Desorption Characteristics of CH4-C2H6 Mixed Gas in Heavy Hydrocarbon-Rich Coal Seams.

The gas desorption characteristics of coal are closely related to the gas content of the coal seam. The gas in heavy hydrocarbon-rich coal seams contains CH4 and C2H6 heavy hydrocarbons. However, most current research on the gas desorption characteristics of coal seams focuses on CH4 analysis, ignoring the influence of the C2H6 heavy hydrocarbon gas. To accurately determine the gas content of a heavy hydrocarbon-rich coal seam, methods based on CH4 analysis are inadequate and the desorption characteristics of CH4-C2H6 mixed gas must be clarified. This work experimentally and theoretically studies the desorption characteristics of single-component gas and CH4-C2H6 mixed gas from coal samples. The results show that increasing the adsorption-equilibrium pressure was found to increase the desorption quantity and desorption speed of single-component gas and increase the desorption quantity, desorption ratio, and diffusion coefficient of mixed gas. Under the same adsorption-equilibrium pressure, the desorption quantity and rate of single-component CH4 gas exceeded those of C2H6. The quantity and speed of mixed gas desorption increased with rising CH4 concentration and decreased with rising C2H6 concentration. The change in the mixed gas concentration during desorption reflects the distribution characteristics of light hydrocarbon components on the outer surface and heavy hydrocarbon components on the inner surface of coal. From the desorption characteristics of mixed gas, desorption models of mixed gas were obtained at different concentrations, laying a theoretical foundation for accurate determinations of gas contents in heavy hydrocarbon-rich coal seams.

Anisotropic Damage Mechanism of Coal Seam Water Injection with Multiphase Coupling.

After coal seam water injection, coal mechanical properties will change with brittleness weakening and plasticity enhancement. Aiming at the problem of coal damage caused by the coal seam water injection process, based on nonlinear pore elasticity theory and continuum damage theory, a nonlinear pore elastic damage model considering anisotropic characteristics is proposed to calculate and analyze the gas-liquid-solid multiphase coupling effect with the fully coupled finite element method during the coal seam water injection process. The research results indicate that the wetting radius of calculated results by the model agrees well with the in situ test results, and the relative errors are less than 10%. Water saturation and induced damage of the coal body in the parallel bedding direction are greater than that in the vertical bedding direction during the coal seam water injection process, which exhibits significant anisotropic characteristics. With the increasing water injection time, the induced damage of the coal body also increases near the water injection hole. Considering the inherent permeability arising with damage, it has a significant impact on both water saturation and induced damage, which also indicates that there is a strong interaction between water saturation and induced damage. The theoretical model reveals the coal damage mechanism of gas-liquid-solid multiphase coupling after coal seam water injection and provides a theoretical prediction of coal containing water characteristics in engineering practice.

Experimental Study on Coal and Gas Outburst Risk under Different Water Content Rates in Strong Outburst Coal Seams.

To investigate the alleviation potency of coal seam water infusion on coal and gas outburst, this paper focuses on the Qidong coal mine outburst coal seam, where outburst accidents have occurred many times, and obtains the impact of water content on outburst prediction parameters by studying the features of outburst parameters and gas desorption law under different water content rates. How water content affects outburst was also researched through the use of a self-made outburst simulation test system, and the relationship between water content and outburst intensity and critical gas pressure was studied. It can be concluded that with the rise of water content, the initial velocity of gas diffusion, the gas desorption index of drilling cuttings, and the adsorption constant a decrease, but the firmness coefficient (f) increase, and these indicators are exponentially related to the water content. Meanwhile, as the water content raises, the outburst pressure threshold increases, the outburst intensity gradually decreases, and the less likely outburst occurs. Under 0.5 MPa pressure, as the water content arose from 2.02 to 5.14%, the outburst intensity was significantly weakened, while no outburst occurred as the water content reached to 10.25%. Fitting analysis of the influence curve of outburst parameters and comparing the vital values of outburst prediction indexes finally determined that the water content rate of 5.14% could be used as a key index for water injection measures for coal and gas outburst prevention coal seam in Qidong coal mine no. 9. This research offers a guiding significance for the outburst prevention measures of water infusion in outburst coal seams and gives a feasible scheme for the safe mining of outburst coal mines.

Study on the Effect of Pore Structure on Desorption Hysteresis of Deep Coking Coal under High-Temperature and High-Pressure Conditions.

Pore space is the main desorption space for methane in coal; to study the effect of changes in pore structure on the desorption hysteresis effect of methane in coal under high-temperature and high-pressure conditions, the coking coal from Pingdingshan Twelve Mine was taken as the research object, and the isothermal adsorption and desorption curves were obtained and quantitatively analyzed at different temperatures and pressures by the help of isothermal adsorption and desorption experiments, combined with the pressed mercury experiments and the low-temperature liquid nitrogen adsorption experiments to test the pore structure of the coal samples before and after the adsorption and desorption tests. The pore structure of coal samples before and after the adsorption and desorption tests was tested by combining the mercury pressure test and the low-temperature liquid nitrogen adsorption test, and the influence of the change in the pore structure of coal samples after the high-temperature and high-pressure adsorption and desorption tests on the hysteresis effect of methane desorption was studied. The results showed that under the same pressure, the pore volume of coal samples increased with the increase in temperature, the pore-specific surface area showed a tendency to decrease, and the fractal dimension could well characterize the relationship between the pore structure and the pore surface of coal, in which the fractal dimension of the pores in the large pore size section gradually increased with the increase of temperature, and the fractal dimension in the small pore size section gradually decreased; there was a good correlation between the pore structure of the coal samples after the high-temperature and high-pressure adsorption and desorption tests and the hysteresis coefficient of desorption. The structural characteristics of the coal samples after adsorption and desorption hysteresis coefficient at high temperature and high pressure showed good correlation, i.e., the pore volume, the fractal dimension of the large pore size section, and the desorption hysteresis effect were negatively correlated, while the specific surface area, the fractal dimension of the small pore size section, and the desorption hysteresis effect were positively correlated.

Study on Adsorption Characteristics of Deep Coking Coal Based on Molecular Simulation and Experiments.

To study the effect of high temperature and high pressure on the adsorption characteristics of coking coal, Liulin coking coal and Pingdingshan coking coal were selected as the research objects, and isotherm adsorption curves at different temperatures and pressures were obtained by combining isotherm adsorption experiments and molecular dynamics methods. The effect of high temperature and high pressure on the adsorption characteristics of coking coal was analyzed, and an isothermal adsorption model suitable for high-temperature and high-pressure conditions was studied. The results show that the adsorption characteristics of deep coking coal can be well characterized by the molecular dynamics method. Under a supercritical condition, the excess adsorption capacity of methane decreases with the increase of temperature. With the increase of pressure, the excess adsorption capacity rapidly increases in the early stage, temporarily stabilizes in the middle stage, and decreases in the later stage. Based on the classical adsorption model, the adsorption capacity of coking coal under high-temperature and high-pressure environments is fitted. The fitting degree ranges from good to poor. The order is D-R > D-A > L-F >BET > Langmuir, and combined with temperature gradient, pressure gradient, and the D-R adsorption model, it can be seen that after 800 m deep in Liulin Mine and 400 m deep in Pingdingshan Mine, the adsorption capacity of coking coal to methane decreases with the increase of depth.

Research on Factors and Influencing Correlation of Coal Core Temperature during Coring.

The core-tube method is a common method to measure the coal seam gas content (CSGC). However, cutting heat and friction heat will be generated in the core-tube coring process, which will increase the coal core temperature and the coal core gas loss, thus resulting in a large error in the determination of the gas content. The accuracy of the gas content determination is closely related to the temperature variation of coal core during core-taking. Based on this, the team developed the "thermal effect simulation device of coal core in the core-taking process" and carried out the temperature change test experiment of the coal core in the core-taking process under different conditions. The results show that the temperature variation of the coal core during the core-taking process shows four stages: constant temperature, rapid temperature rise, slow temperature rise, and temperature drop. The temperature rise rate, temperature rise duration, and temperature rise peak of the coal core increase with the increase in rotate speed, coal strength, friction area, and frictional load. In the axial direction, the closer to the upper end of the core pipe, the higher the core temperature. In the radial direction, the closer the core is to the wall of the core pipe, the higher the core temperature is. Under the influence of cutting heat and friction heat in the process of core-taking, the maximum heating rate of the core-taking tube wall within 8 min is 20 °C/min, the peak temperature is 158.4 °C, the average temperature of the wall is above 100 °C, and the average temperature rise of the coal core reaches 55.7 °C. Within 60 min, the average temperature of the coal core remained above 50 °C. The order of influence of coal core temperature from large to small is as follows: rotate speed, frictional load, friction area, and coal strength. It can provide a reference for accurately determining CSGC using the core-tube method or designing a coring device to eliminate or reduce the thermal effect during coring.

In Situ Volume Recovery Method for Non-Seal Gas Pressure Measurement Technology: A Comparative Study.

Coal seam gas pressure is one of the basic parameters for coalbed methane resource exploitation and coal mine gas disaster prevention. However, the present coal seam gas pressure measurement technology requires harsh field measurement conditions and a long testing period. In this study, a novel non-seal gas pressure measurement technology is proposed, and this technology is mainly aimed at three different changes before and after the collection of coal samples and realizes the real gas pressure measurement through the compensation of gas leakage, in situ volume recovery of the coal core, and reservoir temperature simulation. The technique not only can measure the original gas pressure of coal seam quickly and accurately but also does not need to seal the measuring hole. This paper focuses on the study of a key factor that affects the accuracy of non-seal gas pressure measurement: the restoration of in situ volume. Based on this, the influence of four different in situ volume recovery methods on the measurement accuracy is compared with the self-developed non-sealing gas pressure measuring system. Experimental results show that the in situ volume of the coal core cannot be completely restored by stress loading. Although the contact injection method can restore the original volume of the coal core, the pressure recovery error is large due to the replacement and displacement of the gas effect of water and the inclusion of the coal body effect of oil. Interestingly, the combination of stress loading and contact oil injection can not only restore the original volume of the coal core but also minimize the pressure recovery error, which is only less than 10%. Finally, based on the abovementioned experimental results, the in situ volume recovery method of non-seal gas pressure measurement technology is improved. Therefore, the research results of this paper provide a scientific basis for the field application of non-seal gas pressure measurement technology.

Temperature of the Core Tube Wall during Coring in Coal Seam: Experiment and Modeling.

Temperature is the primary factor affecting the law of coal gas desorption. When the core method is used to measure the coal seam gas content (CSGC), the temperature of the coal core sample (CCS) will increase because the heat generated by the core bit cutting and rubbing the coal is transferred to the CCS through the core tube. To solve the above problems, the temperature of the core tube wall during coring at core depths of 10, 20, and 30 m was measured by a self-designed temperature measuring device. The thermodynamic models of the core bit and the core tube during coring were established. The thermal flux of the system at different stages was inverted numerically by the dichotomy method. The reliability of the model was verified by comparing the numerical simulation results with the field measurement results. The main influencing factors during coring were studied by numerical simulations. The results show that the temperature change of the core tube wall goes through four stages: slowly rising, fast rising, slowly rising, and slowly falling, which correspond to the process of pushing the core tube, drilling the CCS, and the early stage and later stage of withdrawing the core tube, respectively. The maximum temperature of the core tube wall appears in the first 5 min of withdrawing the core tube and increases with the increase of core depth. When the core depth is 30 m, the maximum temperature of the core tube wall reaches 105.17 °C. The temperature of the measuring point at the end of drilling the CCS and the maximum temperature during coring linearly increase with the core depth, friction heat generated while pushing the core tube, and coal strength. This study can provide a basis for further research on the dynamic distribution characteristics of temperature in the CCS during coring, which is of profound significance to calculate the gas loss amount and CSGC.

Measurement and Modeling of Spontaneous Capillary Imbibition in Coal.

Coal is a typical dual-porosity medium. The implementation process of water invasion technology in coal is actually a process of spontaneous imbibition of external water. To obtain a model of spontaneous capillary imbibition in coal, the spontaneous imbibition of water in coal samples with different production loads is conducted experimentally. Due to the coal particle deformation and the cohesive forces, the porosity and maximum diameter decrease gradually with increasing pressing loads. Due to the filling effects and occupying effects, the proper particle grading can reduce the porosity and tortuosity. The Comiti model can be used to describe the tortuosity. The tortuosity increases with decreasing porosity. The smaller the porosity, the smoother the surface of the coal sample. The contact angle is negatively correlated with the surface roughness. The fractal dimension decreases with increasing pressing load. The difference in the pore characteristics between particles is the main reason for the difference in the fractal dimension. The proposed model of spontaneous capillary imbibition in coal is consistent with the experimental data. The implications of this study are important for understanding the law of spontaneous imbibition in coal and the displacement of gas by spontaneous capillary imbibition in coal, which is important for optimizing the parameters of coal seam water injection.

Experiment and Modeling of Permeability under Different Impact Loads in a Structural Anisotropic Coal Body.

A mount of bedding and cleat in a coal body causes that the mechanical property and gas permeability are anisotropic in a coal seam, partly. To reveal the permeability change law of the impacted coal, a self-developed vertical split Hopkinson pressure bar (SHPB) device is used to carry out the dynamic impact mechanical property tests of coal samples in three different coring directions under five impact loads and then the permeability of the impacted coal samples is measured by a permeability measuring instrument under different gas pressures. Finally, a calculation model for the anisotropic coal permeability is established to analyze the permeability distribution law in any direction with different angles to the bedding plane. The results show that with an increase in impact height the dynamic peak stress of coal samples increases gradually, which shows a linear growth relationship. The permeability of the impacted coal samples is much larger than that of raw coal samples, and the bigger the impact load, the larger the permeability. Moreover, under the same impact load and gas pressure, the permeability is the largest in parallel to the bedding direction, followed by that in oblique 45° to the bedding direction, and the smallest in perpendicular to the bedding direction. The permeability calculated by the anisotropic model in oblique 45° to the bedding direction is in good agreement with the measured results, and the errors are no more than 10%, which will provide a theoretical basis for the permeability distribution law of the coal seam after deep-hole blasting.

Freezing method for rock cross-cut coal uncovering I: Mechanical properties of a frozen coal seam for preventing outburst.

A comprehensive technology is proposed to realize fast and safe rock cross-cut coal uncovering (RCCCU) based on artificial freezing engineering method. This comprehensive technology includes four steps, namely, drilling a borehole, wetting the coal body by water injection, gas drainage and freezing the coal seam by liquid nitrogen injection. In this paper, the compressive strength, tensile strength and shear strength of frozen coal specimens are tested to obtain the mechanical parameters of the specimen. Then, for RCCCU under freezing temperatures, the outburst prevention effects are calculated and quantitatively analysed with regard to three aspects, namely, the enhancement of coal the mechanical properties, the reduction in the coefficient of outburst hazard (COH) in the distressed zone and the reduction in the interfacial elastic energy ratio (IEER) between the coal seam and the roof/floor. The results show that a considerable improvement in the mechanical properties of frozen coal and that the coal mechanical parameters, such as the compressive strength and the tensile strength, increase linearly with decreasing temperature. The coefficient of outburst hazard in the distressed zone decreases rapidly and drops from above 0.8 to below 0.3. The interfacial elastic energy ratio is greatly reduced from dozens of times of that of the roof/floor before freezing to several times of that of the roof/floor after freezing, which effectively weakens the sudden change of the elastic energy at the coal-rock interface.