Hydro-mechanical Model Research Articles

Hydromechanical behavior analysis of fractured vuggy carbonate rocks based on homogenization theory

Fractured vuggy carbonate rocks are important for underground water and geo-energy reservoirs due to their significant contribution on water and hydrocarbon reserves and production. A vug is a small cavity in a carbonate rock, which is relatively larger than the intergranular pore space. The presence of multiscale fractures and vugs makes the hydromechanical behavior of rocks different from that of most geological materials. The objective of this work is to develop an upscaling method to analyze the hydromechanical behavior of fractured vuggy carbonate rocks based on homogenization theory. To this end, at first a novel conceptual model named discrete fracture-vug network (DFVN) model was proposed to describe the hydromechanical behavior on the fine scale. The matrix and fractures are poroelastic domains in which Biot equations are applied. The vugs are free fluid domains governed by Stokes equations. Two domains are coupled with extended Beavers–Joseph–Saffman interface conditions. Then, an upscaled hydromechanical model was developed via two-scale asymptotic homogenization. The model consistent with classical Biot equations, but the model coefficients possess explicit formulations which can be determined by three periodicity cell problems. Subsequently, efficient numerical solutions of cell problems are provided using finite element methods. Herein, the discrete fractures are modeled as lower-dimensional interfaces between matrix elements. The proposed model and method are verified through several numerical examples and experimental data. The results show that the storage coefficient and Biot coefficient increase with the presence of fractures and vugs. The equivalent elastic stiffness of a fractured vuggy rock is majorly affected by the vugs' volume ratio. The connectivity of DFVN has an important impact on the equivalent permeability.

Exploring the stability of unsaturated soil slope under rainfall infiltration conditions: A study based on multivariate interrelated random fields using R-vine copula

Estimating the instability of soil slopes due to rainfall in highly heterogeneous materials poses a considerable challenge. In this study, a general framework for coupled hydro-mechanical modeling of rainfall-induced instability in unsaturated slopes with multivariate random fields is developed. The R-vine copula is introduced to simulate the intricate non-Gaussian dependencies between soil parameters. UMAP is utilized for visualizing these dependencies. The study compares the fitting performances of R-vine and Gaussian copulas using LOWESS Regression. Subsequently, deterministic model computations are conducted in Abaqus, along with batch random field model analysis based on the R-vine copula. The instability probability of soil slopes under rainfall infiltration conditions is evaluated through direct Monte Carlo simulations, and statistically investigate the relationships between groundwater level, sliding volume, plastic zone volume, and safety factor. The findings indicate that: 1) The R-vine copula model proficiently captures the non-Gaussian dependencies in soil data, demonstrating superior fitting performance over the Gaussian copula; 2) deterministic simulations might overestimate the safety factor at certain instances; 3) as rainfall progresses, a growing negative correlation is observed between groundwater levels and slope instability; 4) continuous rainfall leads to a re-equilibration of the sliding mass, heightening the influence of the sliding volume on stability.

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Fully coupled hydro-mechanical modeling of two-phase flow in deformable fractured porous media with discontinuous and continuous Galerkin method

Discrete fracture and matrix modeling of coupled flow and geomechanics is instrumental for understanding flow in fractured media for various geoengineering applications such as enhanced geothermal energy systems and groundwater remediation. We employ the governing equations for two-phase flow in deformable fractured porous media with a stress-dependent porosity and permeability model of matrix, and variable fracture aperture and the corresponding permeability. A finite element framework is presented, in which fractures are regarded as low-dimensional objects, to discretize the coupled two-phase flow and geomechanics in fractured porous media. A hybrid method, combining discontinuous Galerkin (DG) and continuous Galerkin (CG) finite element methods (FEM), is utilized to solve for the two-phase flow, while the solid deformation is approximated using a discontinuous Galerkin FEM approach. Several benchmark cases are utilized to examine the accuracy of the proposed hybrid DG-CG FEM method. Further validation is performed using more complex, realistic fracture configurations. As the mechanical characteristics of the fracture and the surrounding matrix differ, the simulation results demonstrate that displacement and stress are discontinuous on both sides of the fracture. While the case with low permeability fractures exhibits pressure jump across the fractures, the pressure changes are reasonably smooth for the conduit fractures.

Permeability evolution and production characteristics of inclined coalbed methane reservoirs on the southern margin of the Junggar Basin, Xinjiang, China

The thick and steeply inclined coal seams of the Junggar Basin of Xinjiang, China, are unique with dip angles generally >50° but over the range 0°–85°. Initial and evolving permeability and pressures change drastically around wells down-dip within the steeply inclined reservoir as a result of the depth differential. Hence, the evolution of permeability and fluid pressures during drainage exhibits significant differences from those of flat-lying or even slightly inclined reservoirs. We apply a hydro-mechanical model to evaluate the interaction of two-phase flows of gas and water in the inclined system. The influence of different reservoir inclinations (15°, 30°, 45°, 60°, and 75°) on the evolution of permeability, reservoir pressure, and gas production are explored through finite element modeling of this system. The results show that: 1) Reservoir inclination induces differences in permeability, reservoir pressure, gas content and methane production between the shallower updip reservoir and deeper downdip reservoir. The difference in permeability between the updip and downdip reservoirs is amplified as the dip angle increases and as drainage proceeds in the presence of the varying stress gradient. 2) An apparent asymmetric distribution of reservoir pressures results for wells along dip. The difference in reservoir pressure between the updip and downdip reservoirs intensifies as the inclination increases but lessens with the progress of drainage. The larger the dip angle, the smaller the final reservoir pressure. 3) The pressure reduction in the updip reservoir is larger than that in the downdip reservoir, resulting in the unsynchronized desorption of methane in the updip and downdip reservoirs. Methane within the updip reservoir desorbs preferentially over that in the downdip reservoir. For reservoir dip angles <45° a single peak in methane production rate is apparent but this is supplanted by dual peaks for inclinations >45°. The time gap in gas desorption between the updip and downdip reservoirs results in the “dual-peak” on gas production profile. 4) A larger well spacing along the dip of a more highly inclined reservoir results in more efficient water drainage and gas production. An inverted trapezoidal well pattern is recommended to facilitate the drainage and gas production of reservoirs with significant dip angles.

A hydromechanical material model for compacted bentonite

This paper presents a new hydromechanical model, developed with the intention to be used for predicting the mechanical evolution of compacted bentonite components in repositories for spent nuclear fuel. The model is based on a thermodynamic relation for the chemical potential of the clay water, and uses empirical functions which are parametrized and calibrated with data on water retention properties at free swelling conditions, swelling pressure, and shear strength; the latter two are central for defining technical requirements for the bentonite buffer components. This results in a model which at its core incorporates all these properties, as well as the hysteresis behaviour. The validity of the model was investigated by analysing nine experimental test conditions, using COMSOL Multiphysics®, with a single set of parameter values. Model results were compared with experimental data for both saturated conditions (free swelling retention test; oedometer tests and triaxial compression tests) and for unsaturated conditions (swelling pressure build-up; swelling or shrinkage at constant load; compression at constant water or constant suction and unconfined compression tests). These comparisons gave a broad demonstration of the predictive capabilities of the model. Some limitation of the model regarding the representation of shrinkage, the shear strength at lower water contents, and the path dependence can nevertheless be noted.

On the use of a mass lumping technique compatible with a fully‐coupled hydromechanical model for the case of strong discontinuities with the extended finite element method

AbstractIn the present article, we introduce a new approach that uses a standard lumping technique, also known as the row‐sum technique, in the extended finite element method (XFEM) in order to stabilize a fully‐coupled hydromechanical model in the presence of a strong discontinuity. Typically, numerical oscillations can be observed where high gradients appear (pressure shocks) due to the violation of the maximum principle. In order to remediate this abnormal behavior, it is of common use to lump the mass matrix associated with the discretized form of the diffusion equation, a parabolic partial differential equation. Several techniques are available to diagonalize the mass matrix in XFEM in the context of dynamic crack growth. However, these techniques were designed for a wave‐like partial differential equation, a hyperbolic partial differential equation. The direct use of such techniques with a fully‐coupled hydromechanical model does not conserve the mass and leads to the violation of the maximum principle. The approach proposed in this work fulfills the features of a parabolic partial differential equation. It is also adapted to the nonlinear case for which the mass density is given by a constitutive law with a distribution that depends on the position inside the domain but which can be different on each side of the discontinuity. In order to test the robustness of our XFEM formulation, only discontinuous enrichments are considered in this article and the discontinuity is supposed impermeable in order to recover two distinct materials on each side of the boundary‐controlled interface for which we can exhibit independent analytical solutions.

Geomechanical in situ testing of fault reactivation in argillite repositories

Abstract. Pressurization of natural faults as a result of repository-induced effects can lead to their reactivation and permeability generation in case such features are present near disposal tunnels. Potential driving forces for such pressurization are the temperature increase caused by heat-producing high-level radioactive waste and the generation of hydrogen and other gases due to corrosion of engineered materials. We are primarily concerned about pressurization and fault activation in host rocks and faults that have very low natural permeability for pore pressure increases to dissipate, such as the argillite rocks currently investigated in Switzerland, France and other countries. This presentation discusses a series of in situ experiments of fault activation by fluid injection conducted in the argillite rock (Opalinus clay) at the Mont Terri underground research laboratory in Switzerland. A multi-model monitoring test bed was installed at Mont Terri that includes distributed fiber optics for strain, temperature, and acoustics; local fault pore pressure and three-dimensional displacement sensors; active seismic imaging; and passive seismic monitoring. The fault experiments (and their subsequent analysis via hydromechanical modeling) provide a new fundamental understanding of the coupling between pore pressure, fault deformation and permeability generation as a function of time and allow exploring how seismic and aseismic events may impact the integrity of faulted argillite host rock. Our experimental observations demonstrate that significant flow (and transport) can occur along the initially impermeable argillite fault when rupture is activated. However, the fault permeability decreases to almost its pre-activation value when fluid injection ceases and fluid pressure drops. Dilatant slip on the fault plane alone does not explain the observed increase in fault permeability; pressure-induced fault opening also plays a role, favored by the softness of the shale along with the fact that the structure of the fault zone prevents fluids from diffusing into the adjacent damage zone. Rupture initiation and permeability generation is initially aseismic, associated only with an increase in noise level and emerging tremors. Micro-earthquakes are initiated later in the experiments and typically occur away from the fluid-pressurized area. Hydromechanical models show that stress transferred from the initial aseismic deformation can build up to stress criticality and later induce seismic rupture. After presenting the experimental results, we will close the presentation with an outlook to future experimental campaigns using the fault test bed at Mont Terri. We are currently planning a controlled thermal stimulation of the fault, where instead of fluid injection as a trigger mechanism we will heat up the nearby rock volume and measure potential effects on fault stability. Lessons learned from our past and future experiments will help inform the safety assessment of geologic disposal in argillite host rock.

Multiphysics analysis of tramway geotechnical infrastructure: numerical modelling

Under realistic field conditions, geotechnical infrastructures are usually influenced by complex interactions of mechanical behaviour under the action of an internal water flow. This mechanism could be the main origin of damage to embankments. This study develops a new hydro-mechanical (HM) approach based on Darcy's law model and Biot's poroelastic concept to investigate the behaviour of soil with and without geotextile under realistic conditions. The problem's numerical solution is carried out using a finite-element method. The proposed two-dimensional model was implemented in Comsol Multiphysics Software. Under coupled HM behaviour, the stress in porous materials causes a volumetric change in strain, which causes fluid diffusion. Consequently, pore pressure dissipates through the pores. To discuss the advantages of coupled HM modelling and evaluate the geotextile performance, volumetric strain, pore-water pressure, storage and displacement are compared for a mechanical, hydraulic and HM model. These analyses were undertaken in connection with a tramway embankment project in a marshy area in Sidi Bel Abbes. The simulation results show better results for geotextile and embankment in HM coupled modelling.

Physics-Based Modeling to Understand and to Propose Forecasting Methods of Induced Seismicity

Abstract Induced seismicity compromises the widespread deployment of geoenergy applications that contribute to mitigate climate change. In particular, the development of Enhanced Geothermal Systems (EGS) has been hindered by the risk of induced seismicity, mostly caused by hydraulic stimulation aimed at enhancing the permeability of deep hot crystalline rocks. Injection-induced seismicity has been traditionally attributed to fluid pressure buildup, which destabilizes fractures and faults. However, the largest seismic events commonly occur after the stop of injection, when pore pressure drops and both the magnitude and frequency of induced seismicity is expected to decrease. This counterintuitive phenomenon is not well understood. Yet, understanding the triggering mechanisms is the key to reliably forecast and manage induced seismicity. Here, we investigate the triggering mechanisms of co- and post-injection seismicity using coupled hydromechanical models, considering both a homogeneous and a fault-crossed domain, based on the case of Basel EGS (Switzerland). We find that the combination of pressure diffusion, poroelastic stressing, and static stress transfer explains the occurrence of induced seismicity, especially after the stop of injection, significantly better than the pore pressure alone. Considering a fault zone, which is more permeable and deformable than the surrounding rock, amplifies pressure diffusion along the fault and causes anisotropic variations of the stress field that lead to an increase in the seismicity rate that is orders of magnitude larger than for the homogeneous domain. These results point out that identifying the main geological structures through subsurface characterization is key to improve physics-based induced seismicity forecasting.

Seismic swarms as intermittent quasi-static ruptures driven by pore pressure variations due to the water reservoir impoundment

Seismic swarms are characterized as time-space clustered sequences of earthquakes without a dominating strong event. The occurrence of earthquake swarms has been observed in various environments and found out to be closely related to underground migration of fluids. Mechanical interactions between fluid-conduit properties of fault and rupture determine the spatio-temporal complexity of rupturing process and earthquake occurrence. Often, seismic swarms are triggered by Water Reservoir Impoundment (WRI). Although many studies have investigated the relationship between the pore pressure changes due to WRI and observed seismicity, hydromechanical models that explain the observed processes are limited. Here, we investigate the role of hydromechanical interactions in triggering earthquake swarms in WRI conditions, using data from the Song Tranh 2 Reservoir in Vietnam as an example. We found that seismic swarms evolve due to stress redistribution and subsequent induced fluid flow. Swarm sequences form interconnected rupture increments and their aggregated behaviour is similar to a single large earthquake that ruptures the fault segment at once. The evolution of friction with the mean slip retrieved from the swarm earthquakes exhibits both, slip weakening and strengthening stages, resembling linear slip-weakening friction. The effective slip over which the weakening is completed, is modulated by the pore pressure diffusion. These results provide insights into the physical mechanisms of seismic swarms and have implications for seismic hazard.

Hydromechanical modelling of salt caverns subjected to cyclic hydrogen injection and withdrawal

The present work is devoted to the study of underground hydrogen storage in salt caverns. Based on the hydromechanical characteristics of rock salts obtained from some laboratory experiments, we propose a novel model that includes short- and long-term mechanical behaviour. Specifically, the short-term part incorporates the elastoplastic and instantaneous damage mechanisms. Concerning the long-term behaviour, in addition to the primary and secondary creep phases, the tertiary phase takes into account a delayed damage mechanism.As application, we performed hydromechanical modelling of two vertical salt caverns with different depths based on existing hydrogen gas caverns, which are subjected to cyclic hydrogen injection and withdrawal (seasonal and daily scenarios). Gas transport is also modelled taking into account diffusion and advection mechanisms. Mechanical results indicate that the stability problem of a very deep cavern is more worrying in comparison with a shallow cavern, as expected. However, the gas extension is the same for both caverns because the gas flow is mainly by diffusion transport, while the permeability does not significantly increase. Since field data at very depths are limited, a sensibility analysis of material properties was carried out to provide insight into key mechanisms that may occur. Typically, a decrease in mechanical properties increases the extent of the damage around deep cavern but did not lead to significant increase in the extent of gas leakage. Under the assumptions made, these findings suggest that the use of salt caverns for green hydrogen storage, even with aggressive operating conditions to regulate variations between renewable energy production and peak power demands, should not significantly affect the stability of salt cavern nor promote an increase in hydrogen loss.

Reducing uncertainties in hydromechanical modeling with a recently developed Rosetta 3 podeotransfer function

Stability analysis of unsaturated landslide deposits requires reliable estimates of soil moisture and pore water pressure. However, modeled soil moisture and pore water pressure contain substantial uncertainties due to imperfect information on soil hydraulic properties. Due to the relatively high dimensionality, commonly used parameter optimization strategies can be significantly affected by equifinality problems. This study investigates the effectiveness of reducing parameter estimation dimensionality using soil pedo-transfer functions. Specifically, we first estimated soil hydraulic parameters using the traditional Generalized Likelihood Uncertainty Estimation (GLUE) method, with parameters randomly drawn from the entire space (refer to as GLUE-random). In a second strategy, we use the Rosetta 3 pedotransfer function to constrain soil hydraulic parameters (refer to as GLUE-Rosetta). The two methods were tested in a typical landslide deposit with in-situ measured soil moisture dynamics for inverse modeling. The GLUE-random estimated soil hydraulic parameters contained substantial uncertainties –resulting in poorly constrained soil water retention curves (SWCC) and hydraulic conductivity functions (HCF). As a result, the uncertainty bands of pore water pressure and slope stability can cross values with several orders of magnitudes. In contrast, GLUE-Rosetta provided well-constrained SWCC and HCF, which significantly reduce the uncertainties in pore water pressure and slope stability estimates. These results suggest that the Rosetta 3 pedotransfer function can significantly improve the reliability of soil hydraulic parameters by reducing the dimensionality of the optimization problem and high-quality prior information of soil hydraulic properties. In conclusion, Rosetta 3 can enhance the reliability of soil parameters estimates and the reliability of subsurface hydrology, which may benefit the development of landslide early-warning systems.

Stability of loess high-fill slope based on monitored soil moisture changes

This paper aims to assess the influence of moisture content changes during rainfall on the stability of loess high-fill slopes by taking a loess high-fill slope in Lanzhou City as an example. First, according to the moisture content monitoring data collected from a slope online monitoring system, direct shear tests were performed on soil samples of different moisture content to determine the relationship between the shear strength parameters and moisture content. Next, a coupled hydro-mechanical model with soil shear strength related to moisture content was established and used to analyze 16 working conditions with various rainfall intensities and durations for two cases: shear strength parameters from the site exploration report and those from this study. Finally, the results from the two cases were compared regarding the changes in stresses and displacements after rainfall infiltration to analyze the influence of moisture content on the stability of loess high-fill slopes. The conclusions are as follows: (1) Segmental relationship equations of cohesion and angle of internal friction were established for loess with various moisture content. (2) Under the conditions of different rainfall intensities, significant differences were observed in the trends of slope stress and displacement changes. (3) The slope displacement occurred in the shallow soil layer within about 12.5 m from the slope top, and the plastic strain concentrated in the soil layer within about 6 m from the slope top. (4) The results of slope stability analyses based on moisture content monitoring data are more in line with the observed.

3D coupled hydro-mechanical modeling of multi-decadal multi-zone saltwater disposal in layered and faulted poroelastic rocks and implications for seismicity: An example from the Midland Basin

Knowledge of subsurface coupled pore pressure and stress perturbations due to saltwater disposal (SWD) is crucial for understanding and managing induced seismicity. Current studies choose Biot poroelasticity to implement fluid-rock coupling and perform numerical modeling in relatively simple settings. In this study, we first introduce a general poromechanical framework to capture a broader range of hydro-mechanical coupling and then apply it to model real-world SWD problems with geological and operational complexities. We demonstrate a viable workflow through the case study of a 2500 km2, layered, faulted, and recently seismically awaken area that has undergone multi-decadal multi-zone injection by nearly 200 SWD wells in the Midland basin. The example provides a unique opportunity to study the respective impact of shallow and deep injections across depths and the role of inter-zone interactions, among others. Our modeling suggests over 500 and 250 psi increases in pore pressure and Coulomb stress, respectively, in the deep disposal zone in the last decade, which were 5 to 10 times greater than those accumulated over the past four decades in the two shallower disposal zones. Vertically, shallower injections impacted mostly shallower zones, whereas the deep injection produced changes across depths and dominated especially in the deep zone and the upper segment of the basement. Towards the mid-segment of the basement, changes due to shallow and deep injections were comparable, with the latter 2 to 5 times higher. Laterally, in-zone changes were dominated by pore pressure diffusion in the near field and poroelastic stressing in the far field in shallower zones but were dominated by pore pressure diffusion everywhere in the deep zone, owing to their different hydraulic diffusivities. Faults acted as pore pressure and Coulomb stress sinks for disposal zones and sources for non-disposal zones, and fault-zone permeability structure had a minor impact. Finally, seismological modeling suggests that seismicity in the area was most likely driven by the deep injection.

PS-InSAR data, key to understanding and quantifying the hydromechanical processes underlying the compaction of aquifer systems. Case of West- and East-Flanders, Belgium

Until the advent of remote sensing techniques, the study of regional subsidence was hampered by the rarity and sparsity of field measurements of ground displacement, which led to a significant uncertainty regarding the representativeness of the calibration data used in the modelling of these phenomena and the reliability of derived predictions. This paper explores the unique potential of Persistent Scatterer Interferometry with Synthetic Aperture Radar (PS-InSAR) in the hydromechanical modelling of a large-scale, long-term subsidence occurring in the provinces of West- and East-Flanders, Belgium, related to the overexploitation of the deep aquifer system in the region. PS-InSAR has been used, not only to describe and characterise the ground subsidence and identify representative locations, but also to calibrate the hydromechanical parameters of the ground and validate the results of a numerical model. PS-InSAR has proven to be essential to improve the calibration and reach a set of parameters (i) coherent with the hydrogeology of the study area, (ii) coherent between different deforming zones and (iii) able to reproduce the dynamics of the observed ground subsidence. Thus, PS-InSAR enables to accurately reproduce, understand and simulate the compaction of an aquifer system and, ultimately, design adequate and optimised management policies.

An XFEM-based hydro-mechanical model to consider the non-Darcy flow in complex fracture networks

Geotechnical instability and failure are closely related to the hydro-mechanical coupled behavior of fractured rocks, and the existing studies often regard the fracture seepage as linear Darcy flow. Due to the neglect of non-Darcy effect caused by rough rock wall and high flow velocity, the estimated rock deformation and seepage flow can lead to incorrect assessment of geotechnical risks. Hence, this study proposed an efficient and robust hydro-mechanical (HM) coupled model to consider non-Darcy flow in complex fracture networks. The fractured rock mass's mechanical response, including the fracture surface's frictional contact, was calculated using the XFEM solver, and the widely used Forchheimer's law was adopted to describe the non-Darcy flow along the fractures. The fracture segments cut by mechanical element edges are considered one-dimensional flow elements, and the Newton-Raphson iteration method is adopted to solve the Forchheimer equation directly. To consider the influence of fracture deformation on the conductivity of fractures, the Barton-Bandis model was adopted to determine the hydraulic aperture of the fractures when the fracture was closed. Subsequently, a staggering Newton-Raphson method can be developed to decouple the fracture deformation and fluid flow process. Experimental results were adopted to validate the accuracy of the numerical model, and several numerical cases were used to illustrate the efficiency and robustness of the proposed model. The effects of inlet pressure, confining stress, and non-Darcy effect on the HM-coupled behavior of fractured rock were investigated. Numerical results suggested that the distribution of fluid pressure and failure process of rock mass based on Darcy assumption could be dramatically changed after considering the nonlinear seepage behavior.

Causal mechanism of Gotthard Base Tunnel-induced ground deformation: Insights from 3D fully-coupled hydro-mechanical simulation and comparison to field measurements

The Gotthard Base Tunnel (GBT), constructed between 2000 and 2011, is a 57 km long and up to 2.5 km deep high-speed railway tunnel located in the Swiss Alps. Significant ground surface displacements reaching about 10 cm were observed during and after the tunnel construction. To gain a better understanding of the causal mechanism of such conspicuous ground deformations, we develop a three-dimensional (3D) fully-coupled hydro-mechanical model to simulate GBT-induced groundwater drainage, stress redistribution, rock mass consolidation, and fault zone deformation at the regional scale. First, we construct a geological model with topographical features, lithological units, and natural faults realistically represented. We constrain the material properties of fault zones and rock masses based on available extensive laboratory testing results and site characterisation datasets. We then simulate the tunnelling process over time, with the resulting coupled hydro-mechanical responses of faulted rock masses well captured and ground surface/subsurface displacements quantitatively analysed. The simulation results in general show a good agreement with the field monitoring data of ground surface displacement, subsurface tunnel settlement, and groundwater inflow into the tunnel. Our model indicates that ground surface displacements originate from GBT-induced water drainage and rock mass consolidation in the deep subsurface. Our results also show that the GBT construction could trigger faults to shear via drainage-induced pressure diffusion and poroelastic stressing. The research findings from our work have important implications for many groundwater drainage-related geoengineering activities such as underground excavation in alpine mountains and fluid withdrawal in subsurface reservoirs.

A hydro-mechanical semi-analytical framework for hollow cylinder sanding tests

In this work we propose a novel semi-analytical hydro-mechanical framework for modeling sand production in the context of the hollow cylinder test, based on a kinematic formulation of the hydro-mechanical models of Vardoulakis et al. (1996) and Papamichos et al. (2001). We aim at the construction of a simple and useful tool which allows for quick estimates of the relevant quantities and can be efficiently used to study different forms of the postulated laws regarding the mechanics, hydrodynamics and degradation of the rock. In particular, this framework can be used to systematically calibrate the sand production coefficient λ as a function of the external conditions of the experiment, such as the external stress, which still is a major unknown in the hydro-mechanical modeling of the erosion process. As a first approximation we restrict ourselves to the case where pressure drawdown is small compared to the external stress, which is applicable in certain laboratory experiments. We illustrate the application of the framework by studying the effect of different forms of the hydrodynamic law, modified in the low porosity regime and the degradation law with respect to the non-linear dependence of cohesion and friction angle on the porosity. We use this framework to calibrate the dependence of λ on the external stress using the data of the experiment of Papamichos at al. (2001). We find that the sand production coefficient exhibits a power law modified by a decreasing exponential dependence as has been suggested in a recent work by the authors. The model is also applied in a different sanding experiment with varying external stress and flow rate exhibiting good agreement with the laboratory dataset.

Coupled Hydromechanical Modelling of a Vertical Hydraulic Sealing System Based on the Sandwich Principle

For the shaft sealing of a repository for radioactive waste, a Sandwich sealing system was developed by KIT-CMM consisting of bentonite-based sealing segments (DS) and sand mixture-based equipotential segments (ES). To demonstrate the functionality of the Sandwich sealing system, various laboratory tests (MiniSandwich tests and semi-technical scale experiments) have been carried out before a large-scale experiment has been implemented in situ at the Mont Terri Rock Laboratory (CH). An important coupled process in the Sandwich system is the swelling deformation of the DS while aqueous fluid penetrates into the system. Consequently, the interparticle porosity (effective porosity) of the DS decreases by swelling strain, resulting in a reduction in the permeability of the DS. Pore space of the ES also decreases slightly due to swelling stress in the adjacent DS, which also leads to a reduction in the permeability of the ES. To understand the coupled hydromechanical processes of the Sandwich sealing system, a numerical model was developed to interpret the experimental observations from the MiniSandwich tests and to parameterize different components. A linear swelling model for DS and empirical functions for the swelling deformation-induced permeability change for both DS and ES segments were introduced into the coupled model with Richards’ flow and elastic model. Sensitivity analysis with parameter variations of the most important parameters reduces the uncertainty in the system behavior.