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With the accelerated development of surface well cementing technology, the engineering site has higher requirements for the early fluidity and coagulation performance of surface well cementing water slurry. In order to improve the top-off efficiency of cement injection operation in cementing engineering and improve the application of cementing engineering field operation, the method of nanoalumina particles modified cementing cement was proposed, and the NMR technique was used to study the change characteristics of the hydration reaction of nanoalumina-modified cement slurries (with the water-cement ratio of 0.44) with the mass fractions of 0.00% wt, 0.05% wt, 0.10% wt, 0.15% wt, and 0.20% wt at different hydration ages were investigated to characterize the changes in hydration reactions, and to probe the aqueous phase distribution, changes in the total signal amount of physically bound water, peak shape index, and migration rate of the nanoalumina-modified cements. The results showed that the T2 patterns of nano-alumina-modified cement pastes at the hydration stage would show three relaxation peaks, corresponding to the flocculated structure-filled water (0.1−10 ms), capillary water (10 ms), and free water (800−1000 ms), respectively; and the total signal of the physically bound water decreased gradually with the increase of the mass fraction of nanoalumina, in which the nanoalumina-modified cement pastes with the mass fraction of 0.20%wt. Alumina-modified cement paste has the fastest reduction rate of signal; the peak shape index shows the trend of increasing and then decreasing, when the hydration reaction is carried out to 600 min, the modified cement paste with mass fraction of 0.20%wt has the fastest rate of change, and the rate of the relaxation peak moving to the short relaxation direction is accelerated; the combination of the migration rate of the nanoalumina-modified cementing cement hydration reaction stage division, and the hydration mechanism of its hydration mechanism respectively acted in the four Hydration period: ① initial hydration period (5−60 min), the hydration rate did not change significantly; ② accelerated hydration period (60−600 min), nano-alumina to make the physical binding water in the cement paste into chemical binding water in a shorter period of time, the hydration process is accelerated to achieve the effect of promoting the hydration of the cement; ③ stabilized hydration period (600−1200 min), the cement system tends to stabilize; ④ Delayed hydration period (1200−1800 min), the rate of conversion of physically bonded water to chemically bonded water slows down, and the hydration process of cement slows down. It was concluded that the hydration period of nanoalumina-modified cement paste can be divided into four stages, and nanoalumina plays the roles of physical filling, accelerating, stabilizing, and retarding for the cement paste, respectively. In the accelerating period, nanoalumina promotes the early flow of cement, which is conducive to the pumping and flow of cement paste, and in the retarding period, nanoalumina retards the further hydration process of cement, which is conducive to the development of early strength. Among them, the nano-alumina modified cement paste with a mass fraction of 0.20% wt has the greatest influence on the cement hydration process, promotes the conversion of filler water to bound water, and accelerates the hydration reaction of cement paste to the greatest extent during the accelerated hydration period, which serves as the basis for providing guidance for the preparation and application of cementing cements in the field.

Abstract The method of segmented hydraulic fracturing in the coal seam roof has proven to be an efficient technique for coalbed methane exploitation. However, the behavior of hydraulic fractures in multilayer formations with significant differences in mechanical properties is still unclear. This paper studied the variation in hydraulic fracture width at the coal-rock interface by employing experimental method with a true triaxial hydraulic fracturing experimental system and numerical simulation method. Results revealed that the hydraulic fracture more likely to expanded along the coal-rock interface instead of break through it with the small horizontal stress difference and low flow rate injection condition. And improving the injection flow rate lager than a critical value, the hydraulic fracture tends to break through the coal-rock interface. Hydraulic fractures in both mudstone and coal beds exhibited a trend of increasing and then decreasing of fracture width at the interface. Since the strength of the coal seam was lower compared to that of the mudstone, maintaining high pressure was no longer necessary when the hydraulic fracture crossed the interface and entered the coal seam, leading to a reduction in fracture width within the mudstone. During the later stages of fracturing, the entry of proppant into the coal seam became challenging, resulting in a phenomenon characterized by excessive fluid but insufficient sand. The time required for the fracture width to traverse the proppant was found to be inversely proportional to the difference in horizontal ground stress and the flow rate of the fracturing fluid. And it was directly proportional to the modulus of elasticity, permeability of the coal seam, and interface strength. The interface strength has the greatest influence on the width of hydraulic fractures. In conclusion, this study provides valuable insights into the behavior of hydraulic fractures in multilayer formations with varying mechanical properties. The findings contribute to a better understanding of the factors affecting hydraulic fracture width within coal seams, which can ultimately enhance the efficiency of coalbed methane exploitation.

The three-field (stress, displacement and fracture field) evolution laws of the inclined coal seam group are complex, which are important for the transport and storage of pressure-relief gas. In order to investigate the three-field evolution law of inclined coal seam group, the study carried out a similar simulation experiment of multiple mining in inclined coal seam group with the 1930 coal mine in Xinjiang as an object. The collapse pattern of the overlying strata was analyzed, the stress evolution characteristics of overlying strata was obtained, the overlying strata displacement distribution and movement direction characteristics were analyzed, and the characteristics of mining-induced fracture distribution were elucidated. The influence of the three-field evolution law on gas migration was further investigated, and a directional borehole gas extraction field test was carried out for veri-fication. The study results shown that, the rectangular ladder platform of mining-induced fracture shown obvious asymmetric characteristics under multiple mining of inclined coal seam group. The overlying strata stress of the lower side was more variable, and the pressure-relief effect was more obvious with increasing mining frequency, while the overlying strata stress of the higher side was less variable. Combined with the gravity-dip effect, the overlying strata of the higher side was more prone to be damaged, and the collapse order was preferred with asymmetric characteristics. The displacement distribution of overlying strata was asymmetric, with significant displacement on the high side and large changes in the movement direction. The frequency of mining-induced fractures in the high side fractured area was significantly higher than that in the low side. The high side fracture area had more fracture distribution and fracture aperture was bigger. The mining-induced fractures shown the asymmetric characteristics of “high expansion-low compression”. The multiple mining made the asymmetric characteristics of three-field more significant. In addition, there was a “slow decreasing-fast decreasing” in the overlying strata penetration. Based on the relationship between three-field evolution characteristics and gas mi-gration, the mechanism of preferential gas extraction targeting was revealed. Combined with the experimental results, the process of determining the target area for gas extraction in the fractured zone based on the three-field evolution law was constructed. The gas extraction effect in the field was great, which ensured the safe and efficient recovery of the working face. The results of this study provide a theoretical reference for the accurate extraction of pressure-relief gas in the inclined coal seam group, aiming to improve the gas extraction from the inclined coal seam group, prevent gas overlimit in the upper corner, and achieve safe and efficient mining of the inclined coal seam group.

In order to clarify the key technologies for the coordinated development of coal and coalbed methane that are compatible with the mining conditions of inclined broken soft coal seams. Based on the characteristics of gas drainage and asymmetric mining of inclined crushed soft coal seam group gas drainage and coal seam group, a matching coordinated development mode of coal and coalbed methane was formed in the spatiotemporal synergy of “three holes, four zones and five quantities” of asymmetric pressure relief, and the coordinated development strategy of coal and coalbed methane under asymmetric mining was clarified, and the spatiotemporal synergy mechanism of coordinated development of combined coal and coalbed methane was clarified. In view of the problem of difficulty in eliminating the first mining seam of inclined crushed soft coal seam, the construction process of long drilling under inclined crushed soft coal seam was optimized, and a wireless measurement system of electromagnetic wave while drilling of drill bit was developed, which realized higher precision anti-biasing of drilling trajectory. Aiming at the problem of lack of pertinence of surface well protection in asymmetric mining area, the temporal and spatial evolution characteristics of asymmetric mining-induced pressure relief failure were clarified, the evolution characteristics of mining-induced fractures under asymmetric mining were obtained, the safe position of wells in the surface well of inclined coal seam was determined, and a three-open casing structure in the mining area was developed that could take into account the stability and extraction efficiency of surface wells. The application results show that the “three-hole, four-zone and five-quantity” mode of spatiotemporal synergy can realize the efficient extraction of coalbed methane and ensure the balance of extraction and mining of inclined coal seam groups. The progressive extraction method of downward drilling of inclined coal seam expands the application scope of progressive extraction and solves the problem of time incoordination in the extraction process of inclined coalbed methane. The optimized stability maintenance structure of the mining surface well can adapt to the asymmetric mining effect, and realize the efficient extraction of coalbed methane from the surface well in the mining area. The above results have been popularized and applied in the Aiweigou mining area of Xinjiang Coking Coal Group, and the key technology and typical model of coalbed methane extraction that conform to the characteristics of inclined coal seams in major high-gas mining areas in Xinjiang have been initially formed.

Abstract Accurately determining the precursors of coal and rock failures is crucial for preventing associated risks in coal mines. In this study, based on the values of the four parameters amplitude, duration, counts and energy of acoustic emission (AE) signals, the AE signals of coal and sandstone samples failure were clustered by the two‐step clustering method. And the precursory AE signals of sample failure under uniaxial compression were analyzed and identified. Results show that the peak of AE amplitude is before that of AE duration, counts, and energy. AE signals of both coal and sandstone samples can be classified into five types. Values of the parameters of Types I–IV AE signals of the sandstone sample are lower than that of the coal sample. The duration, counts, and energy of Type V AE signals of the sandstone sample are higher than that of the coal sample. Type V AE signals, which are middle amplitude (hundreds of mV), high duration (hundreds of thousands of μS), high counts (about 10,000), and high energy (tens of thousands of mV*mS), account for the lowest proportion of all AE signals, at approximately 0.1%. And they began to occur at 87% σc of the coal sample and at 97% σc of the sandstone sample, respectively. Type V AE signals can be used as precursors of coal and sandstone samples failure under uniaxial compression.

The coal pillar is an important structure to control the stability of the roadway surrounding rock and maintain the safety of underground mining activities. An unreasonable design of the coal pillar size can result in the failure of the surrounding rock structure or waste of coal resources. The northern Shaanxi mining area of China belongs to the shallow buried coal seam mining in the gully area, and the gully topography makes the bearing law of the coal pillar and the development law of the internal fracture more complicated. In this study, based on the geological conditions of the Longhua Mine 20202 working face, a PFC2D numerical model was established to study the damage characteristics of coal pillars under the different overlying strata base load ratios in the gentle terrain area and the different gully slope sections in the gully terrain area, and the coal pillar design strategy based on the fractal characteristics of the fractures was proposed to provide a reference for determining the width of the coal pillars in mines under similar geological conditions. The results show that the reliability of the mathematical equation between the overlying strata base load ratio and the fractal dimension of the fractures in the coal pillar is high, the smaller the overlying strata base load ratio is, the greater the damage degree of the coal pillar is, and the width of the coal pillar of 15 m under the condition of the actual overlying strata base load ratio (1.19) is more reasonable. Compared with the gentle terrain area, the damage degree of the coal pillar in the gully terrain area is larger, in which the fractal dimension of the fracture in the coal pillar located below the gully bottom is the smallest, and the coal pillar in the gully terrain should be set as far as possible to make the coal pillar located below the gully bottom, so as to ensure the stability of the coal pillar.

Studying pore structures of disturbed coal and their influences on adsorption characteristics is conducive to in-depth understanding of occurrence and migration of gas in reservoirs in areas prone to coal and gas outbursts. A mercury porosimeter and a high-pressure gas adsorption instrument were separately used to investigate pore structures and measure adsorption characteristics of disturbed coal and undisturbed coal in Ningtiaota Coal Mine and Xigu Coal Mine (Shaanxi Province, China). In addition, pore structures and gas adsorption characteristics of coal samples were studied. The Menger’s sponge model was adopted to calculate fractal dimensions of coal samples, to estimate influences of pore structures and fractal features on the gas adsorption characteristics of disturbed and undisturbed coal. Results show that the pore volume of undisturbed coal is mainly contributed by micropores and transitional pores, while that of disturbed coal arises mainly from macropores and mesopores. Micropores and transitional pores account for large proportions of the specific surface area of pores in both disturbed and undisturbed coal. The adsorption isotherms of disturbed and undisturbed coal conform to the Langmuir equation and tectonism increases the limiting adsorption quantity of coal. The fractal dimensions D1 of the four types of coal samples in the experiments are in the range of 2.7617–2.9961, while the fractal dimensions D1 and D2 of disturbed coal are both larger than those of undisturbed coal, indicating that disturbed coal is more likely to collapse under high pressure. The total pore volume, total specific surface area of pores, and fractal dimensions are positively correlated with the adsorption constant a, while they have U-shaped correlations with the adsorption constant b of coal samples. The adsorption constant a of disturbed coal is always greater than that of undisturbed coal, while no obvious trend is observed between the adsorption constant b and tectonism. The research results can provide theoretical basis for further study of gas occurrence in disturbed coal seams.

Coal and gas outburst is one of the major disasters that restrict the safe production of coal mine. The coal seam occurrence environment in China is complex and changeable. Outburst disasters occur from time to time. In order to further improve the accuracy of outburst prediction and early warning, some progresses in the mechanisms of outburst were reviewed, and the three key elements, which are gas, crustal stress and coal mechanics, for the prevention of outburst were pointed out. The development status of outburst prediction was summarized. The prediction methods mainly include single index method, comprehensive index method and multi-attribute index method. The main shortcomings of prediction methods are small prediction range, non-continuous prediction, poor adaptability, etc. The key progress of outburst early warning was analyzed. Based on the changes of crustal stress, gas and coal in the process of outburst preparation, the early warning methods of outburst mainly include acoustic emission (AE), electromagnetic radiation (EMR), micro-seismic (MS), gas concentration, and AE-EMR-Gas comprehensive monitoring and early warning methods. The purpose of real time early warning is realized by judging the dangerous values of monitoring parameters. At present, the field application effect is affected by the low accuracy of monitoring data and the low reliability of early warning results. Based on the current situation of outburst prediction and early warning, as well as the demand for intelligent coal mine safety, the future research prospects were proposed. The outburst prediction should develop fine and quantifiable indexes about starting criteria and intensity prediction. Outburst early warning should track the nonlinear changes of indicators, develop trend early warning models based on theoretical indicators, empirical early warning models based on accident matching, and precursor recognition early warning models based on monitoring data mining. Through combining early warning models, combining qualitative and quantitative early warning methods, a combined early warning model based on theory, experience, and data was formed to further improve the accuracy of early warning. At the same time, the digital twin construction of mines should be developed to form an integrated, continuous and accurate visual intelligent early warning of coal mine outburst disasters.

With the global transition to a low-carbon energy structure, coupled with the depletion of coal mine resources, gas disasters, and other problems, the number of closed/abandoned coal mines is increasing rapidly. After the coal mine closes, the residual methane in the goaf escapes to the ground continuously, becoming an essential source of greenhouse gas emissions. In view of a series of issues such as methane residual stock, emission rate and emission reduction measures in closed/abandoned coal mines, the number of closed/abandoned coal mines and the distribution of high-gas mines at home and abroad were clarified through a large number of literature review, and the sources and estimation methods of methane residual stock were summarized. At the same time, the theory and research method of methane geological leakage in the field of natural gas accumulation was used for reference, and the emission mechanism and monitoring means of residual methane were analyzed. Finally, the countermeasures and challenges of residual methane emission reduction were put forward. The study found that there are many closed/abandoned coal mines in Shanxi, Guizhou, Chongqing, Hunan, Jiangxi, etc., and the residual coal is mainly anthracite with strong methane adsorption capacity, resulting in a large amount of residual methane in the mine, which will become a critical methane emission source. Methane from closed/abandoned coal mines is desorbed and released to the goaf, and then discharged to the atmosphere through channels such as wellhead and mining-induced fractures. Methane monitoring in coal mines can be realized by means of satellite remote sensing, flux chamber method, geochemical probe method, micro-meteorological technology, etc. Based on the methane emission prediction model of closed/abandoned coal mines, the methane emissions from closed/abandoned coal mines may account for more than 20% of total methane emissions from coal mining operations by 2050, so it is urgent to solve the problem of methane emissions from closed/abandoned mines. Therefore, the countermeasures of emission reduction, such as extraction and utilization, in-situ deflagration power generation, microbial degradation of methane, water flooding, and methane emission channel closure are put forward. Considering the limitations of cost, treatment time, groundwater contamination, and other limitation factors, it is concluded the mineralized remediation method can be used to seal large-scale mining-induced fractures in overlying rocks, which can achieve the methane emission reduction goal of closed/abandoned coal mines at a low cost.

Surface wells are prone to deformation and even instability under the mining action caused by coal seam mining. This is also a key issue that restricts the application of gas drainage technology in surface wells in mining pressure relief areas. Based on the mining of high-incline coal seam group in Xinjiang 1930 Coal Mine, this paper carried out similar simulation experiments of surface well deformation under high-incline coal seam group mining, by monitoring overlying rock movement, coal seamstress changes, and the axial and circumferential directions of different positions of surface well pipelines. Deformation reveals the law of deformation of surface wells under the mining of large-dip coal seams. Research shows that: first, surface wells are in a state of sheer, compression, and tension during the mining process. Most of the time, the shear deformation in the axial direction is greater than the tensile deformation in the axial direction, that is, the shearing action on the surface well is dominant. At the same time, during the mining process, the shear deformation in the circumferential direction and the axial expansion and contraction deformation of the surface well is negative to a certain extent. The correlation, to a certain extent, shows that the shearing and tensioning, and compressing effects of surface wells are mutually restrictive; secondly, the axial deformation of surface wells shows an increasing trend as a whole, but the phenomenon of tension and shrinkage alternates in the process, and there is a big difference in the constant increase in the repeated mining of the near-level coal seam, and the changing trend of the circumferential shear deformation is similar to that of the near-level coal seam. The direction of displacement is always the inclination direction of the coal seam; the final study also found that the surface well defamation law in the upper part of the main key layer showed an overall “increase-decrease” alternating three times, and “increase-decrease” in the main key layer and lower part. Repeat four times alternately. The research results can provide certain theoretical support for the engineering application of surface wells in high-dip multiple mining areas.

Coal and coalbed methane (CBM) coordinated exploitation is a key technology for the safe exploitation of both resources. However, existing studies lack the quantification and evaluation of the degree of coordination between coal mining and coalbed methane extraction. In this study, the concept of coal and coalbed methane coupling coordinated exploitation was proposed, and the corresponding evaluation model was established using the Bayesian principle. On this basis, the objective function of coal and coalbed methane coordinated exploitation deployment was established, and the optimal deployment was determined through a cuckoo search. The results show that clarifying the coupling coordinated level of coal and coalbed methane resource exploitation in coal mines is conducive to adjusting the deployment plan in advance. The case study results show that the evaluation and intelligent deployment method proposed in this paper can effectively evaluate the coupling coordinated level of coal and coalbed methane resource exploitation and intelligently optimize the deployment of coal mine operations. The optimization results demonstrate that the safe and efficient exploitation of coal and CBM resources is promoted, and coal mining and coalbed methane extraction processes show greater cooperation. The observations and findings of this study provide a critical reference for coal mine resource exploitation in the future.

Abstract Ad‐/desorption and diffusion processes are the key factors controlling methane production from coal. The experiments used samples from methane‐producing coal seams with different contents of carbonate mineral and different ranks. Acid treatment using 1.0 mol/L acetic acid (CH3COOH) and oxidant treatment using 1.0 mol/L ammonium persulfate ((NH4)2S2O8) were conducted on these coal samples to observe the changes in surface functional groups that act as primary sorption sites and pore structure that determines diffusivity by Fourier transform infrared spectrometer, in situ scanning electron microscopy (SEM) imaging and energy dispersive spectroscopy. The results showed that the acidizing‐related mass loss of coal samples is in the range of 3.78%–7.57% due to the complete dissolution of pore‐ and fracture‐filling carbonate minerals. For the coal samples after oxidizing treatment, dissolution evidence can be observed in both carbonate minerals and organic matter, with a mass loss ranging from 2.01% to 3.0%. Acidizing or oxidizing induced dissolution can destroy aliphatic functional groups and the cross‐linked structure of coal, resulting in a decrease in methane adsorption capacity that is beneficial to methane desorption. More opened pores and interconnected fractures can be observed in SEM images due to the dissolution of carbonate minerals, suggesting an alteration of methane diffusion pathways in the coal matrix. Although only some of the carbonate minerals were dissolved following oxidant treatment, coal organic matter is partially oxidized and generates some dissolution pores, which provides more methane diffusion pathways in the coal matrix. Therefore, all the treated coal samples showed a reduction in methane adsorption capacity and an enhancement of desorption capacity and diffusivity. This result suggests that chemical treatment in coal seams using acetic acid or oxidant has the potential to improve coalbed methane extraction.

Low-index coal and gas outburst (LI-CGO) is difficult to predict, which seriously threatens the efficient mining of coal. To predict the LI-CGO, the Support Vector Machine (SVM) algorithm was used in this study. The Particle Swarm Optimization (PSO) algorithm was used to optimize the parameters of the SVM algorithm. The results show that based on the training sets and test set in this study, the prediction accuracy of SVM is higher than that of Back Propagation Neural Network and Distance Discriminant Analysis. The prediction accuracy of the SVM model trained by the training set T2 with LI-CGO cases is higher than that of the SVM model trained by the training set T1 without LI-CGO cases. The prediction accuracy gets better when the SVM model is trained by the training set T3, made by adding the data of the other two coal mines (EH and SH) to the training set T2, that only contains the data of XP and PJ. Furthermore, the PSO-SVM model achieves a better predictive effect than the SVM model, with an accuracy rate of 90%. The research results can provide a method reference for the prediction of LI-CGO.

Coal mine gas accidents will cause great economic losses and casualties. It is of great significance to find out the essential causes of coal mine gas accidents and put forward measures to prevent them. In this paper, 110 coal mine gas accidents which occurred in China from 2001 to 2022 are selected to analyze the causes of the accidents by extracting the keywords of human factors, equipment factors, environment factors, and management factors from the accident investigation reports. Firstly, the accident statistical analysis is carried out from three dimensions of factor frequency, accident type, and accident grade. Secondly, the Apriori algorithm is used for data mining to obtain frequent item sets and association rules of coal mine gas accident factors. Finally, the coal mine gas accident cause chains which are obtained by using the association rule. The frequent terms of 9 factors, 23 association rules, and 3 coal mine gas accident cause chains are obtained. The results show that the production of coal mine enterprises by illegal organizations is an important reason for the occurrence of coal mine gas accidents. The lack of good management culture easily leads to habitual violations of personnel and decision-making errors, and then causes equipment problems, reflected in the ventilation system which is not perfect, resulting in gas accumulation. The occurrence of coal mine gas accidents can be prevented to a large extent by preventing the absence of good management culture in enterprise management and the occurrence of illegal production behaviors.

Coal mine methane (CMM) emissions from underground coal mines is becoming a global problem since it produces greenhouse gas contributing to global warming. It is critical to reduce low-concentration CMM(LCM) and ventilation air methane (VAM) emissions, which has become the focus to be solved urgently. One of the methods is utilizing this VAM as a fuel by mixing of CMM with the help of the porous burning system. In order to optimize the performance of a porous burning system composed of alumina particles, a series of complex porous media burners composed of two layers of alumina particles were proposed, after which a laboratory experimental system based on these burners was built. The lean-burn limits, lean-burn stabilization temperature and lean-burn fluctuation patterns of the burners built in this study were tested and reported in this paper. Results show that cylindrical burner with equal segment length of 3 mm diameter alumina particles at the upstream and 6 mm diameter alumina particles at the downstream is suitable for lean-burn with low flow rate, and cone burner with equal segment length of 3 mm diameter alumina particles at the upstream and 6 mm diameter alumina particles at the downstream is more suitable for lean-burn with high flow rate and low methane concentration, and the lean-burn limits for cone burner with volume methane concentration of 1.5% is obtained, which is the lowest lean-burn limit have reported at present.

The method of improving permeability of coal seams by freeze-fracturing is presented to accelerate the diffusion of coalbed gas into exploited wells. The pore structure of coal sample changes caused by freeze-fracturing were characterized by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), and a permeability-calculation model was employed to estimate the permeability of coal sample. The pore size distribution, porosity, and permeability of coal before and after freezing were analyzed. Results show that freeze-fracturing can complicate the coal pore structure, and its complexity is positively correlated with the freezing temperature and coal moisture content. The main manifestation is the growth and expansion of coal pores, with higher pore connectivity, generation of gas seepage pores with pore diameter > 100 nm, and expansion of original pores into larger ones. By comparing the connectivity and seepage space of the pore structure of coal before and after freezing, it is concluded that freeze-fracturing can promote the coal seam permeability significantly, which is beneficial for the exploitation of coalbed gas.

To quantitatively describe the distribution characteristics of stress field of coal rock in fault area, a finite element model was established to study the influence mechanisms of different fault parameters on the stress distribution characteristics of coal rock and a prediction model of the tectonic stress field of a fault-containing coal rock was established based on the numerical simulations. The numerical simulation results show that the coal rock in the hanging wall of a normal fault forms an area of disturbance significantly larger than that in the footwall of the normal fault and reverse fault. With the increase in fault throw, the stress concentration factor and fault influence range decrease in both the normal and reverse fault scenarios. Additionally, for both the normal and reverse fault scenarios, with the increase in fault dip angle, the coal rock stress concentration zone gradually collapses to the fault plane and the stress concentration coefficient gradually increases due to the reduction in the area of pressure relief. The prediction model of the tectonic stress field can better describe the quantitative relationship between the fault parameters and the stress of coal rock, which can provide guidance for the excavation design of mining engineering and early warning for possible coal and gas outburst disasters.

Abstract 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.

Abstract 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.

Abstract The increase in extraction height will increase the mining‐induced overlying strata failure height. In this scenario, the strata pressure behavior is strong in a fully mechanized face with large mining height (FMFLMH), which frequently causes coal wall falls, roof falls, and hydraulic support failure accidents (e.g., support closure and hydraulic column damage). The key to solving these issues is to determine support's working resistance of the FMFLMH. In this paper, comprehensive theoretical analysis, numerical simulation, and field observation were applied to determine the support's working resistance in the FMFLMH based on movement type of the first subordinate key stratum (SKS 1). First, six kinds of movement types of SKS 1 in the FMFLMH are found and defined by theoretical analysis and numerical simulation, which are the direct caving movement type of cantilever structure (direct caving), the double‐sided rotation movement type of cantilever structure (double‐side rotation), the quadratic rotation movement type of cantilever structure (quadratic rotation), the alternative movement type of cantilever structure hinged structure (alternate hinged), the voussoir beam structure movement type (voussoir), and the short voussoir beam structure movement type (short voussoir), respectively. Besides, based on this, the support load calculation model of each movement type was established, and a formula for the support working resistance of each movement type was obtained. Finally, the correctness of the formulae for the support working resistance under six types of movement of SKS 1 were verified using measurement data from four FMFLMHs in China. These research results have important guiding significance for reasonable selection of support and ensuring safe mining of the FMFLMH.

Abstract 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.

With mining technology and mechanization degree being improving, fully mechanized caving mining technology (FCM) has become a main method for thick coal seam extraction in China. However, roof-coal caving characteristics in turn restrict its recovery efficiency, especially for the coal seam with complicated structure (CCS), that is, the coal seam comprises hard or soft coal and gangue. In order to explore the key factors influencing the roof-coal caving and recovery characteristics, related research work has been conducted as follows: firstly, a mechanical model of CCS has been established, which indicates the strength of the coal and gangue will directly affect the roof-coal recovery. Meanwhile, based on the geological settings of Qinyuan coal mine, numerical simulation on roof-coal caving law under different thicknesses of hard or soft coal and gangue has been performed using UDEC software. The results show that the maximum principal stress will increase with the increase of mining depth, making the roof-coal to break easily. Furthermore, the range of the plastic zone of the top coal and the damage degree of the top coal increase with the increase of mining depth. Physical modeling results show that when an extraction-caving ratio is 1, the number of times the coal arch forms is 0.43 at every caving, up to a maximum of 3; the number of times coal arch forms with an extraction-caving ratio of 2 is 4.65 times larger than that with an extraction-caving ratio of 1. The probability of coal arch formation with an extraction-caving ratio of 3 is minimal, about 0.4, which is due to that the arch span is large and the curvature is small, so it is difficult to form a stable arch structure. According to the mechanical characteristics of roof-coal in Qinyuan coal mine, deep-hole blasting technique has been used to reduce the fragments of roof-coal crushed. The results show that this technique can effectively improve the recovery of roof-coal.

The isothermal adsorption experiment of coal is an important method to study the mechanism of coal gas desorption. The orthogonal experiment is used to compare the gas desorption mechanism of coal under multifactor conditions, such as the particle size, temperature, pressure, moisture content, and molding pressure. The sensitivity of five factors was used to conduct regression analysis. The sensitivity and influence degree of five factors on the coal gas desorption capacity were analyzed. The results showed the following: (1) the effect of the coal sample particle size, temperature, pressure, moisture content, and molding pressure of coal on the sensitivity of desorption capacity is shown as C (pressure) > B (temperature) > A (particle size) > D (moisture content) > E (molding pressure); (2) the regression analysis of various factors for gas adsorption indicates that the degree of correlation of the multivariate quadratic regression equation is higher compared to that of the multivariate one-time regression equation; and (3) the coal sample particle size, temperature, pressure, moisture content, and type of gas desorption can well represent the gas desorption capacity of a coal sample under various conditions. The smaller the particle size of the coal sample, the higher the ambient temperature, the higher the gas pressure, the lower the moisture content of the coal sample, and the greater the gas desorption per unit mass in 30 min after coal sample gas adsorption equilibrium. The orthogonal test results have important theoretical significance for guiding gas adsorption and desorption tests of coal. For coal and gas outburst prediction, the coal seam gas flow mechanism, coal gas content prediction, and calculation of the mining coal gas emission have important practical significance for gas explosion accident prevention.

Coal and gas outburst is a dynamic phenomenon with violent eruptions of coal and gas from the working coal seam. It has been proved that the rapid desorption within a short period is necessary for the occurrence of an outburst. Due to limitation of the present test condition, gas desorption characterization for the first 60 s has not been researched sufficiently. In the present study, an experimental apparatus with the ability of high-frequency data collection was developed. Initial desorption characterization of methane and carbon dioxide in coal was experimentally studied. Both the initial desorption characterization of methane and carbon dioxide were experimentally studied with different equilibrium pressures. The desorbed gas pressure was measured at desorption time phase of 0–10 and 45–60 s, besides the initial amount of desorbed gas and initial diffusion velocity of coal gas were calculated to assess their risk of outburst. The results show that the gas pressure for both methane and carbon dioxide increases sharply in the initial time and then levels off, and the total amount of desorbed gas increases with the increasing desorption time. Although the amount of desorption methane is slightly larger than that of carbon dioxide at the beginning, the total amount of desorbed carbon dioxide is significantly larger than that of methane at the desorption process. Therefore, it can be concluded that the coal and carbon dioxide outburst is more dangerous than the coal and methane outburst based on the obtained experimental results.

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.

This paper deals with the theoretical aspects combined with stress analysis over the floor strata of coal seam and the calculation model for the stress on the coal floor. Basically, this research presents the relevant results obtained for the rock burst prevention in the floor of roadway driven along next goaf in the exploitation of thick coal seam with large obliquity in deep well and rock burst tendency. The control mechanism of rock burst in the roadway driven along next goaf is revealed in the present work. That is, the danger of rock burst can be removed by changing the stress environment for the energy accumulation of the floor and by reducing the impact on the roadway floor from the strong dynamic pressure. This result can be profitable being used at the design stage of appropriate position of roadway undergoing rock burst tendency in similar conditions. Based on the analysis regarding the control mechanism, this paper presents a novel approach to the prevention of rock burst in roadway floor under the above conditions. That is, the return airway is placed within the goaf of the upper working face that can prevent the rock burst effectively. And in this way, mining without coal pillar in the thick coal seam with large obliquity and large burial depth (over a thousand meters) is realized. Practice also proves that the rock burst in the floor of roadway driven along next goaf is controlled and solved.