Pore Pressure Changes Research Articles

Performance of buried pipes in moving slopes under axial loading – evaluation tools

Buried pipeline systems form a key part of effective and safe infrastructures for the transportation of natural resources such as natural gas and liquid. However, they may be constructed in areas prone to landslides where ground movement may exert excessive strains on the pipe sections causing rupture. This paper presents a methodology for estimating strains in a pipe under axial loading induced by ground movement. This methodology quantifies effects of soil displacement rate, total stress and pore pressure changes as well as time-independent (intrinsic) and time-dependent properties on the strains exerted on the pipe. These factors are not explicitly considered in current guidelines for constructing pipelines through areas prone to landslides. The procedure is based on a simplified analytical approach that can be readily implemented in practical design and analysis. A series of design charts are generated for performance evaluation of buried pipeline systems constructed in active slopes. They can be used to assess the frequency of necessary remediation such as in-situ stress relief procedure to protect the environment and maintain pipeline integrity and operation. For illustration, the proposed methodology is used in a case study to evaluate the potential risk of yielding in the pipe with the given soil displacement rates that occurred in the slope.

Influence of Shear Heating and Thermomechanical Coupling on Earthquake Sequences and the Brittle‐Ductile Transition

AbstractLocalized frictional sliding on faults in the continental crust transitions at depth to distributed deformation in viscous shear zones. This brittle‐ductile transition (BDT), and/or the transition from velocity‐weakening (VW) to velocity‐strengthening (VS) friction, are controlled by the lithospheric thermal structure and composition. Here, we investigate these transitions, and their effect on the depth extent of earthquakes, using 2D antiplane shear simulations of a strike‐slip fault with rate‐and‐state friction. The off‐fault material is viscoelastic, with temperature‐dependent dislocation creep. We solve the heat equation for temperature, accounting for frictional and viscous shear heating that creates a thermal anomaly relative to the ambient geotherm which reduces viscosity and facilitates viscous flow. We explore several geotherms and effective normal stress distributions (by changing pore pressure), quantifying the thermal anomaly, seismic and aseismic slip, and the transition from frictional sliding to viscous flow. The thermal anomaly can reach several hundred degrees below the seismogenic zone in models with hydrostatic pressure but is smaller for higher pressure (and these high‐pressure models are most consistent with San Andreas Fault heat flow constraints). Shear heating raises the BDT, sometimes to where it limits rupture depth rather than the frictional VW‐to‐VS transition. Our thermomechanical modeling framework can be used to evaluate lithospheric rheology and thermal models through predictions of earthquake ruptures, postseismic and interseismic crustal deformation, heat flow, and the geological structures that reflect the complex deformation beneath faults.

The Influence of Rate of Change in Confining and Pore Pressure on Values of the Modulus of Compressibility of the Rock Skeleton and Biot’s Coefficient

This work discusses the results of a study of the influence of rates of change of confining pressure on the result of a drained compressibility tests intended to determine the modulus of compressibility of a rock skeleton Ks. A series of cyclical compressibility tests was performed on samples of sandstone soaked in kerosene, for various rates of compression and decompression of the pressure liquid filling the cell and the pore volume of the sample. The studies showed that the deformability of the tested sample was directly proportional to the rate of change of the confining pressure. As a consequence, the value of the Ks modulus and Biot coefficient α decreased with increasing sample load rate. This phenomenon should be attributed primarily to equilibration of the liquid pressure inside the high-pressure cell with the liquid pressure in the sample pore space, caused by filtration of the pore liquid. These phenomena prove that the filtration process impacts the values of the modulus of compressibility of the rock skeleton Ks and of Biot coefficient α determined on the basis of the experiment. This is significant in the context of the use of Biot equations as constitutive equations for a porous rock medium.

Controls of fault geometry and thermal stress on fault slip modes: Implications for permeability enhancement and injection-induced seismicity

Fluid pressurization within the fault zone generates increasing pore pressure and stress change which is liable to create shear and/or brittle fractures within the reservoir volumes and subsequently generating earthquakes of varying magnitudes. Here, we explored time-dependent fault weakening processes in the fault zone which are dependent on several factors, including the rate of cold-water injection, modes of injection (hydromechanical (HM) and thermo-hydro-mechanical (THM) interactions), and changing fault spatial configurations using data from Niger Delta Basin. The variation in the stability of different fault models in withstanding stresses induced by HM and THM fluid interactions is evident. Fault permeability enhancement and the behaviour of slip event under isothermal and non-isothermal conditions revealed that stress and pore pressure perturbations have a first order control on the rate of fault dilation and compression. It is observed that the progressive cooling of the reservoir induced thermal stress which induced the timing of slip by unloading the fault to earlier seismic rupture in the non-isothermal case, and accelerates the magnitude of the fault reactivation and the accompanied induced seismicity. Owing to increased tendency of shear failure during injection, fracture opening through shear dilation is more enhanced in THM simulation as the fracture permeability is significantly higher than in HM. This effect becomes increasingly more dominant with intermediate fault angle and joint orientation. Certain fault/joint configurations which were resistant to shear failure under isothermal injection had their frictional resistance broken by thermal stress. The results also indicate that there is higher pore pressure build-up in THM than in HM as the injection rate increases and reservoir temperature drops during cold injections.. This study has demonstrated the importance of fully characterizing the fracture geometries and configurations of normal faulting regime in addition to fluid injection conditions when developing fractured reservoirs to mitigate seismic risks and hazards that could result from early fault reactivation.

Geomechanical response due to nonisothermal fluid injection into a reservoir

The geomechanical response of a porous reservoir due to injection of fluid can result from a complex interplay between the changes in porepressure and temperature near the wellbore. As a result, predictions are usually made using either simplified analytical models, which may apply unrealistic assumptions in order to produce a tractable model, or detailed numerical simulations that can be computationally expensive.LaForce et al. (2014a, 2014b) developed a semi-analytical model for the geomechanical response of a reservoir to nonisothermal, multi-phase fluid injection, which has been used in studies of CO2 sequestration. We demonstrate that a numerical solution using the MOOSE software precisely matches the analytical formulae. We then include various effects in the numerical model that relax the simplifying assumptions made in the analytical derivation. We find the analytic and numerical solutions for the fluid and temperature fronts still agree reasonably, while only qualitative agreement is observed for other quantities such as stress and displacement.We conclude the LaForce et al. (2014a,b) solutions are useful for rapid investigation of projects involving injection of cold fluid into warm aquifers. However, the enhancements afforded by MOOSE, such as high-precision fluid equations of state and the ability to more accurately capture geological complexity, along with its computational scalability which greatly reduces runtimes, means that MOOSE should be preferred for more sophisticated analyses. Because validating complex coupled codes is challenging, we propose that the model contained herein can be used as a benchmark for other coupled codes.

How Equivalent Are Equivalent Porous Media?

Geoenergy and geoengineering applications usually involve fluid injection into and production from fractured media. Accounting for fractures is important because of the strong poromechanical coupling that ties pore pressure changes and deformation. A possible approach to the problem uses equivalent porous media to reduce the computational cost and model complexity instead of explicitly including fractures in the models. We investigate the validity of this simplification by comparing these two approaches. Simulation results show that pore pressure distribution significantly differs between the two approaches even when both are calibrated to predict identical values at the injection and production wells. Additionally, changes in fracture stability are not well captured with the equivalent porous medium. We conclude that explicitly accounting for fractures in numerical models may be necessary under some circumstances to perform reliable coupled thermohydromechanical simulations, which could be used in conjunction with other tools for induced seismicity forecasting.

Fault‐Valve Behavior Estimated From Intensive Foreshocks and Aftershocks of the 2017 M 5.3 Kagoshima Bay Earthquake Sequence, Kyushu, Southern Japan

AbstractDetermining fluid migration and pore pressure change within the Earth is key to understand earthquake occurrences. We investigated the spatiotemporal characteristics of the intense foreshocks and aftershocks of the 2017 ML 5.3 earthquake in Kagoshima Bay, Kyushu, southern Japan, to examine the physical processes governing this earthquake sequence. Our relocated hypocenters show the foreshocks moved on a sharply defined plane with a steep dip. The mainshock rupture initiated at the edge of the foreshock seismic gap. The size of the foreshock seismic gap is comparable to that of the mainshock estimated from the source corner frequency, suggesting this seismic gap corresponds to the large slip region of the mainshock. The aftershocks migrated upward along several steeply dipped planes with a seismicity pattern that deviated from the typical mainshock–aftershock type. This deviation of seismicity pattern, together with the hypocenter migrations, suggests aseismic processes, such as pore pressure migration and aseismic slip, affected this earthquake sequence. We established the following hypothesis. First, fluids originating from the subducting slab migrated upward and intruded into the fault plane, reducing the fault strength and causing the foreshock sequence and potentially aseismic slip. Second, the mainshock rupture occurred due to the decreased fault strength and the increased shear stress in an area with relatively high strength. Third, pore pressure increase associated with post‐failure fluid discharge caused the upward aftershock migration. These observations are consistent with the fault‐valve model and show the importance of fluid movement at depth not only in earthquake swarms but also in foreshock–mainshock–aftershock sequences.

Stress and deformation induced in layered media by cylindrical thermo-poro-elastic sources: An application to Campi Flegrei (Italy)

In recent literature thermo-poro-elastic (TPE) inclusions are proposed to contribute to deformation and stress in volcanic and hydrothermal areas. With respect to previous works, the present study implements the modeling improvements requested for a more realistic description of a TPE source and the embedding medium. Firstly we propose a numerical method to represent inclusions with an arbitrary geometry. In the case of cylindrical-shaped sources, the proposed approach allows us to study the effects of the source thickness on both deformation and stress fields. Moreover, we are allowed to consider depth dependent pore pressure and temperature changes within the inclusion, as expected during the transient stage of hot fluid propagation. Secondly, we take into account elastic stratification of the crust, in order to perform unbiased inversion of ground deformation data. The inversion of geodetic data collected during the 1982–84 unrest episode in the Campi Flegrei caldera (Italy), performed in a layered elastic medium, leads to a lower misfit and a deeper deformation source with respect to the one obtained inverting data in a homogeneous half-space. Furthermore, the geometry and location of the thermo-poro-elastic inclusion retrieved from the inversion in the layered medium leads to a shear stress distribution in good agreement with the relocated seismicity observed at Campi Flegrei. Most hypocenters are found to be located near and inside the TPE inclusion boundaries, where the greatest shear stress is predicted. With respect to previous works we also found that vertical gradients of pore pressure and temperature and elastic layering can promote both normal and thrust earthquakes within the inclusion.

An Improved New Convolutional Neural Network Method for Inverting the Pore Pressure in Oil Reservoir by Surface Vertical Deformation

Abstract Average pore pressure in oil formation is an important parameter to measure energy in the formation and the capacity of injection–production. In past studies, average pore pressure mainly depends on pressure build-up test results, which have a high cost and need a long testing time, so it is not conducive to utilize. In this paper, the vertical deformation of the Earth’s surface was used to calculate changes in reservoir pore pressure. We set up the marker stake for measuring ground displacement and measured the vertical deformation at the in situ position. Furthermore, we provided an improved new convolutional neural network (CNN), which adopted image-to-image mode, removed pooling layers and full connection layers, and used a new loss function considering the boundary influence coefficient matrix. Then, the machine learning method was used to invert surface vertical deformation to change pore pressure in the oil reservoir. The method was tested in the block of the Sazhong X development zone, in the Daqing Oilfield of China. The average pore-pressure change values of 20 grids whose area was 900×900 m each were determined by the inversion of the corresponding surface vertical deformation at 27 marker stakes. The pore pressure change accuracy reached 82.34%. This study provides a new method for calculating average pore pressure in terms of an oil block. At the same time, it also provides a new technical method for inversion calculations.

Long-term Seismic Monitoring of Reservoir Dynamics at Decatur

Accurate estimation of the moment magnitude of microseismic event catalogs is essential for proper estimation of moment release as well as for the extraction of source parameters that can reveal variations in the reservoir geomechanics and hydraulic properties. Using Decatur as an example we present how an improper choice of the analysis bandwidth and lack of instrument correction can lead to an underestimation of larger magnitudes which may affect the functionality of traffic light protocols. Spatial variations of Gutenberg’s b-value within selected seismicity clusters at Decatur are associated with seismicity migrating over time along the cluster orientations, suggesting differences in either the medium or fault properties and changes in pore pressure. The different b-value regions are supported by clear systematic difference in frequency-magnitude distributions.

DSHANSEP: Alternative considerations of the SHANSEP approach

Since it was first proposed by Ladd and Foott in 1974, the SHANSEP approach has become a quite useful and convenient approach for the evaluation of the undrained shear strength of clay soils in-situ. SHANSEP is commonly practiced with determination of its parameters using various correlations based on consistency limits. While seemingly handy, such correlations hide some details of the behaviour of clayey soils that are useful for engineering judgement. In this article, an alternative way of looking at the SHANSEP method is presented by examining an integrated framework for strength behaviour of clays. This resulted in a new approach called DSHANSEP, where the D represents the so-called “dilatancy parameter” which measures changes in pore pressure due to changes in shear stresses during undrained loading. The resulting association with dilatancy is logical as this parameter reflects failure mechanisms of the clays which is integral aspect of the undrained shear strength of clays. The implication of this relation is explored by looking at its dependence on the overconsolidation ratio, the mobilized stress ratio and sample quality. Furthermore, a formalized system of classification for clays based on the dilatancy parameter and their overconsolidation ratio is proposed and applied for Norwegian clays along with suggested parameters. Use of DSHANSEP and the proposed classifications are in line with the safety philosophy adopted for stability of slopes involving soft clays in Norway.

Probabilistic analysis of Vette fault stability in potential CO2 storage site Smeaheia, offshore Norway

Reliable assessment of fault stability is key for safe CO2 storage in a saline aquifer in fault-bounded structures. The Alpha structure located in the Vette fault's footwall in the Smeaheia area, offshore Norway, is one of the potential CO2 storage sites with a fault-bounded three-way closure. Assessing fault stability in the Smeaheia area is challenging because of the uncertainties associated with the subsurface fault properties (i.e., fault rock lithologies, strength and geometry of faults, etc.). Besides, CO2 injection-related pore pressure changes is another critical factor for mechanical deformation and potential failure. We employed a stochastic analytical approach to assess the probability of Vette fault failure using the Monte Carlo Simulation (MCS) and First Order Reliability Method (FORM). The possible fault smear scenarios of the Vette fault zone are evaluated by interpreting the seismic section and the detailed geological understanding. Each scenario's likelihood and the corresponding probability of failure are then integrated stochastically using an event tree method. Overall, Vette fault's system reliability shows a good to average performance range, which has a system probability of failure between 10−3 to 10-4. This finding suggests that the Vette fault will likely act as a potential barrier during CO2 injection into the Alpha structure. Moreover, the sensitivity study reveals that the stresses (both horizontal and vertical) and fault rock strength (i.e., cohesion and friction angle) are the most crucial parameters to characterize uncertainty reduction.

Aquifer Monitoring Using Ambient Seismic Noise Recorded With Distributed Acoustic Sensing (DAS) Deployed on Dark Fiber

AbstractGroundwater is a critical resource for human activities worldwide, and a vital component of many natural ecosystems. However, the state and dynamics of water‐bearing aquifers remain uncertain, mostly due to the paucity of subsurface data at high spatial and temporal resolution. Here, we show that analysis of infrastructure‐generated ambient seismic noise acquired on distributed acoustic sensing (DAS) arrays has potential as a tool to track variations in seismic velocities (dv/v) caused by groundwater level fluctuations. We analyze 5 months of ambient noise acquired along an unused, 23 km‐long telecommunication fiber‐optic cable in the Sacramento Valley, CA, a so‐called “dark fiber." Three array subsections, ∼6 km apart, are processed and the stretching technique is applied to retrieve daily dv/v beneath each location. Near the Sacramento river, dv/v variations in the order of 2%–3% correlate with precipitation events and fluctuations in river stage of ∼1.5 m. In contrast, regions away (2.5 km) from the river do not experience large dv/v variations. These observations reveal short‐scale spatial variability in aquifer dynamics captured by this approach. Dispersion analysis and surface wave inversion of noise gathers reveal that seismic velocity perturbations occur at depths of 10–30 m. Rock physics modeling confirms that observed dv/v are linked to pore pressure changes at these depths, caused by groundwater table fluctuations. Our results suggest that DAS combined with ambient noise interferometry provides a means of tracking aquifer dynamics at high spatial and temporal resolutions at local to regional scales, relevant for effective groundwater resource management.

Evaluation on the Effect of Pressure Transients on Rock Joints in Unlined Hydropower Tunnels Using Numerical Simulation

Frequent pressure transients are identified as the cause of block failures in many unlined hydropower tunnels. The primary design objective of such tunnels is to prevent hydraulic jacking at design static pressure and mass oscillation but neglects the effect of short transients, i.e., water hammer. The issue has not been studied from the perspective of hydro-mechanical interactions due to frequent pore pressure changes in the rock mass. This article mainly focuses on the effect of pressure transients at different static heads, or different effective normal stresses across the joints and the effect of time period of pressure transient. Further, the change in such behaviour due to different mechanical properties of rock joints, such as stiffness, friction angle and dilation, is investigated. Numerical simulations of observed pore pressure response in the rock mass during a pressure transient are carried out using distinct element code 3DEC. The results show that relative joint deformation due to short pressure transients are the highest when joint normal stresses are 1.5–2.5 times higher than static water pressure in the tunnel and thus the vulnerability to weakening of such joints by hydraulic fatigue is higher. Further, results show that water hammers can travel up to 4 m into the rock mass even in stiff joint conditions and sufficiently high normal stresses. Results further indicate that the hydraulic impact due to water hammer is smaller as compared to mass oscillation. It is concluded that water hammers, wherever applicable along the waterway, can still contribute to hydraulic fatigue of rock joints in addition to the effect of mass oscillation and cannot be neglected when pressure transients occur frequently. Tunnel filling/dewatering and mass oscillations cause macroscopic joint displacements or block movements over long-term operation which is the major cause of block falls in unlined pressure tunnels.

Reservoir fluid effect on caprock properties in the Horda Platform area, northern North Sea

Upper Jurassic organic-rich shales' caprock properties in the Horda Platform area are crucial due to their importance as primary seals for the Middle Jurassic reservoir sandstones. The reservoir sandstones are Sognefjord, Fensfjord, and Krossfjord formations. These reservoir sandstones are overlain by the Upper Jurassic Draupne and Heather Formation Shales. Caprock failure due to injection-related pore pressure change is one of the critical risks for CO2 storage. The thick gas column in Troll East Field, located in the Horda Platform area, can consider an analog for a CO2 post-injection scenario in the area. Evaluating the caprock properties above the thick gas column in the Troll Field helps to understand the area's caprock integrity qualitatively. This study assesses Draupne and Heather Formation Shales' caprock properties using petrophysical and rock physical analyses. Four exploration wells from the Troll East area consider in this study where wells 31/3-1 and 31/6-1 have gas columns of approximately 224 and 222 m, respectively, and two other wells (31/3-3 and 31/6-3) are water wet. The exhumation correction suggests that Draupne and Heather Formations of four studied wells' maximum burial depth experienced only mechanical compaction. Two rock physics templates, such as Vp versus density and Young’s modulus versus Poisson’s ratio, are used to evaluate the caprock behavior due to HC accumulation-related overpressure. The caprock shales in wells with gas columns show a significantly soft nature compared to the shale in the dry well 31/3-3. However, a dry well (31/6-3) also shows remarkably low stiffness. The pressure changes due to the HC accumulation might change the pressure above the reservoir. The caprocks may behave more ductile due to the generation of non-connected micro-fractures. However, fracture connectivity might initiate the caprock's mechanical failure, which poses a significant risk in CO2 post-injection scenario. Moreover, a significantly thick caprock in well 31/6-3 may have influenced the caprock properties differently compared to the other dry well. The caprock mechanical behavior is complex and influenced by several factors.

Poroelasticity Contributes to Hydraulic‐Stimulation Induced Pressure Changes

AbstractHigh‐pressure fluid injections cause transient pore pressure changes over large distances, which may induce seismicity. The zone of influence for such an injection was studied at high spatial resolutions in six decameter‐scaled fluid injection experiments in crystalline rock. Pore pressure time series revealed two distinct responses based on the lag time and magnitude of pressure change, namely, a near‐ and far‐field response. The near‐field response is due to pressure diffusion. In the far‐field, the fast response time and decay of pressure changes are produced by effective stress changes in the anisotropic stress field. Our experiments confirm that fracture fluid pressure perturbations around the injection point are not limited to the near field and can extend beyond the pressurized zone.

The role of sorption-induced coal matrix shrinkage on permeability and stress evolutions under replicated in situ condition for CBM reservoirs

Matrix shrinkage is a unique property of coal which dictates the sorption induced strain in the constitutive relation. Both permeability and stress profiles are passively modified with continuous depletion and they are intrinsically controlled by both poroelastic and matrix shrinkage properties. The models for quantifying matrix shrinkage related effects including horizontal stress loss, vertical strain variation and permeability evolution were proposed. The matrix shrinkage property was also coupled into the poroelastic relationships to study the potential possibility of local failure. Experimental study was performed to measure the permeability and its dynamic applied horizontal stress under replicated in situ uniaxial strain condition. The effects of sorption induced horizontal stress evolution varies with gas types depending on the sorption intensity. The vertical strain of coal bulk under uniaxial strain condition consists of the sorption induced matrix shrinkage/swelling strain, cleat volume strain and matrix mechanical strain due to changes of pore pressure and external stresses, which was validated against with the experiment data. As gas depletion from 4.14 MPa to 0.55 MPa, matrix shrinkage effects can lead to the permeability ratio nonlinearly increases from 1 to 2.27 (~127%) for methane and from 1 to 4.58 (~358%) for carbon dioxide, respectively. Additionally, the magnitude of the vertical effective stress always increases with continuous gas depletion, but the horizontal effective stress may increase or decrease depending on the intensity of sorption-induced matrix shrinkage. The local deviatoric effective stress tends to increase by considering matrix shrinkage effect, which potentially increase the possibility of local coal failure.

Fluid‐Earthquake and Earthquake‐Earthquake Interactions in Southern Kansas, USA

AbstractAn increase in injection activity associated with energy production in southern Kansas starting in 2013 has been linked to the occurrence of more than 130,000 earthquakes (M −1.5 to 4.9) between 2014 and 2017. Studies suggest that the dramatic increase in seismicity rate is related to wastewater injection into the highly permeable Arbuckle formation. Most of the seismicity is located in the underlying crystalline basement, for which hydrological properties and specific fault geometries are unknown. Additionally, some earthquake clusters occurred relatively far (tens of kilometers) from the main injection wells. Therefore, the effect of pore pressure diffusion may be insufficient to explain the relation between the volume of injected fluids and the spatiotemporal evolution of seismicity. Combining physical models (static stress and poroelasticity) and a statistical cluster analysis applied to a high‐resolution relocated catalog, we analyze the evolution of seismicity in southern Kansas. We find that pore pressure changes (Δp) and Coulomb stress changes (ΔCFS) due to fluid diffusion smaller than 0.1 MPa are enough to initiate seismic sequences, which then evolve depending on their distance from the major injection wells. However, we find that earthquake sequences have different seismogenic responses to Δp and ΔCFS in terms of triggering threshold. In regions located close to disposal wells (Harper area) our cluster analysis suggests that both earthquake interactions and fluid diffusion control the evolution of seismicity. On the other hand, at greater distances (Milan area), where clustering behavior suggests greater earthquake interactions, we find that coseismic ΔCFS are larger than Δp.

Permeability Evolution During Shear Zone Initiation in Low-Porosity Rocks

Using an innovative experimental set-up (Punch-Through Shear test), we initiated a shear zone (microfault) in Flechtingen sandstone and Odenwald granite under in situ reservoir conditions while monitoring permeability and fracture dilation evolution. The shear zone, which has a cylindrical geometry, is produced by a self-designed piston assembly that punches down the inner part of the sample. Permeability and fracture dilation were measured for the entire duration of the experiment. After the shear zone generation, the imposed shear displacement was increased to 1.2 mm and pore pressure changes of pm 5 or pm 10 MPa were applied cyclically to simulate injection and production scenarios. Thin sections and image analysis tools were used to identify microstructural features of the shear zone. The geometry of the shear zone is shown to follow a self-affine scaling invariance, similar to the fracture surface roughness. The permeability evolution related to the onset of the fracture zone is different for both rocks: almost no enhancement for the Flechtingen sandstone and an increase of more than 2 orders of magnitude for the Odenwald granite. Further shear displacement resulted in a slight increase in permeability. A fault compaction is observed after shear relaxation which is associated to a permeability decrease by a factor more than 3. Permeability changes during pressure cycling are reversible when varying the effective pressure. The difference in permeability enhancement between the sandstone and the granite is related to the larger width of the shear zones.

Numerical analysis of coupled hydro-mechanical and thermo-hydro-mechanical behaviour in buffer materials at a geological repository for nuclear waste: Simulation of EB experiment at Mont Terri URL and FEBEX at Grimsel test site using Barcelona basic model

We investigated the applicability of a thermo-elasto-plastic model using a TOUGH2-MP/FLAC3D simulator to reproduce the coupled hydro-mechanical behaviour at the engineered barrier emplacement experiment in Opalinus Clay and the coupled thermo-hydro-mechanical behaviour at the full-scale engineered barrier experiment in a crystalline host rock, as part of the DECOVALEX-2019 project Task D. In the numerical models, we adopted Darcy's law, Fick's law, and Fourier's law for the advective flux of fluid, vapour diffusion, and conductive heat flux, respectively. We also used the mechanical constitutive law, which explicitly considers the effects of temperature, pore pressure, suction, and strain changes using the elastic model and the Barcelona basic model. In general, the numerical models in the simulator successfully represented the coupled behaviour in bentonite buffer materials at the two long-term in situ experiments in terms of evolution and spatial distribution for temperature, relative humidity, total stress, and displacement. The performance of the models for the dismantling of the in-situ experiments is globally satisfactory compared with the post-mortem results for saturations, water contents, and dry density.