Microcracks development and porosity evolution in sandstone, Sichuan basin, China: an experimental approach

摘要

The process of fault rupture is closely related to the weakening of fault rock. The weakening process of fault rock from Sichuan basin was investigated when subjected to different loading levels and subsequently unloaded under uniaxial compression. With nuclear magnetic resonance, the transverse relaxation time spectra (T-2) distribution and porosity characteristics of the sandstone were quantified; meanwhile, optical microscopic experiments were performed to explore the microcrack growth patterns. According to the growth curve of the normalized porosity, the degradation process of sandstone was divided into three phases. During the first phase, the normalized porosity grew at a small accelerating rate. The intergranular microcracks grew slightly, and there was no distinct change in the number of transgranular and transgranular-intergranular microcracks during this phase, suggesting the weakening was mainly caused by the initiation of new microcracks. In the second phase, the growth rate of the normalized porosity increased linearly as a response to the percentage of failure. At this stage, although intergranular microcracks still dominate, there was a significant increase in the density of transgranular-intergranular microcracks and transgranular microcracks. Furthermore, microcracks developed in parallel with principal stress direction with the increase of the microcrack number and length. In the third phase, the normalized porosity increases at a large accelerating rate. Microcracks continued to grow along the major principal stress direction and penetrate each other. It is noteworthy that the crack density of fault sandstone increased exponentially with load level, and the crack density increased sharply at about 75% of peak strength, suggesting the initiation of fault rupturing. Such results contributed to improve the understanding of porosity change and cracking development of rocks in fault zones, which is pertinent to shear fracture nucleation.