Gas-containing Coal Research Articles

A permeability model considering the dynamic effect of stress on gas adsorption under discontinuous compression.

With the continuous advance of coal mining, the normal stress in front of coal seams will continue to increase, resulting in changes of internal structure and permeability of coal seams. During the continuous advance of mining face, the increase of vertical stress somewhere in front of coal seams may not be a transient process, but a gradual change process. Therefore, this paper carried out the seepage test of gas-containing coal under discontinuous loading axial stress. The stress, deformation and permeability of coal specimens were measured and recorded during the test, and the influence of discontinuous loading axial stress on the deformation, internal structure and seepage properties of coal specimens were discussed. On this basis, considering the influence of stress on the gas adsorption state, Klinkenberg effect and other factors, combined with Shi-Durucan permeability model, a new permeability model considering the dynamic adsorption state of coal specimens was developed.

The Effect of Water on Gas-Containing Coal with Cyclic Loading: An Experimental Study

In deep mining engineering, coal often is subjected to the influence of cyclic loading and water. In order to study the effect of water on gas-containing coal with cyclic loading, the pore evolution, permeability and energy dissipation of gas-containing coal under the effects of water and cyclic axial stress were analyzed in depth. The results showed that with the increase of water content, dissipation energy and permeability of coal samples decreased gradually. The presence of water weakened the increase in micropores and mesopores and promoted the increase in macropores during the cyclic loading process. Compared to dry coal samples, the ratio of micropores and mesopores in saturated coal samples decreased by 0.8872%, while the proportion of macropores increased by 0.0341%. On the basis of the above, a new formula for calculating the dissipation energy of gas-containing coal under the effects of water was proposed. A new damage variable based on energy dissipation was proposed. Finally, a theoretical model was derived to describe the permeability of gas-containing coal under the effects of water and cyclic axial stress.

Particle Size Distribution and Energy Dissipation in Drop Hammer Impact Damage of Gas-Containing Coal.

Coal and gas outburst is the mechanical damage behavior of gas-containing coal in a complex environment. In this paper, the established experimental system is used to carry out hammer damage experiments on coal adsorption of different gases under different gas pressures and analyze the mass distribution and surface area distribution characteristics of coal samples in each particle size interval after the hammer damage of coal samples. The energy of the crushing process was analyzed by using the specific work of crushing, mass of newly generated particle size added per unit of energy, newly added surface area per unit of energy, and newly added specific surface area per unit of energy. The results show that the crushing effect of gas-containing coal samples during hammer damage is related to the type and pressure of the adsorbed gas. The stronger the gas adsorption performance and the higher the gas pressure, the weaker the effect of gas-containing coal hammer damage. Meanwhile, the energy required for the mass and surface area of coal samples with newly generated particle size decreased, indicating that the stronger the gas adsorption and the higher the gas pressure, the more serious the energy dissipation in the hammering damage process of gas-containing coal samples. The research results are conducive to an in-depth understanding of the coal and gas outburst processes, which can provide a theoretical basis for the damage characteristics and energy dissipation of coal and gas outbursts.

Research on the Critical Moisture Content of Frost Heaving in Gas-Containing Coal.

As the depth of coal seam mining increases, coal and gas outburst prevention and control face unprecedented challenges. In recent years, freezing outburst prevention technology has received attention and been applied in industry; this method mainly improves the strength of the coal body while reducing its energy storage. Research has shown that due to the influence of moisture content, frost heaving occurs in coal bodies during the cooling process, weakening the coal bodies' strength. Therefore, it is necessary to explore the effect of the moisture content on the coal body frost heave. This paper uses a research method that combines laboratory experiments, theoretical analysis, and numerical simulation to study the critical moisture content of frost heaving in gas-containing coal. The results showed that the coal sample can be divided into three stages during the cooling process: the cold shrinkage stage, frost heave stage, and stabilization stage. When the moisture content is greater than 7%, as the moisture content increases, the amount of ice increases accordingly, which causes an increase in the frost heave deformation of the coal sample and produces an obvious frost heave effect. Utilizing COMSOL simulation software, the deformation of gas-containing coal samples at different moisture levels is simulated. By combining the results with laboratory measurements, the critical moisture content for the frost heave in gas-containing coal is determined to be 6.6%.

Energy dissipation and crack propagation in gas-containing coal under impact loading: An experimental study

Gas-containing coal seams are highly susceptible to coal and gas outburst and composite dynamic disaster due to dynamic loads generated by roof breaks and blasting. This is an energy-driven state destabilization phenomenon, and is intricately correlated to the development of the cracks (i.e. damage evolution) within the coal. To accurately understand the energy evolution and damage principle of gas-containing coal under impact loading condition, we conducted impact compression experiments on the coal body under varying gas pressures. We analyzed the compression energy dissipation characteristics of the coal body under different gas pressures. Additionally, we utilized the CT scanning system to obtain the fracture evolution laws of gas-containing coal under the impact loading condition, and we explored the damage law of the gas to the coal body. In addition, the energy required for crack energy dissipation density (CEDD) is introduced to quantitatively characterize the relationship between energy dissipation and crack propagation.

Experimental and numerical investigation on effects of gas adsorption pressures on damage behaviors, failure characteristics, and energy evolution of coals

Coal and gas outbursts are anthropogenic hazards that can be divided into four stages: preparation, occurrence, development, and termination. Studying the influence of gas on coal damage and energy changes during the formation of outbursts is highly important for investigating the entire hazard. In this paper, laboratory experiments were performed on samples. The energy changes and failure mechanism of coal under different gas pressures were revealed, and a seepage model was established based on compressibility and adsorption to investigate gas-induced coal damage. The results demonstrated that gas pressure caused initial damage within the coal, resulting in nonlinear deformation and strength deterioration of the coal. Gas-induced damage affected the energy evolution mechanism of coal under loading, as pressure caused the premature release of energy at the tips of internal microcracks and reduced the ability to accumulate energy. These changes increased the proportion of dissipative energy under unstable conditions. The combined acoustic emission (AE) parameters—rising angle (RA) and average frequency (AF)—were used to study the fracture mode of gas-containing coal. As the gas pressure increased from 0 to 4 MPa, the RA–AF distribution pattern changed from tensile failure to tensile–shear composite failure, with the proportion of shear cracks increasing from 0.30% to 25.44%. As the complexity of the crack network increased, the randomness of crack propagation increased, and the fracture surface roughness parameters, arithmetic mean height Sa, root mean square height Sq, and maximum height Sz increased by 90.33%, 94.02%, and 81.70%, respectively. These findings could contribute to a comprehensive understanding of the underlying mechanism of coal and gas outbursts and guide for predicting and preventing these hazards.

Study of the mutual coupling characteristics of the oxidation thermal effect and microstructural evolution of gas-containing coal

Besides be affected by coal confining pressure, coal seams are also be affected by the surrounding pressure during mining. To understand the heat release characteristics and microstructural evolution of oxidization within coal under different gas pressures is of great significance. For this reason, a combination of theoretical research and test analysis was adopted, which includes differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and mercury intrusion method (MIP). The influences of gas phase transformation and migration on the oxidation and spontaneous combustion processes of gas-containing coal under different gas pressures were explored. The distributions and variations in heat release, gas derivation, pore structure and functional group characteristics during the oxidation of gas-containing coal were analysed. We clarified the cross-coupling attributes of heat, seepage and chemical properties in the oxidation of gas-containing coal. The experimental results show that the methane within coal migrates outward in pores with the increase of temperature, which inhibits the penetration and adsorption of oxygen, thereby inhibiting the coal‑oxygen composite reaction and delaying the heat accumulation within coal. There is a positive correlation between loose and porous characteristics of coal and gas pressure. With the continuous increase of coal temperature, the pore connectivity of high-pressure gas-containing coal is enhanced, which increases the risk of coal spontaneous combustion. The research results are of great significance to the theoretical research on the prevention and prediction of spontaneous combustion of gas-containing coal.

Experimental Study on the Effect of Gas Pressure on the Pore Structure and Dynamic Mechanical Properties of Coal.

Understanding the effect of gas pressure on coal pore structure and dynamic mechanical properties can better guide the accurate monitoring of stress and gas in gas-containing coal seams in coal mines and efficiently prevent and control coal/rock-gas composite dynamic hazards. In this study, the characterization of the pore structure of the coal body under different gas pressures and three-dimensional impact compression tests were carried out. The findings demonstrate that when the axial static load and confining pressure are fixed, the gas pressure determines the amount of gas adsorbed by the coal samples and its pore structure changes. The effect of gas pressure on the pore structure of the micropores is not obvious, but it has an obvious dilatation effect on the pore structure of the macropores. Within the range of conditions and gas pressures studied in this paper, gas-containing coals' dynamic compressive strength and failure strain decrease linearly with increasing gas pressure. The average dynamic strength deterioration rate of gas-containing coals increases linearly with an increase of gas pressure, which plays a deteriorating role in the dynamic mechanical properties of coal bodies. When the gas pressure increases from 0.7 to 2.8 MPa, the radius of the macropores inside the gas-containing coal increases 0.63 times, and the increased pores and cracks produce a stress concentration effect around the pores and cracks and the shorter time required for instability damage of the coal samples to occur when subjected to dynamic loading. The research results improve the basic theory of gas-containing coal dynamics and provide a theoretical basis for the mine coal/rock-gas composite dynamics disaster.

Study on the energy and damage characteristics of gas-containing coal under confining pressure unloading process

Research on energy and damage evolution patterns of gas-containing coal under constricting pressure unloading conditions is urgently needed in the process of deep underground mining and is crucial for understanding the mechanisms underlying coal and gas composite rockburst occurrences. Prior reaching peak stress, cyclic loading and unloading experiments were carried out on gas-containing coal specimens under varied confining pressures and unloading circumstances. According to the experimental findings, gas pressure significantly degrades the mechanical characteristics of coal specimens, with a higher gas pressure causing worsening of the mechanical properties. The degree of mechanical property deterioration in coal specimens caused by gas pressure steadily reduces as confining pressure rises, indicating that confining pressure has an inhibitory influence on the deterioration of mechanical characteristics brought on by gas. Additionally, the degradation of coal mechanical characteristics is a result of the unloading of confining pressures. The peak stress and elastic modulus of coal decrease as confining pressure unloading increases in magnitude. Prior to the peak stress, the capacity of coal to convert external work into elastic energy is mostly indicated by the compression storage energy coefficient, whereas the peak elastic energy predominantly denotes the upper bound of compressed storage elastic energy for coal. Based on the Weibull statistical damage theory and employing elastic energy as the distribution variable, a statistical damage model is proposed. Comparisons of coal damage evolution curves indicate that both the confining pressures unloading process and gas pressure exert a promoting effect on the damage evolution of coal.

ISCSO-PTCN-BIGRU Prediction Model for Fracture Risk Grade of Gas-Containing Coal Fracture

A multi-strategy improved sand cat swarm algorithm with PTCN-BIGRU is proposed to solve the problem of predicting the risk level of gas-containing coal fracture. Combined with kernel entropy component analysis to downscale the gas-containing coal fracture risk level predictors, TCN is used for feature extraction by parallel convolution operation, and BiGRU is used to further obtain the contextual links of the features. A parameterized exponential linear unit based on the standard TCN is used to improve the linear unit and to enhance the generalization capability of the model. Combined with the sand cat swarm optimization algorithm to determine the optimal BIGRU network parameters, Singer chaos mapping, chaos decreasing factor, and adaptive t-distribution are used to improve the SCSO for optimal risk level prediction accuracy. The results show that the prediction accuracy of the ISCSO-PTCN-BiGRU model is 93.33%, which is better than other models, and it is proved that this paper can effectively improve the prediction accuracy of gas-containing coal fracture risk level. This research adds a theoretical support for the prevention of gas protrusion accidents and a guarantee for the safety of underground production in coal mines.

Study on thermal effects and gases derivation of spontaneous combustion of gas-containing coal

Heat release and gas derivation can reflect the degree of coal spontaneous combustion (CSC) as important indicators. Meanwhile, both physical and chemical properties of gas-containing coal and methane molecules are important factors for CSC. The C600 high-precision microcalorimeter was coupled with a gas chromatograph to conduct oxidation and pyrolysis experiments on gas-containing coal. The thermal effects and gas derivation rules in the process were analyzed. The influence of the physicochemical properties of gas-containing coal on the spontaneous combustion process was clarified. The quantitative relationship between particle size and heat release coal was discussed. The experimental results show that compared with raw coal, the unique characteristics of gas-containing coal are as follows: (1) The oxidation process is slowed down, while high temperature combustion is more adequate, and pyrolysis is more likely to occur and more intense; (2) The heat absorption and release intensity increased to different degrees, and the characteristic temperature shifted forward; (3) With the decrease of oxygen consumption and generation of CO2, CO release increases significantly; (4) As the particle size of gas-containing coal decreases, the oxidation is more likely to occur, while pyrolysis is slowed down. The research results provide an important theoretical basis for the prevention and control of spontaneous combustion of gas-containing coal.

Experimental Study on the Effects of Vibrational Frequency on the Permeability of Gas-Containing Coal Rocks

Experimental Study on the Effects of Vibrational Frequency on the Permeability of Gas-Containing Coal Rocks

Crack instability in deep coal seam induced by the coupling of mining unloading and gas driving and transformation of failure mode

Coal and gas outburst is a violent mechanical instability of gas-containing coal, and its mechanism is crucial for prevention of gas dynamic disasters. The macroscopic instability of gas-containing coal is the result of the expansion and connection of micro cracks driven by unloading induced tensile stress and gas pressure. This study is aimed at solving the problem of crack instability of gas-containing coal during mining unloading. To this end, first, the governing equations of unloading-induced tensile stress and gas pressure evolutions in cracks were built, and the dynamic evolutions and main controlling factors of tensile stress and gas pressure were revealed. Furthermore, the evaluation indexes of crack stability under tensile stress and shear stress were established, and the variation of crack stability during unloading was investigated. Finally, the phase diagram of crack failure mode was drawn, and the transformation of crack failure mode was analyzed. The results show that unloading-induced tensile stress is generated when θ > 54.73°, and it rises gradually with the increase of original stress, unloading rate, unloading time and crack size and dip angle. Overall, gas pressure in cracks increases first and then decreases during unloading. It increases gradually with the increase of initial gas pressure, gas desorption rate and original stress, while it decreases as the unloading rate gets higher (k from 10−4 to 10−2 s−1) and the dip angle gets larger (θ from 50 to 90°). With the increase of unloading ratio, the crack stability declines slowly first and then plunges rapidly until the crack finally becomes unstable. In addition, burial depth of coal seam, unloading rate, gas desorption rate and mechanical strength of coal have significant effects on this process. Different factors have important influences on the crack failure mode. With the increase of burial depth from 100 to 2000 m, the crack failure mode gradually transforms from tensile failure to shear failure, and the higher the unloading rate is, the more easily tensile failure occurs. With the increase of cohesion (0.1–2 MPa), the scope of the shear failure zone shrinks gradually, and the crack is less prone to shear failure. The desorption rate has a notable effect on the scope of the non-failure zone. Under a higher gas desorption rate, the scope of the non-failure zone becomes smaller, which is due to the greater gas pressure in crack for a given time. The research in this paper has certain reference value for further grasping of the instability failure of gas-containing coal and the mechanism of coal and gas outburst.

Influence of uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in micropores of bituminous coal by macromolecular simulation

Heterogeneous and anisotropic coal tends to deform under strong mining disturbances, affecting the microscopic adsorption and diffusion of binary CH4-CO2 mixtures in micropores. Therefore, the grand canonical Monte Carlo and molecular dynamics methods were used to study the influence of the uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in coal micropores. The results show that compression strain has significant effects on adsorption selectivity and thermodynamic factor compared to tension strain, and CH4 displacement efficiency is greater when the strain is less than −0.12. Both compression and tension strains have dual effects on promotion and inhibition of volumetric swelling of the gas-containing coal matrix, micropore development and gas diffusion. Furthermore, the diffusivity of CH4 is significantly larger than that of CO2, and Einstein diffusion is dominated by low-velocity motion. Results provide an important basis for the optimal coalbed methane recovery design in deformed coal reservoirs.

Study on Mechanical Characteristics of Deformation and the Failure of Gas-Containing Coal in the Wuhai Mining Area of China under Different Gas Pressure Conditions

The mechanical properties of gas-containing coal and rock mass play important roles in controlling the occurrence and development of coal and gas outbursts. The gradual increase in mining depth will change the failure mechanism of gas-containing coal and rock mass. In order to further study the failure mechanism of gas-containing coal and rock mass, samples were taken from the gas-containing coal seam in the Wuhushan Coal Mine of the Wuhai Mining area of China. The mechanical parameters of coal samples during the failure process under different gas pressure conditions were measured and analyzed with the SAW-2000 rock mechanics testing machine, the gas-containing coal uniaxial compression device and inflation system. Meanwhile, the failure process and mechanical parameters of coal samples under different gas pressure were simulated by RFPA2D gas plate numerical simulation software. The results show that with increasing gas pressure in the coal there is decrease in Compressive strength, Elastic modulus, Strain, Peak strength and Bearing capacity and increase in Poisson’s ratio. When the failure state appears in the coal, the cracks are longer and wider, more random cracks are genareted, and the damage degree of the coal is greater. The numerical analysis’ results are in good agreement with experimental results. The research results are applicable to the gas bearing coal with the same or similar gas geological conditions. The tests can be carried out repeatedly and reasonable results can be obtained according to the physical and mechanical parameters of the actual coal seam and the occurrence of gas. On this basis, physical experiments and numerical simulations of triaxial compression can also be carried out to further study the mechanical characteristics of deformation and the failure of gas-containing coal under gas pressure and provide technical support for revealing the mechanism of coal and gas outbursts.

Effect of Confining Pressure on Mechanical and Energy Conversion Properties of Gas-Containing Coal under Loads

Confining pressure is closely related to the deformation and failure characteristics of gas-containing coal and is one of the key factors for gas outburst-rockburst coupled dynamic disaster. Firstly, this paper briefly summarized the triaxial compression test results of gas-containing coal under different confining pressures, including deformation and stiffness and failure and strength. Then, based on the second developed MatDEM gas-solid coupling program, the macro- and microdeformation and failure process of coal samples under different confining pressures and constant gas pressure were simulated, and the distribution of particle stress, pore stress, and crack, as well as energy conversion characteristics, was obtained. Finally, the influence mechanism of confining pressure on deformation and failure of gas-containing coal was discussed, and the occurrence conditions of coal and gas outburst, rockburst, and their coupled dynamic disasters were distinguished. The results show that the peak strength and residual strength after failure of gas-containing coal both increase approximately linearly with the increase of confining pressure, while the elastic modulus does not change much or increases continuously, which is closely related to the number of microdefects in the coal sample. The strengthening effect of confining pressure on rock stiffness and strength mainly comes from external support and compaction. The total number of cracks decreases with the increase of confining pressure, while the number of shear cracks increases, and the inclination angle between the formed macroscopic main crack and the horizontal plane gradually decreases. The maximum cumulative elastic energy and residual elastic energy both increase with the increase of confining pressure, which leads to the increase of the impact tendency index of coal samples.

Research on Permeability Enhancement Model of Pressure Relief Roadway for Deep Coal Roadway Strip

Aiming at the problem of permeability enhancement in deep coal roadway strip, it proposed a pressure relief roadway permeability enhancement model, and the model was verified through experiments, theoretical calculations, and field tests. The experiment reveals that the unloading of gas-containing coal can achieve the effect of permeability enhancement. Theoretical calculation shows that with the increase of the distance from the coal seam, the overlying coal seam’s permeability enhancement multiple and the width of the significant permeability enhancement zone gradually decrease, and the width of the general permeability enhancement zone gradually increases and then stabilizes. Field tests show that the pressure relief roadway is arranged 9 m directly below the coal seam; the overlying coal strata has a significant pressure relief and permeability enhancement effect. This research can provide reference value for the high-efficiency permeability enhancement in the deep coal roadway strip.

New Insights into the Drill Cutting Characteristics and Fault Distribution in Gas-Containing Coal Seams

This study is aimed at detecting small faults in coal seams. With coal seams of different buried depths in Luling Coal Mine, Yuenan Coal Mine, and Sihe Coal Mine with obvious difference in hardness and outburst hazards selected as the research objects, the morphological characteristics of a sample particle size such as the Rosin-Rammler distribution function, average particle size, cumulative mass distribution, and mechanical parameters such as particle strength and F value were statistically analyzed through drill cuttings related to sampling points at different positions of faults already known. The results show that the effect of tectonic stress reduces the strength and permeability of the coal after the borehole enters the fault damage zone, resulting in abnormal fluctuations in the morphological characteristics of the drill cuttings. Correspondingly, large-size particles account for a high percentage. The weight analysis discloses that the characteristic particle size of the Rosin-Rammler distribution function has the greatest influence on the determination of fault existence. The analysis of drill cutting characteristics conduces to detecting the distribution of small faults, helps to improve the utilization of mine information, and is of great significance for accident prevention and control.

Study on Rheological Properties of Coal Rock Containing Gas under Disturbance Conditions

In view of the dynamic phenomenon that coal and rock are susceptible to external impact disturbance in the mining process, combined with the rheological hypothesis mechanism of coal and gas outburst, the RLSS-II-type triaxial loading creep test system of gas-containing coal and rock developed by ourselves is used to carry out the conventional triaxial rheological test of gas-containing coal and rock and the rheological disturbance effect test of gas-containing coal and rock under impact disturbance. The strength limit neighborhood of gas-containing coal and rock is determined, and the gas-containing coal and rock entering the strength limit neighborhood are subjected to different impact disturbances. The experimental results show that, (1) under the confining pressure of 0 MPa, 2.5 MPa, and 5 MPa, the longitudinal deformations of gas-bearing coal and rock are 32 mm, 27 mm, and 22 mm, respectively, indicating that the deformation of coal and rock will be affected by confining pressure, and with the increase of confining pressure, the deformation will decrease. (2) In the test, the deformation of coal and rock in the late stage of uniform creep stage can be regarded as a strain threshold. Before this threshold, the strain of coal and rock is not obvious, and the deformation is only 1.1 mm. After exceeding a threshold, the deformation of coal and rock is 9 mm, and the deformation increases significantly. Then, it enters the accelerated creep stage quickly and finally damages. The vicinity of this threshold is called the strength limit neighborhood of coal and rock containing gas. (3) The gas-bearing coal and rock without entering the strength limit neighborhood and entering the strength limit neighborhood are changed by confining pressure and different impact disturbance, respectively. It is found that, whether in the strength limit neighborhood or outside the strength limit neighborhood, the confining pressure has an effect on the strain, but the influence is not large. Under different impact disturbance, the deformation of coal and rock within and outside the strength limit neighborhood is 8 mm and 0.4 mm, respectively, and the deformation changes obviously, indicating that the impact disturbance has a great influence on the deformation of coal and rock within the strength limit neighborhood, and the coal and rock with large impact disturbance are destroyed before the coal and rock with small impact disturbance, indicating that the greater the impact disturbance, the shorter the time required for destruction.

Experimental study on the desorption effect of penetrant on gas-containing coal.

Two non-ionic reagents, polyethylene glycol 4000 and Tween-80, two anionic reagents, sodium dodecyl benzenesulfonate and sodium lauryl sulfate, and a mixture of these non-ionic and anionic reagents were used as penetrants. The processes of replacement desorption and relief-pressure desorption of gas-containing coal were studied, the influence of the penetrant on the amount of gas replacement desorption and relief-pressure desorption was explored, and the change rule of the amounts of gas replacement desorption and relief-pressure desorption was analysed. The results show that the increase rate of the replacement desorption amount of the mixed penetrant is 11.81%-34.75%, and the decrease rate of the relief-pressure desorption amount is 51.68%-72.69%, which are higher values than those with a single penetrant. As the mass fraction of penetrant increases within the range of 0.5%~2%, the capacity of gas replacement desorption and hindering gas relief-pressure desorption will increase. At the same mass fraction, the effect of the mixed penetrant is better than that of the anionic penetrant, which in turn is better than that of the non-ionic penetrant.