花岗岩三轴循环加卸载条件下的气体渗透率

Gas permeability of granite in triaxial cyclic loading/unloading tests

为研究花岗岩在三轴循环加卸载条件下气体渗透率的演化规律,利用岩石多场耦合三轴试验仪分别对花岗岩进行三轴恒下限分级循环加卸载试验和气体渗透试验。试验结果表明:加载曲线与上一次循环卸载曲线形成塑性滞回环,随着循环次数的增加,每个循环应力下限的轴向应变也随之增加;前期几个循环试样体积压缩明显,以轴向压缩变形为主,后期轴向应力达到一定数值时体积由压缩转为扩容,裂纹发展方向偏向于轴向方向,均呈剪切脆性破坏;气体渗透率变化分为稳定下降阶段、缓慢增加阶段、急剧上升阶段3个阶段,试样脆性破坏后气体渗透率均上升2～3个数量级;体积应变曲线拐点与横向应变曲线结合起来可以作为研究岩石渗透率变化规律的一个重要参考因素。

To investigate the evolution of gas permeability of granite in triaxial cyclic loading/unloading tests, the cyclic loading/unloading tests and gas permeability tests were carried out on granite specimens under different confining pressures by using a rock T-H-M-C coupled testing system. The test results show that the plastic hysteresis loops are formed by the loading curve and unloading curve of the former cycle. The lower limit of axial strain in each cycle increases with increasing number of cycles. The volume compression of specimens occurs at the first several cycles, and the main deformation is along the axial direction. Afterwards, the specimen volume changes from compression to dilatancy when deviatoric stress increases to a certain value. All specimens are divided into several parts by macroscopic cracks when shear fracture generates, and the orientation of crack is inclined to axial direction. Gas permeability curves are divided into three stages: a stable decline, a slow rise and a sharp rise. The gas permeability increases by 2-3 orders of magnitude after the brittle failure happens. The inflexion point of volume strain curve and the lateral strain curve can be used as an important reference to predict the change of permeability.