Effect of Grain Size on the Mechanical Behaviour of Granite Under High Temperature and Triaxial Stresses

Abstract

The optimal development of hot dry rock (HDR) geothermal is deep HDR geothermal. Because of different diagenesis environments, the mineral composition and micro-structure of deep granite are quite different than those of shallow granite. To reveal the characteristics of deep granite and guide HDR geothermal development, the difference in thermal and mechanical properties between the granite in Luya Mountain, Shanxi Province, China (coarse-grained granite) and the granite in Shandong Province, China (fine-grained granite) under high-temperature (100–400 ℃) triaxial stress was studied. The results show that the thermal expansion coefficient of the coarse-grained granite increases linearly with increasing temperature, and the thermal expansion coefficient of the coarse-grained granite is 1.52 times that of the fine-grained granite on average, and the difference reaches a maximum at 400 ℃. The elastic modulus of the coarse-grained granite increases slowly first and then decreases sharply with increasing temperature, and its threshold temperature varies with temperature at approximately 300 ℃. The elastic modulus of the fine-grained granite is 1.4–2.6 times that of the coarse-grained granite, and the difference increases with increasing temperature and confining pressure. According to the failure test under triaxial stress (confining pressure (σc) = 25 MPa) and at 400 ℃, for the coarse-grained granite, the peak strength, elastic modulus and its threshold temperature change with temperature are smaller, the peak strain is larger and the elasto-plastic transition occurs easier than those for the fine-grained granite. Micro-observation shows that the larger crystal particles and the extreme heterogeneity of the coarse-grained granite lead to larger thermal deformation and greater deterioration of the mechanical properties, compared with those of the fine-grained granite, further leading to higher permeability of the coarse-grained granite under high temperature and high pressure. The existence of coarse-grained granite provides a good geological foundation for high-efficiency, low-cost and large-scale construction of artificial reservoirs in the process of HDR geothermal development.