Coalbed methane (CBM) recovery is attracting global attention due to its huge reserve and low carbon burning benefits for the environment. Fully understanding the complex structure of coal and its transport properties is crucial for CBM development. This study describes the implementation of mercury intrusion and μ-CT techniques for quantitative analysis of 3D pore structure in two anthracite coals. It shows that the porosity is 7.04%–8.47% and 10.88%–12.11%, and the pore connectivity is 0.5422–0.6852 and 0.7948–0.9186 for coal samples 1 and 2, respectively. The fractal dimension and pore geometric tortuosity were calculated based on the data obtained from 3D pore structure. The results show that the pore structure of sample 2 is more complex and developed, with lower tortuosity, indicating the higher fluid deliverability of pore system in sample 2. The tortuosity in three-direction is significantly different, indicating that the pore structure of the studied coals has significant anisotropy. The equivalent pore network model (PNM) was extracted, and the anisotropic permeability was estimated by PNM gas flow simulation. The results show that the anisotropy of permeability is consistent with the slice surface porosity distribution in 3D pore structure. The permeability in the horizontal direction is much greater than that in the vertical direction, indicating that the dominant transportation channel is along the horizontal direction of the studied coals. The research results achieve the visualization of the 3D complex structure of coal and fully capture and quantify pore size, connectivity, curvature, permeability, and its anisotropic characteristics at micron-scale resolution. This provides a prerequisite for the study of mass transfer behaviors and associated transport mechanisms in real pore structures.

Stress-dominated coal and gas outburst disaster has become one of the main safety problems in deep coal mines. Acoustic emission (AE) or microseismic technology has been viewed as a promising method that can effectively reflect the stress and stability status of rock mass. The AE activity precursor of coal failure is the theoretical basis of this technology. In this study, AE experiments in failure process of coal specimens with different properties and under different stress conditions were performed in laboratory to explore influence factors and their effect of AE activity, and AE activity pattern classification was proposed based on the failure type of coal. The results indicate that the AE activity of different coals under loading are associated with the failure phase, and the evolution pattern of AE activity depends on the failure type of stressed coal. Both the mechanical property and the external stress condition have an important influential effect on the failure type and AE activity pattern in coal failure process. The internal mechanical property decides the inherent tendency of stressed coals to perform brittle or ductile behavior, and the responded AE activity pattern. The contrast of fissure distribution of specimens suggested that fissure structure in coal significantly affects the fracturing mode of coal in uniaxial compression and the AE activity pattern. The external stress condition has a transition effect on AE event energy distribution and AE activity pattern. Under the effect of external stress condition, the energy distribution of AE events was transforming between relative disperse and relative concentration, the failure type and AE activity evolution pattern of coal could appear the brittle-ductile transition. Based on the view of failure type, the pattern of AE activity of coal failure can be classified into three types, i.e., ductile, brittle, and semi-brittle pattern. It is suggested that the high-level AE activity can be viewed as the precursor of brittle instability of coal, and relative quiet phenomenon of AE activity as the precursor of ductile or semi-brittle instability. The research achievement can provide a theoretical base for the prewarning criteria establishment of coal and rock dynamic disasters at depth and improve the insight of AE activity in the coal failure process.

Underground mining in coal mines causes strong disturbance to geological structures and releases a large amount of elastic strain energy. When the roof is a hard and thick rock layer, it is easy to cause dynamic disasters such as rock burst. To analyze the impact of a deep thick and hard roof fracture on the safe mining of thick coal seams, this paper studied the dynamic evolution process of the stress field, displacement field, energy field, and plastic zone of the coal seam and overlying strata during the mining process using FLAC3D numerical simulation. The results show that as the working face continues to be mined, the concentrated stress in the overlying strata first increases and then decreases, and the support pressure in front of the working face continues to increase. When it advances to 100 m, collapse occurs, and the stress increases sharply; the bottom plate undergoes plastic failure, resulting in floor heave. The overlying strata mass in the top plate exhibits downward vertical displacement, while the rock mass in the bottom plate exhibits upward vertical displacement, with a maximum subsidence of 4.51 m; energy concentration areas are generated around the working face roadway, forming an inverted “U” shape. When collapse occurs, the energy density decreases slightly; the direction of the plastic zone changes from “saddle shaped” to complete failure of the upper rock layer, and the overlying strata is mainly shear failure, which expands with the increase in mining distance. The research results have important practical significance for guiding the safe mining of deep thick and hard roof working faces.

With the increase of mining depth, the occurrence mechanism of coal and gas outburst becomes more complex, and stress‐dominated outburst disaster has become one of the major safety problems in deep coal mines. Therefore, stress relief of coal seam has become an urgent technical demand for outburst prevention method in deep mines. In this study, based on zonal disintegration characteristics of deep surrounding rock, a regional strip outburst prevention method, called strip stress relief combined with gas extraction through stress‐relief floor roadway, was proposed, and relevant in‐site investigations were carried out in the Qujiang coal mine in China. Our results show that the zonal disintegration phenomenon occurs in the high‐stress surrounding rock at great depths, and induces the stress‐relief effect on the upper coal seam, in which the permeability and extraction radius of boreholes are increased greatly. The results of deformation and failure of surrounding rock of floor roadway show that from the surface to deep regions, fractured and nonfractured zones were alternatively distributed in the surrounding rock of deep floor roadways, the damage degree of surrounding rock was gradually weakened, and the width of fractured area was gradually reduced. The displacement of surrounding rock also presented the characteristics of alternative distribution of wave peak and trough. Moreover, the deformation and failure of surrounding rock presented a time‐dependent rheological behavior. In the stress‐relief zone, the coal seam permeability was enhanced significantly, with the maximum value being 43 times larger than that of the original coal seam. The effective gas extraction radius of the borehole was improved by 15%‐45%. During the driving period of coal roadways, the outburst risk prediction indexes K₁ and S of the working face were lower than the critical value, and the driving speed of the coal roadway improved from 40 m per month to 100 m per month. This indicates that the proposed strip regional outburst prevention method can simultaneously reduce the ground stress and the gas content, and can effectively eliminate the outburst risk of coal roadway in deep mines.

Real-time drilling depressurization technology is widely used in the prevention and control of dynamic disasters, such as deep-seated rock burst. However, current coal- and rock-loading tests under drilling conditions seldom account for real-time issues associated with drilling, thus failing to fully reflect the actual stress state of the surrounding rock during the implementation of drilling depressurization technology. Therefore, this study designed and implemented a uniaxial loading scheme for coal samples incorporating real-time-drilling characteristics. The results indicate a significant reduction in the uniaxial compressive strength ($R_{\mathrm{C}}$), elastic energy index ($W_{\mathrm{ET}}$), and impact energy index ($K_{\mathrm{E}}$) of the samples post-drilling. These parameters show a clear decreasing trend with increasing axial stress during real-time drilling. The weakening effect of impact tendency following real-time drilling depressurization is significant, and the depressurization effect is pronounced. The $R_{\mathrm{C}}$, $W_{\mathrm{ET}}$, and $K_{\mathrm{E}}$ of each real-time-drilled sample exhibit a notable decrease with increasing drilling stress, with the reduction rate significantly diminishing after the drilling stress reaches 20% of the peak strength.

In order to study acoustic emission (AE) evolution characteristics of coal with different mechanical properties in failure process, uniaxial compression experiments of coals from 4 mines were carried out to analyse fractal feature of AE time series by G-P algorithm. The results indicate that AE parameter series of all 4 different coals have fractal feature, and the fractal dimension value of coal with different properties go through a process of "first rise, then fall". The change of AE fractal dimension value can reflect the cracking evolution in coal failure process, which is closely related with the failure phase. The continuous decline of AE fractal dimension can be viewed as a precursor of impending failure of coal, which could provide theoretical basis for the AE pre-warning model establishment of coal dynamic disaster.