Study On Raw Coal Deformation,Permeability And Energy Evolution Under Cyclic Loading

In this thesis,the coal sample of a coal mine in Sichuan Coal Group was used as the research object.The pore size distribution and porosity of the coal sample were measured by nuclear magnetic resonance equipment,and the coal sample was scanned by CT and three-dimensional reconstruction was carried out.The numerical simulation software RFPA^2D was used to simulate the compression failure of gas-bearing coal under different side pressures,and compared with the triaxial compression physical experiment of gas-bearing coal,the failure form of coal rock was analyzed.Based on thermal fluid-solid coupling test system of coal containing gas,which was used to carry out circulating loading and unloading gas seepage experiment under different confining pressures on the coal sample,and analysis of coal rock deformation,seepage law and energy dissipation.The following main conclusions are obtained:(1)The pore size distribution of coal samples was measured by nuclear magnetic resonance method,the measured porosity provided a basis and reference for the selection of CT thresholds during the three-dimensional reconstruction of coal samples.From the CT three-dimensional reconstruction of coal samples before and after the experiment,it is found that there are some cracks in the coal rock that do not extend to the surface of the specimen,the macro-failure of the coal sample is the shear failure with a certain inclination angle between the main crack and the radial direction of the coal sample,along with secondary cracks intersecting the main crack surface.(2)The RFPA^2D numerical simulation software was used to simulate the compression failure of gas-bearing coal under different side pressures,it is concluded that the failure mode of coal and rock is mainly compression shear failure,and the main cracks form an inclined shear plane at an angle with the radial direction of the model,comparative analysis of the results of numerical simulation and physical experiments,it was found that both of the failure modes belonged to shear failure.(3)Under the effect of cyclic loading of coal and rock,the relative residual strain can be divided into three stages:compression stage,elastic stage and yield to destruction in the whole cycle experiment;the axial relative residual strain increases sharply in the compression stage,while slowly decreases in the elastic stage,and gradually increases in the yield failure stage.As the axial stress increases,the permeability of coal rock generally declines in a"spiral"way,and the permeability will rebound when the coal sample is about to fail.(4)The permeability change rate?K_xis defined,and the change of permeability relative to the initial permeability during the loading and unloading process of coal and rock is quantitatively analyzed,when the rate of change reaches the maximum value,the coal sample is about to be destroyed.(5)A new method considering the influence of confining pressure for calculating the elastic modulus was obtained from the energy point of view,and compared with the calculation results of the traditional elastic modulus,it is found that the loading elastic modulus value obtained by this calculation method is smaller than the traditional loading elastic modulus calculation value,and the unloading elastic modulus value is larger than the traditional unloading elastic modulus value...