坚硬低渗煤层孔周煤体瓦斯径向渗流特性实验研究

Experimental study on radial gas seepage characteristics of coal around boreholes in hard low-permeability coal seams

  • 摘要: 为探究坚硬低渗煤层孔周煤体渗透率演化与孔周径向应力分布、破坏特征之间的耦合机制,揭示坚硬低渗煤层孔周煤体瓦斯径向渗流特性,分析了孔周煤体径向应力分布特征和裂隙发育程度,利用恒速恒压泵加载轴压直至煤样发生破坏,模拟煤样从弹性变形到破坏的全过程,结合稳态法与瞬态法开展了坚硬低渗煤层孔周煤体瓦斯径向渗流特性实验研究,得到了煤样变形破坏全过程的渗透率演化特征。结果表明:① 孔周煤体在径向依次形成破碎区、塑性区与弹性区:破碎区内煤体结构失稳,产生大量相互贯通的裂隙;塑性区内煤体虽产生新生裂隙,但在高应力作用下裂隙开度受限;弹性区内原生裂隙发生弹性闭合。② 孔周煤体沿钻孔径向的渗透率迅速降低,随后缓慢回升,整体呈现明显的“V”型变化趋势。③ 当围压从3 MPa升到4 MPa时,煤样渗透率显著下降,煤样在4 MPa围压下的渗透率仅为3 MPa时的4.57%。④ 孔周煤体沿钻孔径向各应力分区内,破碎区渗透率最高,塑性区内出现应力峰值且渗透率最低,弹性区内应力集中现象逐渐消失且渗透率逐渐回升至煤层原始渗透率。

     

    Abstract: To investigate the coupling mechanism between permeability evolution of coal around boreholes and the radial stress distribution and failure characteristics in hard low-permeability coal seams, and to reveal the radial gas seepage characteristics of coal around boreholes, the radial stress distribution characteristics and fracture development degree of coal around boreholes were analyzed. Axial stress was applied using a constant-speed constant-pressure pump until coal sample failure occurred, simulating the entire process from elastic deformation to failure. By combining steady-state and transient methods, experimental studies on the radial gas seepage characteristics of coal around boreholes in hard low-permeability coal seams were conducted, and the permeability evolution characteristics during the whole deformation and failure process of coal samples were obtained. The results showed that: ① coal around boreholes radially formed a crushed zone, a plastic zone, and an elastic zone in sequence. In the crushed zone, the coal structure became unstable and a large number of interconnected fractures were generated. In the plastic zone, new fractures were generated, but fracture apertures were restricted under high stress. In the elastic zone, primary fractures underwent elastic closure. ② The permeability of coal around boreholes along the radial direction of the borehole decreased rapidly and then increased slowly, showing an overall distinct V-shaped variation trend. ③ When the confining pressure increased from 3 MPa to 4 MPa, the permeability of coal samples decreased significantly, and the permeability under a confining pressure of 4 MPa was only 4.57% of that under 3 MPa. ④ Within each stress zone along the borehole radial direction, the crushed zone exhibited the highest permeability, the plastic zone showed a stress peak and the lowest permeability, and the stress concentration phenomenon in the elastic zone gradually disappeared while permeability gradually recovered to the original permeability of the coal seam.

     

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