Experimental Study on the Fracture Toughness of Granite Affected by Coupled Mechanical-Thermo

Abstract

Abstract Hot dry rock geothermal energy is deep geological energy. Its ability to resist fracture is an important basis for effective reconstruction and scientific evaluation of the stability of geothermal reservoirs. Hot dry rock is typically buried deeply, and the reservoir is often in a high-temperature and high stress environment. There have been limited studies conducted on the effect of different three-dimensional stress and temperature on granite fracture toughness. Thereby, herein an experimental study is conducted on the heat treatment of granite under different external loads and temperatures. The variation in fracture toughness of granite with temperature and pressure is studied using a three-point bending fracture mechanics experiment, scanning electron microscope (SEM) observation, and acoustic wave velocity measurement. The results show that under the joint influence of 25 MPa deviator stress and 200 °C temperature, the Mode I, Mixed mode (I + II), and Mode II fracture toughness of granite show a nonlinear change trend of decreasing and increasing. Among the three modes, the change range of Mode I fracture toughness is not more than 10% which is not significant. Contrarily, the degradation effect of rock mechanical properties caused by the joint action of stress and temperature in Mode II and Mixed mode (I + II) is predominant. The maximum range of Mode II fracture toughness is reduced by 22%, whereas the maximum range of Mixed mode (I + II) fracture toughness is reduced by 18%. However, the compression action of three-dimensional stress causes a slight enhancement in granite mechanical properties, wherein the maximum range of Mixed mode (I + II) fracture toughness is increased by 12%. Furthermore, the change of granite’s ability to resist tensile, shear, and composite (tensile + shear) fracture is not coordinated under the joint action of different temperatures and external loads. This may be due to the small deviator stress effect, which is similar to the early loading stage of uniaxial compression. External loads and thermal stress damage occur in the rock along with the compaction of pore cracks. These mechanisms have different dominant positions under varied temperature and three-dimensional stress coupling conditions, resulting in either the enhancement or weakening of the mechanical properties of granite. The results of this experimental study are conducive to gaining an in-depth understanding of the change law of deep rock mechanical properties and the exploration of hot dry rock reservoir reconstruction.