Thermomechanical properties and fracture behavior of dry-hot granite subjected to cyclic thermal shock and dynamic tensile loading

Abstract

In geothermal engineering, dynamic loads (e.g., drilling, hydraulic fracturing) and thermal shock between working fluids and high-temperature reservoirs significantly affect well-wall stability and exploitation efficiency. This study performed a series of dynamic tensile experiments on granite flattened Brazilian disc (FBD) specimens treated by cyclic thermal shock (CTS) using a split Hopkinson press bar (SHPB) system. The influence of temperature and thermal cycles on the physical and tensile mechanical properties, failure process, and morphological characteristics was analyzed, and the coupled failure mechanism was discussed in combination with numerical simulation. The results show that the P-wave velocity and dynamic tensile strength of granite decrease with increasing CTS temperature and cycles, while porosity increases, and a response surface model for the degradation of physical properties is developed. The central cracking of the FBD specimens is controlled by pure tensile, and the cracking time lags with increasing CTS temperature and cycles. The specimens exhibit tensile-shear composite failure, and the failure forms change from Y-shape to X-shape with increasing temperature, but transition from Y-shape to mono-diagonal shape with increasing thermal cycles. The fractal dimension and joint roughness coefficient (JRC) of the fracture surface are positively correlated with CTS temperature and cycles. Moreover, the CTS-induced thermal cracks increase with temperature and thermal cycles in a polynomial and negative exponential function law, respectively, making the granite transgranular fracture more prominent. The transition from parallel cleavage to exfoliation cleavage of mica may be an important reason for the increase of granite ductility at high temperatures.