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Due to the insufficient understanding of the outburst mechanism, the coal and gas outburst disasters in China are more serious. Gas expansion energy is the main source of energy that causes outburst. In order to explore the distribution law of gas expansion energy in outburst coal seams, a gas-solid coupling equation of outburst coal seams was established. The distribution law of coal stress field, deformation field, gas flow field, and gas expansion energy were simulated and analyzed by using COMSOL Multiphysics. The results showed that from the excavation face to the deep part of coal seam, the stress presented unloading zone, stress concentration zone, and original stress zone. The volumetric strain and permeability reached the minimum, while the gas pressure reached the maximum at the peak value of vertical stress. As time goes on, the gas pressure in the fracture near the working face gradually decreased and was less than the pressure in coal matrix. The total gas expansion energy consists of free gas and desorption gas expansion energy. Affected by the excavation, free gas expansion energy maintained a constant value in the original coal seam and gradually decreased in the area close to the working face. The expansion energy provided by desorption gas was zero in the original coal seam. And it first increased and then decreased rapidly near the working face. Compared with stress and coal seam thickness, gas pressure and initial diffusion coefficient had significant influence on gas expansion energy of coal seam. When the diffusion coefficient was greater than 1e-9 m2/s, the gas expansion energy of the coal seam near the working face was significantly higher than that of the original coal seam, which had the risk of inducing outburst.

Hydraulic fracturing is a key technology for increasing the permeability of coal seams and improving the extraction effect of coalbed methane. Exploring the parameters that represent the infiltration effect is an essential part of the optimization process of hydraulic fracturing. In this work, based on a similarity principle design, a hydraulic fracturing test was carried out on large raw coal samples using an indoor hydraulic fracturing physical simulation test system. Based on the distribution of primary cracks in the coal samples, the law of crack extension during hydraulic fracturing was studied. The stress variations before and after hydraulic fracturing of the coal seams were monitored, and the stress transfer laws during confining pressure loading and hydraulic fracturing were investigated. The experiment showed that the new fissures in the fracturing process are more likely to extend to the primary fissure development area. When the applied coal body stress exceeds the strength of the coal sample body, the internal cracks in the coal sample are completely penetrated. During this complete penetration, the energy accumulated by pressure water injection is continuously released, and the count detected by the acoustic emission increases sharply. The coal is mainly subjected to compressive stress during the confining pressure loading process, whereas the coal body is mainly subjected to a shear stress during the hydraulic fracturing process. Both the stresses can be transferred through the coal samples. When the stress increase during hydraulic fracturing is greater than the confining load, the overall effect is due to different stresses. As the cracks continue to expand and extend during hydraulic fracturing, the stress transfer effect becomes weaker. Through the exploration of the law of crack extension and stress transmission, this experiment showed that large-scale physical simulation experiments in a laboratory setting can help effectively simulate on-site hydraulic fracturing conditions. The test method and results reported herein provide a reference and basis for the design and optimization of on-site hydraulic fracturing parameters.

Acoustic emission (AE) is a popular technique to monitor the process of rock failure during hydraulic fracturing for unconventional natural gas production. It contains abundant information that will be useful to study in-depth the nature of hydraulic fracturing. In this study, we focused on the AE count, energy, peak frequency, crack classification, and location recorded from four rock specimens subjected to a specific triaxial stress condition. We found the multi-frequency-response phenomenon of AE, and proposed the multi-frequency-response index to indicate the moment of the macrohydraulic crack formation. Furthermore, it was found that the power law distribution index of AE energy of non-stratified specimen was bigger than that of stratified specimens during hydraulic fracturing. The tensile crack dominated in all hydraulic fracturing tests. Our results are of significance for understanding hydraulic fracturing in stratified rocks.

Gas emission from the working face in a gas-bearing coal seam impairs safe production. Especially in the coal face, influenced by mining, pressure-relief gas in adjacent seams flows into the working face through the goaf. Moreover, caved overlying strata in different regions show differences in pore and seepage characteristics. Thus, the caving of overlying strata in the goaf of a working face was explored by carrying out physical similarity simulation. Then, based on the characteristics of the geometric shapes of caved overlying strata after mining, a trapezoidal three-dimensional model for gas extraction was established. According to the calculation result, the parameters of field high-level boreholes were optimized. Then, the controllability of gas concentration at the working face after gas extraction was assessed by applying statistical process control. The result showed that after the observed surface of the physical similarity model was lightened, it was more favorable for conducting the test. Moreover, the maximum gas concentration in the goaf was negatively correlated with the diameter of high-level boreholes and the negative pressure for gas extraction. A statistical process control chart revealed that the gas concentrations at the working face were safe after gas extraction based on high-level boreholes, which also validated the feasibility and effectiveness of the model.

During the multiseam mining process, the overlying strata were damaged due to different mining disturbances, where a large number of fractures were produced which were the main seepage paths of gas and water, easy to cause gas or water inrush accidents. Thus, it is necessary to explore the development characteristics of the fractured zone in the overlying strata from multiseam mining. In this paper, laboratory triaxial compression tests were firstly conducted on the rock specimens subjected to different mining disturbances. In these tests, the specimen was loaded to different stress before a regular test to simulate the different damage effects on the rock strata caused by different mining disturbances. And the corresponding values of m and s could be obtained through a calculation based on the test results. Subsequently, the numerical simulation was conducted to simulate the field situation, in which the m and s values were used in the numerical model combined with the Hoek‐Brown–modified criterion, and the results show that the overburden failure height was increased with the mining disturbances from 48 to 120 m, and then, the effects of mining height and interval thickness on the development height of fracture zone were studied by numerical simulation. Finally, the leakage of roof drilling at different stages was tested and calculated in the field. The results showed that the loss of leakage drilling can directly reflect the development degree of roof cracks, and the detection results were close to the development height of the numerical simulation fracture zone. It is verified that the simulation method used in this paper is feasible and can be used as a reference for similar research.

When faced with multi‐seam mining, some coal pillars are left to support the overlying strata when mining the lower seam, where stress concentration usually occurs. When the working face in the lower seam advances to the area influenced by these coal pillars, high ground pressure behavior occurs. Furthermore, the instability of coal pillars under the participation of the roof may induce rock burst, which threaten the safe production of coal mine. To study the roof strata movement and coal pillar instability in multi‐seam mining, a coal mine in Datong was taken as an example. Firstly, the bursting liability of coal and rock stratum was tested. The test results show that the 7#, 8#, and 11# coal seams in the mining area all have strong bursting liability. And the roof was dominated by siltstone and firestone, which also have strong bursting liability. Then, based on the geological conditions of the coal mine, we investigated roof movement and instability of the coal pillar during multi‐seam mining through physical simulation. The simulation results indicate that multi‐seam mining aggravates the roof damage, which leads to the increase in the fracture range of overlying strata, and that the failure height increases from 46 to 121 m. In addition, the detailed distribution of stress on residual coal pillars and its influence on the lower working face were studied through numerical simulation. The results show that, when the working face 8707 of the 8# coal seam advances to a horizontal distance of 20 m from the coal pillar, it enters the influence zone. As the working face advances beneath the coal pillar, the stress reached to 24 MPa. Moreover, when the working face passes by the coal pillar, the coal pillar is cutoff, thus affecting the working face.

To more accurately determine the stress distribution law of coal and rock masses containing faults, the fA42 reverse fault of the Shihao coal mine is taken as an example to carry out an in...

Hydraulic fracturing can evidently improve the coalbed methane production in underground coal mines, but it is difficult to delimit the stimulated area accurately. In order to evaluate the stimulated area, microseismic (MS) monitoring technique is proposed to investigate the seismic responses of the induced fractures of hydraulic fracturing. Three coal seams were targeted to be treated in a coal mine. An array of geophones was set along the underground roadway to detect the MS signals caused by HF. In order to verify the result of MS monitoring, water content of each coal seam has been measured before and after HF treatment. The results showed that a series of MS events were detected during the entire HF process, and a sharp MS event usually occurred during the first hour of HF process. The energy of the sharp MS event had higher magnitude than others. The MS distribution exhibited complex morphological features. The directionless MS response was distributed over a radius of less than 40 m but tended to be significantly conjugated with a radius of more than 40 m. HF could stimulate both the coal seam and the rock layers nearby. The achievable stimulated area in the coal seam was determined to be 50 m × 50 m according to the MS density and water content. The stimulated area in terms of MS density was easily found to be broader than the area of water direct intrusion. The present study indicated that MS monitoring technique could potentially be used for evaluation of HF in underground coal mine.

Complex civil structures require the cooperation of many building materials. However, it is difficult to accurately monitor and evaluate the inner damage states of various material systems. Based on a convolutional neural network (CNN) and the acoustic emission (AE) time-frequency diagram, we used the transfer learning method for classifying the AE signals of different materials under external loads. The results show the CNN model can accurately classify cracks that come from different materials based on AE signals. The recognition accuracy can reach 90% just by re-training the full connection layer of the pre-trained model, and its accuracy can reach 97% after re-training the top 2 convolutional layers of this model. A realization of cracking source identification mainly depends on the differences in mineral particles in materials. This work highlights the great potential for real-time and quantitative monitoring of the health status of composite civil structures.

Understanding the pore wetting process is of great significance for determining fluid wetting behavior in porous media such as coal, and for the development and utilization of natural gas reservoirs, however, most studies on the process simplify porous media as glass capillaries, which may lack practical application. To better understand and quantitatively characterize the actual pore wetting process of coals, a new, low-field nuclear magnetic resonance (LF-NMR) experimental characterization method was developed. The experiment used equal grain sizes of glass beads and coals to yield porous media with variable pores of specific size ranges. LF-NMR was then used to characterize the pore wetting process of a given amount of water in the porous media at different times. Six groups of high-rank coals were also characterized by contact angle experiments to analyze the role of the surface contact angle on pore wetting. The results show that the pore wetting process can be divided into three ideal physical phases: a first touch state, a semi-free water state and an inter-granular diffusion or quasi-static state. The wetting pore size distribution (W-PSD) spectrum obtained by measurement of the transverse relaxation time and the transformation coefficient reflected change in pore size during wetting, with the location of the W-PSD quantified by the equivalent wetting pore size. The equivalent wetting pore size obtained at a final time of 24 h was used to characterize the pore wettability, it was significantly correlated with the surface contact angle with a linearity of up to 0.99. However, the W-PSD or the equivalent wetting pore size is dependent on the surface relaxivity. Overall, the experimental characterization method used in this study is an effective way to study the “black box problem” of the wetting process in porous media, as it is simple and has low sample requirements.

Tight sandstone gas (TSG) reservoirs are developed in large buried depths and complex geo-stress field environments. During exploitation, the surrounding rock of a drill well is often damaged or can collapse under artificial engineering disturbances. Understanding the failure mechanism of tight sandstone under high and complex three-dimensional (3-D) stress states is essential for the safe and efficient exploitation of TSG. In this study, using the stress Lode angle (θσ) as a variable, failure experiments of low porosity sandstone specimens under various 3-D stress paths are performed, and the mechanical responses (e.g., stress–strain behavior, strength, fracture pattern, and acoustic emission characteristics) are analyzed. The results show that as θσ increases, the strength of the specimen as well as the deviatoric stress required its failure decrease linearly, whereas its brittleness increases. The failure of the specimens is primarily due to numerous micro tensile cracks and a few macro shear cracks. θσ significantly affects the cracking mode during failure. Acoustic emission (AE) parameters show that the failure process can be categorized into three stages within the time-to-failure window, among which Stage 2 (acceleration stage) can be regarded as the precursor stage of the ultimate failure of the specimen. The descriptive statistical results of AE energy show that uniaxial stress, hydrostatic stress, and true triaxial stress compression impose different effects on the damage mode of the specimens. Under the high 3-D stresses, the multiple fracture surfaces formed inside the specimen are intertwined to present several “X”-shaped fracture pairs. These findings facilitate the understanding of the failure mechanism of rock mass surrounding the wellbore, and of significance in stability designing in well trajectory of TSG reservoir actual development.

Coalbed gas content measurement is important for both unconventional gas recovery and mining safety. This study experimentally discussed factors affecting coalbed gas content measurement. It is found that small coal particles demonstrated high desorption rate at the early stage of desorption tests. However, the difference in methane desorption rate reduces when the coal particle size is more than 1.0 mm, indicating that a threshold of coal particle size exists in methane desorption in coal. Also, the particle size distribution tends to be stable when coal sample mass is over 15 g, stabilizing at around 1.75 mm. Therefore, the coal sample over 15 g is recommended to use in desorption tests of coalbed gas content measurement. The temperature and gas pressure can enhance the desorption rate, especially at the early stage of desorption. This temperature effect, however, mainly occurred in the first 15 min of desorption test. After 30 min, the desorption rate tends to be stable, slightly changing from around 0.35 mol/kg/min to 0.75 mol/kg/min. Due to endothermic process, absolute isothermal condition does not exist in desorption test, but this temperature variation mainly occurred in the first 15 min. The results also show incorporating the desorption data from the time of exposure to 13 min of desorption tests is essential to typically reflect methane desorption kinetics in coal. The study also shows that the measurement difference in gas content between the case with sample being air-puffed and -flushed and the case with sample exposed to the air is small. Therefore, the air-puffing and flushing during the sample collection has an insignificant effect the gas contact measurement result. This study can help to advance the understanding of impact factors on coalbed gas content measurement method.

The study of stress transfer can delimitate the hydraulic fracturing (HF) range to evaluate the extraction efficiency of coalbed methane (CBM). However, less research has been done to explore the stress change of underground coal seam due to the field test limitations. In this paper, a total of 9 stress monitoring boreholes (SMBs) are designed to investigate the stress evolution of coal seam during HF through field experiments. Although the middle-low pressure fracturing may cause some stress concentrations, the stress could completely release in the period of subsequent mining, which will not cause potential safety hazards. The stress increase direction is more likely to extend to the original fracture area, especially the direction of loose ring in front of coal seam. The range of stress increase is inversely proportional to the distance to the hydraulic fracturing borehole (HFB) in the same direction, except for the geological structure development areas. The plastic ring will expand with an increasing radius R 1 along with the stress increase of coal seam until the radial stress increment Δ σ rp of outer wall of plastic ring decreases to zero. The effective stress affecting range is about 140 m according to the analysis of stress transfer at different time during HF. The stress no longer increases and gradually decreases to stability after fracturing. The stress transfer method is one of ways to represent the effective region of field HF and enrich the evaluation system for HF affecting range.

Pulsed water jets have been widely applied in the field of underground mining because of their unique advantages. Revealing the mechanical mechanism of rock breaking under a pulsed water jet is significant in advancing the application of pulsed water jets in the rock crushing process. Based on continuum mechanics and interpolation theory, this paper establishes a numerical model to simulate the propagation of stress waves in rock by adopting the method of smooth particle hydrodynamics (SPH). According to the simulation results, this paper presents a quantitative approach to schematize the propagation of stress waves in the time-space dimension. The waveforms of stress waves in the selected path and the effective stress history of test particles are obtained to quantitatively describe the propagation of stress waves. Relying on the approach, the effects of jet velocity and rock properties on the propagation of stress waves are investigated. The results show that in the rock model, the influence area, propagation speed and attenuation rate of the stress wave all increase with increasing jet velocity. Moreover, the mean effective stress of a test particle in the numerical model increases with increasing jet velocity. Of the rocks considered in this paper, granite shows the greatest mean effective stress acting on the test particle, propagation speed, and attenuation rate of the stress wave.

Hydraulic fracturing is an efficient technique for enhancing coal seam gas production. Induced stress (or stress shadow) of hydraulic fracturing has been popularly studied recent years. In this study, induced stress was measured in a field during the process of hydraulic fracturing. It was found that the induced stress evolution was closely related to the increase of injection volume. The relation between the average induced stress value and the injection volume was fit by the Michaelis-Menten equation well. When carried out the injection operation, we found that the induced stress increased significantly within the radius of 70 m around the hydraulic fracturing borehole. The induced stress also dropped occasionally during the injection process, which indicated some large fractures’ propagation. When the injection operation was finished, the induced stress began to decrease slowly. The subsequent drilling of gas extraction could release the induced stress. In addition, the induced stresses caused by adjacent fracturing boreholes (spacing was 50 m) substantially influenced each other. The induced stress acted earlier in the upper part of the coal seam than the lower part influenced by the steep incidence of the coal seam. Furthermore, the orientation of the induced stress distribution was eventually parallel to the maximum horizontal principal stress direction controlled by a nearby large fault, although the initial orientation was arbitrary. The results are helpful for hydraulic fracturing evaluation and optimization and numerical simulation research.

Hydraulic fracturing is an effective method to improve the permeability of coal seams and enhance extraction of coalbed methane (CBM). Evolution of cracks in coal seam becomes increasingly complex when one borehole is drilled through multiple coal seams. In this work, physical models with two coal seams having different uniaxial compressive strength (UCS) ratios and height ratios were prepared to study the competitive initiation and extension of cracks based on pump pressure, acoustic emission (AE) source location, and crack morphology. The results showed that, a competitive relationship exists between the two coal seams in terms of crack extension. The pump pressure curves of all specimens presented four stages: (I) fluid-filling stage, (II) pressure rise stage, (III) pressure drop stage, and (IV) pressure stabilization stage. Compared with the pure specimen, the crack initiation pressure of specimens with two coal seams reduced generally. The AE source locations were first generated and mainly distributed in the softer coal seam, resulting in the formation of a complex crack network, whereas only a few cracks were generated in the harder coal seam, as observed after cutting the specimens. In seams with different height ratios, the AE source locations and tracers indicated that cracks preferentially developed and extended in the thinner coal seam. The higher the height ratio of the two coal seams, the more difficult it was for cracks to extend simultaneously in two coal seams. The research results will help improve the permeability of CBM reservoirs when subjecting multiple coal seams to hydraulic fracturing.

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.

In order to release the tension and shear effect of the superjacent rock strata movement during excavation in coal mine, protect the surface borehole case from fracturing fast and make a good use of the surface borehole during goaf methane drawing, a common synthesis tension deformation fracture model was set up based on the synthesis tension effect of the rock strata, and the deformation rule of the surface borehole case with time and space was researched. The results suggest that, to reduce the deformation the surface borehole should be built between the boundary of the stope and the knee of subsidence curve. At the same time, a 3DEC simulation model and an engineering example were carried out to examine the rules of theoretical model. The result suggests that the model and the rules accord to the test and have good building and protection engineering application values to the surface borehole. Keywords: Surface borehole, Methane drawing, Synthesis tension, Time-space rules

In this communication, the experimental parameters and sorption isotherms of CO2 on moisture-equilibrated Argonne premium coal samples provided by Goodman et al. [Goodman, A.L., Busch, A., Bustin, R.M., Chikatamarla, L., Day, S., Duffy, G.J., Fitzgerald, J.E., Gasem, K.A.M., Gensterblum, Y., Hartman, C., Jing, C., Kross, B.M., Mohammed, S., Pratt, T., Robinson Jr., R.L., Romanov, V., Sakurovs, R., Schroeder, K., White, C.M., 2007. Inter-laboratory comparison II: CO2 isotherms measured on moisture-equilibrated Argonne premium coals at 55°C and up to 15 MPa. International Journal of Coal Geology 72, 153–164] were used to investigate the reasons that CO2 sorption on coal diverged significantly among the laboratories at high pressure. We demonstrate that the experimental parameters for manometric equipment affect the CO2 isotherms at high pressure. High adsorption values may be obtained with a higher volume in the reference cell or a lower void volume in the sample cell. Although the impact of experimental parameters for manometric equipment on CO2 sorption on coals is not clear, the investigation of the equation of the Gibbs excess adsorption used to obtain incremental adsorption shows that higher sorption of CO2 can be obtained with a higher ratio of the volume of the reference cell to the void volume of the sample cell for manometric equipment. On the other hand, higher adsorption is difficult to obtain at high pressure with a smaller volume ratio.

Presently many research projects focus on the adsorption and predictions of methane, nitrogen, carbon dioxide and their mixtures on coals under equilibrium gas conditions. It is highly important to develop a method for prediction of CO 2 /CH 4 adsorption on coal under feed (source) gas conditions in the fields of enhanced coal bed methane recovery and CO 2 sequestration in coal bed with CO 2 injection. The objective is to develop reliable models to predict the adsorption behavior of mixtures on coals under feed gas conditions. In this context, the adsorption and predictions of methane, carbon dioxide and three binary mixtures of these gases on two China coals under feed gas conditions have been performed on the experimental data and derived equation. Firstly, the experimental measurements of CH 4 , CO 2 and their binary gas adsorption isotherms to investigate the adsorption behavior and use for prediction of adsorption equilibrium were conducted at 301 K at pressures up to 10 MPa. Secondly, a calculation formula of adsorbed amount of total gas was derived from relation between adsorbed amount and feed gas composition at standard state on condition of unknown equilibrium gas concentration during adsorption. Thirdly, the extended Langmuir, ideal adsorbed solution (IAS) theory in conjunction with Langmuir were used to predict the experimental data of binary gas adsorption based on an iterative algorithm. Finally, the prediction accuracy for experimental data of total and adsorbed amount of component gas and free gas compositions were analyzed. Experimental data indicate adsorbed amounts of the total gas and single-component CO 2 increase, but adsorbed amounts of CH 4 increase at low equilibrium pressure and decrease at high pressure with increase of CO 2 concentration in feed gas. Model predictions based on single-component isotherms and the calculation formula show that the quantitative formula obtained can be used to predict adsorption equilibrium of CH 4 /CO 2 binary gas under feed gas conditions. The binary gas prediction data are quite variable and depend upon the feed gas composition. The IAS–Langmuir provides, on average, a better fit to total gas adsorption but is variable for the single-component adsorption with the feed gas composition. The predictions of total and component gas adsorption and equilibrium gas compositions illustrate similar trends with their experimental data.

CH4/CO2 adsorption isotherm studies on coals samples give adsorption behavior, adsorption ratio, and other influential factors for enhanced coalbed methane recovery (ECBM) and CO2 sequestration by CO2 injection. Yet, they only provide static characteristics, and do not provide information on pressure-driven and concentration-driven during gas recovery and injection processes. The purpose of this study is to construct an experimental apparatus for CH4 displacement with CO2 injection and primarily study the basic procedure of coalbed methane (CBM), ECBM, and CO2 sequestration at driven-conditions. An experimental apparatus was built based on the configuration of one-injection well/one-production well under ideal conditions. The apparatus was used to model CBM with CH4 adsorption, investigate primary production of CBM and ECBM with gases desorption, and study CO2 sequestration with its breakthrough. The change of pressure and gases amount during gases injection and desorption, and residual quantity after gases desorption were employed to ECBM and CO2 sequestration with this equipment. The effect of CO2 injection on CH4 production was investigated on a small scale experimentally. The results indicate that the recovery procedure of ECBM and CO2 sequestration with CO2 injection can be studied with the apparatus and the gases adsorption/desorption characteristics obtained with the apparatus is obviously differ from that obtained with conventional volumetric measurement.