Catherine A. Davy

Effect of gas pressure on the sealing efficiency of compacted bentonite-sand plugs.

This research relates to the assessment of the sealing ability of bentonite/sand plugs when swollen in presence of both water and gas pressures, in the context of deep underground radioactive waste storage. Compacted bentonite/sand plugs are placed inside a constant volume cell, and subjected to swelling in presence of both water and gas: swelling kinetics and effective swelling pressure Pswell are identified. Secondly, the gas breakthrough (GB) characteristics of swollen plugs are assessed to determine their ability for gas migration, which has to be minimal for sealing radioactive waste repositories. We show that gas pressure Pg does not affect significantly Pswell until a threshold Pg>2MPa. When swelling occurs inside a tube with a smooth (turned) inner surface, continuous GB occurs when Pg is equivalent to the effective Pswell (obtained without gas pressure, at 7.32MPa±0.11). When the plug swells inside a grooved tube, continuous GB does not occur up to Pg≥10.5MPa: smooth interfaces are a preferential gas migration pathway rather than grooved interfaces, and rather than water-saturated bentonite-sand plugs. With smooth tubes, in presence of Pg≥2MPa, although Pswell is not affected, gas passes through the sample at significantly lower values than Pswell, due to partial sample saturation. It is concluded that GB pressure is a more accurate indicator of partial sample saturation than swelling pressure Pswell alone.

Gas Transfer Through Clay Barriers

Gas transport through clay-rocks can occur by different processes that can be basically subdivided into pressure-driven flow of a bulk gas phase and transport of dissolved gas either by molecular diffusion or advective water flow (Figure 1, Marschall et al., 2005). The relative importance of these transport mechanisms depends on the boundary conditions and the scale of the system. Pressure-driven volume flow (“Darcy flow”) of gas is the most efficient transport mechanism. It requires, however, pressure gradients that are sufficiently large to overcome capillary forces in the typically water-saturated rocks (purely gas-saturated argillaceous rocks are not considered in the present context). These pressure gradients may form as a consequence of the gravity field (buoyancy, compaction) or by gas generation processes (thermogenic, microbial, radiolytic). Dissolved gas may be transported by water flow along a hydraulic gradient. This process is not affected by capillary forces but constrained by the solubility of the gas. It has much lower transport efficiency than bulk gas phase flow. Molecular diffusion of dissolved gas, finally, is occurring essentially without constraints, ubiquitously and perpetually. Effective diffusion distances are, however, proportional to the square root of time, which limits the relevance of this transport process to the range of tens to hundreds of metres on a geological time scale (millions of years). 2 Process understanding and the quantification of the controlling parameters, like diffusion coefficients, capillary gas breakthrough pressures and effective gas permeability coefficients, is of great importance for up-scaling purposes in different research disciplines and applications. During the past decades, gas migration through fully water-saturated geological clay-rich barriers has been investigated extensively (Thomas et al., 1968, Pusch and Forsberg, 1983; Horseman et al., 1999; Galle, 2000; Hildenbrand et al., 2002; Marschall et al., 2005; Davy et al., 2009; Harrington et al., 2009, 2012a, 2014). All of these studies aimed at the analysis of experimental data determined for different materials (rocks of different lithotype, composition, compaction state) and pressure/temperature conditions. The clay-rocks investigated in these studies, ranged from unconsolidated to indurated clays and shales, all characterised by small pores (2-100 nm) and very low hydraulic conductivity (K < 10-12 m·s-1) or permeability coefficients (k < 10-19 m²). Studies concerning radioactive waste disposal include investigations of both the natural host rock formation and synthetic/engineered backfill material at a depth of a few hundred meters (IAEA, 2003, 2009). Within a geological disposal facility, hydrogen is generated by anaerobic corrosion of metals and through radiolysis of water (Rodwell et al., 1999; Yu and Weetjens, 2009). Additionally, methane and carbon dioxide are generated by microbial degradation of organic wastes (Rodwell et al., 1999; Ortiz et al., 2002; Johnson, 2006; Yu and Weetjens, 2009). The focus of carbon capture and storage (CCS) studies is on the analysis of the long-term sealing efficiency of lithologies above depleted reservoirs or saline aquifers, typically at larger depths (hundreds to thousands of meters). During the last decade, several studies were published on the sealing integrity of clay-rocks to carbon dioxide (Hildenbrand et al., 2004; Li et al., 2005; Hangx et al., 2009; Harrington et al., 2009; Skurtveit et al., 2012; Amann-Hildenbrand et al., 2013). In the context of petroleum system analysis, a significant volume of research has been undertaken regarding gas/oil expulsion mechanisms from sources rocks during burial history (Tissot & Pellet, 1971; Appold & Nunn, 2002), secondary migration (Luo et al., 2008) and the capillary sealing capacity of caprocks overlying natural gas accumulations (Berg, 1975; Schowalter, 1979; Krooss, 1992; Schlomer and Kross, 2004; Li et al., 2005; Berne et al., 2010). Recently, more attention has been paid to investigations of the transport efficiency of shales in the context of oil/gas shale production (Bustin et al., 2008; Eseme et al., 2012; Amann-Hildenbrand et al., 2012; Ghanizadeh et al., 2013, 2014). Analysis of the migration mechanisms within partly unlithified strata becomes important when explaining the 3 origin of overpressure zones, sub-seafloor gas domes and gas seepages (Hovland & Judd, 1988; Boudreau, 2012). The conduction of experiments and data evaluation/interpretation requires a profound process understanding and a high level of experience. The acquisition and preparation of adequate samples for laboratory experiments usually constitutes a major challenge and may have serious impact on the representativeness of the experimental results. Information on the success/failure rate of the sample preparation procedure should therefore be provided. Sample specimens “surviving” this procedure are subjected to various experimental protocols to derive information on their gas transport properties. The present overview first presents the theoretical background of gas diffusion and advective flow, each followed by a literature review (sections 2 and 3). Different experimental methods are described in sections 4.1 and 4.2. Details are provided on selected experiments performed at the Belgian Nuclear Research Centre (SCK-CEN, Belgium), Ecole Centrale de Lille (France), British Geological Survey (UK), and at RWTH-Aachen University (Germany) (section 4.3). Experimental data are discussed with respect to different petrophysical parameters outlined above: i) gas diffusion, ii) evolution of gas breakthrough, iii) dilation-controlled flow, and iv) effective gas permeability after breakthrough. These experiments were conducted under different pressure and temperature conditions, depending on sample type, burial depth and research focus (e.g. radioactive waste disposal, natural gas exploration, or carbon dioxide storage). The interpretation of the experimental results can be difficult and sometimes a clear discrimination between different mechanisms (and the controlling parameters) is not possible. This holds, for instance, for gas breakthrough experiments where the observed transport can be interpreted as intermittent, continuous, capillary- or dilation-controlled flow. Also, low gas flow rates through samples on the length-scale of centimetres can be equally explained by effective two-phase flow or diffusion of dissolved gas.

Water Retention and Gas Migration of Two High-Performance Concretes after Damage

In the context of long-term repository of high-level and long-lived nuclear waste, authors investigate different concrete properties related to fluid transport, in order to determine which is able to detect damage earliest. To this purpose, different protocols are tested, which impose progressive damage to two different Andra high-performance concretes (HPCs), based on pure portland (CEMI) or composed cement (CEMV)-type cements. The properties investigated are pore size distributions and porosity [by mercury intrusion porosimetry (MIP)], water retention curves, relative gas permeability, and gas migration properties (gas breakthrough pressure). Gas breakthrough pressure (GBP) is assessed rather than gas entry, by accurately measuring gas presence on the downstream side of a confined sample subjected to slowly increasing gas pressure on its upstream side. From MIP data, authors show that CEMI concrete has smaller porosity but greater pore sizes than CEMV. For CEMI concrete, all damage procedures significantly affect water retention curves and gas breakthrough pressures, yet they have no effect upon the relationship between relative gas permeability and water saturation. For CEMV concrete subjected to low damage levels, gas breakthrough measurements detect damage, whereas water retention and gas permeability do not.

Gas migration through COx claystone and implications for self-healing

The originality of this contribution is to evidence the nature of progressive passage through initially macro-cracked and water-saturated (i.e. self-sealed) COx claystone of given thickness (9.5 to 30mm), by using a mix of gases detected on the sample downstream side by a mass spectrometer accurate to 2-5ppm. We show that gas passage occurs first, by discontinuous capillary passage, i.e. snap off , followed by continuous breakthrough (i.e. permeation) whether the upstream pressure value is increased or not. Upstream gas pressure kinetics are key to observing gas passage.

Characterization of transport and water retention properties of damaged Callovo-Oxfordian claystone

Abstract In the context of the underground storage of radioactive waste, the aim of this experimental study is to characterize the effect of damage on transport and water retention properties of Callovo-Oxfordian (COx) argillite. The originality of the study is to simultaneously investigate the pore-size distribution, water retention, the dry, effective and relative gas permeability, and the gas breakthrough pressure (GBP) of damaged COx argillite. These different properties are all relevant to characterizing the fluid transport ability of COx argillite. Results show that the damage has a significant impact on the properties of the COx argillite. It induces a decrease in its water retention capacity and GBP, and it increases its gas permeability and apparent porosity available to water owing to the creation of micro-cracks. Another objective is to show which of these properties is the most suitable to detect early damage states in COx argillite, with a potential use being to identify them in situ. GBP appears to be the best ‘detector’ of damage because of its sensitivity to damage even under high confinement pressures. Gas permeability could be a good indicator of damage, as it increases significantly (one or several orders of magnitude) after the damage. Finally, the water permeability curve is a poor indicator of COx argillite damage.

Analyse micromécanique du couplage perméabilité-contrainte d'une argilite fracturée

ABSTRACT A micromechanics-based analysis is developed throughout this paper in order to study stress loading/permeability couplings in cracked porous media. Fracture closure phenomena under macrocopic compressive stresses are addressed for a fracture modelled as an arrangement of parallel cracks. This first multiscale step emphasizes the key role of the cracks connectedness on the macroscopic permeability evolution with respect to the applied confining pressure.

A multiaxial failure criterion for a brittle orthotropic composite

Abstract A multiaxial failure criterion is validated for a brittle orthotropic composite, namely a tridirectional carbon–carbon composite (3D CC). It is aimed at designers of 3D CC structures. The composite is subjected to a triaxial strain state along its reinforcement axes, which is representative of its in-service loading conditions. A multiaxial test rig is designed in order to reproduce this required strain state. Its mock-up dimensions are optimised by combining Taguchi experimental strategy with tridimensional finite element modelling. Comparison of experimental results with numerical modelling shows that failure occurs due to the required strain state; failure is brittle and corresponds to simultaneous breakage of carbon yarns along the three reinforcement axes. Subsequently, the experiment validates a failure criterion, which assumes linear coupling between the three principal strains.

Effect of a machining-induced defect on the tensile strength of a 3D composite material

This paper investigates the effect of finite specimen size upon the tensile failure of a tridirectional carbon-carbon composite along each reinforcement axis. Asymmetry in the position of load-bearing axial yarns across the cross-section is generated randomly by machining. This yields parasitic bending of the specimen, and thereby premature failure of the yarns subjected to the maximum bending stress. However, bending effects become negligible at final failure. Additionally, the composite failure strength σF is determined from the cross-sectional area of the actually load-bearing axial yarns, using both symmetrical and asymmetrical specimens. Results are in good agreement with previous work, and we show that the variability of σF is small.

Assessment of Cancellous Bone Architecture after Implantation of an Injectable Bone Substitute

This paper is aimed at describing the evolution of newly formed bone dens ity and micro- architecture after implantation of an Injectable Bone Substitute ( IBS). Cancellous bone growth (rabbit) is described as dependent of IBS formulation parameters. Pos sible improvements to the IBS formulation are finally discussed.