with the observed permeability changes and can predict the permeability of coal better than the other models. The sensitivity of the new model to changes in the physical, mechanical and adsorption deformation parameters of the coal was investigated. Biot’s coefficient and the bulk modulus mainly affect the effective stress term in the proposed model. The sorption deformation parameters and the factor \(f_\mathrm{m}\) affect the coal matrix deformation term.

Coal and gas outbursts are among the most destructive dynamic disasters. It is very important to study the failure process and mechanism of coal and gas outbursts in coal mines. Based on outburst cases and outburst experiments, the outburst process and failure types of coal are discussed, and the flow process of outburst coal is studied in depth. This paper concludes that the whole process of outburst generally involves four stages (preparation, trigger, development and termination), which include both time and spatial dimensions. There are two types of coal failure: shear failure and tensile failure. Under the effect of the gas pressure gradient, the coal exhibits tensile failure, and the coal ultimately fails in the form of multiple similar layers. After the layer is separated, the pressure difference between the two sides of the layer is found to be stable for a period of time, and a pressure difference coefficient is introduced to characterize the change of this pressure difference before and after the separation of the layer. Furthermore, the flow process of outburst coal is divided into a quasi-static phase, a constant acceleration phase, a freefall phase and a deceleration phase. Accordingly, a simplified model of the coal flow is established, and the model is calculated using the data of outburst experiments. Based on the layered failure of outburst coal, the flow velocity of outburst coal is obtained. The distribution features of outburst coal in both the experiments and the outburst cases are explained as well.

The coal permeability is an important parameter in mine methane control and coal bed methane (CBM) exploitation, which determines the practicability of methane extraction. Permeability prediction in deep coal seam plays a significant role in evaluating the practicability of CBM exploitation. The coal permeability depends on the coal fractures controlled by strata stress, gas pressure, and strata temperature which change with depth. The effect of the strata stress, gas pressure, and strata temperature on the coal (the coal matrix and fracture) under triaxial stress and strain conditions was studied. Then we got the change of coal porosity with strata stress, gas pressure, and strata temperature and established a coal permeability model under tri-axial stress and strain conditions. The permeability of the No. 3 coal seam of the Southern Qinshui Basin in China was predicted, which is consistent with that tested in the field. The effect of the sorption swelling on porosity (permeability) firstly increases rapidly and then slowly with the increase of depth. However, the effect of thermal expansion and effective stress compression on porosity (permeability) increases linearly with the increase of depth. The most effective way to improve the permeability in exploiting CBM or extracting methane is to reduce the effective stress.