含瓦斯砂岩卸围压变形特征与渗透规律试验研究

Experimental study on the deformation characteristics and permeability laws of gas infiltrated sandstone under unloading confining pressure condition

为研究含瓦斯砂岩卸荷破坏过程中的变形及渗流规律,采用自主研发的含瓦斯煤热-流-固耦合渗流伺服试验系统,开展了不同围压和相同瓦斯压力条件下的卸围压渗流试验,建立了基于应变指标的瓦斯渗流模型。得出的主要结论有:(1)含瓦斯砂岩破坏表现出明显的脆性特征,峰值应力点后瞬间,应力、应变及流量均出现突跳;(2)应力-应变及渗透率变化呈现明显的阶段性特征:孔隙、微裂隙压密及弹性变形阶段(Ⅰ、Ⅱ阶段),砂岩渗透性为0;卸荷屈服阶段(Ⅲ阶段),产生新的损伤,渗透性小幅增加;卸荷破坏应力跌落阶段(Ⅳ阶段),应力突降,应变陡增,裂隙贯通后渗透性陡增,是渗透性主导阶段;破坏后阶段(Ⅴ),应力、应变及流量变化均趋于稳定;(3)含瓦斯砂岩破坏阶段,受气体压力和泊松效应影响,使径向应变和体积应变远大于轴向应变;(4)含层理砂岩卸荷破坏形式以产生沿层理面方向的张剪混合裂缝为主。基于Kozeny-Carman方程和裂隙流理论,建立了应变相关渗透率模型,揭示了含瓦斯砂岩损伤破坏渗流机制,对研究瓦斯突出、及在破断岩体中的运移规律及抽采孔优化设计等具有借鉴意义。

In this study, a self-developed ‘THM coupled with triaxial servo-controlled seepage apparatus for gas infiltrated coal＇ was employed to study the deformation and permeability laws of sandstone. Seepage experiments were carried out at the same gas pressure and under different unloading confining pressures. Then, a seepage model was established on the basis of strain parameters of sandstone. The results showed that gas infiltrated sandstone exhibited clear brittle feature during the failure process and sudden jumps occurred among the stress, strain and flow immediately after the peak stress point. Both changes of stress-strain and permeability showed obvious periodical characteristics, which means that the gas permeability was zero during pore and micro-fissure compaction, and elastic deformation stages（Ⅰ and Ⅱ）. During unloading and yield stage（Ⅲ）, new fractures were initiated and the permeability increased slightly. During unloading failure stage（Ⅳ）, the stress dropped significantly, the strain increased sharply, and the gas flow also increased greatly when fractures connected with each other, which indicated that this period was dominated by the permeability. The strain, stress, and gas flow all tended to be steady after failure（Ⅴ）. At failure state of sandstone, the radial strain and volumetric strain were far larger than the axial strain, due to the effects of gas pressure and the Poisson. The unloading failure mode of bedding sandstone was mainly dominated with extension and shear fracture along beddings. Based on Kozeny-Carman equation and fracture flow theory, the permeability model related to strain was established. Finally, the permeation mechanism of gas infiltrated sandstone at different failure states was revealed.