Experimental study of the influence of temperature and cooling method on mechanical properties of granite: Implication for geothermal mining

In geothermal mining, the mechanical properties and micro‐structure of the rock under different temperature or cooling modes are highly relevant to the stability of the artificial reservoir. For reflect the actual working conditions, an experimental study is carried out on high‐temperature granite that has been cooled both naturally and rapidly with water. The following conclusions are reached. (a) The wave velocity, elastic modulus, and uniaxial compressive strength of the test specimen decrease with an increase in its heating temperature under both cooling modes. (b) The characteristics of the stress‐strain curve of granite are different after different temperatures and cooling modes. With natural cooling, the stress‐strain curve features double peaks in the range of 200‐300°C, whereas such phenomenon occurs at temperatures greater than 300°C with water cooling. (c) When the temperature is less than 300°C, no obvious change occurs in the crystal structure of the sample. When the temperature is more than 300°C, the original micro‐cracks gradually expand and eventually develop into larger cracks. These observations show that after being exposed to the action of high temperature and water, the minerals and structures of the granite undergo not only physical but also chemical changes, which cause the surface color of the specimens to change.