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.