Study On Fluid-Solid Coupling Response Mechanism And Permeability Model Of Gas-bearing Coal Under Stress Disturbance

The occurrence conditions of coal seams in China are complex.With the gradual increase of coal mining depth,the impact of mining stress disturbance on coal seam stability and coal seam permeability is more and more significant.At the same time,due to the diversity of mining stress disturbance,coal seams are often affected by different stress paths,Therefore,it is necessary to systematically study the mechanical properties and gas migration law of coal seam under different stress disturbances.In this paper,the mechanical properties and the gas migration law of coal under stress disturbance were studied by means of experimental research,theoretical analysis and numerical simulation.In terms of experimental research,taking the coal samples of a coal mine of Sichuan Coal Group as the research object,the mechanical and seepage tests of coal samples under different stress paths were carried out by using the thermal-fluid-solid coupling triaxial servo seepage test device for the gas-bearing coal and RLW-2000 microcomputer servo triaxial test machine,focusing on the mechanical properties,the changes of pore and fracture structure and the law of gas migration under different stress paths.In terms of theoretical analysis,according to the migration characteristics of gas in coal under different stress conditions,three new permeability models for different stress paths were constructed respectively,and the effect of the model was evaluated by combining with the experimental data.In terms of numerical simulation,by considering the coal as a dual porous medium,combined with the gas mass conservation equation,stress balance equation and permeability model considering dynamic Klinger coefficient,the fluid-solid coupling model of coal was constructed,and it was imported into COMSOL software to study the occurrence and flow characteristics of gas in the coal sample and coal seam.The main research results were as follows:(1)Under the condition of discontinuous axial stress loading,the variation law of deformation and permeability of coal sample was basically consistent with that under continuous loading,but the permeability would not lag behind the stress.The adsorption space of coal sample didn’t have fractal characteristics,but the seepage space had fractal characteristics,and the fractal dimension of seepage space decreased after discontinuous axial stress loading.Based on S-D model,a permeability model considering the internal dynamic adsorption state of coal sample was constructed by comprehensively considering the influence of stress on gas adsorption state and Klinkenberg effect.(2)Under the loading and unloading conditions,the smaller initial value of confining pressure unloading would reduce the peak strength of coal,the smaller axial stress value at the same initial value of confining pressure unloading would also reduce the peak strength of coal,and the higher gas pressure would reduce the peak strength of coal.Based on the entropy theory,the entropy change rate was proposed as the damage characterization parameter,and its rationality was verified by comparing with the change rate of porosity.Based on the basic definition of porosity,combined with the statistical damage model,and comprehensively considering the factors such as gas adsorption expansion and Klinkenberg effect,a permeability model of coal under loading and unloading considering damage was established.(3)Under the condition of constant amplitude cyclic loading of confining pressure,permeability had a strong sensitivity to confining pressure,which gradually decreased with the increase of cycle number.With the increase of the number of cycles,the average permeability damage value generally showed a downward trend.The average permeability damage value of the subsequent cycles was far less than that of the first cycle,while the cumulative average permeability damage value gradually increased,and this increasing trend gradually slowed down,and the cumulative average permeability damage value generally increased in the form of logarithmic function.After the cyclic loading and unloading of confining pressure,the fractal dimension of seepage space was reduced.Based on the S-D model,taking the fracture compressibility C_f as a variable and considering the loading process and unloading process respectively,the permeability model under cyclic loading and unloading of confining pressure is constructed.(4)Under the condition of graded cyclic unloading confining pressure,the cumulative plastic deformation and corresponding permeability increased gradually with the unloading level.At most unloading levels,the acoustic emission signals mainly occurred in the first cycle and less in subsequent cycles.During the cyclic unloading of confining pressure,Kaiser effect and felicity effect actually coexisted,and there was a significant stress memory effect between cumulative acoustic emission signals and confining pressure.According to the principle of effective stress and the law of energy conservation,the calculation formula of dissipated energy of gas-bearing coal was put forward.With the increase of unloading level,the relationship between dissipated energy and cycle number was transformed from logarithmic relationship to linear relationship.Based on energy dissipation and information entropy,two new damage variables were proposed,and obtained that the critical damage value D_c≈0.9.(5)Considering the coal as a dual porous medium,combined with gas mass conservation equation,stress balance equation and permeability model considering dynamic Klinkenberg coefficient,the fluid-solid coupling model of coal was constructed and imported into COMSOL software.With the increase of simulation time,the distribution of pore pressure and fracture pressure in the model of the coal sample gradually tended to be stable and presented a relatively stable pressure gradient in the model of the coal sample.The parametersχ_p represented the change trend of fracture pressure and pore pressure in the simulation process.It was concluded that after gas flows in the model of coal sample for 34 h,the internal fracture pressure and pore pressure basically reached equilibrium.In the simulation process of gas drainage,the gas pressure of fractures around the borehole gradually decreased.The closer to the borehole,the more the gas pressure of fractures decreased,and the farther away from the borehole,the less the gas pressure of fractures decreased.With the increase of extraction time,the decreasing range for the gas pressure of fractures around the borehole gradually increased,and the wider the decreasing area for the gas pressure of fractures around the borehole.Compared with the original stress zone,the reduction range of fracture gas pressure in the concentrated stress zone was smaller,while the ratio of fracture gas pressure to pore gas pressure was always larger.