深部煤层力学各向异性特征与深度效应OA

[Objective]The mechanical properties of coal seams determine the stability,fracability,and fracturing per-formance of deep coal reservoirs.Since the in situ stress applied to coal increases with burial depth,deep coal differs from its shallow counterpart in damage behavior,key mechanical parameters,and failure mechanism.Furthermore,deep coal exhibits pronounced mechanical anisotropy.[Methods]Focusing on the No.9 coal seam in the Linxian area along the eastern margin of the Ordos Basin,this study carried out triaxial compression tests at 60℃ under different confining pressures to determine the mechanical anisotropy of deep coal.By comparing the full stress-strain curves,key mechanic-al parameters,and failure modes of coal specimens with sampling directions parallel to face cleats,parallel to butt cleats,and perpendicular to beddings,this study revealed the confining-pressure effects and directional differences in the mech-anical responses of deep coal.In addition,the anisotropic failure criterion for deep coal was established.[Results and Conclusions]The results indicate that,under confining pressures of 0 MPa and 20 MPa,the coal specimens showed dis-tinct linear stress-strain relationships in the elastic deformation stage and then underwent rapidly growing strain in the plastic deformation stage,with coal failure dominated by tensile splitting.Under confining pressures ranging from 30 MPa to 60 MPa,the slopes of the stress-strain curves increased,with the failure mode shifting from tensile-dominated to shear-dominated failure.Comparison reveals that the deformability of the coal specimens with three sampling directions decreased in the order of bedding-perpendicular,butt-cleat-parallel,and face-cleat-parallel directions,while their strength and stiffness decreased in the order of face-cleat-parallel,butt-cleat-parallel,and bedding-perpendicular direc-tions.The compressive strength,residual strength,and elastic modulus of the coal specimens generally increased as the confining pressure rose,with amplitudes increasing at lower rates under high confining pressures.The Poisson's ratios of the coal specimens showed an overall downward trend but dispersed to some extent.Correlation analysis indicates that the elastic modulus and residual strength of the coal specimens were positively correlated with their compressive strength,while the relationship between the Poisson's ratio and compressive strength of the samples exhibited pro-nounced directional differences.Based on triaxial compression data in the three sampling directions and the Mohr-Cou-lomb shear failure criterion,this study determined the internal friction angles and cohesion in different directions and es-tablished an anisotropic shear failure criterion for deep coal.The results indicate that increasing deep in situ stress would enhance the load-bearing capacity of the coal seam and change its failure mode,while directional mechanical differ-ences remained significant.The results of this study will provide a mechanical basis for the stability analysis,fracability evaluation,fracturing parameter design,and shear failure determination of deep coal reservoirs.