Radial impact fracture characteristics and crack initiation criterion of concentric perforated granite after high temperature-water cycle

Abstract

In this study, the radial impact fracture characteristics and crack initiation criterion of concentric perforated granite after high temperature-water cycle were evaluated based on the engineering environment such as high temperature, water encounter and dynamic disturbance experienced by the wellbore surrounding rock in the energy storage area during deep geothermal energy mining. And the combination of mechanical test, numerical simulation and theoretical analysis was used. The results revealed that the concentric perforated granite treated with high temperature and water soaking under radial impact load successively underwent three stages: transient elastic deformation, stable and unstable expansion of internal cracks. Finally, the tensile failure was produced and the granite was split into four pieces of approximately symmetrical. The main fracture direction of the rock sample was the impact direction revealed by the strain, fracture history and lateral strain clouds of the inner bore wall of the sample. Meanwhile, the tensile cracks firstly sprouted from the inner bore wall of the sample in the impact direction and expanded to the outer wall, and then from the outer wall in the vertical direction to the inner bore wall. Then, based on the test results, the damage variables were defined according to the proportion of damage degree caused by single factor and the law of crack germination inside the rock sample. It was calculated that the damage accumulation rate of concentric perforated granite under radial impact load after high temperature and water immersion experienced three typical stages: “slow-fast-slow”. Finally, the energy structure relationship of the micro-element of concentric perforated granite was constructed. The crack initiation criterion of the rock sample with concentric perforated structure under radial impact load was established by considering the initial damage caused by heating and water immersion and using the parameters such as the energy storage limit of the rock sample and the energy required for crack initiation. In addition, the rationality was also verified. The research can provide a theoretical reference for the formation of heat transfer channel caused by fracture of stored rock mass and the stability control of surrounding rock of geothermal wellbore, which have great engineering practice and scientific significance.