Study On Permeability Evolution Of Deep Coal Under The Combined Influence Of Multi-field

The deep environment is characterized by "three highs" of "high in-situ stress,high temperature and high seepage pressure".At the same time,the coal and rock mass in deep mining also have engineering response of strong disturbance and strong aging.The characteristics of "three highs" have a significant impact on the change of coal permeability in mining.The fracture caused by mining disturbance can effectively improve the permeability of coal seam.Therefore,it is very important to grasp the permeability evolution of deep coal under multi-field coupling effect and mining disturbance for preventing deep coal and gas outburst and promoting simultaneous extraction of coal and gas.However,the advanced achievements in the study of coal permeability mainly focus on the exploitation of coalbed methane at present.Mechanical boundary conditions and physical field changes of permeability in coalbed methane exploitation are different from those in coal mining.As for coal permeability under mining,most studies focus on shallow coal seam or permeability change under the influence of a single factor,and the research on permeability evolution of deep coal seam under multi-field coupling and mining disturbance is less.To solve this problem,the coal in front of deep working face was determined as the research object.Mining stress path of deep mining and the coupling of disturbance stress-gas pressure-temperature were taken as control conditions.The permeability evolution of deep coal under multi-field coupling was studied by means of theoretical deduction,numerical simulation and laboratory test.The main conclusions are as follows:(1)Breaking through the assumptions of uniaxial strain and constant overlying load in previous coal permeability models,two types of coal permeability models under three-dimensional stress were established:exponential model and cubic model.Combining the measured permeability data of changing axial and confining pressures or changing gas pressure,it was proved that the two models of coal permeability under three-dimensional stress have good applicability.But the fitting effect of exponential model is better.Depending on the first two models,considering the effects of adsorption and damage,two kinds of permeability models of deep coal with damage under mining disturbance were constructed:exponential model and cubic model.Based on the measured results of permeability under conventional triaxial loading,mining disturbance loading and unloading or gas pressure change,the applicability of the two models was proved.The fitting result of exponential model is closer to the measured value than that of cubic model.(2)According to the damage and desorption caused by coupling of temperature and disturbance stress,a permeability model of deep coal was established?The model can better reflect the permeability evolution according to the measured results of permeability with changing temperature or changing the coupling condition of disturbance stress and temperature.Under the coupling of temperature and disturbance stress,the coal permeability increases with the increase of internal swelling ratio and decreases with the increase of thermal internal swelling ratio.(3)The factors causing fracture deformation under multi-field coupling were classified as effective stress,gas pressure and temperature.Then the anisotropic permeability model of deep coal under multi-field coupling was established by using anisotropic elasticity theory.Meanwhile,the good applicability of the model was verified by the test results under different effective stress,gas pressure,temperature and their combination.In addition,the anisotropic damage constitutive was derived in accordance with statistical damage mechanics.And the method of determining parameters in damage constitutive was given based on total differential method.Thereafter,an anisotropic permeability model of deep coal considering damage under multi-field coupling was developed.(4)Considering that coal is connected by matrix bridge,the influence of multi-physical field on the internal swelling coefficient was simulated and analyzed by finite element software.Under the boundary condition of constant confining pressure,the distribution of coal matrix horizontal displacement is similar on the condition of increasing pore pressure or temperature.The maximum displacement of coal matrix occurs at both ends of coal.Because of the resistance of matrix bridge,the horizontal displacement of coal matrix near matrix bridge is almost zero.With the increase of pore pressure or temperature,the increment of horizontal displacement increases nonlinearly.The internal swelling coefficient decreases linearly with the increase of pore pressure,which is in good agreement with the theoretical inversion results.Moreover,the internal swelling coefficient increases exponentially with increasing temperature.(5)The peak strength of deep coal in Pingdingshan Coal Mine is between 47.67 MPa and 52.53 MPa under the mining stress path of protective seam mining.And the corresponding stress concentration factor ranges from 1.907 to 2.101.The peak strength and peak strain of deep coal decrease with the increase of gas pressure difference.Moreover,shear failure is the main failure mode of coal sample after loading and unloading under mining stress.Due to the recovery process of the original rock stress in the mining stress path,the initial permeability of deep coal is very lovr,which is in the order of 1×10-18 m2.The evolution of deep coal permeability under mining stress path shows a trend of slow increase at first,and then a sudden rise after the peak stress.