EFFECT OF INJECTED WATER ON HYDRAULIC FRACTURING DEDUCED FROM ACOUSTIC-EMISSION MONITORING

In order to investigate the effects of injected water in hydraulic fra cturing, experiments were conducted on cubic granite specimens, compar ing fracturings induced by conventional water injection with those ind uced by pressurization of a urethane sleeve, thereby realizing ''hydra ulic fracturing'' without the use of fracturing fluid. In both experim ents, a shear type mechanism was found to be dominant in fault plane s olutions of AE events. However, in the case of water injection, cracks extended rapidly with large drops in hole water pressure and bursts o f AE, whereas in pressurization by the urethane sleeve, cracks extende d stepwise with no such large drops in hole pressure and no bursts of AE. The difference in crack extension in the two experiments call be a nalyzed by comparing relations between crack length and stress intensi ty factor of mode I at a crack tip. The observation and analysis indic ate that existence of fracturing fluid like water helps initiated crac ks to extend rapidly and widely in hydraulic Fracturing in actual HDR fields.