An Experimental Study on the Response of Seepage Characteristics of Low Permeability Coal to Its Loading Axis/Confining Pressure

Kun-yun TIAN; Xiao-li LI

doi:10.13718/j.cnki.xdzk.2019.02.018

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In order to understand the laws of permeability variation of low permeable coal samples under changing axle loads and confining pressures, an experimental research was conducted on coal permeability under different confining pressures and axle loads using a self-developed resultant stress-strain triaxial seepage test system based on servo temperature control. The test results demonstrated that permeability decreased with changes in axle load and confining pressure, but confining pressure exerted greater effects on permeability than axle load; that the relation of confining pressure to permeability could be expressed as K=αe^-0.45σ₂, where the fitting coefficient a decreased steadily as the axle load increased; and when the axle load and the confining pressure were within the range of 1~10 MPa, their relation to permeability could be written as K=15.335 89e^-0.45σ₂-0.276 43σ₁e^-0.45σ₂. These results are expected to lay a theoretical basis for the evolution of permeability during mining.

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An Experimental Study on the Response of Seepage Characteristics of Low Permeability Coal to Its Loading Axis/Confining Pressure

Received Date: 24/07/2018

Available Online: 20/02/2019

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Abstract: In order to understand the laws of permeability variation of low permeable coal samples under changing axle loads and confining pressures, an experimental research was conducted on coal permeability under different confining pressures and axle loads using a self-developed resultant stress-strain triaxial seepage test system based on servo temperature control. The test results demonstrated that permeability decreased with changes in axle load and confining pressure, but confining pressure exerted greater effects on permeability than axle load; that the relation of confining pressure to permeability could be expressed as K=αe^-0.45σ₂, where the fitting coefficient a decreased steadily as the axle load increased; and when the axle load and the confining pressure were within the range of 1~10 MPa, their relation to permeability could be written as K=15.335 89e^-0.45σ₂-0.276 43σ₁e^-0.45σ₂. These results are expected to lay a theoretical basis for the evolution of permeability during mining.

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近年来煤炭开采深度不断增加，煤层透气性也随之愈来愈低.现有瓦斯抽采资料表明：我国绝大部分煤层透气性系数较低，为低透气性煤层，透气性直接影响着瓦斯的抽采效果，对瓦斯治理非常关键^[1].瓦斯在煤层隙中的扩散及渗流受煤体内部裂隙、所处的应力场、周围温度场等多因素影响^[2].煤体应力场环境直接决定了瓦斯渗流规律，渗透特征很大程度上直接取决于轴向应力或侧向应力.

针对煤体渗透率变化特征，袁梅等^[3]分析了围压对含瓦斯煤体渗透率演化规律及两次围压升降过程中渗透率最大损失率变化趋势；李树刚等^[4]的研究成果表明应力加卸载过程中煤样渗流率及体积变化呈现出两个不同阶段的函数关系；王登科、朱卓慧等^[5-6]认为煤样试件在应力的加卸载过程中，渗透率随有效应力的变化趋势呈现出近似的“V”型；黄启翔等^[7]则认为，试件所加载的围压对渗透性的影响大于其所受载的轴压，并且渗透率与轴(围)压之间均呈现负指数关系；祝捷等^[8]研究表明，加卸载过程中由于受压而发生的煤体形变对瓦斯渗透性的影响作用较大，形变的大小直接决定了煤体渗透率的大小；C.R.Mckee等^[9]的研究成果从现场实践方面也证明了煤体渗透率与煤体埋藏深度之间呈负指数关系；胡耀青等^[10]基于试验表明体积应力变大，煤的瓦斯渗透率降低；魏建平等^[11]对受载含瓦斯煤渗透性影响因素进行了分析，发现在温度一定情况下，煤样渗透率随有效应力的增大而呈现负指数变化关系；丁宝成等^[12]研究表明：在相同围压条件下，煤样渗透率随孔隙压力增大先减小后增大，变化曲线呈二次函数关系；王广荣等^[13]研究表明渗透率随围压的增大而减小，渗透率在应力-应变峰后呈现急剧上升的趋势；Rongkun Pan等^[14]对煤样不同层理方向进行了加卸载，发现层理结构对煤层中气体渗流规律影响很大，加载过程裂纹破坏很难在卸载过程中得到恢复.

上述有关研究成果均就煤样渗透率与试件所受围压及轴压的关系进行了探讨.本文以义煤集团新安煤矿主采的二₁煤层为研究对象(该煤层透气性系数较低)，在对煤样试件进行轴压及围压加卸载的情况下，测试加卸载过程不同阶段的渗透率，分别就试件所受围压及轴压与对应的渗透率大小做定性分析，并对渗透率与两者之间的关系进行定量耦合，以期对煤层瓦斯抽采过程中的钻孔布置提供一定的理论指导.

3. 结论

利用全应力-应变三轴伺服控温渗流试验系统，在不同轴压及围压作用下对低渗透性煤样试件的瓦斯渗透率进行了实验，结论如下：

1) 试件渗透率在轴压(围压)加载过程中不断降低，前期较为急剧，后期趋于平缓，围压对渗透率影响高于轴压.

2) 围压与渗透率符合K=αe^-0.45σ₂关系，拟合系数α随着轴压的升高呈依次降低的趋势，拟合系数α与轴压符合α=15.426 23-0.261 94σ₁函数关系.

3) 轴压、围压为2~10 MPa时，两者和渗透率满足K=15.335 89e^-0.45σ₂-0.276 43σ₁e^-0.45σ₂的关系.

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An Experimental Study on the Response of Seepage Characteristics of Low Permeability Coal to Its Loading Axis/Confining Pressure

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