Multiscale characterization of shale softening induced by water-based fluids

Characterization of mechanical alterations of shale constituent phases is critical for an in-depth understanding of the underlying mechanisms of shale softening. In this study, a hydro-thermal reaction system is set up to mimic the interactions between shale and water-based fluids under the subsurface environment in shale formations. Using a coupled analysis of grid nanoindentation and in situ mineralogical identification, mechanical alterations of shale constituent mineral phases are revealed. Mechanical degradation of carbonate and clay phases is 10 times greater than quartz, pyrite and organic phases. The KCl additive greatly mitigates mechanical degradation of the clay phase. The high temperature and pressure results in a mechanical degradation of carbonate minerals as much as three times of that occurs at room temperature and atmospheric pressure. Multiscale mechanical models, which are established based on Mori-Tanaka (MT) and self-consistent (SC) schemes, predict more accurate elastic softening of shale composite than the microindentation experiments, due to the microcracks generated in the experiments. Based on the calculation of the multiscale mechanical model, under the subsurface environment of shale formations (e.g. 80 °C and 8 MPa), the carbonate dissolution leads to a reduction in Young's modulus of shale composite by about 30%, while the degradation of clay minerals only causes a reduction by up to 9%.