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

Abstract

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.

Research on the Influence of Friction on the Determination of Rock Fracture Toughness

“…In this study, two test methods for determining the fracture toughness in rock specimens with NSCB configuration and CSTBD configuration are considered. The influence of contact friction in the testing process on the test results of fracture toughness is analyzed in detail, and the following conclusions are drawn as follows: (1) The test results of rock specimens with CSTBD configuration used in the fracture toughness test are less affected by the friction between the rigid plate and specimen surface. For rock specimens with NSCB configuration, the KI of the specimen decreases with the increase of friction coefficient under the same load, indicating that the failure load of the specimen increases with the increase of friction coefficient.…”

“…In addition, some industrial activities such as hydrocarbon extraction and gas storage, shale gas mining, deep geothermal energy development, mining, reservoir water storage can also induce earthquakes, which may bring a series of public safety problems [1][2][3][4][5] . In this case, it is particularly important to make short-term forecasts based on the changing laws of seismogenic factors and various precursor observations in engineering activities.…”

Fluid activity detection in geothermal areas using a single seismic station by monitoring horizontal-to-vertical spectral ratios

“…On the other hand, temporal changes of H/V spectral ratios have been hardly reported, although there are areas where temporal variations in subsurface structures are likely expected, that is, areas abundant in subsurface fluid flow. For example, temporal variations in elastic wave velocities in geothermal areas induced by natural earthquakes as well as operations of fluid extraction and injection have been reported 8 , 9 . Similar reports have been made on carbon dioxide capture and storage sites 10 .…”

“…In Okuaizu geothermal field (OGF), Japan, located in a volcanic front in the western part of Fukushima Prefecture (Fig. 1 a), there is active microseismicity, which is considered to be related to fluid flows within and around the geothermal reservoir 9 , 11 . Since 2015, the Japan Oil, Gas and Metals National Corporation (JOGMEC) has been conducting gravity‐driven water‐injection tests into the reservoir to prevent the reduction of steam production 12 .…”

Prediction of Earthquakes; Considering the Ratio of <i>P-to-S</i> Wave Velocities

“…Seismic waves and their dynamic were studied in depth and the arising stress is the feature of these activities, Aki and Richards [14] and Lowrie [15]. There are abundant studies between seismic waves and earthquakes, Okamoto et al [16] and Takemura et al [17]. The seismic waves propagate in nonhomogeneous and anisotropic media; it is response of the stressed of the medium.…”