循环加卸载作用下砂岩孔隙度与渗透率演化规律试验研究

研究砂岩在多次循环荷载作用下孔隙度及渗透率随荷载的变化规律,利用致密岩石惰性气体渗透率测试系统对砂岩进行了5次循环加卸载试验。试验结果表明:砂岩的孔隙度和渗透率随围压的增大而减小,循环加卸载过程中,砂岩的孔隙度、渗透率在加载阶段与卸载阶段的变化曲线均是不重合的。围压加载阶段孔隙度和渗透率随围压的变化关系均呈指数关系,在围压卸载阶段孔隙度与渗透率随围压变化均呈幂函数关系。砂岩在低围压条件下渗透率随围压变化的程度较大,在高围压条件下渗透率变化程度较小。加载阶段砂岩渗透率变化程度在第1次循环期间较大,从第2次循环开始由于岩样一定程度的压密渗透率随围压变化越来越小,而卸载阶段5次循环过程中渗透率随围压的减小而增大,但是循环次数增加过程中,渗透率的恢复程度相对于第一次循环越来越小。

Experimental study on permeability and porosity evolution of sandstone under cyclic loading and unloading

In order to investigate the evolution of porosity and permeability of sandstone during the cyclic loading and unloading, the cyclic loading and unloading tests on sandstone are carried out by using the test system of gas permeability measurement for compact rock. The test results show that the porosity and permeability of sandstone decrease with the increasing confining pressure. The permeability and porosity curve in the process of loading does not overlap that in the unloading process. In the process of loading, the relationship among porosity, permeability and confining pressure can be well described by an exponential function, and it can be well described by a power function in the process of unloading. The permeability changes faster under low confining pressure than that under high confining pressure. The permeability changes much more largely in the first cyclic loading, and from the second cyclic loading, due to the pre-compaction, the permeability changes with the decreasing confining pressure. But in the process of unloading, the permeability gradually increases with the decreasing confining pressure in the fifth cycles, and compared to that of the first cycle, the recovery of permeability is much smaller with the increase of cycles.