Research On The Mechanism Of Influence Of Stress And Slip Effect On Permeability Characteristics Of Coal And Roc

Coalbed methane（CBM）,an associated energy source during coal mining,has been incorporated as an important vehicle to drive the energy production and consumption revolution.Not only is it efficient and clean,but it is also dangerous and threatens the safe mining of coal.Improving the development and utilization efficiency of coalbed methane can not only ensure the full utilization of energy,but also reduce environmental pollution and ensure mining safety.Coal permeability is a key parameter reflecting the gas flow within a CBM reservoir.As the depth of CBM resource extraction increases,changes in reservoir stress,Klinkenberg effect and matrix sorption expansion/contraction will affect the flow of CBM,resulting in dynamic changes in coal permeability.Therefore,it is of far-reaching significance to investigate the effects of pore pressure,external stress,Klinkenberg effect and matrix sorption expansion/contraction on the permeability characteristics of coal,in order to improve the efficiency of deep CBM extraction and prevent coal bed methane disasters in coal mines.In this paper,coal samples from Songzao coal mine in Chongqing and Sihe coal mine in Jincheng,Shanxi were used to conduct coal seepage tests with elevated and reduced pore pressures at different external stresses and coal seepage tests with elevated external stress at different pore pressures,respectively,using a triaxial servo seepage test device for gas-bearing coal.A combination of experimental and theoretical methods is used to explore in depth the evolution law of coal permeability and its influencing factors under different boundary conditions.The following research results were mainly obtained.（1）Carry out seepage tests of pore pressure increase and decrease under different external stress conditions,respectively.Based on the characteristics of the dual-pore structure medium of coal,the dual-pore permeability model containing matrix and fracture is constructed by considering the mechanical properties of coal,Klinkenberg effect,adsorption expansion and thickness change of adsorption layer during the boosting process.Then,by introducing the correction function L（p）,the change of coal-rock permeability in the process of depressurization is further quantified,and the response mechanism of coal-rock permeability in the process of pore pressure rise and fall is attempted to be explored.（2）The percolation tests of CH₄and CO₂under different pore pressure conditions were carried out separately.Based on the variation of surface free energy,the elastic modulus discount factor R_mis introduced to characterize the sorption deformation equation and analyze the sorption deformation law of coal under two gases.Then,based on the relationship between stress and strain,a basic permeability model is derived by generalized Hooke’s law,and it is developed into a permeability model with three specific boundary conditions（constant pore pressure,constant external stress and uniaxial strain）.The experimental data were used to verify the reasonableness of the model from various perspectives,in an attempt to investigate the evolution of coal permeability under different boundary conditions and its influencing factors.（3）Based on the dual porosity permeability model,the dynamic equations of matrix Klinkenbergpage coefficient and fracture Klinkenbergpage coefficient are derived,and the dual porosity permeability model of coal considering the dynamic Klinkenbergpage effect is established.In this paper,the new permeability model is verified by the experimental data of pore pressure rising stage,so as to reveal the dynamic change law of Klinkenbergpage coefficient and its influence mechanism on the permeability characteristics of coal.