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A decrease in the viscosity of a solution of sodium carboxymethyl cellulose (CMC-Na) is a commonly encountered problem in coalbed methane (CBM) pressure measurement with an apron CMC-Na sealer and capsule CMC-Na sealer. Studies have shown that the factors that can reduce the CMC-Na solution viscosity include the hydrogen ion concentration index (pH), the environmental temperature, the drilling and sealing time, and the high-pressure CBM gas environment. In this study, a homemade instrument for measuring the viscosity of a CMC-Na solution under high pressure was used to determine the change laws of the viscosity under different conditions, and comparative tests were performed to investigate various factors affecting the viscosity. The best solution for stabilizing the viscosity was determined and effectively applied at a coal mine site, where an overall improvement in the viscosity behaviour was observed. [ABSTRACT FROM AUTHOR]

In order to study the gas occurrence regularity and the main controlling factors of Panxi Mine, gas parameters of the mine were determined through the combination of field survey and experiment, and the distribution regularity of gas content in coal seam strike and dip was analyzed. Besides, the fault structure was quantitatively researched based on the fractal theory. The experimental results show that the fractal dimension of the mine mostly ranges from 0.7 to 1.6. The fault structure becomes more complex with the increase of fractal dimension, and the gas content in the region with a larger fractal dimension is higher. The fractal dimension of fault can reflect complexity of geological structure of the mine. The research is targeted to the prevention and control of gas accidents in Panxi Mine, and it has important theoretical and practical significance for promoting safety production and gas development and utilization in coal mines. [ABSTRACT FROM AUTHOR]

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 rapid desorption within a short period is necessary for the occurrence of an outburst. Due to the limitation of the present test condition, gas desorption characterization in coal with different moisture content for the first several seconds (0–60 s) has not been researched sufficiently. In this study, initial desorption characterization of gas in coal with different moisture content is studied by experiments with methane. The most remarkable characteristic of the experimental setup is the application of a self-developed real-time data acquisition system with a time interval of about 10 ms, which achieves the goal of collecting enough pressure data for analysis and calculation. The data is used to study gas pressure variation and calculate the initial amount of desorbed gas and index (ΔP) of initial velocity diffusion of coal gas. From the experimental results, the new proof has been found to verify that coal with lower moisture content and methane outburst is more dangerous than coal with higher moisture content and outburst. The degree of coal and methane outburst is exponentially decaying with increasing moisture content. [ABSTRACT FROM AUTHOR]

Gas occurrence in coal seams shows zonation, and the outburst risk prediction index is significantly affected by coal seam gas. For different coal mine, the sensitivity of prediction index of outburst risk is inconsistent, and the different area of the same coal mine can present different sensitivities to the same index. In this study, we measured the industrial analysis of coals and determined the degree of outburst risk of each coal seam using coal samples with the particle size of 1 ∼ 3 mm. The relationships between the equilibrium gas pressure and gas desorption index of drill cuttings, between the desorption index of drill cuttings and the adsorption pressure, and between the critical values of the indexes of drill cuttings were also analyzed using the fuzzy clustering method. Based on the analysis of the particle size distribution of coal samples, the influence of grain size on the critical value of the drill cuttings desorption index was studied. The results showed that the critical values of the desorption index of drill cuttings were different under different adsorption equilibrium gas pressures, and those critical values increased with increasing adsorption equilibrium gas pressure. The desorption index of drill cuttings and adsorption equilibrium gas pressure had a linear relationship, and the higher the degree of outburst risk was, the greater the slope of the fitting function and the smaller the intercept; under the same gas pressure of adsorption equilibrium, there was a linear relationship between Δh2 and K 1, and the higher the outburst risk of coal seam was, the smaller the slope of the fitting function and the greater the intercept. The drill cuttings desorption index Δh2 was more sensitive than the index K 1. Under the same test pressure, the smaller the particle size of the coal samples was, the greater the value of desorption index of drill cuttings and the higher the outburst risk of coal seam. The research results have significant theoretical importance and practical value for the prediction and prevention of coal and gas outbursts in coal mines with similar occurrence conditions of coal seams. [ABSTRACT FROM AUTHOR]

Since most coalfields in China are commonly characterized by high gas content and low permeability, there is an urgent need to improve coal seam permeability and further enhance coal bed methane (CBM) extraction efficiency. As an emerging fracturing technology, the CO2 gas fracturing (CGF) technology has been widely used because of its advantages of low cost, environmental protection and high fragmentation efficiency. In order to improve the fracturing ability of CGF technique and optimize the release orifices of discharge head, computational fluid dynamics model was used in this paper to simulate the flow fields of dynamic pressure of gas jet released from the orifices with different structures and other geometrical parameters. The results show that the orifice structure has a great influence on the flow field of gas jet, but little influence on the magnitude of the dynamic pressure. Besides, the maximum dynamic pressure of gas jet linearly decreases with the increase in the number of release orifices. Based on a series of simulation results, the discharge head which has single group of orifices with structure c, diameter of 24 mm can be identified as the best choice for fieldwork. Then, two field experiments were conducted in Pingdingshan and Changping coal mines to evaluate the enhanced CBM extraction efficiency by CGF. The results indicate that the CGF can effectively create a large number of cracks in a large range around the fracturing borehole in the coal seam and further significantly improve the permeability. And the CBM extraction efficiency can be improved to a higher level from a lower level and maintained for a long time. Besides, the effective influence radii caused by CGF in Pingdingshan and Changping coal mines are 15.19 m and 12.5 m, respectively. Compared with other fracturing techniques, the CGF technique has a promising application prospect. [ABSTRACT FROM AUTHOR]

The microstructure of coal has a significant influence on the permeability of the coal seam. To study the characteristics of microstructure changes in coal seam under temperature impact, we conducted temperature-impact experiments using a high–low temperature test system, and we studied the coal pores and fissure structure before and after the temperature impact using scanning electron microscopy, industrial micro-computed tomography, and mercury intrusion. Based on the digital image processing technology and thermal stress theory, we qualitatively and quantitatively analyzed the variation of crack width, specific surface area, and pore diameter, and deeply analyzed the failure mechanism of temperature impact on coal seam microstructure. The results showed that the temperature impact caused the macropores to interpenetrate and form macroscopic cracks in the coal sample, which resulted in a relatively small volume of macropores and increased the volume of mesopores and small pores. The maximum thermal stress generated during the temperature impact process was located in the tangential direction of the coal sample surface. The thermal stress generated by the temperature impact exceeded the tensile strength of the coal sample, which directly causes crack initiation, expansion, and mutual penetration. This study provided the technical support necessary for the efficient development of coalbed methane and the improvement of gas drainage rate in the coal seam. [ABSTRACT FROM AUTHOR]

Nowadays, the water jet technique has been more frequently used for coal bed methane stimulation. As the ultimate component of the water jet system, the nozzle has important influence on the rock breaking ability of water jet. To optimize the nozzle, a simplified model for impinging water jet was established. The effects of geometric structure (cylindrical, conical, and conical‐straight) and parameters (outlet diameter d, conical angle α, and length L) on the flow field of the impinging water jet are simulated through computational fluid dynamics software Fluent14.0. Besides, the effects of inlet pressure (Pin) and target distances (S) on the jet dynamics are also discussed based on the simulation results. The results show that the jet dynamic parameters including dynamic pressure, velocity, and impinging pressure vary with different nozzles. The jet dynamic parameters linearly increase with the inlet pressure, while they first increase then decrease with the increase in target distance. Additionally, the optimal nozzle designed based on the numerical simulation was used in the field test, and the results indicate that this technique can effectively improve the gas drainage efficiency. [ABSTRACT FROM AUTHOR]

Gas disasters are a major factor influencing safe production in mines: Gas extraction can reduce the gas content in coal seams, providing a guarantee of safer production. The parameters for gas extraction are the primary factors influencing the effectiveness thereof. Aiming at the creep properties of soft coal, a fluid‐solid coupling mathematical model considering creep properties of coal was established based on dynamic evolution equation for permeability considering the effects of matrix shrinkage and effective stress. Additionally, by utilizing COMSOL Multiphysics software, the gas extractions from a single borehole and multiple boreholes were calculated. Moreover, the parameters for gas extraction were optimized and applied and tested in field conditions. The result showed the reduction in gas pressure around the boreholes was larger than that from a single borehole when conducting gas extraction from multiple boreholes. The borehole spacing when extracting gas in coal seams by drilling multiple boreholes should be more than twice that of the effective drainage radius. The optimal borehole spacing ranged from 3.2 to 4.2 m for gas extraction lasting 180 d. Numerical simulation was carried out to ascertain the distribution of stress on coal around a roadway. The result revealed that the damage radius of the roadway was 11.8 m, and a reasonable hole‐sealing depth was 12 m. On condition that the borehole spacing during gas extraction from multiple boreholes was 4 m, the reasonable pre‐extraction time was 180 days taking the gas pressure being reduced to <0.74 MPa as a critical point. Furthermore, the gas content, the amount of extracted gas, etc, in a working face after the parameters for gas extraction were optimized were measured. The result suggested that the effect of gas extraction after optimizing parameters conformed to industry standards. [ABSTRACT FROM AUTHOR]

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

Abstract Almost all the coal seams in China are characterized by high gas content, extremely low permeability, complicated geological structure, thus, natural disasters such as gas explosive and outburst easily occur. At present, gas extraction is an effective approach for preventing such disasters. In this paper, in order to improve gas extraction efficiency, the Multi-discharge CO 2 fracturing system (Multi CO 2 Frac) was proposed and tested in Changping coal mine. Through analysis of the waveforms from the explosions of 0.5 and 1 kg TNT dynamites and 1 kg liquid CO 2 , it can be obtained that the TNT equivalence of 1 kg CO 2 is about 400–430 g, that is, the explosion of 1 kg liquid CO 2 has same fracturing ability as 400–430 g TNT dynamites. Besides, field test shows that the effective drainage radius caused by Multi-CO2-Frac technique is about 12.5 m, regardless of the number CO 2 discharging sets. However, the gas extraction concentration increases with the number of CO 2 discharges. This phenomenon indicates that more cracks will be created along the axial direction in coal seam drilling boreholes by Multi CO 2 Frac techniques. By comparing gas concentration before and after fracturing, it can be concluded that the Multi CO 2 Frac techniques can change and maintain the uptrend of gas concentration at a higher level in a long period of time. Therefore, in order to extend the duration of high efficiency gas extraction, Multi CO 2 Frac techniques can be used periodically at different position of coal seam. Meanwhile, after gas extraction, the maximum gas emission of coal seam was reduced from 5.5 m3/min to 3.48 m3/min and the driving footage was enhanced from 2.4 m/d to 5.7 m/d. In summary, Multi CO 2 Frac techniques can effectively improve the coal seam permeability and further enhance the gas drainage efficiency, which can meet the requirement of gas extraction in high gas content thick and soft coal seam. Therefore, this technique has a promising application prospect because of its advantages of safety, environmental protection and economy. Highlights • Multi-discharge CO 2 Fracturing System was proposed to improve coal seam permeability and enhance gas extraction efficiency in high gas content thick coal seam. • The explosion 1 Kg liquid CO 2 has same fracturing ability as 400-430 g TNT dynamites. • The effective influence radius of gas drainage by Multi-CO 2 -Frac technique is about 12.5 m. [ABSTRACT FROM AUTHOR]

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

• Based on leakages of underground CBM drainage borehole, a material with dual expansion sources (DE) was proposed. • The expansion ratios (by rubber bag method) and the expansion pressures (under small swelling) of a modified cement (MC) and the DE were tested. • The expansion force of the cement-based material increased first and then tended to become a constant value with time. • The effect of the DE and the bag-grouting sealing method is preferable to that of the MC and the traditional "two sealing and one grouting" method. Coalbed methane (CBM) is not only a hazard to underground coal mines and atmospheric environment, but also an unconventional natural gas. The effect of underground CBM drainage is influenced markedly by borehole sealing quality. This research focused on a novel sealing material and a bag-grouting sealing method for CBM drainage. First, based on leakages of underground CBM drainage borehole, a material with dual expansion sources (DE) was proposed. Then, the expansion ratios (by rubber bag method) and the expansion pressures (under small swelling) of a modified cement (MC) and the DE were tested. Next, a bag-grouting sealing method, characterized by a bag-grouting sealing device, was proposed. Slurry can be grouted into the bag-grouting sealing device continuously, then the borehole can be sealed tightly. Finally, engineering test was performed at No.2 coal mine in Huangling mining area. Laboratory test results show that the DE expands fast at initial hydration stage, then continues to expand slowly. Especially, the expansion ratios of the DE are more than 60%, however, the MC tends to shrink after initial expansion. Besides, the expansion pressures of the DE are above 1.3 MPa, which are much greater than those of the MC. Engineering test results show that the effect of the DE and the bag-grouting sealing method is preferable to that of the MC and the traditional "two sealing and one grouting" method. The average methane concentration of the boreholes, sealed by the bag-grouting sealing method with the DE as sealing material, is 40% within 90 days. Besides, the concentration does not decrease drastically. [ABSTRACT FROM AUTHOR]

Coalbed gas content is the most important parameter for forecasting and preventing the occurrence of coal and gas outburst. However, existing methods have difficulty obtaining the coalbed gas content accurately. In this study, a numerical calculation model for the rapid estimation of coal seam gas content was established based on the characteristic values of gas desorption at specific exposure times. Combined with technical verification, a new method which avoids the calculation of gas loss for the rapid estimation of gas content in the coal seam was investigated. Study results show that the balanced adsorption gas pressure and coal gas desorption characteristic coefficient (Kt) satisfy the exponential equation, and the gas content and Kt are linear equations. The correlation coefficient of the fitting equation gradually decreases as the exposure time of the coal sample increases. Using the new method to measure and calculate the gas content of coal samples at two different working faces of the Lubanshan North mine (LBS), the deviation of the calculated coal sample gas content ranged from 0.32% to 8.84%, with an average of only 4.49%. Therefore, the new method meets the needs of field engineering technology. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) is a popular technique to monitor the process of rock failure during hydraulic fracturing for unconventional natural gas production. It contains abundant information that will be useful to study in-depth the nature of hydraulic fracturing. In this study, we focused on the AE count, energy, peak frequency, crack classification, and location recorded from four rock specimens subjected to a specific triaxial stress condition. We found the multi-frequency-response phenomenon of AE, and proposed the multi-frequency-response index to indicate the moment of the macrohydraulic crack formation. Furthermore, it was found that the power law distribution index of AE energy of non-stratified specimen was bigger than that of stratified specimens during hydraulic fracturing. The tensile crack dominated in all hydraulic fracturing tests. Our results are of significance for understanding hydraulic fracturing in stratified rocks. Unlabelled Image • Multi acoustic emission characteristics were combined to explain the rock failure of hydraulic fracturing. • A stratified formation was particularly studied comparing with the single stratum. • The multi-frequency-response index was proposed to indicated the macrohydraulic crack formation. • The type of ruptured stratum was diagnosed with the statistics of acoustic emission energy. [ABSTRACT FROM AUTHOR]