Assessing the Gas Transport Mechanisms in the Swiss L/ILW Concept Using Numerical Modeling and Supporting Experimental Work

2010 ◽  
Author(s):  
Irina Gaus ◽  
Paul Marschall ◽  
Rainer Senger ◽  
John Ewing ◽  
Joerg Rueedi
Keyword(s):  
Host Rock ◽  
Gas Flow ◽  
Gas Transport ◽  
Gas Permeability ◽  
Low Permeability ◽  
Gas Migration ◽  
Transport Mechanisms ◽  
Transport Capacity ◽  
Two Phase ◽  
Tunnel System

In low/intermediate-level waste (L/ILW) repositories, anaerobic corrosion of metals and degradation of organic materials produce hydrogen, methane, and carbon dioxide. Gas accumulation and gas transport in a L/ILW repository is an important component in the safety assessment of proposed deep repositories in low-permeability formations. The dominant gas transport mechanisms are dependent on the gas overpressures as with increasing overpressure the gas transport capacity of the system increases. The dominant gas transport mechanisms occurring with increasing gas pressure within the anticipated pressure ranges are: diffusion of gas dissolved in pore water (1), two phase flow in the host rock and the excavation damaged zone (EDZ) whereby no deformation of the pore space occurs (2), gas migration within parts of the repository (if repository materials are appropriately chosen) (3) and pathway dilation (4). Under no circumstances the gas is expected to induce permanent fractures in the host rock. This paper focuses on the gas migration in parts of the repository whereby materials are chosen aimed at increasing the gas transport capacity of the backfilled underground structures without compromising the radionuclide retention capacity of the engineered barrier system (EBS). These materials with enhanced gas permeability and low water permeability can supplement the gas flow that is expected to occur through the EDZ and the host rock. The impact of the use of adapted backfill and sealing materials on the gas pressure build-up and the major gas paths were assessed using numerical two-phase flow models on the repository scale. Furthermore, both the gas and water fluxes as a function of time and gas generation rate can be evaluated by varying the physical properties of the materials and hence their transport capacity. Results showed that by introducing seals with higher gas permeability, the modelled gas flow is largely limited to the access tunnels and the excavation disturbed zone for the case of a very low permeability host rock. The bulk of the gas flows through the repository seal and the adjacent EDZ into the tunnel system. In addition to the demonstration of the gas flow in the seal and access tunnel system by numerical models, laboratory results confirm the high gas transport capacity of the sand/bentonite mixtures. In a next step a multi year demonstration scale experiment (GAST) at the Grimsel Test Site is envisioned.

Demonstration of Gas-Permeable Seals for Radioactive Waste Repositories: Laboratory and In-Situ Experiments

2011 ◽  
Author(s):  
Joerg Rueedi ◽  
Paul Marschall
Keyword(s):  
Radioactive Waste ◽  
Large Scale ◽  
Gas Flow ◽  
Two Phase Flow ◽  
Gas Transport ◽  
Phase Flow ◽  
Transport Capacity ◽  
Underground Structures ◽  
Two Phase ◽  
National Cooperative

In low/intermediate-level waste (L/ILW) repositories, anaerobic corrosion of metals and degradation of organic materials produce hydrogen, methane, and carbon dioxide. Gas migration in a L/ILW repository is one of the processes evaluated in the safety assessment of deep geological disposal in low-permeability formations, in particular with respect to the development of gas pressures in the repository caverns which could negatively affect the host rock or the engineered barrier system (EBS). In order to restrict build-up of gas overpressures in the emplacement caverns, Nagra (National Cooperative for the Disposal of Radioactive Waste, Switzerland) has proposed design options aimed at increasing the gas transport capacity of the backfilled underground structures, compromising neither the low hydraulic conductivity nor the radionuclide retention capacity of the EBS (Nagra, 2008). They involve specially designed backfill and sealing materials such as high porosity mortars as backfill materials for the emplacement caverns and sand/bentonite (S/B) mixtures with a bentonite content of 20% to 30% for the seals themselves and for backfilling other underground structures. These increased gas permeability materials can supplement the gas flow that is expected to occur through the excavation damaged zone (EDZ) and avoid the creation of overpressures. Preliminary experimental studies have confirmed the gas transport capacity of the S/B mixtures and demonstrated the ability to design mixtures with specific target permeabilities for water and gas flow (Nagra, 2008). Two-phase flow modelling studies have shown that the gas transport capacity of seals is largely dependent on their permeability and length. More detailed models of sealing elements show a rather complex history of seal saturation during the early saturation phase and the later gas escape phase (Gaus et al., 2010). Note, however, that current modelling approaches are based on parameters and conceptual understanding of small-scale laboratory experiments. Two large(r) scale experiments which aim at validating and, if necessary, improving current conceptual models for the resaturation and gas invasion processes into S/B seals and the determination of up-scaled gas / water permeabilities of S/B seals (i.e. two-phase flow parameters for large-scale models) have been initiated and will be highlighted in the paper. The first one, a mock-up experiment, was set up in 2010 as part of the EU 7th FP project FORGE, aiming at demonstrating seal performance on an intermediate (decimetre scale). The second one is a large-scale experiment (metre-scale), the Gas-Permeable Seal Test (GAST), which was also initiated in 2010 at the Grimsel Test Site (GTS). For GAST, a seal will be emplaced at the GTS to demonstrate the effective functioning of gas-permeable seals on a realistic scale and with realistic boundary conditions (‘proof of concept’).

scholarly journals Characterization of Gas Transport in Low-Permeability Media: Two-Phase Flow Analysis of an In-Situ Experiment

2013 ◽  
Author(s):  
Pierre Gerard ◽  
Jean-Pol Radu ◽  
Rémi de La Vaissière ◽  
Jean Talandier ◽  
Robert Charlier ◽  
...  
Keyword(s):  
Two Phase Flow ◽  
Gas Transport ◽  
Flow Analysis ◽  
Low Permeability ◽  
Phase Flow ◽  
In Situ Experiment ◽  
Two Phase

Gas Flow Through Water-Saturated Shear Zones: Field and Laboratory Experiments and Their Interpretation

1997 ◽  
Vol 506 ◽  
Author(s):  
P. Marschall ◽  
J. Croisé ◽  
U. Fischer ◽  
R. Senger ◽  
E. Wyss
Keyword(s):  
Shear Zone ◽  
Gas Flow ◽  
Two Phase Flow ◽  
Gas Permeability ◽  
Numerical Models ◽  
Shear Zones ◽  
Parameter Estimates ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Processes

ABSTRACTGas threshold pressure tests and gas tracer tests have been performed at the Grimsel Test Site to study two-phase flow processes in a shear zone. In addition, capillary pressure and gas permeability measurements were carried out in the laboratory on drillcore samples. The laboratory investigations were complemented by assessing the pore structure of the shear zone material. The interpretation of the field tests with numerical models indicated that the structural and two-phase flow parameters to be determined are highly correlated with one another and, consequently, the parameter estimates can be rather uncertain. The joint interpretation of field and laboratory results, however, led to a more stringent description of the two-phase flow processes, expressed by a better overall fit of the test data and smaller uncertainty ranges of the estimated parameters. The results showed that the gas mobility in the shear zone was very high even at high water saturation and gas flow was limited to the narrow zones of brittle deformation along the shear zone.

scholarly journals Klinkenberg effect for gas permeability and its comparison to water permeability for porous sedimentary rocks

2006 ◽  
Vol 3 (4) ◽  
pp. 1315-1338 ◽  
Author(s):  
W. Tanikawa ◽  
T. Shimamoto
Keyword(s):  
Pore Pressure ◽  
Water Permeability ◽  
Gas Flow ◽  
Gas Permeability ◽  
Slip Flow ◽  
Sedimentary Rocks ◽  
Water Film ◽  
Klinkenberg Effect ◽  
Two Phase ◽  
Order Of Magnitude

Abstract. The difference between gas and water permeabilities is significant not only for solving gas-water two-phase flow problems, but also for quick measurements of permeability using gas as pore fluid. We have measured intrinsic permeability of sedimentary rocks from the Western Foothills of Taiwan, using nitrogen gas and distilled water as pore fluids, during several effective-pressure cycling tests at room temperature. The observed difference in gas and water permeabilities has been analyzed in view of the Klinkenberg effect. This effect is due to slip flow of gas at pore walls which enhances gas flow when pore sizes are very small. Experimental results show (1) that gas permeability is larger than water permeability by several times to one order of magnitude, (2) that gas permeability increases with increasing pore pressure, and (3) that water permeability slightly increases with increasing pore-pressure gradient across the specimen. The results (1) and (2) can be explained by Klinkenberg effect quantitatively with an empirical power law for Klinkenberg constant. Thus water permeability can be estimated from gas permeability. The Klinkenberg effect is important when permeability is lower than 10−18 m2 and at low differential pore pressures, and its correction is essential for estimating water permeability from the measurement of gas permeability. A simple Bingham-flow model of pore water can explain the overall trend of the result (3) above. More sophisticated models with a pore-size distribution and with realistic rheology of water film is needed to account for the observed deviation from Darcy's law.

scholarly journals Gas transport in granular compacted bentonite: coupled hydro-mechanical interactions and microstructural features

2020 ◽  
Vol 195 ◽  
pp. 04008
Author(s):  
Laura Gonzalez-Blanco ◽  
Enrique Romero ◽  
Paul Marschall
Keyword(s):  
Gas Flow ◽  
Gas Transport ◽  
Initial Conditions ◽  
Gas Injection ◽  
Transport Processes ◽  
Dry Density ◽  
Gas Migration ◽  
Micro Computed Tomography ◽  
Saturation State ◽  
Compacted Bentonite

The initial conditions (dry density and saturation state), the stress state and its history, and the deformation undergone during gas migration, affect the gas transport processes in granular compacted bentonite. Additionally, the sample microstructure set on compaction has a significant influence since gas tends to flow through preferential pathways. This experimental study intends to shed light on the gas transport and their coupled hydro-mechanical interactions with particular emphasis in the changes of the pore and pathway network. Controlled volume-rate gas injection followed by shut-off and dissipation stages have been performed under oedometer conditions. The microstructure of the samples has been characterised with three different techniques before and after the gas injection tests: Mercury Intrusion Porosimetry (MIP), Field-Emission Scanning Electron Microscopy (FESEM) and X-ray Micro-Computed Tomography (μ-CT). The results show a coupling of the deformational behaviour during the gas flow, revealing an expansion of the samples upon the development of gas pathways, which have been detected with the microstructural techniques. The opening of these pressure-dependent and connected pathways plays a major role in gas migration.

scholarly journals Gas migration experiments in bentonite: implications for numerical modelling

2012 ◽  
Vol 76 (8) ◽  
pp. 3279-3292 ◽  
Author(s):  
C. C. Graham ◽  
J. F. Harrington ◽  
R. J. Cuss ◽  
P. Sellin
Keyword(s):  
Gas Phase ◽  
Gas Permeability ◽  
Radioactive Decay ◽  
Phase Flow ◽  
Gas Migration ◽  
Full Understanding ◽  
Two Phase ◽  
Key Features ◽  
Development And Validation ◽  
Further Development

AbstractIn the Swedish KBS-3 repository concept, there is potential for gas to be generated from corrosion of ferrous materials under anoxic conditions, combined with the radioactive decay of the waste and radiolysis of water. A full understanding of the probable behaviour of this gas phase within the engineered barrier system (EBS) is therefore required for performance assessment. We demonstrate key features from gas transport experiments on pre-compacted Mx80 bentonite, under laboratory and field conditions, and discuss their implications in terms of a conceptual model for gas migration behaviour. On both scales, major gas entry is seen to occur close to the sum of the porewater and swelling pressures of the bentonite. In addition, gas pressure at breakthrough is profoundly sensitive to the number and location of available sinks for gas escape. Observations of breakthrough can be explained by the creation of dilatational pathways, resulting in localized changes in the monitored porewater pressures and total stresses. These pathways are highly unstable, evolving spatially and temporally, and must consequently influence the gas permeability as their distribution/geometry develops.Such observations are poorly embodied by conventional concepts of two-phase flow, which do not fully represent the key processes involved. Although dilatancy based models provide a better description of these processes, the paucity of data limits further development and validation of these models at present.

Viscous and Knudsen gas flow through dry porous cometary analogue material

2021 ◽  
Author(s):  
M Schweighart ◽  
W Macher ◽  
G Kargl ◽  
B Gundlach ◽  
H L Capelo
Keyword(s):  
Diffusion Coefficient ◽  
Gas Flow ◽  
Gas Transport ◽  
Gas Permeability ◽  
Knudsen Diffusion ◽  
Flow Measurements ◽  
Knudsen Flow ◽  
Internal Sources ◽  
Trapped Gas ◽  
Knudsen Gas

Abstract According to current theories of the formation of stellar systems, comets belong to the oldest and most pristine class of bodies to be found around a star. When approaching the Sun, the nucleus shows increasing activity and a pressure increase inside the material causes sublimated and trapped gas molecules to stream away from their regions of origin towards the surface. The present work studies two essential mechanisms of gas transport through a porous layer, namely the Darcy and the Knudsen flow. Gas flow measurements are performed in the laboratory with several analogue materials, which are mimicking dry cometary surface properties. In this first series of measurements, the aim was to separate gas transport properties from internal sources like local sublimation or release of trapped gases. Therefore, only dry granular materials were used and maintaining a low temperature environment was unnecessary. The gas permeability and the Knudsen diffusion coefficient of the sample materials are obtained, thereby representing the relative importance of the respective flow mechanism. The experiments performed with air at a stable room temperature show that the grain size distribution and the packing density of the sample play a major role for the permeability of the sample. The larger the grains, the bigger the permeability and the Knudsen diffusion coefficient. From the latter we estimated effective pore diameters. Finally, we explain how these parameters can be adapted to obtain the gas flow properties of the investigated analogue materials under the conditions to be expected on the comet.

Impact of dynamic slippage on productivity of shale reservoirs

2015 ◽  
Vol 12 (5) ◽  
pp. 443-451 ◽  
Author(s):  
Samarth D. Patwardhan ◽  
Niranjan Bhore ◽  
Anirban Banerjee ◽  
G. Suresh Kumar
Keyword(s):  
Gas Flow ◽  
Gas Permeability ◽  
Descriptive Analysis ◽  
Low Permeability ◽  
Flow Mechanism ◽  
Computational Environment ◽  
Reservoir Models ◽  
Permeability Changes ◽  
Flow Mechanisms ◽  
Shale Reservoirs

Ultra low permeability rocks such as shales exhibit complex fracture networks which must be discretely characterized in our reservoir models to evaluate stimulation designs and completion strategies properly. The pressure (Darcy’s law) and composition driven (Fick’s law) flow mechanisms when combined result in composition, pressure and saturationdependent slippage factor. The approach used in this study is to utilize pressure-dependent transmissibility multipliers to incorporate apparent gas-permeability changes resulting from multi-mechanism flows in commercial simulators. This work further expounds on the effectiveness of the theory by presenting a descriptive analysis between two commercially utilized numerical simulators. The applicability of dynamic slippage as an effective flow mechanism governing gas flow mechanisms within the computational environment of two different simulators is attempted in this analysis. Results indicate that slippage-governed flow in modelling shale reservoirs should not be ignored.

Experimental Study of Gas Permeabilities and Breakthrough Pressures in Clays

1996 ◽  
Vol 465 ◽  
Author(s):  
K. Tanai ◽  
T. Kanno ◽  
C. Gallé
Keyword(s):  
Two Phase Flow ◽  
Gas Permeability ◽  
Phase Flow ◽  
Gas Migration ◽  
Two Phase ◽  
Saturation State ◽  
Flow Models ◽  
Water Permeation ◽  
Compacted Bentonite ◽  
Breakthrough Pressure

ABSTRACTIn this study, gas migration experiments in unsaturated and saturated states were carried out to clarify the fundamental gas migration characteristics in compacted bentonite to be used for the geological disposal of high-level radioactive waste. In unsaturated experiments, the gas permeability for Japanese bentonite (Kunigel VI) as a function of degree of saturation was measured to examine the applicability of conventional two-phase flow models to compacted bentonite. The intrinsic permeability obtained in this study was about five orders of magnitude larger than that obtained in water permeation tests with the same density. The difference seems to originate from the change of pore structure due to the swelling phenomenon of the bentonite. Since these effects have not been evaluated quantitatively yet, various relative gas permeability functions of conventional two-phase flow models were applied as a first approximation.Saturated experiments designed to simulate the gas migration phenomenon in a repository for the waste were carried out to obtain relationship between breakthrough and swelling pressures using Kunigel VI and French Fo-Ca clay in saturation state. The reproducibility of the breakthrough pressure was also examined for Kunigel VI bentonite. The breakthrough pressure was almost the same as swelling pressure irrespective of the type of clay. As to the reproducibility of breakthrough pressure, it was observed that first and second breakthrough pressures were almost the same for Kunigel VI specimens with the dry densities of 1.7 and 1.8 g/cm3.

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