Modelling transport pathways of faults with low hydraulic connectivity in mudstones with low swelling capacity

Ono, Hirokazu; Ishii, Eiichi; Takeda, Masaki

Geoenergy (Internet), 2(1), p.geoenergy2023-047_1 - geoenergy2023-047_10, 2024/12

https://doi.org/10.1144/geoenergy2023-047

Transmissivity prediction of the Excavation Damaged Zone fracture around the gallery at 500 m at the Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Ozaki, Yusuke; Tamura, Tomonori; Ishii, Eiichi

Proceedings of 4th International Conference on Coupled Processes in Fractured Geological Media; Observation, Modeling, and Application (CouFrac2024) (Internet), 10 Pages, 2024/11

In high-level radioactive waste disposal, it is crucial to estimate the transmissivity of gallery excavation-induced fractures, i.e., excavation damaged zone (EDZ) fractures, because EDZ fractures can be a radionuclide migration pathway after the backfilling of the facility is completed. From previous research, the transmissivity of the fracture can be estimated through the empirical equation using the parameter ductility index (DI), which corresponds to the effective mean stress normalized to the tensile strength of the rock. In this research, we performed a hydromechanical coupling analysis of a gallery excavation at the Horonobe Underground Research Laboratory to estimate the transmissivity of the EDZ fracture before the excavation. At first, we simulated the gallery excavation at 350 m and showed that the measured transmissivity was within the range of the estimated transmissivity using the DI. After that, we also predicted the excavation of a gallery at 500 m by setting the hydromechanical parameters acquired from the laboratory tests before the excavation. The estimated transmissivity at 500 m was one order of magnitude less than that at 350 m. This result might be related to the closure of the fracture under high-stress conditions and low rock strength.

Constant-head step-injection tests using a conventional straddle-sliding-packer system for investigating the shear capabilities of minor faults

Ishii, Eiichi

Rock Mechanics and Rock Engineering, 57(10), p.8861 - 8878, 2024/10

https://doi.org/10.1007/s00603-024-04010-5

Low-permeability rock is suitable as a host rock of an underground repository for radioactive waste disposal, but small faults may develop there. Investigating the shear capabilities (= shear compliances) of those faults is crucial as they could elastically shear by the waste's thermal effect to damage to the waste's engineered barriers. The present study performed constant-head step injection tests combined with a recently developed packer-pressure-based extensometer method for assessing the applicability of this method as a method to investigate the shear capabilities of small faults. Two neighboring small faults (faults A and B) in a siliceous mudstone were tested. The results showed that the shear capability is high for fault A with centimeters-thick fault breccia but low for fault B with millimeters or less-thick faut breccia despite containing an incohesive fault rock. As for fault A, an elastic shear displacement occurred during injection and reached to 15-66 mm when the test-section pressure increased from 4.1 MPa to 4.3 MPa, where the shear capability was on the order of 10 mm/MPa or more. Fault B had a cohesion, and no shear displacement was detected even when the test-section pressure increased from 4.0 MPa to 6.0 MPa; the shear capability was on the order of 10 mm/MPa or less. The estimated shear capabilities were consistent with results from previous laboratory experiments, and the applied method is useful to investigate the shear capabilities of small faults.

Evaluation of temporal changes in fracture transmissivity in an excavation damaged zone after backfilling a gallery excavated in mudstone

Aoyagi, Kazuhei; Ishii, Eiichi

Environmental Earth Sciences, 83(3), p.98_1 - 98_15, 2024/02

https://doi.org/10.1007/s12665-023-11416-x

The long-term geological disposal of high-level radioactive waste relies on predictions of future changes in a disposal facility's hydro-mechanical characteristics to assess potential leakage through fractures in the excavation damaged zone (EDZ) after backfilling the facility. This study evaluated the transmissivity of EDZ fractures using in situ hydraulic tests around the area of a full-scale, experimental, engineered barrier system in the Horonobe Underground Research Laboratory, Hokkaido, Japan. After their installation, the buffer blocks swelled, altering the stresses within the EDZ fractures. The effects of these changing stresses on the fractures' transmissivity were assessed over a period of 4 years. The transmissivity continuously decreased in this period to about 41% of its value measured prior to the swelling. Using the Barton-Bandis normal-stress-dependent fracture-closure model, the decrease in transmissivity is quantitatively attributed to closure of the EDZ fractures, which was caused by the swelling pressure increasing up to 0.88 MPa. Evidence of fracture closure came from seismic tomography surveying, which revealed a slight increase in seismic velocity in the study area with increasing swelling pressure. The results show that EDZ fractures were closed by swelling of the full-scale buffer material. They also demonstrate the applicability of the Barton-Bandis model to preliminary estimation of the long-term transmissivity of EDZ fractures in facilities for the geological disposal of radioactive waste.

Geochemical modeling of cation exchange reactions in Horonobe mudstone from Northern Hokkaido, Japan

Abe, Takeyasu; Iida, Yoshihisa; Sasamoto, Hiroshi; Ishii, Eiichi

Proceedings of Water-Rock Interaction (WRI-17)/ Applied Isotope Geochemistry (AIG-14), 6 Pages, 2023/08

Cation exchange is one of important elementary reactions among water-rock interactions in sedimentary rocks. In order to model non-constant Na-K-Ca-Mg-H exchange selectivity of sedimentary rocks, exchange isotherms and pH dependence on cation exchange capacity of smectite were analyzed following active fraction model. As the result of comparison of exchangeable cationic composition between measured and calculated, selectivity coefficients determined in this study were preferred to those presented by previous studies. Using the coefficients and groundwater hydrochemistry reported previously, expected compositions of exchangeable cations were calculated. The results suggested that pH and total Na-K-Ca-Mg-H concentration of groundwater are important factors to interpret observed compositional variation in exchangeable base cations.

Verification of the existing hydrogeological model using hydraulic pressure monitoring data during long-term drainage from the Horonobe URL and prediction of the hydraulic response to new excavation of up to a 500-m depth

Ozaki, Yusuke; Ishii, Eiichi

Proceedings of 30th International Conference on Nuclear Engineering (ICONE30) (Internet), 8 Pages, 2023/05

https://doi.org/10.1299/jsmeicone.2023.30.1073

This study aims to verify the estimated hydrogeological model by simulating the monitoring data of pore pressure for longer than 14 years around Horonobe Underground Research Laboratory (URL). A previous study estimated hydrogeological structure around the URL by simulating the monitoring data for 1 year. The subsequent monitoring data of hydraulic head at the same location enabled to validate the hydrogeological structure.

Constant-head step injection tests to quantify the stress dependence of fracture transmissivity in an excavation damaged zone; A Case study from the Horonobe Underground Research Laboratory

Ishii, Eiichi

International Journal of Rock Mechanics and Mining Sciences, 159, p.105229_1 - 105229_17, 2022/11

https://doi.org/10.1016/j.ijrmms.2022.105229

In an underground repository for high-level radioactive waste disposal, fracture transmissivity in an excavation damaged zone (EDZ) along tunnels or deposition holes can decrease during the post-closure period via processes such as self-sealing by clay-swelling at the EDZ's fracture surface or an increase in effective normal stress acting on the fractures owing to swelling of backfilling or buffer materials. Hydromechanical coupling models for the stress-dependence of fracture transmissivity are helpful to estimate the change in an EDZ's fracture transmissivity after closure. The applicability of the applied models should be confirmed by in situ tests at the given site; this appears to be facilitated by using constant-head step injection tests. However, injection testing is rarely applied to EDZ fractures. To investigate the applicability of injection tests, the present study performed them on single, tensile EDZ fractures in the Horonobe Underground Research Laboratory hosted by poorly swelling mudstone. Furthermore, the Barton-Bandis normal stress-dependent fracture-closure model quantified the stress-dependence of EDZ facture transmissivity. The fracture's hydraulic aperture increased gradually during injection, and its variation was well reproduced by fitting the model. Although the model requires the normal stress, this parameter was reasonably estimated by the fitting analyses. Constant-head step injection tests coupled with the Barton-Bandis model is believed to be a convenient method for preliminarily quantifying or verifying the stress dependence of EDZ fracture transmissivity, at least for poorly self-sealed, tensile EDZ-fractures.

Variation in fault hydraulic connectivity with depth in mudstone; An Analysis of poroelastic hydraulic response to excavation in the Horonobe URL

Ozaki, Yusuke; Ishii, Eiichi; Sugawara, Kentaro*

Geomechanics for Energy and the Environment, 31, p.100311_1 - 100311_13, 2022/09

https://doi.org/10.1016/j.gete.2022.100311

This study analyzed the long-term hydraulic pressure data during the excavation of Horonobe URL to estimate the variation of effective-hydraulic-conductivity. We performed the numerical simulation with the poroelastic effect for the estimation because the observed hydraulic pressure is highly affected by the Mandel-Cryer effect. The evaluation of the observed data based on our simulation results showed that the effective-hydraulic-conductivity gradually decreases from 400 m to 500 m in depth and is as low as the intact rock at depths greater than 500 m. Not only the analysis based on our simulation results but also the analysis based on analytical solution indicate the domain with different hydraulic properties in the Wakkanai Formation. These results suggest that the fracture-hydraulic-connectivity changes not abruptly but gradually over several tens of meters around the predicted boundary.

Assessment of the level of activity of advective transport through fractures and faults in marine deposits by comparison between stable isotope compositions of fracture and pore waters

Mochizuki, Akihito; Ishii, Eiichi

Hydrogeology Journal, 30(3), p.813 - 827, 2022/05

https://doi.org/10.1007/s10040-022-02466-9

Assessment of the level of activity of advective transport through faults and fractures is essential for guiding the geological disposal of radioactive waste. In this study, the advective flow (active, inactive) of meteoric water through fractures is assessed by comparing stable isotopes (D and O) between fracture and pore waters obtained from four boreholes in marine deposits in the Horonobe area, Japan. At 27-83-m depth in one borehole and 28-250 m in another, the isotopic compositions of pore and fracture water reflect mixing with meteoric water, with stronger meteoric-water signatures being observed in the fracture water than in pore water of the rock matrix. At greater depths in these boreholes and at all sampling depths in the other two studied boreholes, the isotopic compositions of fracture and pore waters are comparable. These results suggest that the advective flow of meteoric water is active at shallow depths where fossil seawater is highly diluted in the two boreholes. This interpretation is compatible with the occurrence of present or paleo meteoric waters and tritium, whereby present meteoric water and tritium are limited to those depths in the two boreholes. This difference in the level of activity of advective flow is probably because of the glacial-interglacial difference in hydraulic gradients resulting from sea-level change. Although fractures are hydraulically connected to the surface through the sedimentary rock, advective flow through them is inferred to remain inactive so long as sea level does not fall substantially.

Study in the sedimentary processes of organic matter in the Pliocene to Miocene siliceous rocks in the Horonobe district, northern Hokkaido, Japan

Muraoka, Ami*; Chiyonobu, Shun*; Arato, Hiroyuki*; Martizzi, P.*; Ishii, Eiichi

Sekiyu Gijutsu Kyokai-shi (CD-ROM), 87(1), p.86 - 88, 2022/00

no abstracts in English