Triggering processes of microseismic events associated with water injection in Okuaizu Geothermal Field, Japan

A continuous water injection test was conducted to halt the reduction in steam production in the Okuaizu Geothermal Field, Japan. Understanding the factors triggering microseismicity associated with water injection is essential to ensuring effective steam production. We identified possible triggering processes by applying methods based on microseismic monitoring, including a new method to determine the presence of water in local fractures using scattered P-waves. We found that the evolving microseismicity near the injection point could be explained by a diffusion process and/or water migration. We also found that local microseismicity on a remote fault was likely activated by stress fluctuations resulting from changes in the injection rate. A mediator of this fluctuation might be water remaining in the fracture zone. After the injection was terminated, microseismicity possibly associated with the phase transition of the liquid was found. We conclude that a variety of triggering processes associated with water injection may exist.