Study On Multi-field Coupling Mechanism And Gas Migration Law Of Structure Anisotropic Coal Under True Triaxial Mining Stress

Coal seam occurrence conditions are quite different in China.The geological structure is complex,and the internal structure of coal rocks presents obvious anisotropic characteristics.As the intensity and depth of coal mining continue to increase,the mining disturbance effect accompanied by the coupling of multiple fields becomes more pronounced.Under the combined action of tectonic stress and mining stress,it shows a clear true triaxial mining stress state(?₁>?₂>?₃),that is,external stress also appears anisotropic features.The anisotropy of coal rock permeability is the result of the dual anisotropy of the internal structure and external stress of coal rock.Studying the anisotropy of coal rock permeability and mechanical behavior is the key to safe and efficient gas drainage in coal seams.In this thesis,a novel multi-functional true triaxial geophysical(TTG)apparatus independently developed by Chongqing University was used to carry out experimental studies on mechanical seepage of coal with different bedding angles under different true triaxial mining stress paths.Focus on influence of different true triaxial mining stress path or stress state,principal stress,bedding structure or inclination angle,and gas flow direction on the mechanical behavior and permeability characteristics of coal rocks;Based on the results of percolation experiments of bedding coal rocks in true triaxial stress,combined with flat fracture flow theory,the coal rock is simplified into a cube model,and a true triaxial dynamic anisotropic permeability model based on bedding effect is constructed;The coal is regarded as a dual-porous medium model,and a multi-field coupled digital model of coal and rock was constructed from the true triaxial dynamic anisotropic permeability model and the dynamics porosity model,the stress balance equation,and the gas mass conservation equation in the matrix and fractures;A numerical simulation platform was used for numerical simulation experiments of gas drainage under multi-field coupling of anisotropic permeability coal seams.The research results are expected to provide theoretical guidance for the precise layout of coal and gas co-mining drainage boreholes.The key research results of this article are as follows:(1)Under the true triaxial simple stress path,as?₂ increases,coal and rock permeability gradually decreases.In this experiment,the permeability in the parallel bedding direction is about twice the permeability in the vertical bedding direction,and the permeability in the two parallel bedding directions is not much different in general.When the gas percolation direction is parallel to the coal bed bedding plane,?₃ has the highest permeability when the bedding plane is vertical,followed by?₂,and?₁ is the smallest.The permeability anisotropy ratio changes dynamically with changes in stress.As the?₂ increases,the permeability anisotropy ratio reduces.(2)Under the true triaxial mining stress path,the true triaxial loading and unloading seepage experiments were performed on coal with different bedding angles under different stress states,and the deformation,permeability,energy,and damage of coal under the coupling effect of different bedding angles and stress states was discussed.With the increase of bedding angle,the peak strength of coal rocks decreases,and the permeability increases,the dissipation energy increases before failure and instability.The inversion effect of?₂ occurs in the true triaxial stress state(TTSS)A,and?₂<?₃occurs after the failure in TTSS-B.The peak stress in TTSS-A is significantly lower than that in TTSS-B,and the energy consumption is just the opposite.(3)Under the true triaxial cyclic disturbance action,the principal strain corresponding to the direction of the cyclic principal stress is the dominant deformation,and its deformation is relatively large.During the grading cycles?₁ and?₃,?₂ is relatively small and the residual strain changes have the same trend.That is,as the magnitude of the grading cycle load increases,the cumulative maximum principal strain and minimum principal strain are exponential and quadratic parabola respectively.As the function increases,the relative residual maximum principal strain and minimum principal strain are U-shaped and?-shaped,and then decrease first and then increase,while the changes of the cumulative and relative residual intermediate principal strains are small.The cyclic loading and unloading of?₂ may cause the instability and destruction of coal rocks,and the residual intermediate principal strains increase significantly,and the critical failure increases sharply.With the increase of the true triaxial cyclic load amplitude,the total dissipation energy of coal and rock increases exponentially,and the damage variable increases in an"S"shape.(4)Based on the cube model and slab flow theory,an anisotropic unit body percolation model is constructed,the anisotropy coefficient of the initial structure of the coal rock is defined,and the true triaxial stress conditions considering the change of the structure and stress of the coal rock are derived.By using true triaxial experimental data and classic permeability models W-Z(anisotropy),C-B,and S-D permeability models were compared with dynamic anisotropy(D-A)permeability models under true triaxial stress conditions based on bedding effects,and the correctness and superiority of the D-A permeability calculation model under true triaxial stress conditions based on the bedding effect were verified.(5)Treating coal and rock as a dual-porous medium model,based on the true triaxial stress field governing equation and gas flow field governing equation,combined with the dynamic anisotropic permeability model and dynamic porosity model,a true triaxial stress multi-field coupled mathematical model for structural heterogeneous coal seams was constructed.This model takes into account effective stresses under true triaxial stress conditions,heterogeneous characteristics of coal and rock structures,anisotropic characteristics of dynamic permeability of coal and rocks,dynamic changes in diffusion coefficients,matrix shrinkage and bedding.Multiple factors are cross-coupled.A numerical simulation study of single-hole dry gas drainage in anisotropic coal seams was carried out,and the optimal layout scheme of multi-drainage drilling groups was discussed.