In order to accurately evaluate the brittleness and plasticity during hydraulic fracturing of deep shale reservoirs, this study constructs a brittleness evaluation index for deep shales based on the energy evolution features from the complete stress–strain curve. Both the pre- and post-peak elastic energy ratios and the stress drop effect were considered in this index. The brittle–plastic deformation characteristics was fundamentally reflected during rock failure. Due to further comparison between the brittleness index and sample fracture patterns with corresponding stress–strain curves, a quantitative evaluation model (0–1 scale) for deep shale brittleness–plasticity deformation was built. Using this model, the brittle–plastic transition patterns of different shale facies are investigated, creating a three-parameter diagram of the brittleness index, clay mineral content, and depth. The results show that siliceous and carbonate shales undergo a brittle–plastic transition at approximately 4500–5000 m depth, while mixed shales transition at around 3500 m.
Liu et al. (Tue,) studied this question.