EVALUATION OF ROCK FRACTURE TOUGHNESS IN THE PRESENCE OF PRESSURIZED WATER AT ELEVATED TEMPERATURES BY MEANS OF THE AE TECHNIQUE

Fracture toughness tests were performed on four kinds of rock samples of granite, andesite, tuff and mudstone to evaluate the fracture behavior of subsurface rock materials in geothermal areas. The effects of temperature, confining pressure and water environment on the rock fracture toughness were shown through experiments using a pre-notched cylindrical specimen in the presence of pressurized water, ranging from 0.1 to 30 MPa at a maximum temperature of 224°C. The rock fracture toughness (KiAE) is defined by the critical stress intensity factor of crack tip region at the onset of the main crack propagation, which corresponds to an abrupt increase of ΣEAE before the maximum differential pressure. For granite, the value of KiAE at 200°C and 20 MPa decreases by approximately 20% of the value at room temperature and atmospheric pressure. On the contrary, the value of KiAE of andesite is independent of high temperatures and confining pressures provided that the test environment conditions are less than 224°C and 24 MPa. Intermediate value of KiAE between granite and andesite are obtained in the same manner as for tuff and mudstone.