Study On Creep And Seepage Evolution Of Deep Coal Under Multi-Field Coupling

With the decrease of shallow coal reserves,the mining of coal resources tends to be deep.As the geological environment of deep coal resources is more complex,the mining difficulty is increased compared with shallow mining,and the deep scientific problems to be solved also gradually increase.For the mining of coal resources in deep high gas mines,gas treatment,that is,gas pre-extraction,must be carried out first,so that the high gas pressure and gas concentration can be reduced to the safety production standard.During the long-term gas treatment,the deep coal is prone to creep deformation due to the effect of high in-situ stress,and the creep deformation will affect the gas migration.In addition,the temperature of coal seam gradually increases with the increase of the deep mine mining depth,forming a deep high temperature environment.Due to the high temperature environment of deep mine,temperature also has a significant influence on creep deformation and gas migration.Therefore,the research on gas migration in deep coal seam has changed from the traditional gas-solid coupling problem into a more complex multi-field coupling problem.It is necessary to analyze the coal deformation and gas migration from the multi-field coupling perspective,establish a more objective and practical theoretical model,and further provide a more powerful theoretical support for the mining of deep coal resources.Based on the engineering background of coal and gas co-mining in high gas mines,this thesis takes deep coal as the research object,and studies the creep deformation and permeability evolution of deep coal considering multi-field coupling effects,as well as the gas migration characteristics of deep coal seam from three aspects of laboratory test,theoretical modeling and numerical simulation.According to the research on the creep and seepage law of deep coal in this thesis,the following aspects of work have been carried out:(1)In order to investigate the mechanical response of deep coal to achieve better gas control,a fractional visco-elastoplastic constitutive model is established by combining the fractional Burgers model based on conformable derivative with a non-associated Drucker-Prager elastoplastic model.Based on the open-source numerical platform FEni CS and MFront,the fractional constitutive model is discretized within the implicit finite element method.The numerical model is verified by comparing to analytical solutions and experimental data,the numerical model parameters are determined by experimental data.In addition,the numerical model is used to carry out the numerical study of deep coal under triaxial compression condition,and further realize the engineering scale simulation of roadway in deep coal seam.(2)The thermal damage variable is established based on the Weibull distribution and continuum damage mechanics theory.The constitutive equations of mechanical elements considering the influence of temperature are derived,and the new constitutive equations are substituted into the viscoelastic body and viscoplastic body of Nishihara model.Combined with the triaxial stress and pore pressure of deep coal,a triaxial creep model of deep coal is established considering temperature effect based on fractional derivative.The accuracy and validity of the triaxial creep model proposed in this thesis are verified by combining with the experimental data of deep coal considering temperature effect.The fitting analysis shows that the proposed model can well describe the creep deformation of coal samples under the influence of temperature,especially the accelerated creep stage.(3)Based on the contribution of matrix swelling deformation by gas adsorption to the change of bulk volume and fracture volume,an internal swelling coefficient is defined to characterize the degree of matrix-fracture interaction.The new expression of internal expansion coefficient is established and introduced into a classical stress-strain constitutive equation in porous media.In order to study the influence of creep deformation on permeability,a permeability model(CMFI model)is established considering the effects of creep deformation and matrix-fracture interaction(internal expansion coefficient represents matrix-fracture interaction)in deep coal under triaxial stress.The proposed permeability model is validated by experimental data-based permeability estimated by the fractional derivative transient pulse method.It is indicated that the permeability decreases gradually in the primary and secondary creep stage,consistent with the physical process in which the initial fracture and pores are gradually compacted during the creep process.The effects of creep deformation and internal swelling coefficient on the permeability model are discussed.(4)In order to grasp the gas migration in deep coal seam with the multi-field coupling effects,the creep-seepage experiments and theoretical models are studied under different temperature conditions.The experimental results show the decrease of permeability induced by the increase of matrix deformation during the creep process.The gas adsorption capacity is weakened with the rise of experimental temperature,the matrix swelling deformation decreases,and permeability decline is inhibited,resulting in the higher the temperature and the lower the change of permeability.Combined with the experimental results of creep deformation and permeability evolution,a new permeability model is developed with the effect of temperature during the matrix-fracture interaction and creep process.A coupling internal swelling coefficient is adopted to characterize the matrix-fracture interaction induced by gas adsorption and thermal expansion.The influence of matrix deformation induced by creep deformation on permeability is dominant.However,the influence of matrix-fracture interaction and temperature is still cannot be ignored.(5)The gas migration around the borehole is analyzed quantitatively by using COMSOL Multiphysics.In combination with the field data,the variation of gas seepage field with or without time-dependent effect is compared,and the gas migration is simulated in deep coal seam during long-term extraction.Compared with the calculation results of the model without considering the time-dependent effect,it can be seen that the porosity and permeability considering time-dependent effect are significantly smaller than the models without considering the time-dependent effect.With the increase of extraction time,the viscoelastic elements of creep model make the coal body denser,and the time-dependent effect of deep coal seam becomes more obvious,and the difference increases gradually between the simulation results of porosity and permeability considering time-dependent effect or not.The seepage field around the borehole is analyzed by using the permeability model considering the time-dependent effect.Under the same extraction time,the permeability shows an increasing trend and the gas pressure shows a decreasing trend with the decrease of the distance from the borehole center.When the extraction time gradually increases,the permeability increases at the same position,and the gas pressure gradually decreases.