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Low gas permeability of coal seams is a key factor restricting effective prevention and control of gas disasters and high-efficient exploitation of gas resources. Hydraulic slotting technology can ...

The coal’s permeability plays a crucial role in coalbed methane (CBM) extraction and coal seam CO2 sequestration. An accurate understanding of permeability rebound and recovery is therefore essential. This study establishes an improved fully coupled gas migration model for CBM extraction. The permeability rebound and recovery times as well as rebound values are proposed to accurately quantify permeability evolution during CBM extraction. The evolution of these three parameters under the influence of different factors are evaluated in detail, such as initial gas pressure, the diffusion coefficient, and the permeability. The results show that the permeability rebound and recovery times increase along with initial gas pressure and the amount over time rises rapidly under high gas pressures. As the initial gas pressure increases, the permeability rebound value decreases. However, initial diffusion coefficient and permeability have a negative trend in permeability rebound, recovery time, and rebound value. These tendencies are particularly large for low initial permeabilities and diffusion coefficients, yet the change in rebound time is smaller than the one in recovery time. Finally, inspired by the relationship between permeability rebound and gas pressure change during CBM extraction, the evolution of coal seam permeability under different CO2 injection method is discussed. A stepwise increasing-pressure CO2 injection method is also proposed, which could effectively increase the volume of CO2 sequestered and reduce project costs. Therefore, our findings shall shed light on improving coal mine safety production and reducing greenhouse gas emission.

It is very important and effective to characterize the failure mechanism of coal and rock materials from the perspective of energy. Energy dissipation and release are important causes of rock mass failure. To qualitatively and quantitatively characterize the energy evolution process of gas-bearing coal and the energy evolution mechanisms of coal and gas outburst, experimental studies were conducted on gas-bearing coal under different stress paths. The research results demonstrate that the energy evolution of gas-bearing coal has nonlinear characteristics and that the energy density is a quadratic function of stress. The total energy density is mainly stored as elastic energy density and is unrelated to loading paths. According to the distribution characteristics of abutment pressures in a stope, the energy distribution can be divided into an energy dissipation and release zone, an increasing-energy zone, and a stable energy storage zone. During coal mining, abutment pressure in front of the coal wall is an important factor causing energy accumulation, and the stress concentration area is a key area to prevent and control dynamic disasters of coal and rock masses. Furthermore, based on the principle of energy conservation, this study established an energy balance model for coal and gas outburst and derived energy criteria for coal and gas outburst. When $$\Upsilon > 1$$ , there is a risk of coal seams have outburst. If $$\Upsilon = 1$$ , the system is in a state of limit equilibrium, and mining disturbances could trigger outburst.

On the premise that the point of the maximum abutment pressure is the demarcation point of gas migration, an experimental research was conducted on gas-bearing coal by using a thermo-fluid–solid coupling test system. The research results demonstrate that irreversible strain and elastic modulus increase with increase in unloading level. The damage variable defined based on change laws of permeability rises with unloading level. More importantly, the evolution process of coal-and-gas outburst according to gas migration laws of each structure can be re-described after the gas migration laws are structured from the perspective of permeation, and the corresponding criteria for initiation of outburst are established. Besides, the factors influencing outburst can be classified into basic, key and induction factors. The basic factors degrade the strength of coal mass, the key factors make coal and rock mass to be a critical mechanical equilibrium state, and the induction factors break through such an equilibrium state and induce outburst.

With increasing demands for coal resources, coal has been gradually mined in deep coal seams. Due to high gas content, pressure and in situ stress, deep coal seams show great risks of coal and gas outburst. Protective coal seam mining, as a safe and effective method for gas control, has been widely used in major coal-producing countries in the world. However, at present, the relevant problems, such as gas seepage characteristics and optimization of gas drainage borehole layout in protective coal seam mining have been rarely studied. Firstly, by combining with formulas for measuring and testing permeability of coal and rock mass in different stress regimes and failure modes in the laboratory, this study investigated stress–seepage coupling laws by using built-in language Fish of numerical simulation software FLAC3D. In addition, this research analyzed distribution characteristics of permeability in a protected coal seam in the process of protective coal seam mining. Secondly, the protected coal seam was divided into a zone with initial permeability, a zone with decreasing permeability, and permeability increasing zones 1 and 2 according to the changes of permeability. In these zones, permeability rises the most in the permeability increasing zone 2. Moreover, by taking Shaqu Coal Mine, Shanxi Province, China as an example, layout of gas drainage boreholes in the protected coal seam was optimized based on the above permeability-based zoning. Finally, numerical simulation and field application showed that gas drainage volume and concentration rise significantly after optimizing borehole layout. Therefore, when gas is drained through boreholes crossing coal seams during the protective coal seam mining in other coal mines, optimization of borehole layout in Shaqu Coal Mine has certain reference values.

Energy is an important research parameter in rock mechanics. To explore the law of energy evolution of coal, a one‐time loading and unloading test under uniaxial compression was conducted on coal taken from four different coal mines. By utilizing the area integral method, the total input, elastic, and dissipated energy densities in coal at different unloading levels were calculated. The correlation between various energy densities and the evolution law of energy density with different unloading levels was attained. The peak strengths of the coal slowly declined with an increasing unloading level, which conforms to the relation of a linearly decreasing function. The energy dissipation rate has nonlinear characteristics, and the shape of the dissipation rate fitting curve changed from an upper concave to a downward concave with increasing strength. In all the coal samples, the energy density grew nonlinearly with the unloading level. Moreover, the growth rate of the total energy density was the highest, followed by the growth rates of the elastic and dissipated energy densities. All ratios of the elastic and dissipated energy densities to the total input energy density and the ratio of the dissipated energy density to the elastic energy density were constant. As the strength increased, the input energy and the elastic energy density increased at a faster rate, and they observed the same law. There is an insignificant relationship between the degree of destruction of the coal and the level of unloading. Energy is a major factor that drives the failure of a test piece, but this is not the main factor that determines the degree of damage to the test piece.

Sealing of boreholes for gas drainage is crucial for ensuring efficient gas drainage in coal seams. In this paper, the damage to coal masses where roadways were excavated and boreholes were drilled was separately simulated to determine the distribution ranges of pressure-relief plastic and elastic zones. Furthermore, the factors influencing air leakage of boreholes were analyzed to determine three patterns of air leakages-air leakage, air leakage, borehole fissure due to roadway fissure zones, borehole fissure zones and materials in borehole-sealing sections, respectively. In addition, a model for the air leakage of boreholes was established, and the principle underlying the sealing of boreholes with sealing–isolation integration was illustrated to separately determine the reasonable borehole-sealing depth, the position and depth of slots, and the grouting parameters. The borehole-sealing techniques for different parameters were compared to verify the applicability of the borehole-sealing technique with sealing–isolation integration. The gas-drainage concentrations after drilling for 30 days showed that, compared with the test borehole sealed using the traditional technique, the initial gas-drainage concentration in the boreholes sealed with sealing–isolation integration increased by 1.5–2 times, and the average gas-drainage concentration increased by approximately 2–3.5 times.

The pore and microcrystalline structure of coal are studied by combining field emission-scanning electron microscopy (FE-SEM), nuclear magnetic resonance (NMR), and Raman spectroscopy to better understand the physicochemical evolution of bituminous coal treated by high-voltage electric pulses (HVEP) under different capacitance conditions. Experimental results show that the capacitance has no significant impact on the breakdown voltage of the coal body during HVEP treatment. However, increasing the capacitance can effectively increase the porosity of coal cores. In particular, the influence of increasing the capacitance on the macropores is obviously more significant than that on the micropore; besides, increasing the capacitance improved connectivity between the mesopores and macropores. In addition, following HVEP treatment, the microcrystalline defect of coal became smaller and pore arrangement more orderly. Thus, the larger the capacitance is, the more is the energy injected into the coal and the more obvious the change of the coal microcrystalline structure.

Gas permeability in coal characterizes the migration capacity of gas in coal, and it is an important parameter influencing the gas flow in a coal seam. Energy dissipation and energy release are important causes of rock mass failure. By utilizing a THM-2-type thermo-fluid–solid coupling test system for gas-bearing coal developed at Chongqing University, China, experimental research was conducted on raw coal samples from the Jinjia Coal Mine, Guizhou Province, China, under two different cyclic loading paths. The experimental results demonstrated that the overall change trends of permeability were consistent under the different loading and unloading paths and decreased exponentially with increasing stress. As the unloading level increased, the permeability hysteresis loops changed from sparsely to densely distributed and from spindle-shaped to crescent-shaped and ladder-shaped. The change laws of the upper and lower limits of permeability were closely related to the stress paths. The change trends of the lower limit of permeability under the two paths were consistent with that of the overall permeability and decreased exponentially with the increase in energy density. When the lower limit of unloading stress was constant, the upper limit of permeability gradually increased exponentially with increasing energy density, indicating that the gas permeability in the coal samples had strong hysteresis characteristics. The energy densities under the different paths nonlinearly increased with increasing stress; these relationships could be fitted by quadratic functions. The total input energy density showed the fastest rate of increase, followed by the elastic energy density, while the dissipated energy density had the slowest rate of increase, demonstrating that the energy evolution showed nonlinear characteristics. The experimental results were in good agreement with the evolution characteristics of the permeability of the stress loading and unloading zones in front of the working face. Understanding permeability from the perspectives of different stress loading and unloading paths and energy densities provides certain insights into the safe production and extraction of coalbed methane from coal mines.

Coalbed methane (CBM) production in the overlying strata of coal reservoirs is often hampered by the unknown distribution of the mining-induced fractures. Mining-induced fractures are CBM migration pathways in the fractured overlying strata, and the excavation of coal seams within a mine causes the CBM in adjacent coal seams to flow into the overlying strata. The mining-induced fracture field in the overlying strata is the best place from which this CBM is drained. Here, to better understand the distributions of vertical and horizontal fractures caused by excavation, we propose a novel approach to quantify the dimensions of vertical and horizontal fractures in fractured zones. In addition, we demonstrate that there are negligible changes in the dimensions of horizontal fractures and great changes in the dimensions of vertical fractures when there is an increase in the height of the fractured zone. We further demonstrate that mining-induced angles mainly concentrate on 0°–10°, 50°–70°, 110°–120° and 170°–180°, and larger width fractures exist in both sides and top due to the de-stressed effect and fractures in the middle of model close under mining-induced stress. The approach described here could be used to improve the accuracy of cross-measure borehole positioning and the efficiency of CBM drainage.

Low permeability is the main constraint on the high-efficiency coalbed methane recovery in deep coal seams. Hydraulic slotting has been proved to be a favorable method to stimulate low-permeability coal seams. In this paper, the coal samples with various slot inclination angles and borehole-slot ratios were used to investigate the weakening effect of slot inclination angle and borehole-slot ratio on the mechanical property of the pre-cracked coal. Besides, the crack patterns of the slotted coal specimens were identified to reveal the slot weakening mechanism. It is revealed that the variations in compression strength, elastic modulus and Poisson’s ratio with the slot inclination angle generally conform to a Boltzmann function, logistic function and quadratic function, respectively. With the increase in the borehole-slot ratio, the curve clusters of compression strength and elastic modulus show the horizontal “V” with left opening, and the curve clusters of Poisson’s ratio show the trend of rapid increase after slow increase. Compared with elastic modulus, the slot weakening degrees of compression strength and Poisson’s ratio are more significant. Moreover, the tensile and shear cracks mainly appear in the coal samples with small and large slot inclination angles, respectively, which verify the fact that the slot weakening effect on mechanical property of the slotted coal samples with small slot inclination angles is more significant. The research achievements are attributed to the improvement in the efficiency of the hydraulic slotting-based enhanced coalbed methane recovery.

Mining-induced fracture plays a key role in gas drainage for gas burst-prone underground coal mines, especially for closely multilayered coal seams. The layout and location of the designed gas drainage boreholes have a huge influence on the performance of coal and gas extraction. The Shaqu coal mine exploits the #2 and #4 coal seams, which are gas burst-prone coal seams with low concentration of drained gas. The method of borehole camera observation was used in this study to investigate the evolution of fractures field in order to obtain an optimal borehole design. In addition, a method of pixel extraction and gray value calculation for borehole images was developed to obtain 3D gray value distributions for recorded borehole images. Based on the 3D gray value distribution for a borehole image, the evolution of borehole fractures was identified. Based on the analysis, an area of high-density mining-induced fractures within relatively stable overlying strata was found about 25–30 m above the working face of the protected coal seam. Therefore, it is considered that concentration, amount and stability of gas drainage should be improved if boreholes reach the fracture area to enhance gas drainage efficiency and improve mine safety in extraction of coal and gas.

Cyclic loading widely exists in coal mining activities, and it can significantly change the mechanical and seepage characteristics of coal. In this study, raw gas-bearing coal with different coal rank was mechanically tested under three stress paths: cyclic loading with stepwise increased peak stress (path 1), with step-by-step increased peak stress (path 2) and with crossed peak stress (path 3) using a tri-axial seepage testing machine. The acoustic emission (AE) signals under different loading and unloading paths indicate different mechanical properties of the coal sample. The Kaiser point is not a good indicator of the stress history of coal. The ratios of the quiet effect of the three coal samples under the three stress paths show that loading path 1 can increase defects such as micro-cracks in the coal samples (the AE quiet period decreases), while the other two paths decrease the number of defects (the AE quiet period increases). The cumulative dissipated energy of the coal shows an exponential growth with axial effective stress. The damping coefficient of coal first decreases then increases under cyclic loading. The damage variables can be used to predict the failure of coal samples, regardless of the stress path. Our results provide theoretical support and insight into the permeability increase mechanism and strengthened permeability increase mechanism of coal seams based on cyclic-loading-induced fracturing (repetitive hydraulic fracturing) under multiple protections and gas drainage engineering.

With the extensive application of fully mechanized top coal caving technology with a large mining height, a comprehensive investigation of the ignition mechanism of gas in a goaf induced by the caving of hard and thick sandstone roofs is of great importance. In this paper, the mineral compositions of the roofs of the coal seams in the working face were analyzed, the motion modes during the caving of hard roofs were investigated, and the friction force was calculated. Besides, the generation law of sparks induced by the friction of different rock combinations, the distribution law of gas in the goaf, and the flammable and potential explosive zones were investigated and determined. The research results indicate that the range of impact velocity and friction force induced by a roof caving are large under different roof caving modes. The spark generated by interactive friction during the caving of rocks is an important ignition source, and the presence of pyrite can significantly improve the temperature of the contact surface, thereby increasing the intensity of the sparks. In the flammable and explosive zone, gas combustion or explosion is likely to be triggered when high-temperature sparks are generated by an impact occurring during the caving of the hard roof. The results derived can provide an important theoretical basis for the safe and efficient production of coal mines under the similar conditions of Xinji No.2 Coal Mine.

In recent years, high-voltage electric pulse (HVEP) technology has been suggested to improve the permeability of coal seams. However, the effect of moisture content on the structure evolution of coal subjected to HVEP is not clear, which restricts the wide spread application of this technology. In this study, the breakdown voltage of coal samples with different moisture content was tested, and an exponential function relationship was established between the average breakdown field strength and the moisture content of bituminous coal samples. We investigated the changes in pore structure by combing scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) results, to better understand the pore structure evolution characteristics of coal with different moisture content. Furthermore, changes in the chemical structure of the bituminous coal samples with different moisture content subjected to HVEP were investigated by Fourier transform infrared spectroscopy (FTIR). The results show that many mesopores and macropores are formed in the coal body under the action of HVEP, and the connectivity between the mesopores and the macropores is very good. In general, the higher the moisture content of coal body, the more pores and cracks will be formed in the process of electric pulse breakdown. The total porosity of coal samples with moisture content of 1%, 1.5%, and 2% is significantly higher than that of coal samples with moisture content of 0% and 0.5% after electric pulse breakdown. Therefore, the increase of moisture content is beneficial to improve gas permeability in coal body. The FTIR spectral analysis indicates that oxidation occurs on the plasma channel surface in the breakdown process of coal samples with different moisture content. Furthermore, the increase of moisture content in coal favors the generation of more oxygen-containing functional groups, which will facilitate the desorption of methane.

Based on the Langmuir adsorption model, the adsorption constants of eighteen coal samples from the same coal mine with outburst risk, weak outburst risk, and non‐outburst risk were tested by high‐pressure capacity method under different temperature conditions. The results show that the adsorption constants a and b monotonically decrease with the increase in temperature. The relationship between a and temperature shows three stages: accelerated decreasing stage, decelerated decreasing stage, and stable stage. The b decreases with the increase in temperature also shows three stages, namely relative stability, slow decrease, and accelerated decrease. The a × b for the outburst coal seam decreases with increasing temperature, which is similar to b. The a × b for the weak and non‐outburst coal seams decreases with increasing temperature, which is similar to a. Furthermore, the a of the outburst coal seam decreases with the increase in temperature, and the decrease of b is the largest. The a of the weak and non‐outburst coal seams decreases with the increase in temperature, and the magnitude is relatively large. The b decreases slightly, with a smaller magnitude. The a × b of weak outburst seam is smaller than that of non‐outburst seam and is larger than that of outburst seam. The achievements can provide guiding significance for coal and gas outburst prevention and control.

Broken coal and rock masses are the major part of the goaf. The compaction characteristics of coal and rock masses and the breakage law of whose particles during compaction exert an important influence on various aspects including control of strata motion, prediction of surface subsidence, and backfill mining. In this paper, the triaxial compaction experiment on broken coal‐rock masses with different mixing ratios was carried out. The test results showed that with the increase of stresses, the strain of coal‐rock masses gradually rose while the porosity, bulking factor, and degree of compaction gradually declined. During the compaction of coal‐rock masses, the fitting curves of the strain, porosity, bulking factor, and degree of compaction with stresses of coal samples all appeared as a cubic function of stresses. The breakage behavior of coal particles underwent three stages: structure re‐arrangement and breakage of particles, particle breakage, and compression‐induced deformation of particles. With increasing stress, the crushing amount of particles gradually grew while the increase rate of the crushed particles gradually decreased and the larger the particle strength was, the lower the increase rate of the crushing amount. Additionally, in the compaction process of samples, particle breakage mainly appeared before the stress reached to 8 MPa while the coal and rock particles were hardly crushed after the stress was larger than 8 MPa. With increasing stresses, the particle size gradation of samples gradually became reasonable and the lower the particle strength of samples was, the more reasonable the particle size gradation of compacted samples. The particle size gradation of various compacted and crushed samples showed a favorable fractal characteristic. In the stage with a low stress, the value of fractal dimension D rapidly grew and the fractal dimensions D of various samples tended to be stabilized after the stress reached to a high level.

Coal‐like sampling obtained through compression molding is an important application of powder compression molding technology in mining engineering. To obtain ideal coal‐like samples for the revelation of the seepage property of low‐permeability soft coals, gas seepage studies, which utilized the Taguchi method, were performed on coal‐like materials with different particle sizes, activated carbon weight, Portland cement weight, and forming pressure. The effect of a single factor on the fluid‐solid coupling property of coal‐like materials was analyzed. The results indicate that the permeability and axial stress curves that correlated with strain in the conventional triaxial tests can be divided into three clear phases, and that layered damage appears in all tested specimens. The stress‐permeability relationship model of coal‐like materials is proposed. The influence of process parameters on the strength and permeability of coal briquettes during gas seepage tests was experimentally investigated. The Taguchi method and gray correlation analysis were integrated to determine the best combination of input factors through the key indicator of the gray relational grade, which is required to satisfy multiple quality goals in gas seepage coal‐like materials. The contribution percentage of the input factors to the outputs was determined using analysis of variance; it indicated that coal particle size was the prominent influencing parameter followed by activated carbon, forming pressure, and Portland cement.

In this study, adsorption tests of methane on granular coal were performed to investigate the effects of various factors (temperature, particle size, pressure, and water content) on the adsorption ...

High-voltage electrical pulses (HVEP) can potentially be used to increase the yield of low-permeability coalbed methane (CBM) reservoirs. This study highlights the pore change and permeability evolution characteristics of anthracite coal samples using HVEP treatment. An experimental system was developed to study the variation of coal core permeability by subjecting the coal samples to HVEP treatment under confining conditions. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses were adopted to gain a clearer insight into the changes in pore structure. SEM images revealed that the surface morphologys of the raw coal samples were complete and smooth, whereas those of the coal samples subjected to HVEP developed many pores and cracks. The pores and cracks generated enhanced the permeability of HVEP-treated coal samples. MIP test results showed that the cumulative pore volume of the coal samples subjected to HVEP was significantly greater than that of raw coal samples. Importantly, the HVEP technology had a significant effect on mesopores and macropores, compared with micropores. Because the mesopores and macropores are critical for gas transport, the technology will be helpful for enhancing CBM recovery. Seepage experiments results showed that with the variation of CH4 injection pressure from 0.4 MPa to 1.2 MPa, the permeability of both raw coal samples and coal samples subjected to HVEP showed a decline at a particular confining stress. The permeability of Yangzhuang (YZ) coal sample subjected to HVEP was 1.6–2.2 times that of YZ raw coal under a constant confining pressure, and the permeability of Guhanshan (GHS) coal sample subjected to HVEP is 1.8–2.4 times that of the GHS raw coal sample. At the same confining pressure and CH4 injection pressure, the permeability of the coal samples subjected to HVEP was obviously higher than that of the raw coal samples.

Coal mines in China are characterized by low permeability, and hydraulic slotting is an effective technique to increase the gas permeability of coal. However, the slot depth cannot be accurately determined, which results in the layout parameters of the slotted borehole being empirically designed. In this paper, based on the law of momentum conservation, the velocity distribution law at the cross section of the water jet was derived. Based on the linear relationship between water jet impact power and the erosion volume, the law of pressure attenuation of water jet and Mohr–Coulomb criterion, changes of the slot depth with time and the influence range of water jet slot is confirmed. The maximum slot depth prediction model is also established. The results of field test show that the maximum error between the theoretical calculation value and the measured value is less than 10%, which better validates the prediction model of the slot depth. Keywords: Gas drainage, Hydraulic slotting, Impact power, Slot depth

We have established an experimental system to investigate waterjet impact capacity and coal relaxation behaviors, with reference to hydraulic slotting. The results indicate that after waterjet impact, cracks with good connectivity can be observed on the surfaces of the coal specimen. Vibration acceleration can intuitively display waterjet impact capacity. The disturbance capacity of waterjet impact is significantly larger than that of drilling. The waterjet disturbance degree is defined by the vibration amplitude to quantitatively describe the waterjet impact capacity. The waterjet disturbance degree presents a decrease tendency after an initial increase with the increase in waterjet impact angle, presents a gradual increase tendency with the increase in the operating pressure, and presents an overall decrease tendency with the increase in the lateral coefficient. Based on the variations in axial and lateral pressures , we also proposed axial and lateral relaxation degree to quantitatively describe the coal behaviors under coupling static and waterjet impact loads . The axial relaxation degree gradually decreases and the lateral relaxation degree presents an opposite tendency with the increase in waterjet impact angle, and presents a gradual increase tendency with the increase in lateral coefficient. The research achievements can provide certain references for the application of hydraulic slotting in underground enhanced CBM recovery.

Chinese coal seams are characterized by high gas content and low permeability. The permeability of coal seams should be improved to achieve maximum extraction of coalbed methane. This study explore...

With the increasing consumption of conventional oil and gas reservoirs for gas recovery/production, unconventional reservoirs, such as coalbed methane, shale gas, and gas hydrate, have become very popular in recent times. In this regard, hydraulic fracturing is an effective technique commonly used for enhanced coalbed methane recovery. In previous studies, the fracture morphology was described by comparing the fracture morphology before and after hydraulic fracturing from a macroscopic perspective. Because fracture initiation and subsequent networks of fractures are formed instantly when coal mass is subjected to hydraulic fracturing, it is almost impossible to acquire complete information about fracture initiation by only analyzing the change in hydraulic pressure and fracture morphology. In this paper, a triaxial experimental system was developed to simulate hydraulic fracturing using raw coal and briquette coal samples, respectively. The borehole wall strain observed during hydraulic fracturing was plotted (borehole wall strain curves) and the acoustic emission response was also obtained. In addition, the fracture behaviors during hydraulic fracturing were analyzed. Our results show that the response of coal subjected to hydraulic fracturing can be divided into the following four stages: microcrack formation, fracture initiation, unstable crack propagation, and fracture closure. The borehole wall strain curves effectively reflected the deformation and failure of borehole wall. Acoustic emission response can thus be utilized to identify the orientation of fractures during hydraulic fracturing. The combination of the two methods offers an effective option for clarifying the fracture initiation and instability mechanism near the borehole subjected to hydraulic fracturing.

Chinese coal seams are characterized by high gas content and low permeability. The permeability of coal seams should be improved to achieve maximum extraction of coalbed methane. Hydraulic slotting has been extensively used in China for underground enhanced coalbed methane (ECBM) recovery. The coal strength after hydraulic slotting has a significant effect on the efficiency of this method. In this work, stress-strain curves characteristics of pre-cracked similar material were investigated. Three types of stress-strain curves are identified with the increase in slot inclination angle: bimodal curve with two obvious peaks, transitional curve with unobvious second peak, and unimodal curve with only one peak. In general, the first peak strength increases with the increase in slot inclination angle, whereas the second peak strength shows an opposite tendency. The former is mainly affected by initiation and propagation of wing cracks, and the latter is mainly influenced by frictional sliding between slot surfaces. The study results provide certain guidance for the selection of slot inclination angle in field tests.

Injecting CO2 in deep coal seams for enhanced coalbed methane recovery (ECBM) also benefits the environment through simultaneous CO2. Most studies have concentrated on the changes of pore structures that result from sustained supercritical CO2 (ScCO2) treatment. Conversely, the effects of cyclic ScCO2 treatment have been rarely investigated. In this study, a high-pressure reactor system was used to perform cyclic ScCO2 treatment of long-flame coal. Nuclear magnetic resonance techniques and low-pressure nitrogen gas adsorption were employed to determine the fractal dimensions of coal samples and quantitatively characterize their pore structures before and after cyclic treatment. The results demonstrated that the porosity and proportion of macropores increased significantly after treatment. Therefore, new pores were formed, and some small pores might have been converted into macropores. As the treatment duration increased, the daily average porosity rate exhibited an increasing trend that was presumably caused by the effects of coal matrix fatigue. Further, a decreasing tendency was captured in both the pore-surface fractal dimensions of adsorption pores and the pore-volume fractal dimensions of seepage pores, while increasing the cyclic treatment. This phenomenon after treatment indicated that pore roughness and complexity were decreased. A conceptual model was proposed to explain the mechanisms underlying the evolution of coal-pore structures during the treatments, and the effects of the cyclic injection on the CO2-ECBM field were also analyzed. Therefore, our findings have important guiding significance for selecting suitable CO2 injection methods for CO2-ECBM projects.

A proper understanding of liquid CO2 phase transition fracturing is essential, because the technique can not only reduce CO2 emission, but also can increase production of coalbed methane (CBM). In order to improve the speed of gas drainage and increase the gas permeability of a low permeability coal seam in the driving face of an outburst-prone coal seam, this paper introduced the technology of pressure relief and antireflection by liquid CO2 phase transition in the driving face of an outburst-prone coal seam. At first, the research theoretically analyzed the fracturing processing through the liquid CO2 phase transition and established a mathematical model for the fracturing pressure of the coal seam and the influence range of fracturing. Then, the CO2 phase transition fracturing was numerically simulated. Numerical simulation results showed that the influence radius of fracturing was 2.55–2.70 m. Finally, the field test on liquid CO2 phase transition fracturing was conducted in the driving face based on the modeling results. The results of field experiments show that the amount of head-on gas emission increased by about 2 times after blasting, and the quantity of gas extraction from a single borehole controlled by fracturing was increased by about 4 times. After 8 h of blasting, the amount of gas emission and drainage decreased to the level before blasting. The outburst suppression effect is obvious after a comparison of the gas desorption index of drilling cuttings within 20 m before and after blasting in the heading face.

With a focus on different CO2 pressures and H2O, the influences of the ScCO2–H2O coupling effect on the microstructures of low-rank coal samples were compared and analyzed, offering further analysis of the CO2 sequestration capacity in coal seams rendered unworkable owing to the effect of water. By using nuclear magnetic resonance (NMR) and X-ray powder diffraction (XRD), the changes in porosity, pore size, pore size distribution (PSD), fractal dimension, and minerals in coal samples of the two states were compared and analyzed. XRD analysis revealed that a large number of carbonate rocks (calcite) and aluminosilicate minerals (clay minerals) were found in coal. ScCO2 presented the optimal dissolution effect in the water-saturated samples. NMR analysis showed that compared with a single CO2 fluid, the pore structures of the coal samples varied more remarkably under the coupling effect of CO2 and H2O. Moreover, under the supercritical state of CO2, the effect on the coal sample was the greatest. With the increase in pressure during the CO2 treatment, the porosity of the coal samples increased, and the proportion of macropores grew, indicating the transformation of small pores into large pores. The roughness of adsorption pores increased, whereas the complexity and heterogeneity of seepage pores in the coal samples after ScCO2 treatment declined. Generally, the ScCO2–H2O coupling effect on coal samples caused further changes in the pore structures, promoting mineral dissolution in the coal samples.

Enhanced coalbed methane recovery using high-voltage electrical pulses (HVEP) has been a popular research topic in recent years. However, the breakdown process and fragmentation characteristics of anthracite subjected to HVEP treatment remain unclear. This limits the popularization and application of this technology. In this work, Guizhou anthracite samples were subjected to electrical pulse tests with different breakdown voltages. This was performed using a high-frequency oscilloscope to obtain the voltage and current waveforms during coal breakdown. Furthermore, the pore structure and evolution of the microscopic cracks in the coal were analyzed via field emission-scanning electron microscopy, nuclear magnetic resonance, and X-ray computed tomography. Our results reveal that the breakdown of anthracite by the electrical pulse occurs mainly in two stages: thermal breakdown and electrical breakdown. Most of the energy stored in the capacitor is injected into the plasma channel of the coal in the electrical breakdown stage, during which the coal body displays apparent damage. Additional mesopores and macropores are produced as the breakdown voltage increases. The growth rate of seepage pores is significantly higher than that of adsorbed pores under a fixed breakdown voltage. In addition, the seepage pores of coal samples after HVEP treatment exhibit significant fractal characteristics. Meanwhile, several radial tensile microscopic cracks are formed inside the coal body. In general, the larger is the breakdown voltage, the more abundant are the tensile microscopic cracks. The mineral impurities in a coal body affect the expansion of the discharge channel. This, in turn, affects the direction of microscopic crack expansion.

The geological sequestration of CO2 in unmineable coal seams has gradually become one of the most effective means of responding to the global greenhouse effect. Presently, the cyclic injection of CO2 into coal seams is utilized in some enhanced CBM recovery projects with CO2 sequestration; therefore, a proper understanding of the effects of the cyclic saturation of supercritical CO2 (ScCO2) on coal is essential. Here, we present a series of uniaxial compressive strength (UCS) tests on bituminous coal subjected to both sustained and cyclic ScCO2 saturation. Nuclear magnetic resonance and acoustic emission (AE) are used to investigate changes in the pore structure distribution and porosity, as well as the fracture propagation. The results indicate that ScCO2 saturation enhances the continuity of the pore volume distribution, while cyclic saturation has a stronger influence on the pore structure. Furthermore, samples subjected to cyclic saturation exhibit significantly greater decreases in UCS and elastic modulus than the sustained-saturation samples, owing to mechanical fatigue caused by the cyclic saturation. The AE results show that cyclic saturation produces multiple signal releases in the form of unstable crack propagation, reducing crack closure and enhancing stable crack propagation. We also analyze the failure mechanism of coal samples under cyclic ScCO2 saturation in terms of the pore structure and mechanical changes experienced and discuss the influence of the cyclic injection of CO2 into coal seams. Hence, the results of this study are expected to provide a reference for the selection of appropriate CO2 injection methods and safety assessments for field projects involving coal-seam CO2 sequestration.

In this paper, the influences of path control effects on characteristics of gas migration in coal samples with different sizes were investigated by using self-developed test device. Moreover, the mathematical model regarding path control effects on the gas migration was established. The results show that the curves of gas diffusion velocity in the coal samples with different sizes gradually decrease and then flatten with diffusion time. At the initial stage of diffusion, the gas diffusion velocity in coal samples with a small geometric size (short migration path) is larger than that in the coal samples with a large geometric size (long migration path). However, with the test proceeding, the phenomenon of velocity transcendence appears. The cumulative gas diffusion from the coal samples increases sharply with the rise of gas diffusion velocity, and its growth rate decreases gradually with the decrease of gas diffusion velocity. Finally, the gas diffusion curve tends to be flat. Although there is a phenomenon of transcendence in the gas diffusion velocity, transcendence in the cumulative gas diffusion has not been found. By equating the free space around the coal samples into a large fracture connecting to the coal samples, a fracture system was formed together with pore structures in the coal samples to affect gas migration process.

The mechanical properties and seepage characteristics of gas-bearing coal evolve with changes in the loading pattern, which could reveal the evolution of permeability in a protected coal seam and allow gas extraction engineering work to be designed by using the effect of mining multiple protective seams. Tests on gas seepage in raw coal under three paths (stepped-cyclic, stepped-increasing-cyclic, and crossed-cyclic loading and unloading) were carried out with a seepage tester under triaxial stress conditions. The permeability was subjected to the dual influence of stress and damage accumulation. After being subjected to stress unloading and loading, the permeability of coal samples gradually decreased and the permeability did not increase before the stress exceeded the yield stage of the coal samples. The mining-enhanced permeability of the coal samples in the loading stage showed a three-phase increase with the growth of stress and the number of cycles and exhibited an N-shaped increase under the stepped-cyclic loading while it linearly increased under the other two paths in the unloading stage. With the increase of peak stress and the accumulation of damage in coal samples, the sensitivity of the permeability of coal samples to stress gradually declined. The relationship between the damage variable and the number of cycles conformed to the Boltzmann function.

The safe and efficient operation of coal mines is inseparable from rational production deployment. As for outburst-prone coal mines, the production deployment is more complicated due to the introduction of gas disaster control processes, which can easily cause continuity tension and even coal and gas outburst accidents. Therefore, it is necessary to establish an evaluation system for the rationality of production deployment of outburst-prone coal mines. In this paper, based on the production experience of coal mines in Chongqing, China and the summaries of experts, a production deployment evaluation system with eleven indices for outburst-prone coal mines is established. In addition, a Bayesian network is used to establish a corresponding evaluation model. The result shows that the evaluation index system for the production deployment rationality of outburst-prone coal mines, which is composed of the excavation advance indices and the validity indices of regional measure engineering, can systematically diagnose the rationality of each link of the production deployment for outburst-prone coal mines. The results of the case study show that the established Bayesian network model can be used to evaluate the deployment rationality of outburst-prone coal mines. The sensitivity analysis shows that the development coal reserve is the most sensitive to the reasonable deployment of outburst-prone coal mines. The accident prediction and cause diagnosis through the posterior probability reasoning indicate that in the absence of other evidence, the most likely reason for the unreasonable mine deployment is the insufficient development coal reserve. The observations and findings in this research have considerable practical significance for the smooth production of outburst-prone coal mines with similar geological conditions.

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered.

Tectonic coals in coal seams may affect the process of enhanced coalbed methane recovery with CO2 sequestration (CO2-ECBM). The main objective of this study was to investigate the differences between supercritical CO2 (ScCO2) and intact and tectonic coals to determine how the ScCO2 changes the coal’s properties. More specifically, the changes in the tectonic coal’s pore structures and its gas desorption behavior were of particular interest. In this work, mercury intrusion porosimetry, N2 (77 K) adsorption, and methane desorption experiments were used to identify the difference in pore structures and gas desorption properties between and intact and tectonic coals after ScCO2 treatment. The experimental results indicate that the total pore volume, specific surface area, and pore connectivity of tectonic coal increased more than intact coal after ScCO2 treatment, indicating that ScCO2 had the greatest influence on the pore structure of the tectonic coal. Additionally, ScCO2 treatment enhanced the diffusivity of tectonic coal more than that of intact coal. This verified the pore structure experimental results. A simplified illustration of the methane migration before and after ScCO2 treatment was proposed to analyze the influence of ScCO2 on the tectonic coal reservoir’s CBM. Hence, the results of this study may provide new insights into CO2-ECBM in tectonic coal reservoirs.