Experimental study on the mechanical behavior of different lithologic shales in Qingshankou group under in-situ geological conditions

To investigate the mechanical behavior of shale under in-situ geological conditions, a novel experimental methodology called in-situ stress restoration triaxial compression (ISRTC) was proposed for testing shale under in-situ geostress. Then the ISRTC and conventional triaxial compression (CTC) tests were conducted on four different lithological shale core samples from the Qingshankou Formation reservoir to explore the differences in shale mechanical behavior between conventional laboratory conditions and in-situ geostress conditions. Compared to CTC tests, shale samples subjected to ISRTC tests demonstrated a noteworthy decrease in pre-peak elastic deformation, alongside an intensified and increased nonlinear deformation. These findings indicated that in-situ geostress restoration led to reduced brittleness and increased ductility of shale. Moreover, shale cores in the ISRTC test showed higher peak strength, residual strength, Young's modulus, and Poisson's ratio compared to CTC tests. The elastic energy, dissipative energy, and total energy in ISRTC tests were also greater than those in CTC tests. Furthermore, in-situ geostress restoration promoted numerous microcracks in shale, resulting in the release of more energy during failure, leading to a more complex failure mode. In CTC tests, shale damage concentrated in the stage of unstable crack propagation, whereas in ISRTC tests, the damage developed progressively throughout the entire loading stage. Additionally, the abundance of hard-phase minerals and scarcity of weak-phase minerals in shale had a smaller influence of in-situ stress restoration on peak strength and residual strength but a greater influence on the elastic modulus. This study provides valuable insights into the in-situ mechanical characteristics of different lithologies of Gulong shale in the Qingshankou group and serves as a scientific basis for the efficient development of shale oil.