| Title | Spatiotemporal estimation of injected water migration in reservoirs from microseismicity using statistical analysis: A case study in the Okuaizu geothermal field, Japan |
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| Authors | Naoki Aoyagi |
| Year | 2026 |
| Conference | World Geothermal Congress |
| Keywords | Reservoir monitoring, Microseismic observation, Statistical seismology |
| Abstract | In geothermal fields, water injection into geothermal reservoirs is conducted to recover and stabilize steam production and can induce microseismicity. The spatiotemporal distribution of microseismicity induced by this process can be utilized to estimate fluid flow activity and the creation of the geothermal reservoir. However, distinguishing between microseismic events caused by fluid flow associated with water injection and other events is essential. The Epidemic-Type Aftershock Sequence (ETAS) model is employed as a statistical approach to clarify the tendency of microseismicity caused by fluid flow among the detected events. The ETAS analysis models observed microseismic events as independent events following a Poisson process (µ) and superposition of aftershock sequences governed by a modified Omori law. In geothermal fields, where fluid activity is significant, µ-value can be considered an approximation of the probability of microseismicity related to fluid flow. We investigate the spatiotemporal variations of the µ-value in the ETAS model, which indicates microseismicity related to fluid flow in the Okuaizu geothermal field. A higher µ-value is estimated in two main areas: the southern region of the injection well and the northeastern region of the injection well (i.e., near the Sarukurazawa fault). This suggests that fluid flow may be active in these areas. Future studies will apply this knowledge to investigate the spatiotemporal variations in ETAS parameters within geothermal fields. |