• Tunnel and Underground Space
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• v.33 no.5
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• pp.399-413
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• 2023

The present study conducted a numerical modeling of CO₂ injection at the Janggi Basin using the TOUGH-FLAC simulator, and examined the hydro-mechanical stability of the aquifer and the fault. Based on the site investigations and a 3D geological model of the target area, we simulated the injection of 32,850 tons of CO₂ over a 3-year period. The analysis of CO₂ plume with different values of the aquifer permeability revealed that assuming a permeability of 10^-14 m² the CO₂ plume exhibited a radial flow and reached the fault after 2 years and 9 months. Conversely, a higher permeability of 10^-13 m² resulted in predominant westward flow along the reservoir, with negligible impact on the fault. The pressure changes around the injection well remained below 0.6 MPa over the period, and the influence on the hydro-mechanical stability of the reservoir and fault was found to be insignificant.

• Tunnel and Underground Space
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• v.25 no.2
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• pp.155-167
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• 2015

The thermal-hydrological-mechanical (T-H-M) behavior of rock mass surrounding a high-temperature cavern thermal energy storage (CTES) operated for a period of 30 years has been investigated by TOUGH2-FLAC3D simulator. As a fundamental study for the development of prediction and control technologies for the environmental change and rock mass behavior associated with CTES, the key concerns were focused on the hydrological-thermal multiphase flow and the consequential mechanical behavior of the surrounding rock mass, where the insulator performance was not taken into account. In the present study, we considered a large-scale cylindrical cavern at shallow depth storing thermal energy of $350^{\circ}C$. The numerical results showed that the dominant heat transfer mechanism was the conduction in rock mass, and the mechanical behavior of rock mass was influenced by thermal factor (heat) more than hydrological factor (pressure). The effective stress redistribution, displacement and surface uplift caused by heating of rock and boiling of ground-water were discussed, and the potential of shear failure was quantitatively examined. Thermal expansion of rock mass led to the ground-surface uplift on the order of a few centimeters and the development of tensile stress above the storage cavern, increasing the potential of shear failure.

• Tunnel and Underground Space
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• v.30 no.5
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• pp.446-461
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• 2020

Numerical simulations of CIEMAT column test in Spain are performed to investigate the coupled thermo-hydro-mechanical (THM) behavior of MX80 bentonite pellets using TOUGH2-FLAC3D. The heater power and injection pressure of water in the numerical simulations are identical to those in the laboratory test. To investigate the applicability of the thermo-hydraulic (TH) model used in TOUGH2 code to prediction of the coupled TH behavior, the simulation results are compared with the observations of temperature and relative humidity with time. The tendencies of the coupled behavior observed in the test are well represented by the numerical models and the simulator in terms of temperature and relative humidity evolutions. Moreover, the performance of the models for the reproduction and prediction of the coupled TH behavior is globally satisfactory compared with the observations. However, the calculated stress change is relatively small and slow due to the limitations of the simple elastic and swelling pressure model used in numerical simulations. It seems that the two models are insufficient to realistically reproduce the complex coupled THM behavior in the bentonite pellets.

• Tunnel and Underground Space
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• v.28 no.5
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• pp.400-425
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• 2018

This study presents the research results and current status of the DECOVALEX-2019 project Task B. Task B named 'Fault slip modelling' is aiming at developing a numerical method to simulate the coupled hydro-mechanical behavior of fault, including slip or reactivation, induced by water injection. The first research step of Task B is a benchmark simulation which is designed for the modelling teams to familiarize themselves with the problem and to set up their own codes to reproduce the hydro-mechanical coupling between the fault hydraulic transmissivity and the mechanically-induced displacement. We reproduced the coupled hydro-mechanical process of fault slip using TOUGH-FLAC simulator. The fluid flow along a fault was modelled with solid elements and governed by Darcy's law with the cubic law in TOUGH2, whereas the mechanical behavior of a single fault was represented by creating interface elements between two separating rock blocks in FLAC3D. A methodology to formulate the hydro-mechanical coupling relations of two different hydraulic aperture models and link the solid element of TOUGH2 and the interface element of FLAC3D was suggested. In addition, we developed a coupling module to update the changes in geometric features (mesh) and hydrological properties of fault caused by water injection at every calculation step for TOUGH-FLAC simulator. Then, the transient responses of the fault, including elastic deformation, reactivation, progressive evolutions of pathway, pressure distribution and water injection rate, to stepwise pressurization were examined during the simulations. The results of the simulations suggest that the developed model can provide a reasonable prediction of the hydro-mechanical behavior related to fault reactivation. The numerical model will be enhanced by continuing collaboration and interaction with other research teams of DECOLVAEX-2019 Task B and validated using the field data from fault activation experiments in a further study.

• Journal of Soil and Groundwater Environment
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• v.22 no.3
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• pp.42-49
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• 2017

TOUGH2 was used to simulate the migration of $CO_2$ injected into a sandy aquifer. A series of numerical simulations was performed to investigate the effects of a low-permeability layer (LPL) embedded in the aquifer on the injection rate and the pressure distribution of $CO_2$. The results show that the size and location of the LPL greatly affected the spread of $CO_2$. The pressure difference between two points in the aquifer, one each below and above the LPL, increased as the size of the LPL increased, showing a critical value at 200 m, above which the size effect was diminished. The location of the LPL with respect to the injection well also affected the migration of $CO_2$. When the injection well was at the center of the LPL, the injection rate of $CO_2$ decreased by 5.0% compared to the case with no LPL. However, when the injection well was at the edge of the LPL, the injection rate was decreased by only 1.6%. The vertical distance between the injection point and the LPL also affected the injection rate. The closer the LPL was to the injection point, the lower the injection rate was, by up to 8.3%. Conclusively, in planning geologic storage of $CO_2$, the optimal location of the injection well should be determined considering the distribution of the LPL in the aquifer.

• Tunnel and Underground Space
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• v.31 no.4
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• pp.221-242
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• 2021

A numerical study of the performance assesment of coupled thermo-hydro-mechanical (THM) processes in improved Korean reference disposal system (KRS⁺) for high-level radioactive waste is conducted using TOUGH2-MP/FLAC3D simulator. Decay heat from high-level radioactive waste increases the temperature of the repository, and it decreases as decay heat is reduced. The maximum temperature of the repository is below a maximum temperature criterion of 100℃. Saturation of bentonite buffer adjacent to the canister is initially reduced due to pore water evaporation induced by temperature increase. Bentonite buffer is saturated 250 years after the disposal of high-level radioactive waste by inflow of groundwater from the surrounding rock mass. Initial saturation of rock mass decreases as groundwater in rock mass is moved to bentnonite buffer by suction, but rock mass is saturated after inflow of groundwater from the far-field area. Stress changes at rock mass are compared to the Mohr-Coulomb failure criterion and the spalling strength in order to investigate the potential rock failure by thermal stress and swelling pressure. Additional simulations are conducted with the reduced spacing of deposition holes. The maximum temperature of bentonite buffer exceeds 100℃ as deposition hole spacing is smaller than 5.5 m. However, temperature of about 56.1% volume of bentonite buffer is below 90℃. The methodology of numerical modeling used in this study can be applied to the performance assessment of coupled THM processes for high-level radioactive waste repositories with various input parameters and geological conditions such as site-specific stress models and geothermal gradients.

• Tunnel and Underground Space
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• v.31 no.5
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• pp.309-332
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• 2021

Natural barrier systems surrounding the geological repository for the high-level radioactive waste should guarantee the hydraulic performance for preventing or delaying the leakage of radionuclide. In the case of the behavior of a crystalline rock, the hydraulic performance tends to be decided by the existence of discontinuities, so the coupled hydro-mechanical(HM) processes on the discontinuities should be characterized. The discontinuum modelling can describe the complicated behavior of discontinuities including creation, propagation, deformation and slip, so it is appropriate to model the behavior of a crystalline rock. This paper investigated the coupled HM processes in discontinuum modelling such as UDEC, 3DEC, PFC, DDA, FRACOD and TOUGH-UDEC. Block-based discontinuum methods tend to describe the HM processes based on the fluid flow through the discontinuities, and some methods are combined with another numerical tool specialized in hydraulic analysis. Particle-based discontinuum modelling describes the overall HM processes based on the fluid flow among the particles. The discontinuum methods that are currently available have limitations: exclusive simulations for two-dimension, low hydraulic simulation efficiency, fracture-dominated fluid flow and simplified hydraulic analysis, so it could be improper to the modelling the geological repository. Based on the concepts of various discontinuum modelling compiled in this paper, the advanced numerical tools for describing the accurate coupled HM processes of the deep geological repository should be developed.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.12 no.4
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• pp.267-274
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• 2014

The first Low- and Intermediate-Level Waste (LILW) disposal facility with 6 silos has been constructed in granite host rock saturated with groundwater in Korea. A two-dimensional numerical modeling on gas migration was carried out using TOUGH2 with EOS5 module in the disposal facility. Laboratory-scale experiments were also performed to measure the important properties of silo concrete related with gas migration. The gas entry pressure and relative gas permeability of the concrete was determined to be $0.97{\pm}0.15bar$ and $2.44{\times}10^{-17}m^2$, respectively. The results of the numerical modeling showed that hydrogen gas generated from radioactive wastes was dissolved in groundwater and migrated to biosphere as an aqueous phase. Only a small portion of hydrogen appeared as a gas phase after 1,000 years of gas generation. The results strongly suggested that hydrogen gas does not accumulate inside the disposal facility as a gas phase. Therefore, it is expected that there would be no harmful effects on the integrity of the silo concrete due to gas generation.

• Tunnel and Underground Space
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• v.30 no.4
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• pp.359-381
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• 2020

Within the framework of DECOVALEX-2019 Task D, full-scale engineered barriers experiment (FEBEX) at Grimsel Test Site was numerically simulated to investigate an applicability of implemented Barcelona basic model (BBM) into TOUGH2-MP/FLAC3D simulator, which was developed for the prediction of the coupled thermo-hydro-mechanical behavior of bentonite buffer. And the calculated heater power, temperature, relative humidity, total stress, saturation, water content and dry density were compared with in situ data monitored in the various sections. In general, the calculated heater power and temperature provided a fairly good agreement with experimental observations, however, the difference between power of heater #1 and that of heater #2 could not captured in the numerical analysis. It is necessary to consider lamprophyre with low thermal conductivity around heater #1 and non-simplified installation progresses of bentonite blocks in the tunnel for better modeling results. The evolutions and distributions of relative humidity were well reproduced, but hydraulic model needs to be modified because the re-saturation process was relatively fast near the heaters. In case of stress evolutions due to the thermal and hydraulic expansions, the computed stress was in good agreement with the data. But, the stress is slightly higher than the measured in situ data at the early stage of the operation, because gap between rock mass and bentonite blocks have not been considered in the numerical simulations. The calculated distribution of saturation, water content, and dry density along the radial distance showed good agreement with the observations after the first and final dismantling. The calculated dry density near the center of the FEBEX tunnel and heaters were overestimated compared with the observations. As a result, the saturation and water content were underestimated with the measurements. Therefore, numerical model of permeability is needed to modify for the production of better numerical results. It will be possible to produce the better analysis results and more realistically predict the coupled THM behavior in the bentonite blocks by performing the additional studies and modifying the numerical model based on the results of this study.