A hydraulic fracturing study for the green-tuff reservoir

JNOC and Teikoku Oil Co. have studied hydraulic fracturing for a naturally fractured deep volcanic reservoir in Niigata prefecture facing the Japan sea since 1988. First two year of the project was spent for pre-fracturing study. Data obtained from mini-frac, such as step rate test, pump in/flow back test, and pump in/shut in test, were used to design the main fracturing job. Well logging date were used to determine the stress distribution of the formation. In 1990, the main fracturing job was done, succeeded in the improvement of the well production rate about twice as much as that before fracturing, but couldn't be injected the scheduled amount of proppant, because of screen out. The high net pressure and two kinds of closure pressure, which were analyzed from mini-frac, indicate the possible presence of natural fractures. Laboratory experiments have been made to inspect the simulation model. Water is injected to acrylic resin or mortal block, which is put in the true triaxal confining pressure apparatus. Fracturing pressure and surface strain can be measured under various confining pressures. Acoustic emission event detection has also tried to investigate formed fracture geometry.