Modeling Approach To Creep-Seepage-Temperature Coupling In Deep Coal

The physical and mechanical behavior of deep gas-bearing coal is the result of multi-phase coexistence and multi-field coupling.In high stress environment,coal creep becomes a non-negligible factor.The model of coupling creep-seepage-temperature in deep coal is not only an important foundation for deep rock mechanics,but also provides theoretical support and security for deep coal and associated resource exploitation.As the intermediate variable of multi-field coupling process,the evolution of coal permeability should be considered under creep-seepage conditions.Based on the mining conditions of 1000 mdeep coal in Pingmei ShenIa Group,this paper proposes the disturbance fe tor of deep coal based on mining stress.The Abel dashpot is introduced into the creep element model.Based on the fractional calculus theory and the disturbance factor is used to eharacterize the damage degree in the aceelerated creep stage,the fractional derivative creep constitutive equation describing the three stages of rheology is derived.The fitting analysis and sensitivity study of the parameters are carried out by creep experiments under unloading confining pressure.Then,the creep-seepage experiments of stepped unloading confining are eonducted,and it is found that the permeability under unloading confining pressure decreased first and then increased with the volume strain from compressionto expansion after the dilation point.Based on the hypothesis that the real permeability network can minimize the energy consumption of fluid,a hyperbolic permeability equation is established,which can conprehensively reflect the influence of various faetors such as volume strain,mining stress,temperature change and adsorption and desorptioa The equilibrium equation of deformation field,the state equation of seepage field and the energy conservation equation caused by ten?erature change are deduced,and the modeling approach to creep-seepage-temperature coupling in deep coal is established.The deformation and seepage characteristics of coal around the extraction borehole are simulated by numerical method.Research work mainly in tihe following aspects:(1)Considering the triaxial stress experiment under mining stress path,the stress-strain relationship of coal samples is mainly a nonlinear law during the pre-peak stage,and the maximum compressive volume strain is smaller than that under conventional triaxial compressioa When the stress state does not reach its peak value,the volume of coal sample will change from compressive deformation to expansive deformation.Based on the damage evolution equation,the mining stress disturbance factor of deep coal is proposed.The mining stress disturbance factor can not only reflect the influence of the coal depth(ie.,the in situ rock stress state)on the mining stress disturbance,but also the disturbance of different stress states under the same geological environment.Under mining stress conditions,the greater the Principal stress difference,the greater the disturbance characteristics of the coal and the smaller the deviator,the smaller the disturbance level of the coal The numerical results of mining disturbance in deep coal shows that after the protective layer is mined,the stress relief of the disturbed coal is obvious,showing regional stress disturbance;the coal without the mining effect of the upper protective layer maintains a high stress state,and is affected by the layout of the coal seam roadway,showing a local strong disturbance characteristic.The mining stress disturbance actor is positively correlated with the disturbance intensity.(2)Aiming at the creep characteristics in deep coal,one-dimensional creep constitutive model is derived based on the fractional derivative,thus,a three-dimensional creep constitutive equation of fractional derivative is established,and the volumetric creep characteristics are considered in the model.According to in situ stress state in deep coal,the creep characteristics under the condition of unloading confining pressure are carried out by laboratory experiments.The axial and radial strains of the coal samples are similarly changed with time.However,the tvio strain values are different under each stage of unloading and creep conditions;Under the condition of equal diffeential stress,as the confining pressure increases,the axial and radial strains decrease with time.(3)Analyzing the conventional triaxial seepage experimeni and the seepage experiment under the condition of unloading confining pressure,we can see that the shape of the stress-strain curve and the volume-strain-axial strain curve are similar under different stress paths,but the permeability-volume strain curve is different.With the volumetric strain dilation point as the boundary,under the mining stress path,when the coal reaches the dilation point,the process in which the permeability first decreased and then increased as the volumetric strain from compression to expansion.Based on the hypothesis that the real permeable network in coal can minimize the energy consumption of fluid flow under the mining stress path,a hyperbolic permeability equation is established with the volumetric strain as the independent variable.According to the experinental data of unloading confining pressure-seepage and experimental data of creep-seepage,the hyperbolic function type expression of permeability is fitted.The results show that the model can simulate this evolution that the permeability decreased first and then increased with volumetric strain(axial strain),and it can reflect the influence of kinds of factors such as volume strain,mining stress,temperature change and adsorption and desorption.(4)The scientific connotation of strong timeliness characteristics is expressed.Strong tineliness means that the mining rock mass in the deep environment has obvious creep effects related to the mining activities,and has a coupling effect on multi-field multi-phase seepage.The aging characteristics of the rock include three stages of rheology,brittle-extension transformation,and weakening of rock physics and mechanics.The critical damage threshold eriterion and the stability criterion of creep differential equations with strong aging characteristics are proposed.The critical danage threshold criterion can be determined by inelastic volumetric strain,porosity,or acoustic emission data.The stability of creep differential equations can be determined according to the Lyapunov stability theorem The initial position of strong timeliness occurred is determined by the triaxial creep experiment of coal,and the correlation equation between strain acceleration and strain rate under strong timeliness conditions is given Through creep experiments of the unloading confining pressure,it is found that the permeability will increase sharply at the strong timeliness stage.(5)Deep in-situ uniaxial compression test is carried out,and it is found that the axial stress of coal presents slow step loading without mining effect.Based on this,the coal in front of the working face is divided into strain softening zone,strong aging rheological zone,graded disturbance rheological zone and original rock stress rheological zone.Combining the fractional creep constitutive model with the stress field equation to derive the deformation field equation characterizing the creep process,using the proposed permeability equation,and the creep-seepage-temperature coupling model of deep coal is established.With the help of the finite element software COMSOL,the numerical analysis of drainage borehole in deep coal is built,and the multi-field coupling model considering the creep effect will increase the coal deformation and reduce the permeability.