Multi-scale study on the mechanism of shale degradation induced by dry and wet cycles

The outcrop shale bank slope is prone to geological disasters such as caving and bank collapse under the influence of the variation of reservoir water level, which poses a threat to the safety of reservoir area. To enrich the understanding of the mechanism of shale degradation induced by dry and wet cycles, the outcropping shales of Silurian Wufeng Formation-Longmaxi Formation in Changning area of Sichuan Province were studied. Laboratory gas penetration tests and uniaxial compression tests were carried out. Microscopic analysis techniques such as SEM, EDS and XRD were used to reveal the mechanism of shale seepage degradation induced by dry and wet cycles from a multi-scale perspective. The results show that: ① the increase of dry-wet cycles and osmotic pressure difference can promote the permeability of shale, whereas confining pressure can inhibit it. ② The dry-wet cycle has a weakening effect on the compressive properties of shale, and the number of dry-wet cycles is approximately exponentially correlated with the elastic modulus and peak strength. ③ The influence of wet and dry cycles on the microstructure of shale is mainly reflected in the aspects of block thinning, block hydration, pore minimization, sharp edge smoothing, etc. The proportion of hydrophilic minerals is greater than that of non-hydrophilic minerals and organic matter, and the roughness of flat surface increases with the treatment process. ④ The mechanism of shale seepage degradation induced by dry and wet cycling can be revealed by the combined effects of dry and wet cycling on the joint surface of shale, the change of seepage channel, the development of micro-defects and mineral hydration. The degradation mechanism of shale in multiple scales is expounded combined with the test results, the different responses of shale blocks and joints to the dry-wet cycle are analyzed, correlating the degradation phenomena at macro and micro scales. It is considered that the formation of weak joint surface is the decisive factor causing the degradation of shale properties.