As an environmentally friendly means of mine disaster prevention and control, the existing technology of coal seam water injection has not fully integrated the seepage theoretical model and mathematical evaluation method, which makes it difficult to accurately quantify and classify the difficulty of water injection. First, a seepage model of coal seam water injection was constructed by introducing the corrected actual length of the seepage capillary channel and taking the fracture aperture and surface roughness into consideration. The main controlling factors were screened by Sobol sensitivity analysis, and a water injection capacity evaluation system associated with all main controlling factors was constructed by pair analysis, with the key index defined as P to rank the water injection difficulties of coal seams. Finally, the reliability of the constructed evaluation system was assessed using the permeabilities of six coal samples measured by seepage experiment. The results show that the water injection seepage quantity increases with the increases of fracture aperture, porosity, and pore throat radius and decreases with the increase of surface roughness, dynamic viscosity coefficient of the fluid, and contact area. Porosity has the greatest impact on seepage quantity, with the sensitivity of 0.1006, and its influence is much greater than those of other seven factors, followed by fracture aperture with the sensitivity of 0.0905. The minimum pore throat radius and fluid dynamic viscosity coefficient are the least important influencing factors of seepage quantity, showing the sensitivity of 0.0005 and 0.0109, respectively. The water injection difficulties of coal seams can be ranked into four levels, nonwater injection (0 < P < 0.25), difficult water injection (0.25 ≤ P < 0.5) interval), relatively easy water injection (0.5 ≤ P < 0.75), and easy water injection (0.75 ≤ P < 1). The evaluation system orders the water injection difficulties of the six coal samples from easy to difficult as TC > SM > YC1 > NM > ZG > YC2. Through the test results of permeability of six coal samples, it is found that the order of permeability is basically consistent with the evaluation system. The seepage model and difficulty evaluation system of coal seam water injection constructed in this paper offer a novel strategy to identify the water injection capacities of coal seams and provide support for further improving the theoretical understanding of coal seam injectability.

To optimize the selection of surfactants for coal seam water injection, this study examines the effects of ionic surfactants (DTAB and SDS) on water seepage in low-rank long-flame coal. A three-dimensional Eulerian two-phase flow model is established via ANSYS Fluent software to simulate the surfactant migration and is then validated through triaxial seepage experiments. The following crucial results are obtained: For long-flame coal from Houwenjialiang Coal Mine in Ordos City, Inner Mongolia Autonomous Region, China, the seepage coefficient increases with higher water injection pressure when using two surfactant solutions. Subjected to the water injection pressure of 2 MPa, the seepage coefficients of SDS and DTAB surfactants in order are obtained as 0.06 and 0.08, respectively, and the enhancement effect of cationic surfactant DTAB on coal seam water injection is superior to that of anionic surfactant SDS. In the presence of external pressure, the liquid seeps upwards along the pressure direction and with a steady flow, and seeps around along the pores and cracks of the coal body. The obtained results also reveal that the adsorption effect of SDS surfactant on coal is strong, resulting in a weaker water injection effect with increased water injection time into the coal seam.

The near vertical coal seam adopts a segmented mining method to form a layered composite goaf, and its spontaneous combustion law is more complex. In order to understand the evolution law of temperature and oxygen after injecting low-temperature nitrogen into the goaf, the changes in oxygen and temperature before and after injecting low-temperature nitrogen into the goaf were analyzed through on-site observation and CFD numerical simulation. The results showed that after injecting low-temperature nitrogen into the goaf, the oxidation zone decreased by 10.7 m, and the return air corner temperature of this layer and the upper layer decreased by 16.6 K and 12 K, respectively, CO decreased to 0 mg/L; As the injection time of low-temperature nitrogen gas increases, the influence range of low-temperature nitrogen gas gradually increases, and the temperature and oxygen concentration in the goaf continue to decrease to below 283 K. The highest oxygen concentration in this layer's goaf is about 6 %, while the upper layer's goaf is less affected by low-temperature nitrogen gas than this layer's goaf, and the oxygen concentration decreases to 6.8 %. This study provides theoretical support for the prevention and control of coal spontaneous combustion in goaf areas near vertical coal seams.

Hydraulic fracturing generates fractures in a coal seam, which has been extensively used in coal mining and disaster management. To study the influences of water injection pressure and time on the propagation of hydraulic fracture, we conducted true triaxial hydraulic fracturing experiments under acoustic emission monitoring. Five briquette specimens were prepared by molding the mixture of coal powder, cement, river sand, and distilled water under 60 MPa to simulate the properties of coal seam. True triaxial loading was used to simulate the in situ stress environment of the coal seam. Hydraulic fracturing experiments on the test specimens were conducted under different water injection pressures and times. The following conclusions have been drawn. At the fixed injection time, increasing water injection pressure promotes the propagation of hydraulic fracture and the migration of water in the coal seam after the fractures are connected. The number of fractures increases from 3 to 8 as the water injected pressure increased from 3 to 9 MPa. Under the constant water injection pressure, the increase in longitudinal wave velocity in the test specimen decreases with the prolongation of water injection time. When the water reaches the action boundary of the water injection pressure, the water stops moving. At this moment, the water injection time has no effect on the longitudinal wave velocity anymore. The fracturing influence range can be increased by increasing the water injection pressure to produce a fracture network of a large radius with many fractures. Appropriately prolonging hydraulic fracturing time can ensure that the fracturing fluid fully moistens the coal seam and thus effectively reduces dust pollution.

To study the synergic effects of surfactants and nanoparticles on the seepage of water injected into coal seams, we prepared three chip models with complex cracks using glass etching technology based on the real conformation of cracks in coal mass and set up a visual experimental platform for microscopic seepage. Based on these models, we carried out a series of seepage experiments on gas expelling with different solutions and investigated the effects of surfactants, nanoparticles, and flow velocities on seepage characteristics. The results showed that all tested anionic, non-ionic, and cationic surfactant solutions could reduce flow resistance, showing a downward trend with the rise of the mass fractions of these surfactants. The combination of nanoparticles with these surfactant solutions further reduced flow resistance, showing the best drag reduction effects with hydrophobic nanoparticles at all tested flow velocities. Moreover, all three tested surfactants could displace gas and reduce gas residue in the crack networks. The higher the mass fraction of the surfactant solution is, the more significant the gas displacement effect. Furthermore, the gas displacement effect of the surfactant solution is positively proportional to the flow velocity. The higher the flow velocity is, the more significant the gas displacement effect.

The geological structure and stress distribution of a coal body directly determine its hydraulic fracture initiation pressure that is one of the key factors affecting the disaster prevention by hydraulic fracturing. Based on the geological structure characteristics of steeply inclined coal seam and the stress distribution characteristics, the influences of hydraulic fracturing on the top coal of steeply inclined coal seam were explored by a series of hydraulic fracturing experiments with varied bedding directions, triaxial stresses, and water injection rates and the presence/absence of temporary plugging balls in this study. The results show that the vertical stress is the minimum principal stress, and the horizontal stress is the maximum principal stress. The water injection pressure first maintains constant at 0 MPa, gradually increases, and then drops rapidly. The fracture initiation pressure shows no obvious relationship with the horizontal stress if the bedding direction is parallel to the water injection direction, but the node pressure increases with the increase of horizontal stress. If the bedding direction is perpendicular to the water injection direction, both fracture initiation pressure and node pressure gradually decrease with the increase of horizontal stress. The presence of temporary plugging balls increases the fracture initiation pressure if the bedding direction is same as the water injection direction, but reduces the secondary fracture initiation pressure if the directions are perpendicular. The fracture initiation pressures of steeply inclined coal seams are in the range of 10 MPa-21 MPa, much higher than those of horizontal coal seams. The results of this study help to understand the hydraulic fracture initiation pressure of steeply inclined coal seam, which is of great significance to the disaster control by hydraulic fracturing during the mining of steeply inclined coal seam.

Aiming at the problem of pulverized migration and plugging propped fractures during coal seam fracturing, we experimentally studied the pressure changes and pulverized coal blocking characteristics with deionized water and solutions of three surfactants including 1227 (C21H38ClN), SDS (C12H25SO4Na) and TX-100 (C34H62O11). A device capable of visualizing propped fractures was established, and simulation experiments were conducted with solutions of different surfactants at different injection flow rates. The obtained images were binarized and analyzed to quantify the pulverized coal blockage degrees of facture under different conditions. The experimental results show that: (1) The higher the injection flow rate, the higher the inlet pressure. (2) All three surfactants can lower the injection pressure, as compared with water alone. SDS decreases the injection pressure more obviously at low injection flow rates, and the other two perform better at high injection flow rates. (3) Similar to their effects on inlet pressure, the ratio of pulverized coal in SDS solution is lower at low injection flow rates, while TX-100 and 1227 solutions show lower ratios of pulverized coal at high injection flow rates. Our work has provided a theoretical support for coal blockage removal and pressure reduction in propped fractures during coal seam fracturing to improve coal seam permeability and further improves the dust prevention effect of coal seam water injection.

Coal seam water injection can effectively prevent dust pollution and coal mining disasters, such as coal bumps and coal seam fires. However, the quantitative determination of the range where water can reach during the water injection remains a challenge and there is no effective method reported for the measurement yet. Herein, we introduce the fluorescent tracers widely used in oil field into the wetting range detection of coal seam water injection. Three commonly used fluorescent tracers, fluorescein sodium, calcein disodium, and fluorescein isothiocyanate (FITC), were examined for their performances in detecting the wetting area of coal seam water injection. Considering the two migration stages, wetting and seepage, of the water injected in coal seam, the bottom-up imbibition and seepage experiments were respectively conducted on the fluorescent tracers, and the fluorescence intensities of the tracer solutions before and after the seepage experiment were measured. The results suggest that both fluorescein sodium and calcein disodium promote, but FITC inhibits the wetting stage. Fluorescein sodium promotes the seepage, but calcein disodium and FITC only do at low concentrations. Due to the retention effect, the permeability coefficients of all of the tracers show decreasing trends as the experiment proceeded.

Coal seam water injection is one of the main coal mine dust control measures. To study the effects of ultrasonic excitation on the pore structure of water-bearing coal in depth, ultrasonic excitation experiments were conducted with varied variables including ultrasonic excitation time, ultrasonic power, and fluid pressure on coal. The effects of sodium dodecyl sulfate (SDS) treatment and ultrasonic excitation were also compared. The coal samples treated under different conditions are characterized by low-temperature nitrogen adsorption and scanning electron microscopy (SEM) respectively for the quantitative analysis of pore volume and specific surface area and qualitative analysis of surface pore and fracture structure. The results show that ultrasonic excitation can significantly increase the pore volume and specific surface area of coal and damages the inner wall surfaces of the pore to different extents, making the pore inner surfaces rougher. However, the effect of ultrasonic excitation becomes weaker after the pore volume increases to a certain extent, resulting in more uniform pore size and volume distributions. Ultrasonic excitation time affects the pores with smaller size more significantly, and there is a time threshold effect. The volume and specific surface area of large pores and fractures show limited responses (or increases) to as the excitation time increased. The energy is mainly used for the fracturing and expanding the pores with smaller size. The effect of ultrasonic excitation on pore expansion is more obvious under the fluid pressures lower than 4 MPa. High fluid pressures weaken the fracturing and expanding effect of ultrasonic excitation on the pore structure of water-bearing coal. The ultrasonic power threshold effect is also observed. Once exceeding a certain value, further increasing the ultrasonic power does not dramatically affect the pore structure of water-containing coal. Ultrasonic excitation can enlarge the primary pores and fracture and generate secondary pores and fracture in coal. In contrast, the 0.85% SDS treatment is only able to clean the primary small pores, and no new pore structures are formed.

Coal fire remains one of the main hazards of underground work. Spontaneous coal fires cause serious casualties and property losses. At present, most of the studies on coal spontaneous combustion have been conducted on working faces shorter than 200 m. However, the ultra-long working face gob of shallow buried coal seam is much larger, the distribution of its flow field is more complex, and, thus, risk of spontaneous combustion in the gob is higher. Exploring the evolution law of the gob flow field of ultra-long working face to quickly determine the range of the coal spontaneous combustion hazardous zone is of great significance to the safe production of similar mines. In this study, the gas flow field distribution in the gob of an ultra-long working face was measured by buried pipeline method and oxygen concentration was used as the index. It is found that the oxygen concentration decreases with the advance of the working face. Based on the flow field distribution, the oxidation zone of the gob was determined. Meanwhile, a three-dimensional (3D) numerical model of the working face was established, and the overlying stratum collapse and porosity evolution in the gob were simulated using the particle flow software, PFC3D discrete element software, for the porosity distribution law of the gob. The obtained porosity data were then imported into FLUENT using the custom function UDF to construct a 3D grid model. The flow field distribution in the gob was then numerically simulated for the seepage and migration law of the wind flow in the gob. The results reveal an arch-shaped wind flow field distribution with a swirl shape on the intake airway side. In the strike direction, the wind flow gradually becomes weaker with the advance of the working face. In the dip direction, the wind flow seepage range on the return airway side is obviously higher than that on the intake airway side. In the vertical direction, the wind flow range in the upper gob is larger than that in the middle and lower gob. The spontaneous combustion and oxidation zone of the gob is determined to be at 140.4–313.3 m on the intake airway side, 201.2–351.6 m in the middle of the gob, and 153.2–328.1 m on the return airway side. Finally, the residual coal distribution was superimposed onto the oxygen concentration distribution to obtain the spontaneous residual coal combustion hazardous zone in the gob.

为了掌握急倾斜煤层水力压裂造缝规律及作用机理,为现场煤层水力压裂工艺提供技术指导,采用实验室实验的研究方法,以乌东煤矿作为研究对象,以三向应力、煤层层理为实验变量进行了实验室真三轴实验.研究结果表明:水力压裂效果主要取决于克服滤失效应的憋压过程;当层理与最大主应力方向垂直时,随着三向应力的增大,滤失效应减弱,有利于水力压裂;当层理与最大主应力方向平行时,随着三向应力的增大,滤失效应增强,不利于水力压裂.研究结果为急倾斜煤层水力压裂工艺提供了借鉴,完善了矿井水力压裂领域的研究内容.

Coal seam water injection is one of the important technical means to prevent mine disasters. It can effectively change the mechanical properties of coal, prevent dust, coal and gas outburst, and reduce gas concentration. In order to expand the range of coal seam water injection, make coal seam water injection more effective in wetting coal mass, and determine the effective type of ionic surfactant required for coal seam water injection, triaxial seepage experiments and spontaneous imbibition experiments are conducted on the long flame coal samples from Houwenjialiang Coal Mine in Ordos City, Inner Mongolia Autonomous Region, China in this paper. Moreover, the effects and mechanisms of different ionic surfactants on the seepage and wetting stages of coal seam water injection are studied, which provides a theoretical basis for on-site coal seam water injection. After analysis and discussion, it is found that the influence of cationic surfactants on the seepage stage of coal seam water injection first decreases and then increases while the influence of anionic surfactants on the seepage stage of coal seam water injection shows a gradually increasing trend. Furthermore, both cationic and anionic surfactants show more significant wetting effects as the concentration of the surfactants increases. In addition, the adsorption of cations to coal molecules is stronger than that of anions, and the water solubility of cations is better than that of anions.

In order to systematically study the rheological and morphological characteristics of foam fracturing fluid, hydroxylpropyl guar (HPG) was selected as the foam stabilizer, sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) were selected as the foaming agents, and foam fracturing fluid used for coalbed methane (CBM) stimulation was prepared. The viscosity, quality and half-life of foam fracturing fluid under different additive concentration levels were tested, and the bubble morphology and its relationship with half-life were observed and quantitatively analyzed with microscopy. In addition, the influence of pulverized coal on foam stability was also studied. The results show that the influence of pulverized coal on the basic parameters of the foam fracturing fluid correlated with the particle size and concentration of the pulverized coal. The small particle size of pulverized coal increased the liquid phase viscosity and half-life of the foam, but reduced the foam quality. With increasing pulverized coal concentration, the effects were enhanced. The average bubble diameter decreased with increasing HPG concentration, and the changing trend of the bubble diameter conformed to the gamma distribution. The foam half-life had a positive and negative correlation with the bubble morphology parameters α and β, respectively. The results can deepen the understanding of the influence of foaming agents, foam stabilizers, and coal particles on the properties of foam fracturing fluids, and provide a reference for the optimization of foam fracturing fluid formulations.

Active water fracturing is one of the commonly used technologies in area for enhanced coalbed methane (CBM) extraction. However, after the fracturing process, active water residues may block the pore of coal and ultimately reduce the production of CBM. To address this limitation, pore structures of Dongtan (DT) and Houwenjialiang (HW) coal samples were subjected to active water fracturing effect using active water with different concentrations of PAM prepared using polyacrylamide (PAM). Methane adsorption experiment was conducted to probe the effect of active water on the adsorption of methane in coal samples. Additionally, low-temperature nitrogen adsorption experiments (LT-NAT) and nuclear magnetic resonance (NMR) were probed to characterize the discrepancies in the pore structure and chemical composition surface functional groups of the two coal-sample types. The results showed that considerable discrepancies existed in the pore structures of the two coal sample types; however, their surface functional groups were similar. The active water treatment had contrasting effects on the adsorption behavior of methane features of the two coal-sample types, that is, as concentration of PAM in the active water increased, the methane adsorption of the treated HW coal sample increased whereas that of the treated DT coal sample gradually decreased. The PAM molecules in the active water residue could adsorb methane, and blockage of pores by the residue will reduce the amount of methane adsorption. The combined effect of the two dominated the influence of active water fracturing on methane sorption. Further optimizing the active water formulation to improve the flowback effect can be a very useful channel of reducing damage to the coal seam.

Deep mines are greatly affected by changes in natural wind pressure because of their large buried depths and long ventilation paths. Changes in natural wind pressure do affect the air flow of the underground ventilation system, and even change the direction of individual branches. If the dynamic changes of natural wind pressure are not monitored constantly, it is very likely to cause disasters such as gas overrun and may even lead to heavy casualties. In this paper, the changes of natural wind pressure and the air volume entering the mine are measured on-site in the 630 mining area in the south wing of Tangkou Coal Mine, Then, compare the change law of natural wind pressure with the change law of ventilation air volume. Finally, through numerical simulation by FLUENT, the change of internal flow in the gob where there is a loosely closed condition is simulated. Through research, the annual natural wind pressure change and the change of air intake in the 630 mining area of the south wing of Tangkou Coal Mine were obtained; The influence of changes in external conditions on the ventilation air volume of deep mines is obtained; The importance of the influence of natural wind pressure on the stability of the deep mine ventilation system is verified.

To improve the efficiency of coal seam water injection, the influence of nanofluids on coal surface wettability was studied based on the nano drag reduction and injection enhancement technology in the field of tertiary oil recovery. The composition optimization and performance evaluation of nanofluids with nano-silica and sodium lauryl sulfate as the main components were carried out, and the effects of the nanofluid with the optimal ratio on coal wettability were studied through spontaneous upward imbibition experiments. The results show that the composite nanofluid has a lower surface tension, and the lowest value of the interfacial tension is 15.79 mN/m. Therefore, the composite nanofluid can enhance the wettability of coal. However, its effects on coal samples with different metamorphic degrees is different, that is, low rank coal is the largest, middle rank coal is the second, and high rank coal is the least. In addition, a functional relationship between time and imbibition height is found for pulverized coal with different particle sizes. When the particle size of pulverized coal is 60–80 mesh, the wettability of nanofluid to coal is best. The findings in this paper provide a new perspective for improving the water injection efficiency for coal seams with low permeability.

Coal seam water injection has a significant effect on downhole dust resistance. However, during the operation of coal seam water injection, the seepage of the solution in the coal fractures is impacted by the roughness of coal fractures. Therefore, in this study, distilled water and a sodium lauryl sulfate surfactant were used as seepage solutions, kennel coal was used as the research subject, and four coal samples with different roughness coefficients were prepared for seepage experiments. After the analysis and discussion of the experimental results, it is found that the surface roughness of coal fractures hinders the seepage effect of coal seam water injection. The greater the surface roughness of coal fractures, the smaller the permeability coefficient. Furthermore, increasing the injection pressure and fracture aperture can reduce the influence of coal fracture surface roughness on the permeability coefficient. In addition, after sodium lauryl sulfate is added, the permeability coefficient of the coal sample is reduced. This further reveals the seepage of water injection into coal seams and provides certain guidance for the development of coal seam water injection technology.

As the conventional inert gas used for coal firefighting, mainly CO2 and N-2, experience certain deficiencies when used in coal firefighting under particular geological conditions, composite inert gas can be a feasible choice in such situations. Nevertheless, the fire inhibition characteristics and application potential of composite inert gas have not been explored in depth. To expedite the wider use of composite inert gas in preventing and controlling coal spontaneous combustion, the fire inhibition characteristics and application potential of composite inert gas have been evaluated by using temperature programmed experiment and engineering analogy for the first time. The results show that the fire inhibition effect falls in the order of as N-2 < composite inert gas < CO2, and as the result of the competitive adsorption, the composite inert gas exhibits better performance than N-2 and more resembles CO2 in the fire inhibition; Moreover, since the composite inert gas has lower toxicity and cost than CO2, the cost of composite inert gas is only 57.1% of CO2, it offers an overall more balanced performance taking into account fire protection effect, safety, and fire protection cost. The unique advantages of composite inert gas allow broad application in the prevention and control of the coal spontaneous combustion under some special geological environments.

Driven by high pressure, slickwater may intrude into the pores of coal seams, causing changes in the pore structure, and ultimately affecting the flow of Coalbed Methane (CBM). In this study, slickwater prepared with different concentrations of polyacrylamide (PAM) is used to soak coal samples from Inner Mongolia under high pressure to explore the effect of slickwater fracturing on coal seam pores. To study the evolution characteristics of the pore structure of coal samples treated with slickwater, low temperature nitrogen adsorption experiments and methane adsorption experiments are combined. The experimental results show that under the action of external pressure, slickwater invades the pore structure of coal, resulting in a significant decrease in pore volume and specific surface area. Furthermore, with the increase of the pressure and viscosity of the slickwater, the slickwater residue blocks the micropores in the coal pore structure more severely. The damage to methane adsorption by residue is more serious than that to nitrogen adsorption, reflecting that more residue remains in the micropores of coal samples. The development of a gel breaker suitable for slickwater can promote the degradation of PAM polymer molecules and the reduction of residual liquid viscosity after fracturing, and improve the flowback effect. This may be an effective way to reduce reservoir damage.