Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have ...investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies.
Enhanced Geothermal Systems (EGS) have the potential to make a significant contribution to the world energy inventory. One controversial issue associated with EGS, however, is the impact of induced ...seismicity or microseismicity, which has been the cause of delays and threatened cancellation of at least two EGS projects worldwide. Although microseismicity has in fact had few (or no) adverse physical effects on operations or on surrounding communities, there remains public concern over the amount and magnitude of the seismicity associated with current and future EGS operations. The primary objectives of this paper are to present an up-to-date review of what is already known about the seismicity induced during the creation and operation of EGS, and of the gaps in our knowledge that, once addressed, should lead to an improved understanding of the mechanisms generating the events. Several case histories also illustrate a number of technical and public acceptance issues. We conclude that EGS-induced seismicity need not pose a threat to the development of geothermal energy resources if site selection is carried out properly, community issues are handled adequately and operators understand the underlying mechanisms causing the events. Induced seismicity could indeed prove beneficial, in that it can be used to monitor the effectiveness of EGS operations and shed light on geothermal reservoir processes.
1)The 17 november mw 5.4 Pohang, South Korea, main shock could have been either triggered or natural.2)The pohang main shock was not induced by the nearby EGS project.3)The dominant source of seismic ...hazard in and around the city of pohang is natural crustal strain accumulation.
Understanding the cause of the November 2017 Pohang main shock is of considerable importance to the future of the geothermal industry because of its large magnitude compared to prior expectations based on case histories of other projects involving underground fluid injection. Of the three possibilities – induced, triggered or natural, “induced” can be ruled out based on the disproportionately large seismic moment of the main shock. Whether natural or triggered, the source of seismic hazard at Pohang was tectonic strain accumulation, not fluid injection. Arguably, the most timely indicator of seismic hazard and risk in the environs of Pohang was the September 2016 Mw 5.4 Gyeongju earthquake, which was natural and located about 40 km south of Pohang along the same active fault system.
Understanding the cause of the November 2017 Pohang main shock is of considerable importance to the future of the geothermal industry because of its large magnitude compared to prior expectations ...based on case histories of other projects involving underground fluid injection. Of the three possibilities – induced, triggered or natural, “induced” can be ruled out based on the disproportionately large seismic moment of the main shock. Furthermore, whether natural or triggered, the source of seismic hazard at Pohang was tectonic strain accumulation, not fluid injection. Arguably, the most timely indicator of seismic hazard and risk in the environs of Pohang was the September 2016 Mw 5.4 Gyeongju earthquake, which was natural and located about 40 km south of Pohang along the same active fault system.
Water injection into geothermal systems has often become a required strategy to extend and sustain production of geothermal resources. To reduce a trend of declining pressures and increasing ...non-condensable gas concentrations in steam produced from The Geysers, operators have been injecting steam condensate, local rain and stream waters, and most recently treated wastewater piped to the field from neighboring communities. If geothermal energy is to provide a significant increase in energy in the United States (the US Department of Energy goal is 40,000
MW by 2040), injection must play a larger role in the overall strategy, i.e., enhanced geothermal systems (EGS). Presented in this paper are the results of monitoring microseismicity during an increase in injection at The Geysers field in California using data from a high-density digital microearthquake array. Although seismicity has increased due to increased injection, it has been found to be somewhat predictable, thus implying that intelligent injection control may be able to control large increases in seismicity.
A multidisciplinary research team has conducted a field‐scale bacterial transport study within an uncontaminated sandy Pleistocene aquifer near Oyster, Virginia. The overall goal of the project was ...to evaluate the importance of heterogeneities in controlling the field‐scale transport of bacteria that are injected into the ground for remediation purposes. Geochemical, hydrological, geological, and geophysical data were collected to characterize the site prior to conducting chemical and bacterial injection experiments. In this paper we focus on results of a hydrogeological characterization effort using geophysical data collected across a range of spatial scales. The geophysical data employed include surface ground‐penetrating radar, radar cross‐hole tomography, seismic cross‐hole tomography, cone penetrometer, and borehole electromagnetic flowmeter. These data were used to interpret the subregional and local stratigraphy, to provide high‐resolution hydraulic conductivity estimates, and to provide information about the log conductivity spatial correlation function. The information from geophysical data was used to guide and assist the field operations and to constrain the numerical bacterial transport model. Although more field work of this nature is necessary to validate the usefulness and cost‐effectiveness of including geophysical data in the characterization effort, qualitative and quantitative comparisons between tomographically obtained flow and transport parameter estimates with hydraulic well bore and bromide breakthrough measurements suggest that geophysical data can provide valuable, high‐resolution information. This information, traditionally only partially obtainable by performing extensive and intrusive well bore sampling, may help to reduce the ambiguity associated with hydrogeological heterogeneity that is often encountered when interpreting field‐scale bacterial transport data.
•A relationship between hydraulic stimulations and microseismicity is observed.•Conceptual and numerical analyses are developed to explain such a connection.•Results point out a key stimulation ...target defined as Shearing Target Fault (STF).
A series of stimulation treatments were performed as part of the Engineered Geothermal System (EGS) experiment in the shallow open-hole section of Desert Peak well 27-15 (September 2010–November 2012). These injections at variable wellhead pressures, both below and above the magnitude of the least horizontal principal stress (Shmin), produced injectivity gains consistent with hydraulically induced mechanical shear and tensile failure in the surrounding rock. A conceptual framework for the overall Desert Peak EGS experiment is developed and tested based on a synthesis of available structural and geological data. These data include down-hole fracture attributes, in situ stress conditions, pressure interference tests, geochemical tracer studies, and observed induced seismicity. Induced seismicity plays a key role in identifying the geometry of large-scale geological structures that could potentially serve as preferential flow paths during some of the stimulation phases. The numerical code FLAC3D is implemented to simulate the reservoir response to hydraulic stimulation and to investigate in situ conditions conducive to both tensile and shear failure. Results from the numerical analysis show that conditions for shear failure could have occurred along fractures associated with a large northeast-trending normal fault structure located ∼400m below the injection interval which coincides with the locations of most of the observed micro-seismicity. This structure may also provide a hydrologic connection between EGS well 27-15 and injection/production wells further to the south–southwest.
Injection and movement/saturation of carbon dioxide (CO
2) in a geological formation will cause changes in seismic velocities. We investigate the capability of coda-wave interferometry technique for ...estimating CO
2-induced seismic velocity changes using time-lapse synthetic vertical seismic profiling (VSP) data and the field VSP datasets acquired for monitoring injected CO
2 in a brine aquifer in Texas, USA. Synthetic VSP data are calculated using a finite-difference elastic-wave equation scheme and a layered model based on the elastic Marmousi model. A possible leakage scenario is simulated by introducing seismic velocity changes in a layer above the CO
2 injection layer. We find that the leakage can be detected by the detection of a difference in seismograms recorded after the injection compared to those recorded before the injection at an earlier time in the seismogram than would be expected if there was no leakage. The absolute values of estimated mean velocity changes, from both synthetic and field VSP data, increase significantly for receiver positions approaching the top of a CO
2 reservoir. Our results from field data suggest that the velocity changes caused by CO
2 injection could be more than 10% and are consistent with results from a crosswell tomogram study. This study demonstrates that time-lapse VSP with coda-wave interferometry analysis can reliably and effectively monitor geological carbon sequestration.
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