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  • Mathematical Models
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  • 1
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.70(7), pp.3363-3380
    Description: As part of the HG-A experiment in the Mont Terri Rock Laboratory, large-scale in situ water/gas injection experiments was conducted in a microtunnel. This research work focuses on the numerical analysis of the experimental data and the in situ observations. Concerning a temporary change of the hydromechanical properties of Opalinus Clay during experimental operations, three phases were numerically interpreted. These included the generation of excavation damaged zone during tunnel excavation, in which highly permeable flow paths around the tunnel have been formed; the self-sealing effect during water tests; and the pressure evolution during a long-term gas injection test. A coupled two-phase flow and mechanics model, taking into account the strong anisotropic properties of Opalinus Clay, was developed to interpret the measured data. The hydraulic anisotropy was described by a transversely isotropic permeability tensor. An elasto-plastic model was established to consider both stiffness anisotropy and strength anisotropy. Anisotropic plasticity was studied using the microstructure tensor method.
    Keywords: HM coupling ; Anisotropy ; In situ injection experiment ; Mont Terri Rock Laboratory ; EDZ
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 2
    Language: English
    In: Engineering Geology, 23 November 2015, Vol.198, pp.94-106
    Description: In a common, designed disposal system for high-level radioactive waste, bentonite is used worldwide as a buffer material due to its favorable thermodynamic properties. In-situ experiments at different scales are being conducted in several underground laboratories to investigate the long-term coupled thermo-hydro-mechanical (THM) and chemical behavior of bentonite. Simultaneously, numerous laboratory experiments under well-defined conditions were performed to determine the material properties of bentonite. Since 2012, a laboratory heating and hydration test on MX-80 bentonite has been performed by CIEMAT ( , ). The obtained data during the test were analyzed by different research teams within the international DECOVALEX project in order to understand the coupled THM behavior of bentonite and furthermore to determine the thermal-mechanical properties of bentonite. This paper presents a numerical model for fully-coupled THM processes in bentonite based on the finite element method program OpenGeoSys (Kolditz et al., 2012a). In this model, the description of heat conduction is based on Fourier's law, and advection is also considered in the heat transfer. As an important coupling factor from hydraulic process to thermal process, the dependency of thermal conductivity on the water saturation is intensively analyzed. A multiphase flow model considering water evaporation and vapor diffusion is used to describe the hydraulic process. Special attention was also paid on the analysis of the water retention behavior. An elastic constitutive model based on generalized Hook's law was applied to describe the material's mechanical behavior complemented with consideration of the thermally induced strain and the swelling deformation. Good agreement between the calculated and measured data has been achieved concerning temperature, relative humidity, total stress, and water intake. The main coupling processes and bentonite behavior in the experiment could be captured and well analyzed in this model. However, further investigations are also needed, especially for the long-term THM process with respect to water intake.
    Keywords: Radioactive Waste Disposal ; MX-80 Bentonite ; Thm Coupling ; Multiphase Flow ; Thermal Effect ; Opengeosys ; Engineering
    ISSN: 0013-7952
    E-ISSN: 1872-6917
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  • 3
    Language: English
    In: Transport in Porous Media, 2011, Vol.90(2), pp.545-573
    Description: Predicting fluid replacement by two-phase flow in heterogeneous porous media is of importance for issues such as supercritical CO 2 sequestration, the integrity of caprocks and the operation of oil water/brine systems. When considering coupled process modelling, the location of the interface is of importance as most of the significant interaction between processes will be happening there. Modelling two-phase flow using grid based techniques presents a problem as the fluid–fluid interface location is approximated across the scale of the discretisation. Adaptive grid methods allow the discretisation to follow the interface through the model, but are computationally expensive and make coupling to other processes (thermal, mechanical and chemical) complicated due to the constant alteration in grid size and effects thereof. Interface tracking methods have been developed that apply sophisticated reconstruction algorithms based on either the ratio of volumes of a fluid in an element (Volume of Fluid Methods) or the advective velocity of the interface throughout the modelling regime (Level set method). In this article, we present an “Analytical Front Tracking” method where a generic analytical solution for two-phase flow is used to “add information” to a finite element model. The location of the front within individual geometrical elements is predicted using the saturation values in the elements and the velocity field of the element. This removes the necessity for grid adaptation, and reduces the need for assumptions as to the shape of the interface as this is predicted by the analytical solution. The method is verified against a standard benchmark solution and then applied to the case of CO 2 pooling and forcing its way into a heterogeneous caprock, replacing hot brine and eventually breaking through. Finally the method is applied to simulate supercritical CO 2 injected into a brine saturated heterogeneous reservoir rock leading to significant viscous fingering and developement of preferential flow paths. The results are compared with to a finite volume simulation.
    Keywords: Two-phase flow ; Hybrid analytical numerical ; CO sequestration ; Caprock integrity ; Front tracking
    ISSN: 0169-3913
    E-ISSN: 1573-1634
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  • 4
    Language: English
    In: Environmental Earth Sciences, 2011, Vol.62(6), pp.1197-1207
    Description: The present work compares the performance of two alternative flow models for the simulation of thermal-hydraulic coupled processes in low permeable porous media: non-isothermal Richards’ and two-phase flow concepts. Both models take vaporization processes into account: however, the Richards’ model neglects dynamic pressure variations and bulk flow of the gaseous phase. For the comparison of the two approaches first published, data from a laboratory experiment are studied involving thermally driven moisture flow in a partially saturated bentonite sample. Then a benchmark test of longer-term thermal-hydraulic behavior in the engineered barrier system of a geological nuclear waste repository is analyzed (DECOVALEX project). It was found that both models can be used to reproduce the vaporization process if the intrinsic permeability is relative high. However, when a thermal-hydraulic coupled problem has the same low intrinsic permeability, only the two-phase flow approach provides reasonable results.
    Keywords: Non-isothermal two-phase flow ; Richards’ approximation ; Porous media ; CTF1 experiment ; DECOVALEX task D
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 5
    Language: English
    In: Environmental Earth Sciences, 2012, Vol.67(2), pp.481-495
    Description: Three commonly used thermal equations of state for carbon dioxide, as well as the ideal gas law, have been compared against a large number of measurement data taken from the literature. Complex equations of state reach a higher accuracy than simple ones. The inaccuracy of the density function can cause large errors in fluid property correlations, such as heat capacity or viscosity. The influence of this inaccuracy on the results of numerical simulations have been evaluated by two examples: The first one assumes isothermal gas expansion from a volume, while the second one considers heat transport along a fracture. For both examples, different equations of state have been utilized. The simulations have been performed on the scientific software platform OpenGeoSys. The difference among the particular simulation results is significant. Apparently small errors in the density function can cause considerably different results of otherwise identical simulation setups.
    Keywords: Equation of state ; Compressible fluid flow ; Carbon dioxide ; Numerical simulation ; Gas storage ; Supercritical fluid
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 6
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.469-477
    Description: Over the course of hydrological research projects often a large number of heterogeneous data sets are acquired from sources as diverse as boreholes, gauging stations or satellite imagery. This data then need to be integrated into models for the simulation of hydrological processes. We propose a framework for exploration of geoscientific data and visually guided preparation of such models. Data sets from a large number of sources can be imported, combined and validated to avoid potential problems due to artefacts or inconsistencies between data sets in a subsequent simulation. Boundary conditions and domain discretisations for surface and subsurface models can be created and tested regarding criteria indicating possible numerical instabilities. All data sets including simulation results can be integrated into a user-controlled 3D scene and aspects of the data can be enhanced using a number of established visualisation techniques including thresholding and user-defined transfer functions. We present the application of this framework for the preparation of a model for simulation of groundwater flow in a river catchment in southwest Germany investigated in the scope of the WESS project.
    Keywords: Data exploration ; Hydrology ; Simulation ; Visualisation ; OpenGeoSys
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 7
    Language: English
    In: Environmental Earth Sciences, 2016, Vol.75(20), pp.1-21
    Description: To understand the complex thermo-hydro-mechanical (THM) processes and their evolution in a clay-based engineered barrier systems (EBS) during the closure phase of a geological repository for radioactive waste, a 1:2 scale in situ heating experiment (HE-E experiment) has been ongoing since 2011 in the Mont Terri Rock laboratory. Based on the experimental data, a fully coupled THM 3D simulation using the finite-element program OpenGeoSys was carried out. The main objectives of the simulation were to interpret the experimental observations, to understand the thermally induced THM interactions and to analyse the different material properties. Five experimental phases were numerically interpreted, in order to take into account the changes in the temporary material properties during experimental operations. These included the tunnel excavation, the ventilation, the emplacement of the bentonite EBS, the heating phase and the phase after shutdown of the heater. A non-isothermal Richards’ flow model was used to take into account the evaporation and vapour diffusion during the heating. The material behaviours of EBS in association with the saturation-dependent thermal conductivity and the water retention behaviour under high temperatures were analysed in detail. The strong thermal, hydraulic and mechanical anisotropic properties of the Opalinus Clay were described by a transversely isotropic model.
    Keywords: Disposal of radioactive high-level waste ; THM coupling ; Thermal effects ; Clay-based material ; OpenGeoSys
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 8
    Language: English
    In: Environmental Earth Sciences, 2012, Vol.67(6), pp.1573-1586
    Description: Since the 1980s, high-nitrate concentration in one of the groundwater sampling wells at the Nankou site, northwest of the Beijing Plain, has become a major concern for the local water authority. In a previous study (Sun et al. in Environ Earth Sci 64(5):1323–1333, 2011), a hydrogeological structural model was developed based on the borehole logs of this area and the steady, as well as transient groundwater-flow models, were calibrated using the measured hydraulic heads. In this paper, the potential pollution sources in this area are investigated. The chemical analysis of the groundwater is also presented. The results demonstrate that the most likely pollution source is the untreated wastewater discharge from a nearby fertilizer factory. Furthermore, a mass transport model is developed to reproduce the nitrate transport process in the aquifer at the Nankou site under different pollution sources, i.e., a fertilizer factory, river with wastewater and an agriculture field. The combined effects of the river and agriculture fields present a better understanding of the nitrate transport in the local aquifer. In addition, a pumping scenario is designed to clean up the current nitrate concentration. The pumping rate and the well location are first estimated with 2-D analytical solutions of the type curves method. Then a 3-D numerical model is used to calculate the nitrate-concentration changes after the pumping activities start. In the downstream direction of the regional groundwater flow, three pumping wells are set up for the clean-up strategy. The calculated pumping rate in each well is about 1,500 m 3 /day. After 1 year, the nitrate concentration in the observation well recedes to 68 mg/l from the initial value of 72.9 mg/l, and it will be lower than the limitation value (20 mg/l) after 5,400 days of groundwater extraction. The data assessment and clean-up scenarios reported in this paper are fundamental for the contaminated aquifer management in the future.
    Keywords: Nitrate transport modeling ; Pollution sources assessment ; Remediation scenarios ; Nankou
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 9
    Language: English
    In: Environmental earth sciences, 2013, Vol.70(8), pp.3585-3602
    Description: Fluid flow in low-permeable carbonate rocks depends on the density of fractures, their interconnectivity and on the formation of fault damage zones. The present-day stress field influences the aperture hence the transmissivity of fractures whereas paleostress fields are responsible for the formation of faults and fractures. In low-permeable reservoir rocks, fault zones belong to the major targets. Before drilling, an estimate for reservoir productivity of wells drilled into the damage zone of faults is therefore required. Due to limitations in available data, a characterization of such reservoirs usually relies on the use of numerical techniques. The requirements of these mathematical models encompass a full integration of the actual fault geometry, comprising the dimension of the fault damage zone and of the fault core, and the individual population with properties of fault zones in the hanging and foot wall and the host rock. The paper presents both the technical approach to develop such a model and the property definition of heterogeneous fault zones and host rock with respect to the current stress field. The case study describes a deep geothermal reservoir in the western central Molasse Basin in southern Bavaria, Germany. Results from numerical simulations indicate that the well productivity can be enhanced along compressional fault zones if the interconnectivity of fractures is lateral caused by crossing synthetic and antithetic fractures. The model allows a deeper understanding of production tests and reservoir properties of faulted rocks. ; p. 3585-3602.
    Keywords: Wells ; Case Studies ; Mathematical Models ; Drilling ; Carbonate Rocks
    ISSN: 1866-6280
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  • 10
    Language: English
    In: Environmental Earth Sciences, 2012, Vol.67(2), pp.311-321
    Description: The joint research project CLEAN was conducted in the years 2008–2011 by a German research and development (R&D) alliance of 16 partners from science and industry. The project was set-up as pilot project to investigate the processes relevant to enhanced gas recovery (EGR) by the injection of CO 2 into a subfield of the almost depleted Altmark natural gas field. Despite the setback that permission for active injection was not issued by the mining authority during the period of the project, important results fostering the understanding of processes linked with EGR were achieved. Work carried out led to a comprehensive evaluation of the EGR potential of the Altmark field and the Altensalzwedel subfield in particular. The calculated safety margins emphasize that technical well integrity of the 12 examined boreholes is given for EGR without a need for any further intervention. The laboratory and field tests confirm that the Altensalzwedel subfield is suitable for the injection of 100,000 t of CO 2 . Numerical simulations provide sound predictions for the efficiency and safety of the EGR technology based on the CO 2 injection. The development and testing of different monitoring techniques facilitate an improved surveying of CO 2 storage sites in general. The CLEAN results provide the technological, logistic and conceptual prerequisites for implementing a CO 2 -based EGR project in the Altmark and provide a benchmark for similar projects in the world.
    Keywords: Enhanced gas recovery ; EGR ; Altmark ; Gas field ; CO storage
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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