Mechanical characteristics and fracture propagation patterns of deep coal-rock assemblages:A case study of the Wuxiang block,Qinshui Basin
The rock mechanical properties of coal seams and their roofs and floors are critical factors influencing the initial cracking and fracture propagation of coal reservoirs during fracturing.However,a limited understanding of these properties has constrained the large-scale commercial production of deep coalbed methane(CBM).This study aims to investigate the mechanical characteristics and fracture propagation patterns of coal-rock assemblages during fracturing.Based on the acoustic emission signals monitored,we perform uniaxial compressive stress tests and finite element analysis(FEA)on coal-rock assemblages from the Wuxiang block in the east-central Qinshui Basin,with the impacts of different coal/rock thickness ratios,lithologies,and coal-rock interfacial angles on the compressive strength,elastic modulus,features of acoustic emission signals,and breakup characteristics of the coal-rock assemblages delved into.Additionally,we explore the fracture propagation behavior under varying coal-rock configurations.The results indicate that the compressive strength and elastic modulus of the coal-rock assemblages decrease with an increase in the coal-rock ratio in thickness and the coal-rock interfacial angle.The coal-rock assemblages with varying lithologies exhibit elastic modulus and compressive strength ranging from 1.75 to 5.44 GPa and from 1 1.20 to 20.60 MPa,respectively,with the coal-argillaceous shale and coal-sandstone assemblages displaying the weakest and strongest mechanical properties,respectively.A higher coal-rock thickness ratio tends to exacerbate the breakup of the coal-rock assemblages and results in more fractures within.At a given coal-rock ratio in thickness,the energy released from coal fracturing would continue to diffuse to adjacent layers,culminating in the breakup of the whole coal-rock assemblage.The susceptibility to failure among the various coal-rock assemblages decreases in the order of coal-sandstone,coal-limestone,and coal-argillaceous shale assemblages.The coal-rock assemblages are increasingly prone to failure,instability,and fracturing as the interfacial angle increases.As the coal thickness increases,fractures in the coal-rock assemblages evolve from tensile fissures to X-shaped shear fissures while gradually propagating from within coals to the entire assemblages.In contrast,with an increase in the mechanical strength of the coal-rock assemblages of diverse lithologies,fractures in the assemblages tend to evolve from X-shaped shear fissures to tensile fissures,with the propagation range shrinking from the entire assemblages to within coals.Besides,an increase in the interfacial angle between coals and rocks in the coal-rock assemblages leads to larger X-shaped shear fissures within,exacerbating the assemblage failure.
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