Research On Permeability Characteristics During Coal And Rock Damage Evolution Under Different Mining Conditions

Coal permeability is an important index to evaluate the extraction effect of CBM,and it is closely related to the evolution of coal damage under mining behavior.Especially,for the deep coal mining operation,the stress environment of coal is more complex.The mechanical and permeability properties of coal are affected not only by complex stress condition but also by high temperature.Therefore,it is of great practical significance to study the permeability characteristics in coal damage evolution under different mining conditions for CBM extraction and prevention and control of coal and gas outburst disasters.In this paper,coal samples were collected from the No.3 coal seam of Zhaozhuang colliery in the southeast of Qinshui Basin,China.The characteristics of coal pore distribution,adsorption/desorption,coal damage evolution and permeability under conventional triaxial stress state and different stress paths are studied in depth by means of laboratory experiments and theoretical studies.Some progresses were made in the following aspects:1)The isothermal adsorption tests at different temperatures,triaxial seepage tests at different temperatures and triaxial seepage tests at different stress paths were carried out.Then,the characterization relationship of coal adsorption characteristics,damage evolution and permeability under different temperature and stress paths are discussed respectively.The seepage mechanism in coal damage evolution process under different mining conditions is revealed.2)Coal damage is divided into thermal damage and mechanical damage,and the thermal damage increases with the increase of temperature.The established coal damage constitutive model can accurately match the total stress-strain curves at different temperatures.Based on the mathematical idea of segmentation and symmetry function,the concept of correction factor L is proposed,and a permeability model considering thermal damage and adsorption effect is established.The rationality of the model is verified by experiments.The model can characterize the evolution of permeability at different temperatures.3)Based on the relationship between fractal dimension of pore-fracture and stress,the Weibull distribution parameters of fractal dimension are derived.According to the Mohr-Coulomb criterion and the Poiseuille law,the coal damage constitutive model and the coal permeability model considering the fractal effect are constructed respectively.The proposed damage constitutive model and permeability model are in good agreement with the test results.4)Based on the previous coal damage constitutive model,the coal damage constitutive model and permeability model are established under different mining conditions considering the test conditions of different stress paths.The coal damage model can accurately reflect the stress-strain relationship in the process of loading and unloading,and the coal permeability model can accurately reflect the evolution of coal permeability under different mining conditions.