5:15 PM - 6:45 PM
[SSS07-P15] Seismic velocity and attenuation structure variations in response to a water recharge test in the Okuaizu geothermal field, Fukushima, Japan
Keywords:Microseismic, Geothermal development, Water injection, Tomography
A water recharge test has been conducted by JOGMEC in the Okuaizu geothermal field (Fukushima, Japan) since 2015 to stabilize steam production. As part of the test, the monitoring of subsurface fluid flows using microseismic monitoring has also been carried out. In this study, we performed a time series of seismic wave travel time tomography and attenuation tomography using microseismic data from 2017 to 2018 (prior to the start of the continuous water recharge test in operation) and after 2022 (during the continuous water recharge test). For those tomography methods, we employed a double-difference tomography (Zhang and Thuber, 2003) and a double-difference seismic attenuation tomography (Guo and Thurber, 2021).
As a result, we identified a region near the recharge well that exhibited increased seismic velocity and decreased attenuation strength in line with the recharge test. Drawing on a study in the Geysers geothermal field in the U.S. by Guo and Thuber (2022), this finding may suggest that rock pores, originally in a vapor-dominated condition in this field, became saturated with the recharged water. Additionally. We observed regions showing decreased seismic velocity and attenuation strength relatively away from the injection well. The decreased seismic velocity may be attributed to the creation of new microcracks and an increase in pore pressure whereas the decreased attenuation strength may be derived from fluid saturation of rock pores.
In this study, we report the possibility of estimating reservoir behavior in response to water recharge by analyzing variations in seismic velocity and attenuation structures, as discussed above. More precise insights into reservoir behavior, such as fluid flow paths and the phase of pore water, can be gained by utilizing information on microseismic distribution, Vp/Vs, and other parameters.
This study was partly supported by the “Technology to evaluate and manage geothermal reservoirs” project sponsored by the Japan Oil, Gas and Metals National Corporation (JOGMEC). We are grateful to the Geothermal Energy Research & Development Co., Ltd. (GERD) and the Okuaizu Geothermal Co., Ltd. (OAG) for greatly helping with the construction and maintenance of the microseismic monitoring system and facilitating valuable data and discussions.
As a result, we identified a region near the recharge well that exhibited increased seismic velocity and decreased attenuation strength in line with the recharge test. Drawing on a study in the Geysers geothermal field in the U.S. by Guo and Thuber (2022), this finding may suggest that rock pores, originally in a vapor-dominated condition in this field, became saturated with the recharged water. Additionally. We observed regions showing decreased seismic velocity and attenuation strength relatively away from the injection well. The decreased seismic velocity may be attributed to the creation of new microcracks and an increase in pore pressure whereas the decreased attenuation strength may be derived from fluid saturation of rock pores.
In this study, we report the possibility of estimating reservoir behavior in response to water recharge by analyzing variations in seismic velocity and attenuation structures, as discussed above. More precise insights into reservoir behavior, such as fluid flow paths and the phase of pore water, can be gained by utilizing information on microseismic distribution, Vp/Vs, and other parameters.
This study was partly supported by the “Technology to evaluate and manage geothermal reservoirs” project sponsored by the Japan Oil, Gas and Metals National Corporation (JOGMEC). We are grateful to the Geothermal Energy Research & Development Co., Ltd. (GERD) and the Okuaizu Geothermal Co., Ltd. (OAG) for greatly helping with the construction and maintenance of the microseismic monitoring system and facilitating valuable data and discussions.