Research On Fluid-solid Coupling Mechanism And Gas Migration Law Of Coal Under The Influence Of Mining

In this paper,a combination of experimental research,theoretical analysis and numerical simulation is used to study the fluid-solid coupling mechanism and gas migration law of coal under the influence of mining.In the physical experiment,taking coal rock samples from a coal mine of Sichuan Coal Group as the main research object.Combined with mercury injection method,low temperature nitrogen method and nuclear magnetic resonance method,the micro pore and fracture structure of coal samples were tested;Based on triaxial servo-controlled seepage equipment for thermo-hydro-mechanical coupling of coal containing methane(THM),the loading and unloading experiments of coal are carried out,and the laws of seepage,deformation and energy evolution of coal are analyzed.In the aspect of theoretical analysis and numerical simulation,the influence of gas and stress on coal permeability is analyzed;The evolution model of coal permeability under the action of gas and stress is deduced,and the model is validated by combining previous experimental results;The coal is regarded as a dual-porous medium model,Combined with gas mass conservation equation,stress balance equation and dynamic permeability model of coal,a multi field coupling model of coal rock under the influence of mining is established.and numerical simulation is carried out with the help of COMSOL numerical software.Combining the results of the above physical experiments,theoretical analysis and numerical simulation research,the following conclusions are obtained:(1)According to the experimental results of mercury injection,low temperature nitrogen method and nuclear magnetic resonance,the isotherm adsorption curve of coal sample belongs to the second type,and the adsorption desorption curve is separated greatly,and there is no lag ring phenomenon;the total porosity of coal sample is about4%,among which the porosity of micropores and small holes is 3.4%,accounting for 85%of the total porosity,and the porosity of medium and large holes is 0.6%.(2)Two kinds of change relations of the fracture deformation under the action of the lateral stress-linear and exponential type are derived.The influence of the lateral stress on the fracture deformation is the same as that of the normal stress,and the permeability expression of the coal body under the action of the three-dimensional stress is established.Based on previous experimental data,global optimization nonlinear fitting was performed using 1st Opt software,and the rationality of the established expression was verified.(3)Under the loading and unloading conditions,the axial plastic strain and radial plastic strain of the coal sample are approximately linear,and the change of the dilatancy angle of the coal sample with the plastic shear strain fluctuates up and down within a certain range from the beginning of yielding to the residual stage.Based on the relationship between plastic volume strain and crack opening,the relationship between coal permeability and damage variables and stress was established with dilatancy angle as the bridge.The damage evolution process of coal samples is closely related to the energy conversion mechanism,and has different energy conversion characteristics in different loading stages.The cumulative dissipation energy ratio η changes with the axial strain in an "～" shape,and its change trend can be divided into five stages.(4)Considering the influence of gas pressure,coal adsorption deformation and stress on coal permeability,a dynamic permeability evolution model of coal is established;Treating coal and rock as a dual-porous medium model,combined with gas mass conservation equation,stress balance equation and coal rock dynamic permeability model,the coal rock fluid solid coupling model under the influence of mining is constructed,and the model is implanted into the COMSOL numerical software to study the influence of burial depth and gas pressure on the gas migration of coal seam.