Characteristics of Stress Corrosion Cracking of Granite under Simulated Geothermal Conditions

Abstract

A knowledge of stress corrosion cracking of rocks under hydrothermal conditions is required to evaluate longterm geothermal reservoir performance. In this study, stress corrosion cracking tests were carried out on Granite in high temperature water environments from 150°C to 350°C using Pre-notched thick-walled cylindrical specimens. The inner bore of the specimen was hydraulically pressurized up to a predetermined level to give an initial stress intensity factor K_i to the notch tip. This caused radial flow of hot water through the specimen. The pressure was kept constant until the specimen failed, and the time to failure t_f was measured. During the tests, the variation in permeability with time was measured by collecting water drained from the specimen. The following results were obtained. (1) The mechanism of fracture changed with test conditions. Specimen failure was generally caused by a straight crack extending from the notch tip to the outer wall. On the other hand, under the conditions of low K_i and high temperature (≥300°C), a crack initiated at the notch tip branched out during the extension and developed into multiple cracks. (2) Crack extension by stress corrosion occurred preferentially in the direction parallel to the lift plane where pre-existing microcracks are oriented preferentially, because the microcracks had higher sensitivity to stress corrosion. (3) Permeability measurements showed that pre-existing microcracks, which were paths of water, grew and widened markedly owing to stress corrosion and rock dissolution. The developing rate was enhanced in high temperature conditions, especially above 200°C. (4) Under the conditions of high temperature, stress corrosion cracking rate of pre-existing microcracks in the vicinity of a main crack tip might be enhanced up to about that of a main crack with low K_i. Development of multiple cracks was thought to be a result of marked extension of the microcracks and cupling of the extended microcracks with the main crack. (5) Stress corrosion cracking laws were deduced from relationships between K_i and t_f. Results revealed that stress corrosion cracking rate was strongly enhanced by water permeation and high tem-perature, especially above 200°C.