Characterization of 4.2-km-Deep Fractured Granodiorite Cores from Pohang Geothermal Reservoir, Korea

Kwon, Saeha; Xie, Linmao; Park, Sehyeok; Kim, Kwang Il; Min, Ki‐Bok; Kim, Kwang Yeom; Zhuang, Łi; Choi, Jaiwon; Kim, Hanna; Lee, Tae Jong

doi:10.1007/s00603-018-1639-2

Cited by 47 publications

(27 citation statements)

References 26 publications

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“…1) and X-ray diffraction analyses ( Figure S2) indicate the occurrence of epidotization and chloritization along the fracture surfaces. In addition, the borehole cuttings at depths between 3500 and 3800 m also contain up to 20 wt.% chlorite (Lee et al, 2019b), which is consistent with previous work showing that epidote and chlorite are common metamorphic minerals within geothermal systems (Bird & Spieler, 2004;Kwon et al, 2019). Outcrop samples of granite obtained from the Gonghe Geothermal site in China also show the presence of epidote and chlorite as shown in Fig.…”

Section: Introductionsupporting

confidence: 89%

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

Zhang

Min

et al. 2020

Preprint

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Abstract Occurrence of earthquakes related with geothermal reservoirs highlights the possibility that subsurface fluid injection may reactivate critically stressed faults and trigger seismicity. We report on laboratory experiments conducted at T = 100-250 °C, σc = 110 MPa and Pf = 42-63 MPa and show that two prevalent minerals, epidote and chlorite, impact frictional properties and fault stability under conditions relevant to typical geothermal reservoirs. Numerous geothermal reservoirs worldwide exhibit metamorphic epidotization and chloritization. Shear experiments on simulated epidote-rich fault gouges indicate potentially unstable frictional behavior - more pronounced at elevated temperatures and pore pressures. Increased proportions of chlorite in fault gouges stabilize the faults, which indicates that gouge composition exerts significant control on fault stability. Our results imply a high potential for induced seismicity on faults containing epidote found in many geothermal reservoirs.

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Section: Introductionsupporting

confidence: 89%

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

Zhang

Min

et al. 2020

Preprint

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“…We utilized every episode of pressure build-up and drawdown as single-well tests to compute hydraulic conductivity. Single-well test data are not well suited for deriving specific storage; we use specific storage values from laboratory tests of the Pohang EGS 4.2 km deep granitic rock core 24 . Data from PX-2 stimulations yield a hydraulic conductivity range of 2.0 × 10 −10 to 1.0 × 10 −7 m s −1 (Fig.…”

Section: Resultsmentioning

confidence: 99%

“…A Poisson's ratio of 0.25 and two values of Young's modulus (50 and 80 GPa) were used. Laboratory tests on core samples from the site indicated an average Young's Modulus of 33.5 GPa while a larger Young's modulus of~45 GPa was measured using P-and Swaves 24 . The discrepancy between laboratory measurements and in situ measurements of rock properties is often recorded and can be attributed to stressrelief damage 66 when the cores are extracted.…”

Section: Methodsmentioning

confidence: 99%

“…The K values in this study were derived from measured pressure data, and, therefore, should reflect the in-situ condition at the injection depth. The compressibility values we used in computing S s were from mechanical lab tests 24 . In addition, the S s change estimated due to water compressibility change 61 over the above-mentioned temperature range is minimal.…”

Section: Methodsmentioning

confidence: 99%

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Causal mechanism of injection-induced earthquakes through the Mw 5.5 Pohang earthquake case study

Yeo

Brown

et al. 2020

Nat Commun

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Causal mechanisms for fluid injection-induced earthquakes remain a challenge to identify. Past studies largely established spatiotemporal correlations. Here, we propose a multiprocess causal mechanism for injection-induced earthquakes through a case study of the 2017 M w 5.5 induced earthquake near Pohang Enhanced Geothermal System, Korea, where detailed hydraulic stimulation and on-site seismicity monitoring data provide an unprecedented opportunity. Pore pressure modeling reveals that pore pressure changes initiate seismicity on critically stressed faults and Coulomb static stress transfer modeling reveals that earthquake interactions promote continued seismicity, leading to larger events. On the basis of these results, we propose the following causal mechanism for induced seismicity: pore pressure increase and earthquake interactions lead to fault weakening and ultimately triggering larger earthquakes later in the process. We suggest that it is prudent that pore pressure change, initial seismicity locations, and Coulomb static stress transfer from seismicity earlier in the sequence are assessed in real-time.

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“…Each variable is measured or obtained except Φ, resulting in that the helium flow rate is a function of porosity (Φ). The porosity of Yeongnam Massif granodiorite underneath the study area is 0.48% [49]. The porosity measured by wireless logging along PX2 at depth is 5.2% [49], which includes the void volume of fractures.…”

Section: Heliummentioning

confidence: 98%

Mantle-Derived Helium Emission near the Pohang EGS Site, South Korea: Implications for Active Fault Distribution

Kim

Lee

et al. 2020

Geofluids

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An Mw 5.5 earthquake occurred in Pohang, South Korea on November 15, 2017, resulting in a great impact on society. Despite a lot of controversy about the cause of the earthquake in relation to the enhanced geothermal system (EGS), the location of earthquake-related active faults is poorly known. Here, we first report the results of the geochemical and isotopic analyses of dissolved gases in groundwater in the Heunghae, Yeonil, and Sinkwang areas. According to the N2-Ar-He relationship, samples from the Heunghae and Yeonil areas are contributed to the mantle, except for the Sinkwang area, where all samples are atmospheric. The Pohang samples consist mainly of N2 and CO2, and some samples of the Heunghae and Yeonil areas contain substantial CH4. Stable isotope compositions of N2 (δ15N=0.2 to 3.6‰), CO2 (δ13C=−27.3 to−16.0‰), and CH4 (δ13C=−76.1 to−70.0‰) indicate that these components are derived from organic substances in sedimentary layer of Pohang Basin. On the other hand, helium isotope ratios (3He/4He, up to 3.83 Ra) represent the significant mantle contribution in the Heunghae and Yeonil areas. Through the distribution of high 3He/4He ratios, we propose that the Heunghae, Namsong, and Jamyeong faults are the passage of mantle-derived fluids. Computed 3He fluxes of the Heunghae (120 to 3,000 atoms cm-2 sec-1), Namsong (52 to 1,300 atoms cm-2 sec-1), and Jamyeong (83 to 2,100 atoms cm-2 sec-1) faults are comparable to other major active faults around the world, reflecting either high porosity or high helium flow rates. Therefore, our results demonstrate that there are active faults near the EGS facilities, which can provide the basis for future studies.

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Characterization of 4.2-km-Deep Fractured Granodiorite Cores from Pohang Geothermal Reservoir, Korea

Cited by 47 publications

References 26 publications

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

Causal mechanism of injection-induced earthquakes through the Mw 5.5 Pohang earthquake case study

Mantle-Derived Helium Emission near the Pohang EGS Site, South Korea: Implications for Active Fault Distribution

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