Integrated In Situ Stress Estimation by Hydraulic Fracturing, Borehole Observations and Numerical Analysis at the EXP-1 Borehole in Pohang, Korea

Kim, Hanna; Xie, Linmao; Min, Ki‐Bok; Bae, Seongho; Stephansson, Ove

doi:10.1007/s00603-017-1284-1

Cited by 41 publications

(17 citation statements)

References 34 publications

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“…We focus here on those results most pertinent to stress in the vicinity of the Pohang EGS site and at depths comparable to the depth of the 15 November earthquake. The state of stress at shallow depths within ~10 km of the EGS site was investigated using borehole data by Kim et al (2017) and Lee et al (2017a), who derived SHmax estimates of approximately 130° at depths of ~700 m and inferred the stress state to be strike-slip.…”

Section: Regional Stress Statementioning

confidence: 99%

Triggering of the Pohang, Korea, Earthquake (Mw 5.5) by enhanced geothermal system stimulation

Ellsworth¹,

Giardini²,

Townend³

et al. 2021

Preprint

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© 2019 Seismological Society of America. All rights reserved. On the afternoon of 15 November 2017, the coastal city of Pohang, Korea, was rocked by a magnitude 5.5 earthquake (Mw, U.S. Geological Survey). Questions soon arose about the possible involvement in the earthquake of the Republic of Korea's first enhanced geothermal system (EGS) project because the epicenter of the earthquake was located near the project's drill site. The Pohang EGS project was intended to create an artificial geothermal reservoir within low-permeability crystalline basement by hydraulically stimulating the rock to form a connected network of fractures between two wells, PX-1 and PX-2, at a depth of ∼4 km. Forensic examination of the tectonic stress conditions, local geology, well drilling data, the five high-pressure well stimulations undertaken to create the EGS reservoir, and the seismicity induced by injection produced definitive evidence that earthquakes induced by high-pressure injection into the PX-2 well activated a previously unmapped fault that triggered the Mw 5.5 earthquake. Important lessons of a general nature can be learned from the Pohang experience and can serve to increase the safety of future EGS projects in Korea and elsewhere.

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Section: Regional Stress Statementioning

confidence: 99%

Triggering of the Pohang, Korea, Earthquake (Mw 5.5) by enhanced geothermal system stimulation

Ellsworth¹,

Giardini²,

Townend³

et al. 2021

Preprint

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“…@AB , with principal stress components `> a > b > 0) under Andersonian stress. Previous studies examined the azimuth of maximum horizontal stress using different methods, such as borehole and seismological techniques, e.g., stress inversion of focal mechanisms (Kim et al, 2017;Lee, Hong, and Chang, 2017;Soh et al, 2018;Korean Government Commission, 2019;. Soh et al (2018) inferred ?…”

Section: Fault Strength and Loading Stressesmentioning

confidence: 99%

“…However, the earthquakes closest (~40 km) to the Pohang EGS site used in their analysis are the 2016 Gyeongju event and its aftershocks. Based on borehole data, Kim et al (2017) and Lee, Shinn, et al (2017) determined that ? @AB at shallow depths (700 m to 1000 m) within a 10 km radius from the Pohang EGS is about 130°.…”

Section: Fault Strength and Loading Stressesmentioning

confidence: 99%

Dynamic fault interaction during a fluid-injection induced earthquake: The 2017 Mw 5.5 Pohang event

Palgunadi¹,

Gabriel²,

Ulrich³

et al. 2020

Preprint

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The November 15th, 2017 Mw 5.5 Pohang earthquake (South Korea) has been linked to hydraulic stimulation and fluid injections, making this the largest induced seismic event associated with an Enhanced Geothermal System (EGS). To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction and 1D subsurface structure. A guided fault reconstruction approach that clusters spatio-temporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersects under a shallow angle of 15°. Based on simple Mohr-Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify preferred two fault plane scenarios that well reproduce observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, while local stress conditions constrained by borehole logging generate the observed thrust faulting component. Our preferred model is characterized by overpressurized pore fluids, non-optimally oriented but dynamically weak faults and a close to critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection induced earthquakes and that local stress perturbations dominate over the regional stress conditions. These findings, therefore, have important implications for seismic hazard in active geo-reservoir.

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“…A wide range of uncertainty, between N65°E and S50°E (or 065-130°), was estimated for the azimuth of σ H , reflecting uncertainty in the analysis, which nonetheless indicates σ h < σ V < σ H . This analysis extrapolated in situ stress measurements made in shallower boreholes to greater depths; the data include the σ H azimuth (with the intermediate principal stress vertical) of 135 ± 3°(S45 ± 3°E) at~700 m depth from hydraulic fractures and 136 ± 4°(S44 ± 4°E) at~800 m depth from drilling-induced fractures [58], these measurements being made in Early Miocene tuff (Table 1) in a borehole~4 km SW of the Pohang EGS site. However, other case studies indicate that such extrapolation cannot be done with confidence, as the stress tensor orientation may change significantly with depth (especially between contrasting lithologies) (e.g., [21,59]), a phenomenon recognized in Korea [57].…”

Section: State Of Stress the Maximum Principal Stress Beneathmentioning

confidence: 99%

“…Many case studies of seismicity associated Figure 2: Locations of earthquakes at Pohang in November 2017: foreshocks (red circles), the 15 November mainshock (red star), and aftershocks (black circles). Also shown are stations of the local seismograph network (blue triangles), the EGS site (green square), the thermal spa resort (red + symbol; coordinates from [97]), and the borehole that yielded the in situ stress measurements discussed by [58] (red × symbol). Focal mechanisms of the mainshock and five principal aftershocks, each labelled by M W , are drawn as standard lower (or back) focal hemisphere projections with compressional quadrants shaded.…”

Section: Introductionmentioning

confidence: 99%

Fault “Corrosion” by Fluid Injection: A Potential Cause of the November 2017 MW 5.5 Korean Earthquake

Westaway

Burnside

2019

Geofluids

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The November 2017 MW 5.5 Pohang earthquake is one of the largest and most damaging seismic events to have occurred in the Korean peninsula over the last century. Its close proximity to an Enhanced Geothermal System (EGS) site, where hydraulic injection into granite had taken place over the previous two years, has raised the possibility that it was anthropogenic; if so, it was by far the largest earthquake caused by any EGS project worldwide. However, a potential argument that this earthquake was independent of anthropogenic activity considers the delay of two or three months before its occurrence, following the most recent injection into each of the wells. A better understanding of the physical and chemical processes that occur following fluid injection into granite is thus warranted. We show that hydrochemical changes occurring while surface water, injected into granite, reequilibrates chemically with its subsurface environment, can account for time delays for earthquake occurrence of such duration, provided the seismogenic fault was already critically stressed, or very close to the condition for slip. This candidate causal mechanism counters the potential argument that the time delay militates against an anthropogenic cause of the Pohang earthquake and can account for its relatively large magnitude as a consequence of a relatively small-volume injection. The resulting analysis places bounds on combinations of physical and chemical properties of rocks, injected volume, and potential postinjection time delays for significant anthropogenic seismicity during future EGS projects in granite.

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Integrated In Situ Stress Estimation by Hydraulic Fracturing, Borehole Observations and Numerical Analysis at the EXP-1 Borehole in Pohang, Korea

Cited by 41 publications

References 34 publications

Triggering of the Pohang, Korea, Earthquake (Mw 5.5) by enhanced geothermal system stimulation

Triggering of the Pohang, Korea, Earthquake (Mw 5.5) by enhanced geothermal system stimulation

Dynamic fault interaction during a fluid-injection induced earthquake: The 2017 Mw 5.5 Pohang event

Fault “Corrosion” by Fluid Injection: A Potential Cause of the November 2017 MW 5.5 Korean Earthquake

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