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Saturation characteristics of water and capillary pressure are important for hydrofracturing or water flooding, which is widely adopted in dust suppression, methane displacement, and gas and coal outburst elimination. In this paper, we have studied the water saturation characteristics of coal samples with different coal ranks and their effect on capillary pressure. It was found that the isothermal adsorption of water vapor from coal samples can be divided into three processes: monolayer adsorption, multilayer adsorption, and capillary condensation. The dent isothermal adsorption model can well describe water vapor adsorption of coal samples, indicating water molecules were first adsorbed on the first‐order adsorption site of pore surfaces at low relative humidity and then on the second‐order adsorption site with increasing relative humidity. An obvious hysteresis occurred in the process of water imbibition and drainage. A capillary pressure of ~325 MPa or even larger was required to displace the water from the coal. Besides, micro‐ and mesopores play a more important role in capillary pressure. The capillary pressure of coal samples was positively correlated with wettability at lower water saturation, but it is not significant at higher capillary pressures. Therefore, the article's results provide a new direction for solving the water‐locking effect in coal seams. [ABSTRACT FROM AUTHOR]

Coal deformation caused by gas adsorption is a well‐known phenomenon and has played an important role in many engineering fields. For example, the bulk coal deformation induced by gas is a primary factor in coal and gas outbursts, and the changes in coalbed permeability are primarily determined by the evolution of the coal fracture volumetric strain during gas recovery. However, the current models of coal deformation induced by gas do not fully consider the coal seam field conditions or the deformation factors. To better fit coal seam field conditions and provide a basis for engineering applications, in this work, based on coal's physical structure, coal seam field conditions, the energy balance approach, poroelasticity, mathematical differentiation, and previous scholars' research results, the volumetric strain models of bulk coal, coal fracture, and coal matrix under coal seam field conditions are established. In addition, based on the analysis of the effect of gas pressure on coal deformation, the difference in gas pressure effects on the bulk coal deformation between experimental conditions and coal seam field conditions revealed that when the bulk coal deformation is tested in a laboratory, the gas pressure exerts a compressive effect on the bulk coal. However, under coal seam field conditions, the gas pressure has a swelling effect on the bulk coal. Additionally, gas pressure exerts an compression on the coal matrix and a swelling on the coal fracture under coal seam field conditions. At the end of the paper, the established models were analyzed and discussed from multiple perspectives. It was shown that the coal deformation models established in this paper are all accurate and reliable. [ABSTRACT FROM AUTHOR]

Coal permeability is a key parameter that affects the drainage of coal bed methane (CBM). Owing to the lower original permeability of coal seams in China, more attention was paid to the study of damage‐induced permeability laws. However, the theory of damage‐induced permeability still needs to be improved. In this paper, the permeability evolution experiments during the complete stress–strain process of coal were carried out firstly. The results are shown that with the increase in axial strain, the coal permeability decreases slowly at first, then increases rapidly, and finally increases slowly or even remains unchanged, and the peak stress point can be regarded as the extreme point of permeability. Then, the equivalent plastic strain was used to describe the damage of coal. Based on the above results, a new damage‐induced permeability model was developed, and the new model can match the experimental data of permeability very well. In addition, solid–gas coupling models of coal were developed based on the new permeability model, and the numerical results show that there is a region of sudden increase in permeability around the borehole where plastic failure occurs, which indicates that the damage‐induced permeability model is reasonable. Finally, the results of numerical simulation of gas predrainage in combination of soft coal and hard coal under different cases were used to optimize the borehole layout of gas predrainage in the field. [ABSTRACT FROM AUTHOR]

Coalbed methane (CBM) is an important resource of energy. For CBM recovery, sorption‐induced coal deformation can cause significant reservoir permeability change. Moisture content and coal pore morphology affect the gas adsorption capacity and can alter the coal deformation of the coal seam. Therefore, it is crucial to establish a coal gas moisture‐coupled model for CBM production prediction. However, there are currently insufficient data available for quantitative analyses. In this paper, a series of typical sorption‐induced strain experiments were carried out during methane adsorption and desorption on coal samples with different metamorphic degrees and moisture content. The pore morphology and adsorption capacity of coals were measured to analyze the reason for different deformation of coals with various pore structures and moisture. Results show that the adsorption capacity and deformation is corresponding to the specific surface area of micropore, which first decreases and then increases with coal ranks. The deformation and adsorption gas content of dried coals is greater than that of natural moisture coals, which means that moisture can reduce the sorption capacity of coals, resulting in the decrease of gas adsorption‐induced coal deformation. There is also residual deformation of coal samples after gas desorption caused by the residual adsorbed gas in coals. This paper quantitatively investigates the effects of pore characteristics and moisture on coal deformation and the internal mechanism. This work will provide essential information for building out a fully cross‐coupled model of coal gas‐moisture relationships for CBM production prediction. [ABSTRACT FROM AUTHOR]

Hydraulic methods have been extensively employed to improve the production rate of commercially viable coalbed methane and prevent potential mining disasters resulting from the production of this methane by increasing the permeability of coal seams. In this paper, an experimental system with a stirrer was utilized to study the effects of water injection on the gas desorption characteristics of coal seams. Pressure‐released desorption values and total gas desorption values under conditions of different adsorption equilibrium pressures and water contents were determined. The results show that both the free gas and the adsorbed gas could be replaced by the injected water, causing the system pressure to increase significantly. When releasing the system pressure, the gas desorption value of the water‐injected coal sample was reduced. After water injection, the total desorption value of the coal was increased. Thus, water injection could help to promote coal gas desorption. [ABSTRACT FROM AUTHOR]

Protective coal layer (PCL) mining is a preferred regional method for preventing coal and gas outbursts; stress‐relief of PCL mining is the prerequisite of pressure‐relief gas drainage. In order to study the stress‐relief effect of PCL mining on protected coal seams and determine the protection scope of PCL mining under different inclinations, this paper uses physical simulations to study the stress‐relief law and deformation characteristics of the strata during upper PCL mining with a gentle inclination (25°), an intermediate inclination (45°), and a steep inclination (65°). The results show that as the coal seam dip increases, the degree of deformation and failure of the overburden strata gradually decreases, the severe subsidence and expansion area of the overburden strata tends to gradually develop in the uphill direction of the working face, and the floor strata underlying the protective seam significantly expand toward the goaf. However, the absolute value of the upward movement of the floor strata is much smaller than that of the roof strata; and the greater the dip angle, the greater the ratio of floor displacement to roof subsidence. The stress‐relief curve of the protected layer is approximately U‐shaped. As the inclination of the coal seam increases, the stress‐relief range decreases gradually. The maximum relief‐stress and the stress release coefficient of the protected layer initially increase and then decrease. The stress concentration coefficient of the protected coal seam initially increases first and then decreases in both uphill and downhill of the working face in the PCL. As the inclination increases, the stress‐relief angle of the downhill boundary initially increases and then decreases, while the stress‐relief angle of the uphill boundary decreases gradually. Our research results provide guidance for the delimitation of the protected scope of PCLs with different dip angles. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a potentially fatal risk during the mining of gassy coal seams, which seriously threatens the safe mining of collieries. To understand the outburst mechanism and evolution rules, a new apparatus (LSTT) was developed to conduct simulated experiment. In the context of an outburst accident in Dingji coal mine, the authors launched an authentic outburst experiment to replay the outburst accident. Experimental apparatus, similar criterion, coal‐like materials and gas sources, and experimental design were discussed systematically in this paper. Experimentally, the study analyzed the geo‐stress has significant influence on the outburst evolution. At the driving face, the stress concentration possibly caused gas outburst, under the influence of mining‐induced stress. After the outburst occurred, the stress balance of the coal changed, resulting in the instability of the coal. Furthermore, the elastic energy, gas enthalpy, and gravitational potential energy were released rapidly. The experimental result stated that outburst coal has the sorting characteristics, in line with the field outburst law. The intensity prediction model has been built based on the energy model. Moreover, the factors that impact outburst intensity were analyzed. In the process of coal and gas outburst, the gas enthalpy of gas and the elastic potential of coal are the main energy sources. This study provides guidance for the development of the outburst mechanism and outburst mine management. [ABSTRACT FROM AUTHOR]

This paper proposes a coupled model of waterjet cutting coal containing gas with the Arbitrary Lagrange‐Euler algorithm assistance. The dynamic evolution of coal damage and the cutting performance under different jet conditions, including jet pattern (submerged/nonsubmerged state), jet velocity, nozzle diameter, and the initial standoff distance, are investigated by tracing the number of failure elements and the stress distribution. Afterward, the gas flow model is built to study interactions of the discharged coal amount and the effective drainage radius at different periods with COMSOL software. The numerical results show the coal cutting performance of nonsubmerged jets is much better than that of submerged jets. Under submerged conditions, the penetration length increases with jet velocity and nozzle diameter, but decreases with the initial standoff distance. Moreover, the effective drainage radius of nonsubmerged waterjet is substantially greater than that of submerged jet. The increasing trend of the effective drainage radius slows down after 3 months with gas flow attenuation. Under the special submerged circumstances, the difference of coal cutting performance caused by the two jet patterns should be fully considered in order to reasonably arrange the spacing of boreholes. [ABSTRACT FROM AUTHOR]

Gas pressure is one of the major factors to characterize the outburst danger of coal seams. Generally, the process of gas pressure determination is complicate and cannot ensure the accuracy of results, which may cause a wrong identification of outburst danger. This paper discussed and revealed a positive correlation between initial gas emission and gas pressure, which provides a new approach to identify the outburst danger by initial gas emission. Through theoretical analysis and numerical simulation, the liner equation of gas pressure and initial gas emission was deduced. Laboratory experiments under adsorption and desorption of CO2 and N2 were performed using coal samples from Weishe coal mine. The data of initial gas emission under different gas pressure were recorded, and the fitting curves were obtained. The results show a positive correlation between initial gas emission and gas pressure, and the curve of N2 was below the CO2. Especially, while the adsorption gas pressure was under 0.663 and 0.675 MPa, the difference of corresponding initial gas emission was only 0.01 L/s, which indicates the high degree of correlation. The results validated the theoretical and practical feasibility to identify the coal and the gas outbursts through initial gas emission. [ABSTRACT FROM AUTHOR]

China is vigorously promoting the development of coalbed methane (CBM) resources because CBM is cleaner than coal and a hazardous gas in coal mining. However, the CBM production in China is significantly lower than that in other coal‐rich countries, such as Australia and the United States. In this paper, the main problems hindering CBM development in China are discussed to provide management suggestions for the government. These key problems include exploration and development techniques, the overlap of mining rights, management, financial supporting policy, investment environment, regulations, and market conditions. The results of this study indicate that to improve the development of the CBM industry in China, national and local governments must enact specific and exercisable regulations to increase financial support, improve technology, increase investment in CBM exploration, strengthen public project establishment, and improve the reform of CBM exploration and development policies. [ABSTRACT FROM AUTHOR]

At present, the coordinated exploitation of coal measures‐associated resources is facing many problems, such as wide range, complex technology, and lack of in‐depth theoretical research. To study and solve the coordinated exploitation of coal and oil‐type gas‐associated superimposed resources, taking Huangling No.2 mine in Ordos Basin as an example, a fluid–solid coupling finite element model for coal mining in associated mines is constructed, and the variation laws of axial stress, displacement, stability and oil‐type gas pressure of each gas reservoir at different advancing distances of coal face are numerically simulated and the disturbance characteristics of the mining coal seam to the gas reservoir are obtained. The fluid–solid coupling finite element model of associated mine extraction is constructed, and the variation laws of axial stress, displacement, stability, and oil‐type gas pressure of the gas reservoir and coal seam when extracted from different gas reservoirs are numerically simulated. The disturbance characteristics of the surface‐extracted gas reservoir to the coal seam are obtained. The study results show that the larger the interval between the gas reservoir and the mining coal seam, the smaller the degree of disturbance, and the pressure relief effect of the oil‐type gas in the gas reservoir is positively related to its displacement; the surface drilling extract oil‐type gas, the oil‐type gas in the underlying gas reservoir will migrate and supplement to the extracted layer, and the plastic area around the extraction well basically appears in the mudstone layer and coal seam, and the gas reservoir near the coal seam has good pressure relief effect. Combined with the mutual disturbance characteristics between gas reservoir extraction and coal seam extraction and the field practice, disturbance control strategies for coordinated exploitation of superimposed resources of coal and oil‐type gas in Ordos Basin are proposed. [ABSTRACT FROM AUTHOR]

Any disturbance in the ventilation system of a mine during a coal and gas outburst can lead to secondary disasters. This is because, on the one hand, the expansion power of the outburst source makes the airflow in the ventilation system countercurrent, causing the gas in this system to exceed the allowable limit. On the other hand, the airflow density changes because of the outburst and the consequent airflow mixing in the mine roadway, thereby changing the natural wind pressure. From this, the concept of natural gas wind pressure is proposed, and a calculation method for this pressure in a 3D model mine ventilation system is derived. For the "11.10" major coal and gas outburst that occurred in Shizhuang Coal Mine in Qujing, Yunnan Province, the entire process of the counter flow and gas dispersion flow in the main and auxiliary shafts is analyzed using the TF1M 3D simulation program, including the dynamic change in the natural wind pressure in the mine in each stage. The simulation shows that during the gas outburst period, the natural gas pressure of the countercurrent circuit is greater than that of the main fan. Between 140 s and 225 s following the outburst, the natural wind pressure once overcomes the fan pressure and reverses the airflow in the 1824 transport roadway, and the gas is withdrawn from the 1727 service point. Evidently, the natural pressure of the gas produced by the outburst affects the mine ventilation system. In the event of a coal and gas outburst in a private village coal mine, if the natural gas wind pressure can be reasonably utilized and the main fan stopped or the entire mine reversed in time, casualties may be minimized. Scholars studying coal and gas outburst mines should pay attention to mine airflow disorder due to the varying natural gas wind pressure when an outburst occurs and perform simulation drills of the outburst beforehand. Thus, a scientific emergency management plan for mine disaster prevention and reduction can be formulated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The high‐pressure gas expansion‐induced deformation and dynamic fracture of coal are important parts of coal and gas outburst. To better understand the law of this process, laboratory experiments and numerical simulation are used to study the law of damage. A cavity with different pressures of CH4 or N2 was destroyed by a jack to achieve the rapid expansion of the gas and coal fracture inside. The particle size distribution of the coal particles before and after the experiment was measured, and the breakage ratio and the newly added surface area were calculated. The experimental results indicate that during the gas expansion process, the breakage ratio of coal and the newly added surface area clearly increase with the increase in gas pressure. Finally, a numerical model based on peridynamic theory was developed to simulate crack generation and the propagation of coal induced by the expansion of gases at different pressures. The numerical simulation results show that the higher the initial gas pressure is, the higher the number of failure units. Moreover, only when the gas pressure is large enough will the coal crack in various directions at the same time. [ABSTRACT FROM AUTHOR]

Predrainage of gas by cross‐boreholes is an important method to prevent coal and gas outbursts in the process of a coal roadway excavation. Limitations of technologies and poor effects of preventing coal and gas outbursts occur in the coal seam with a soft layer. In this study, a new hydraulic caving technology for cross‐boreholes has been proposed to solve this problem. A CT equipment was used to observe the development of coal fissures around cross‐boreholes in the coal seam with a soft layer. The plastic zone range and permeability distribution of ordinary hydraulic slotting technologies and the new hydraulic caving technology also were compared by numerical simulation. The field test of hydraulic caving was taken in the Xuehu Mine (China). The results show that the fracture development degree of soft coal is greater than that of hard coal at the same distance from cross‐boreholes. The fracture development degree in the plastic zone is greater than that in the elastic zone. The hydraulic caving technique can achieve the same effect as the ordinary hydraulic slotting with less workload. The mass of coal discharged from a cross‐borehole was 1.15 t/m and the average methane concentration of single drainage borehole increased by 1.09‐4.1 times, and the average methane extraction amount of single drainage borehole increased by 6.06 times. Elimination of coal and gas outburst had been completed after 110 days of extraction in the area of hydraulic caving, and Q and S values are lower than the critical value during roadway excavation. It is conclusion that hydraulic caving technique can effectively improve the permeability and rapidly eliminate coal and gas outburst in the coal seam with a soft layer. [ABSTRACT FROM AUTHOR]

We report laboratory experiments to investigate the dynamic failure characteristics of outburst‐prone coal using a split Hopkinson pressure bar (SHPB). For comparison, two groups of experiments are completed on contrasting coals—the first outburst‐prone and the second outburst‐resistant. The dynamic mechanical properties, failure processes, and energy dissipation of both outburst‐prone and outburst‐resistant coals are comparatively analyzed according to the obtained dynamic compressive and tensile stress‐strain curves. Results show that the dynamic stress‐strain response of both outburst‐prone and outburst‐resistant coal specimens comprises stages of compression, linear elastic deformation, then microfracture evolution, followed by unstable fracture propagation culminating in rapid unloading. The mechanical properties of both outburst‐prone and outburst‐resistant coal specimens exhibit similar features: The uniaxial compressive strength and indirect tensile strength increase linearly with the applied strain rate, and the peak strain increases nonlinearly with the strain rate, whereas the elastic modulus does not exhibit any clear strain rate dependency. Differences in the dynamic failure characteristics between outburst‐prone and outburst‐resistant coals also exist. The hardening effect of strain rate on outburst‐prone coal is more apparent than on outburst‐resistant coal, which is reflected in the dynamic increase factor at the same strain rate. However, the dynamic strength of outburst‐prone coals is still lower than that of outburst‐resistant coals due to its low quasi‐static strength. The dissipated energy of outburst‐prone coal is smaller than that of outburst‐resistant coal. Therefore, the outburst‐prone coal, characterized by low strength, high deformability, and small energy dissipation when dynamically loaded to failure, is more favorably disposed to the triggering and propagation of gas outbursts. [ABSTRACT FROM AUTHOR]

Creep behavior is one of the most important mechanical behaviors of coal. In the present study, we systematically carried out a series of creep experiments of briquette samples under triaxial compression and gas pressure. The experimental results showed that the creep deformation characteristics of coal were related to deviatoric stress and gas pressure. The accelerating creep stage initiated when the deviatoric stress reached or exceeded the yield stress until failure took place. In contrast, when the deviatoric stress was less than the yield strength, only transient and steady creep stage occurred. Besides, gas pressure played a positive role in the creep process of coal. The axial strain of coal slightly increased with gas pressure (range from 0.13 to 0.40 MPa) when the effective triaxial stress remained constant. For example, in the steady creep stage (σ1 = 9 MPa, p = 0.13 MPa), the axial strain of sample S1 is 2.710% and that of sample S2 is 3.361%, that is, coal samples with high gas pressure have higher axial strain. Moreover, gas pressure has two main effects on the mechanical behavior of coal. One is that the coal adsorbs gas molecules resulting in swelling strain and swelling stress, and the other is that the gas pressure may compress the coal particles. Considering the swelling stress and gas pressure, we intensively studied the effective stress formula of coal containing gas. Then, the effective stress was taken into consideration in the nonlinear Nishihara creep model. Furthermore, the tertiary creep factor n was introduced in the nonlinear model to control the creep deformation evolution with time so that the nonlinear model could accurately describe the full creep stage of coal. In contrast, due to the lack of such a controlling parameter, the Nishihara model failed to reproduce the accelerating creep stage. [ABSTRACT FROM AUTHOR]

To explore practical approaches for the evaluation of favorable shale gas areas, a structural decomposition analysis with respect to the index system, evaluation criteria, weight determination, and evaluation methods was carried out. Five types of indexes were determined from 25 subindexes that formed the index system, and improved whitenization weight functions are proposed to establish evaluation criteria. The index weights calculated using entropy weight and Euclidean distance methods are consistent, whereas the weights determined by gray relational analysis are significantly affected by the relative number of indexes and evaluation objects, selected mother factor, and resolution coefficient. The significant linear positive correlation between the relative approach degree values calculated using TOPSIS and the scores obtained using fuzzy comprehensive evaluation (FCE) is indicative of a consistent evaluation result, but it should be noted that extreme indicator values can have strong effects on the evaluation using the fuzzy similarity method. Finally, an exploration strategy for China's shale gas is suggested: Areas with weak reformation and relatively low maturity are expected to be favorable for marine shale production in South China, and continental and transitional shale‐bearing basins are promising for the prospecting multiple‐origin shale gas and coal measure gas, respectively. [ABSTRACT FROM AUTHOR]

The water content distribution in a coal body is closely related to the gas distribution. It is difficult to directly determine the water content distribution in the process water imbibition. To retrieve the water content by the resistivity value and establish a relationship between the resistivity and water content, the response characteristics of the resistivity and water content of remolded coal formed under different loads are studied by a self‐designed resistivity experimental system. The results show that the resistivity of the remolded coal decreases over time during the imbibition process, including a rapidly decreasing stage and a slowly decreasing stage. The resistivity of the remolded coal is related to the water in the macropore in the rapidly decreasing stage. In the slowly decreasing stage, the resistivity is related to the contact between coal particles. The pressing load, imbibition range, and imbibition speed are negatively correlated with porosity. The resistivity of the steady state is also negatively correlated with the water content and pressing load. The relationship between the resistivity of the steady state and the water content fit a power function. This study reveals the law of resistivity and the water content during the imbibition process and provides an effective methodology to bridge the gap for measuring water content distribution during the imbibition process. [ABSTRACT FROM AUTHOR]

To further more deeply understand the influence of cyclic loading‐unloading on coal seams during mining and reduce the rock burst and coal and gas outburst disasters and ensure safety during coal mining, triaxial cyclic loading‐unloading tests were performed on coal samples. The permeability recovery rate, residual deformation, and dissipated energy ratio were defined to analyze the evolution in the characteristics of permeability, deformation, and energy observed during the triaxial cyclic loading‐unloading test. The results show as following: (a) The absolute permeability recovery rate first gradually decreases and then increases before failure, while the relative permeability recovery rate decreases sharply during the compaction stage, remains stable during the elastic phase, and gradually increases before failure; (b) The cumulative residual deformation increases with increase number of cyclic loading‐unloading, while relative residual deformation first decreases gradually, then stabilities, and then rises sharply before failure; (c) With increases in deviatoric stress, the total energy of coal samples increased as an exponential function. The dissipated energy ratio gradually decreased in the initial stages of cyclic loading‐unloading, then tended to stabilize in the elastic stage and then increase gradually before failure. This study provides precursor conditions for the early warning of rock burst and coal and gas outburst disasters, which has great importance for reducing coal mine dynamics disasters. [ABSTRACT FROM AUTHOR]

In China, one or several ultrathin soft coal sublayers are widely developed in the coal seam. Therefore, a method of using hydraulic flushing in soft sublayer to enhance the gas extraction in these particular coal seams is developed in this work. We first established a new fully coupled gas extraction model by combing gas diffusion, gas flow, and a permeability model that considers the effect of stress change and plastic failure. By adopting this model, the gas extraction enhancement mechanism and its main influence factors were studied using the numerical simulation method based on the engineering and geological background in the Yangquan No.5 coalmine. Thereafter, a hydraulic flushing equipment, which could move freely in the underground coalmine, was developed to apply the hydraulic flushing method in soft coal sublayers used in the 8402 working face. Its gas extraction effect was systematically investigated. Our simulation results match well with the field results, suggesting that our model is feasible. Meanwhile, after adopting this method, the gas extraction condition in this coalmine improves significantly. The borehole number decreases by 66.7%, while the gas extraction rate and gas extraction concentration increase by 1.33 times and 3 times, respectively. Moreover, during the coal mining process, the gas adsorption index of drilling cuttings, the quantity of drilling cuttings, and the CH4 concentration also decrease dramatically. [ABSTRACT FROM AUTHOR]

Coal and gas outburst disasters are prone to occur within rock cross‐cut coal uncovering in deep underground coal mines. To reduce the risk of coal and gas burst under rock cross‐cut coal uncovering, a technique of permeability enhancement using hydraulic fracturing sand‐carrying is proposed. Field test was conducted using HF sand‐carrying approach, and the effect of permeability enhancement was investigated. The results show that the evolution of cracks has been subjected three stages: energy slowly increasing and crack initiation, damage localization of coal seam and gradually failure, and crack instability expansion and formation of a complex fracture network system. In addition, it is found that "water‐sand" type injection of high‐pressure fluid carried sand into HF cracks, which creates a support force on crack surface and prevents crack closure. A fully developed fracture network system with high flow conductivity capacity is created and thus increases gas permeability largely. Furthermore, a clearly increasing trend associated with the HF sand‐carrying is found for the treated coal seam area. The permeability coefficient is about 21.5‐30.5 times higher than that from the raw coal seam. A good prevention effect of coal and gas outburst is achieved using HF sand‐carrying method. [ABSTRACT FROM AUTHOR]

Coal seams in China are mainly characterizsed by low permeability and high risk of coal and gas outburst. Therefore, the elimination of coal and gas outburst risk of low‐permeability coal seam remains a hard challenge. Taking Shihuatian Coal Mine in Guizhou Province, China as an example, boreholes were slotted using high‐pressure water jet to relieve the gas pressure inside the coal seam and increase the permeability of coal mass, which thus improves the gas drainage efficiency and eliminate the outburst risk. The results show that the gas drainage efficiency of the slotted boreholes significantly increases. Compared with the conventional boreholes, the average gas concentration and flow velocity improved by 1.6 and 7.5 times, respectively. Moreover, the prediction index of outburst risk is less than the critical value after high‐pressure water jet slotting, which indicate that the coal and gas outburst risk has been effectively eliminated. The measured values of gas desorption index of drilling cuttings are all lower than the critical value after the application of the high‐pressure water jet slotting technology. The research achievements could provide a practical reference for the effective gas disaster prevention and control in low‐permeability coal seams. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The permeability characteristics of gas‐containing coal under different radial stress, different axial stress, and gas pressure were studied by orthogonal experiments, using a self‐developed three‐axis servo fluid infiltration system with a gas‐solid coupling of gas‐containing coal, on the basis of a single‐factor influence on the permeability of gas‐containing coal. By considering the effective stress, three kinds of relationships between permeability and radial stress, permeability and axial stress, as well as permeability and gas pressure were established. The results show that radial stress, axial stress, and gas pressure have a great influence on the permeability characteristics of gas‐containing coal: (a) The influence of radial stress on gas permeability is significant, whereas the influence of axial stress is negligible. The degree of influence of radial stress, gas pressure, and axial stress on the permeability decreases in turn; (b) The permeability decreases following a power function with the increase of the radial stress; (c) The permeability gradually increases with the increase of axial stress. With the increase of axial stress, the permeability increases following a power function; (d) The increase in gas pressure will reduce the effective stress on the coal, and the number of pores and cracks inside the coal body will decrease. This will increase the effective seepage flow and gas flow rate of the gas and eventually lead to the increase of coal permeability. There is a quadratic function relationship between the permeability and gas pressure. The influence of radial stress on gas permeability of gas‐containing coal is significant, whereas the influence of axial stress is negligible. The permeability of coal sample increases with the radial stress and it decreases following a power function. With the increase of axial stress, the permeability of coal samples increases following a power function. There is a quadratic function relationship between the permeability of gas‐containing coal and gas pressure. [ABSTRACT FROM AUTHOR]

Abstract: Post‐underground coal gasification (UCG) sites hold attractive prospects for geological storage of carbon dioxide. For the successful commercial implementation of UCG with carbon capture and storage (CCS), site‐selection is crucial, and a careful techno‐economic feasibility analysis is essential to systematically assess the site related parameters aside from evaluating the environmental risk. This study is related to one of the important aspects of site selection‐ the coal type. Specifically, this work investigates the influence of coal properties and gasification conditions on the adsorption capacities of CO2 on gasified coal chars. For this purpose, four coals of diverse ranks varying from lignite to bituminous were selected and subjected to CO2 gasification at atmospheric pressure for 10 min at 800, 900, and 1000°C under a low heating rate of 5°C/min. Subsequently, the gasified chars, as well as the raw coals, were tested for their adsorption capacity in a purpose built volumetric adsorption apparatus at 45.5°C and pressures up to 90 bar. Also, complementary coal and char analysis were carried out for determining the surface area, pore size distribution, and surface morphology. The CO2 storage capacity was observed to be a strong function of the coal properties and gasification conditions. Among the samples examined, the highest adsorption capacity was observed for chars of the sub‐bituminous coals. The CO2 adsorption capacity at 80 bar and 45.5°C on the sub‐bituminous char samples was 2.08, 2.43, and 1.95 mmole/g that were prepared at 800, 900, and 1000°C, respectively. The experimental adsorption isotherms were fitted to the Dubinin‐Radushkevic (DR) and the Dubinin‐Astakhov (DA) models. [ABSTRACT FROM AUTHOR]

Abstract: This study investigated the steam gasification of Indonesian Adaro coal and Japanese cedar mixed in a 1:1 ratio with the physical addition of 10 wt% Fe2O3 in 50 vol% H2O at ambient atmospheric pressure and 800°C for 1 h. The primary objective of this work was to examine the effectiveness of an iron catalyst on the interaction between Indonesian Adaro coal and Japanese cedar. The study demonstrated that the H2 evolution amount for co‐gasification of Japanese cedar and Adaro coal (with 1:1 ratio in weight) without a Fe catalyst was 100 mmol/g‐char. However, the H2 evolution amount for co‐gasification with the addition of Fe2O3 was 152 mmol/g‐char. The increase in the co‐gasification for H2 evolution was based on a change in the char structure during pyrolysis and gasification. [ABSTRACT FROM AUTHOR]