Microseismic monitoring for recharge injections in Okuaizu Geothermal Field

Abstract

　Various research and development of technology that realizes optimization and stabilization of steam production are being conducted for geothermal fields in Japan. One of the means is water recharge by injection, which is a kind of EGS (Enhanced / Engineered Geothermal System). This technology attempts to recover the amount of steam by artificial injection of river water into geothermal reservoirs using a well specially prepared for the injection in addition to the production and reinjection wells that are used for regular plant operations. Recharge injection tests were conducted in Okuaizu Geothermal Field, (Yanaizu-town, Fukushima, Japan) from 2015, as part of Geothermal Reservoir Evaluation and Management Technology Project conducted by of Japan Oil, Gas and Metals National Corporation (JOGMEC). In this project, National Institute of Advanced Industrial Science and Technology (AIST) conducts the observation of microseismic events. For the purpose of accurately illuminating the effect of water injection in real time. In this study, we conducted a semi-real time analysis of the number of micro seismic events and hypocenter distribution regarding the water injection tests based on the data observed at seismographs installed in boreholes as well as on the surface. It leaded to monitoring the behavior of the injected water. Recharge water injection test s were conducted once in 2018 and twice over from 2019 to 2020. In the water injection test in 2018, the number of micro seismic events during the water injection period did not increase compared to the pre-injection term. Particularly, the number near the well did not increase as well. In the former water injection test over 2019 to 2020, an increase in the number of microseismic events was observed during the water injection period compared to the pre-injection term. At that time, microseismic events evolved toward deeper depths than those during the water injection test in 2018. In the letter water injection test over 2019 to 2020, although the number of microseismic events did not increase compared to the pre-injection term unlike with the previous water injection test, the evolution of microseismic events toward deeper depths was confirmed again. From those microseismic monitoring results, we concluded that the relationship between water injection and micro seismic events was not clear in the 2018 water injection test. On the other hand, the injected water well spread over deeper depths in the water injection tests over 2019 to 2020.