Study On The Gas Desorption And Seepage Flow Law Of Coal Under Compressive Stress

The gas migration in coal seam is a complex process coupled by multi-physical fields and related to gas adsorption, desorption, and permeability evolution, etc. It is important in probing the law of gas quick release on site and improving coal seam gas drainage method. The underground mining operation changes the original gas reservoir and the force status of the coal body containing gas. The change of stress on coal in a specific region is mainly reflected in two aspects:(1) generation of stress concentration area, like around the borehole or on the front and both sides of heading face;(2) aging of stress, such as changes of coal body stress in front of heading face caused by stress moving in tunneling process or periodic weighting. For the characteristic of stress variation in the coal mining activities, we need to focus on the effects of stress on gas desorption and permeability when we study the law of gas migration underground.Gas mainly exists as adsorbed phase in coal. When stress loading on coal body, not only free gas forced out from the micro-pore structure of coal, but also the gas molecules released from adsorption interface due to the stress. Therefore, desorption velocity increases with the stress, leading to an increasing desorption amount finally. In this paper, we designed a gas adsorption and desorption system of granular coal under stress and studied the influence of stress on different gas desorption processes in different adsorption equilibrium pressures.The permeability of coal is closely related to the development of the coal fracture. The permeability of coal changes much when the stress acting on the coal is large enough to make the coal produce new fractures. In order to study the process, this paper studied the permeability evolution of coal from stress-free to broke-down under three axial stresses. If the stress is not large enough to produce a new penetrating crack, the stress will affect the original cleats and also change the ability of adsorption and desorption of coal, which influences the adsorption deformation and makes coal cleats opening change, resulting in change of the flow process in coal. In this paper, according to Hol adsorption model, an M adsorption correction factor was introduced to characterize the effect of stress on the ability of adsorption of coal. Combining the internal expansion factor fa and the partition coefficient fe, the anisotropic permeability model of coal under stress was established and validated. Finally, through the mechanics of porous media, fluid solid coupling models which were respectively with M and without M were established and their numerical calculation was carried out.The main research contents and conclusions of paper are as follows:(1) The mechanism analysis of stress influence on the gas adsorption capacity of coalUsing the thermodynamic theory, this paper analyzed the mechanism of stress influence on coal gas adsorption capacity, and introduced the concept of adsorption correction factor, M. The result showed that stress could cause changes on both adsorption interface and the gas molecules stress status of adsorption interface. So it reduced the coal interface potential energy making gas molecules more likely to get rid of coal, i.e. stress reduced the gas adsorption capacity and prompted the desorption of gas in coal. According to the Hol adsorption thermodynamic model, the paper introduced the concept of M adsorption correction factor, and it was used to calculate gas adsorption quantity of coal under stress.(2) The establishment of anisotropic permeability model with the change of adsorption capacityBy introducing the coefficient fe showing the ratio of the crack open degree and the variation of matrix length and another coefficient fa showing the ratio of the variation of the crack open degree and the variation of matrix length, an anisotropic permeability model was established combining with the adsorption correction factor M, in which the change of adsorption capacity was considered. The specific research contents are as follows: Through the parameters sensitivity analysis, it was found that M had a significant impact on the calculation results of permeability within a certain range of parameters as M showed the change of the adsorption capability in permeability model, indicating the importance of introducing the adsorption correction factor. Then, the anisotropic permeability model was established considering the change of adsorption capacity by combining the M with the coefficient fa and the coefficient fe. The model was validated by using the previous experimental data.(3) Design and set up the gas ad/desorption experiment system in granular coal under stressAccording to the related standards, the gas ad/desorption experiment system in granular coal under stress was designed and set up. The experimental system consisted of vacuum degassing system, coal sample storage and stress loading system, gas adsorption system, gas desorption system and data acquisition system. Using sealed jar in stainless steel cell to implement load stress on coal particles and O-rings sealing form to solve the sealing problem between flanges in high pressure. The system can measure the gas adsorption and desorption quantity in granular coal under stress.(4) Experiments on desorption characteristics of different gases under stress.Under different gas pressures, we presented the desorption characteristics of N2, CH4 and CO2 applying different stress to granular coal. The specific contents were as follows: we exerted different stress stages(0-16MPa) to coal particles which adsorption equilibrium pressures were 0.6MPa and 1.7MPa respectively to determine desorption curves. By using Langmuir desorption equation to match desorption curves, we found that Langmuir equation could describe desorption curves of coal particle under stress very well, especially in the initial time, which the fitting effect became worse in subsequently time. Through analyzing the limiting desorption amount and other parameters, we found that stress could promote desorption of different gases, i.e., with the increase of the stress, the desorption amount of three gases increased. By fitting desorption rate, the rate increased gradually after 1 minute, meanwhile the decreasing rate of desorption rate also increased gradually. If the other desorption conditions of the gas were the same, the initial desorption rate of the three gases was CO2>CH4>N2, the decay rate of desorption rate of the three gases was similar, and the desorption rate of the three gases was CO2>CH4>N2.(5) Determination of permeability of different gas under stress.Based on determination of permeability of three-axis experiment,we studied the influence on permeability when there were changes of fracture in coal under three-axis stress. The changes of structure of fracture were mainly detected by acoustic emission device. After monitoring the number of acoustic emission and the energy of acoustic emission by the device,we used the change of fracture during loading process and relationship between the change of gap structure and permeability to get the controlling mechanism of stress influence on permeability. Stress determined the initiation, development, extension and opening degree of fractures. Acoustic emission signals could reflect the fractures in coal very well. By analyzing the change of acoustic emission and permeability in the process of loading,we found that the change of permeability had a good relationship with acoustic emission and the development of fracture in coal is reflected by acoustic emission, which meant the fracture structure in coal played a leading role in the permeability of gas in coal. Stress could affect the permeability by controlling the crack in coal.(6) The establishment and numerical calculation of fluid-solid coupled model of coal containing gas.Based on the anisotropic permeability model in which the change of adsorption capacity was considered, combined with elastic mechanics of porous media, Darcy law and mass conservation law, a fluid-solid coupled model of coal containing gas was established by considering the adsorption correction factor M and without considering M. And through COMSOL Multiphysics, numerical calculation of the fluid-solid coupled model was carried out.(7) Field verification of the control mechanism of stress on gas desorption and seepage.By combining with the variation of underground mining disturbance(working face advancing speed, working face weighting, tunneling blasting) and gas emission, the corresponding relationship between the change of stress and gas emission was analyzed and the control mechanism of stress on gas desorption and seepage was verified.Working face advancing, working face weighting, tunneling blasting etc. all had great influence on the stress distribution of working face. By monitoring and analyzing the relationship between advancing speed and air exhaust gas amount of working face, the working resistance of hydraulic supports and air exhaust gas amount of working face, and the variation of gas concentration tunneling working face after blasting, it’s found that there was a good relationship between the stress disturbance caused by mining and the variation of the gas emission in the working face, which indicates that the mining activities could affect the gas desorption and seepage process in coal seam by changing the stress state of the gas bearing coal, and then control the gas emission law of the working face. Through the analysis of the field monitoring data, the control mechanism of stress on coal gas desorption and seepage has been further verified.