坚硬低渗煤层孔周煤体瓦斯渗流特性实验研究

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

  • 摘要: 为探究坚硬低渗煤层钻孔孔周煤体的渗流特性,本研究首先分析孔周煤体径向应力分布特征和裂隙发育程度,在此基础上,采用“不同围压下渐进加载轴向载荷”的方式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系统开展坚硬低渗煤层孔周煤体瓦斯渗流特性实验研究,得到煤样变形破坏全过程的渗透率演化特征,揭示了坚硬低渗煤体孔周不同应力分区的瓦斯渗流规律。研究结果表明:① 在应力作用下,孔周煤体先经历弹性变形、再进入塑性变形阶段,当应力超出其强度极限时发生破碎,从钻孔壁由近及远在径向依次形成破碎区、塑性区与弹性区。② 随应变增大,煤样渗透率呈先减小后增大的“V”字型演化特征。同时,实验发现围压对煤样渗透率具有强抑制作用,因此深部开采高应力条件下应优先采用卸压技术提高其渗透率。③ 两种渗透率测试方法的适用性存在差异:稳态法适用于测量松散或高渗透率煤样;瞬态法则更适用高应力状态或致密煤样的渗透率测试。

     

    Abstract: To investigate the seepage characteristics of coal around boreholes in hard, low-permeability coal seams and enhance gas extraction efficiency in mines, this study analyzed the radial stress distribution and fracture development degree around boreholes. Using a triaxial seepage experimental system with zonal loading, the complete process from elastic deformation to failure of hard, low-permeability coal samples was simulated by progressively increasing axial load under different confining pressures. Combining steady-state and transient methods, experimental research was conducted on the gas seepage characteristics of coal around boreholes in hard, low-permeability coal seams. The permeability throughout the entire deformation and failure process of coal samples was obtained, revealing the gas seepage patterns in different stress zones around the boreholes.The research results indicate: ① Under stress influence, the coal around boreholes gradually undergoes elastic and plastic deformation. Once the strength limit is exceeded, fracturing begins. Radially, from near to far, the areas are categorized as the fractured zone, plastic zone, and elastic zone. The permeability first rapidly decreases from its maximum value to a minimum and then slowly recovers to the original permeability of the coal seam, overall presenting a "V" shape. ② Experiments revealed that confining pressure strongly inhibits permeability. When the confining pressure increased to 4 MPa, the permeability of the coal sample was only 4.57% of that at 3 MPa. Therefore, under deep mining conditions, pressure relief techniques should be prioritized for gas extraction. ③ The steady-state method can measure the permeability of loose or highly permeable coal samples, while the transient method yields higher permeability results and is suitable for measuring samples under high stress or those with inherently dense structures.

     

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