Research On Damage Behaviors And Permeability Characteristics Of Deep Coal

In deep coal seam group mining,as the working face advances,the coal body is subjected to cyclic loads resulting from mining-induced stress,which will inevitably lead to the dynamic change of the internal damage and permeability of the fractured rock mass.In order to analyze the energy evolution and permeability characteristics of coal during the coal mining process,the triaxial cyclic loading and unloading tests were carried out by controlling confining pressure constant,and the permeability was measured.However with the increase of coal seam mining depth,deep coal will be in a more complicated conditions,specifically,high ground stress,high temperature,high pore pressure,so it is very important to study the influence of temperature and pore pressure on coal permeability.In the process of gas extraction,the borehole will cause stress concentration around,therefore it will produce different deformation and damage from that far away from the borehole.It is of guiding significance to study the flow law of gas in different locations in the process of gas extraction.In this paper,the damage process and permeability evolution of deep coal seam are analyzed by experimental research,theoretical derivation and numerical simulation.Firstly,taking deep coal mining as the research background,triaxial cyclic loading and unloading seepage tests were carried out.The damage process of coal body is analyzed from the perspective of energy evolution,and damage variable is established based on accumulated dissipated energy density.Permeability was calculated by fractional derivative transient pulse method and the relationship between permeability and damage variable is discussed.Then,considering the influence of temperature and pore pressure,a new coal permeability evolution model(ESTD model)is proposed by introducing nonuniform deformation coefficient,and verified by field data and laboratory experimental data.Finally,with gas extraction as the research background,anomalous diffusion model is introduced,and a dual-zone model is proposed for gas extraction to describe gas flow characteristics in drainage damage zone and non-damaged zone,and A series of finiteelement numerical simulations based on proposed models are carried out in gas extraction process.The work of dissertation is mainly performed from the following aspects:(1)The seepage experiments are carried out under cyclic loading condition seepage test by keeping confining pressure constant and loading and unloading axial pressure.The evolution of elastic parameters(elastic modulus,Poisson’s ratio)is analyzed,the input energy density,the elastic energy density and dissipation energy density are calculated,and the evolution trend of proportions of the elastic energy and dissipation energy are obtained.The energy consumption ratio is introduced to characterize the damage state of coal,which is regarded as an important criterion for the instability and destruction of coal.At the same time,the effect of confining pressure in evolution of energy density,energy ratio and energy consumption ratio evolution are discussed.The results show that the elastic modulus increases first and then decreases during loading and unloading,and it reaches the maximum value at the point of yield stress.The input energy density and elastic energy density increase slowly in the compaction stage and linear elastic stage,while the dissipated energy density is at a very low level.After entering the yield stage,the growth trend of the three is obviously accelerated,and the elastic energy density reaches the peak at the peak stress.The input energy density and dissipation energy density reach their own peak values in the first cycle after the peak and tend to be stable in the residual stage.The proportion of elastic energy increases gradually in the compression and linear elastic stages and reaches the peak at the yield stress.After entering the yield stage,the proportion of dissipated energy begins to increase and reaches the peak at the post-peak stage.The energy consumption ratio decreases first and increases gradually after entering the yield stage,and its minimum value means the criterion of coal instability.The limit of energy storage,maximum dissipated energy density and residual elastic energy density of coal increase with the increase of confining pressure.The ratio of dissipated energy and the peak value of energy consumption ratio decrease with the increase of confining pressure,which is due to the hindering effect of confining pressure on the deformation and failure of coal body.(2)The permeability of deep coal is calculated by using the traditional transient method and the transient method based on fractional derivative,and a comparative analysis is made.For the whole stress-strain process of coal,a modified damage variable is proposed from the perspective of cumulative dissipated energy,and its evolution with axial strain is analyzed.The rate of change of damage variable is introduced to characterize the speed of damage evolution so as to predict the occurrence of coal rupture.The piecewise evolution law of permeability with damage variable is explored by taking volume dilation point as subsection point,and the relationship between permeability and damage variable is quantitatively analyzed by fitting formula.The results show that the permeability evolution is consistent with the volumetric strain with axial strain,and the permeability evolution with volumetric strain can be fitted by two exponential functions.With the increase of confining pressure,the initial permeability,minimum permeability and maximum permeability gradually decrease,indicating that confining pressure inhibits permeability evolution.The permeability obtained by transient method based on fractional derivative is more accurate than that obtained by traditional transient method.Moreover,the permeability calculated by fractional method is larger,and the difference is more obvious with the increase of confining pressure.Consistent with the evolution trend of cumulative count obtained from acoustic emission tests,the damage variable proposed based on cumulative dissipated energy density increases slowly in the compaction stage and linear elastic stage,then increases rapidly in the yield stage and failure stage,and finally increases slowly in the residual stage and gradually becomes stable.When the axial stress approaches the peak value,the change rate of the damage variable reaches the maximum value,which can be used as a criterion of coal failure.The permeability evolution with the modified damage variable can be fitted by two piecewise functions: in the volume compaction stage,permeability decreases in the form of a negative exponential function;In the volumetric expansion stage,they can be fitted as a positive power function.(3)Considering the comprehensive influence of temperature and pore pressure in permeability of deep coal,the non-uniform deformation coefficient is introduced to represent the contribution of matrix deformation to the fracture,and a piecewise evolution model of permeability is established in the whole stress-strain process of coal according to the permeability evolution trend of decreasing firstly and increasing gradually.The influence of temperature and pore pressure on mechanical properties and permeability of coal is obtained through permeability experiment under cyclic loading and unloading condition of coal at different temperature and pore pressure.Experimental data are used to verify the permeability model,and the variations of permeability with temperature,pore pressure,axial stress and axial strain are analyzed.At the same time,the influence of non-uniform deformation coefficient and permeability-damage coefficient on permeability evolution is discussed.The results show that the fracture connectivity obtained from fractional order can reflect the development of coal fractures,which is consistent with the evolution trend of permeability.With the increase of temperature and pore pressure,the elastic modulus of coal decreases gradually.Under the condition of low stress,the initial permeability of coal decreases with the increase of temperature,because under the confining pressure,high temperature causes the matrix to expand inward,resulting in the compression of cracks or seepage channels.When the pore pressure decreases,the effective stress increases and the internal fractures are further compressed,so the decrease of pore pressure will lead to the gradual decrease of permeability.The ESTD model is better than traditional models when considering permeability evolution with pore pressure.It can describe and predict permeability evolution with temperature and effective stress in elastic stage.What’s more,the new model can characterize piecewise evolution of permeability with axial strain for the whole stress-strain process of deep coal.The increase of the non-uniform deformation coefficient will accelerate the decline rate of permeability with temperature.Permeability-damage coefficient has little influence on permeability evolution in compression stage,but its increase will make the increase trend of fracture permeability in expansion stage more obvious.(4)Taking the deep coal mines in Pingdingshan area No.12 as the engineering background,the seepage field,stress field and diffusion field of gas extraction simulation process are established by using the permeability model based on triaxial strain hypothesis and the anomalous diffusion model derived by conformable derivative,and the model is imported into the multi-field coupling numerical simulation software COMSOL Multiphysics.The process of gas extraction in coal seam is numerically simulated.Considering the damage caused by drilling extraction,this paper puts forward a dual-zone gas flow model,including the drainage damage zone(DDZ)and the nondamaged zone(NDZ).Gas pressure evolution law with time about different measuring points,the variation trend of permeability with distance and the evolution trend of the area of drainage damage zone with time are analyzed.The anomalous diffusion model is compared and verified by field test data,and the effects of fractional derivative order on gas diffusion,non-uniform deformation coefficient and permeability-damage coefficient on fracture permeability of coal are discussed.The results show that the anomalous diffusion model is more accurate than the normal diffusion model.Moreover,with the increase of fractional order,declining of gas pressure intensifies.With the progress of gas extraction,the gas pressure decreases rapidly at first and then gradually tends to be stable,and the nearer to the borehole,the faster the gas pressure declines.With the decrease of the distance from the borehole,the fracture permeability increases rapidly from the initial value.For the non-damaged zone,the permeability first decreases and then stabilizes with the increase of the distance from the borehole.Furthermore,with the increase of the extraction time,the area of the damaged zone becomes larger obviously.The increase trend of fracture permeability is more obvious with the increase of non-uniform deformation coefficient.The influence of permeability-damage coefficient on the permeability evolution trend of fractures depends on variation of mechanical deformation and gas pressure with gas drainage time.