Abstract A geological disposal facility (GDF) is the widely accepted long-term solution for the management of higher-activity radioactive waste. It consists of an engineered facility constructed in a suitable host rock. The facility is designed to inhibit the release of radioactivity by using a system consisting of engineered and natural barriers. The engineered barriers include the wasteform, used to immobilize the waste, the waste disposal container and any buffer material used to protect the container. The natural barrier includes the rocks in which the facility is constructed. The careful design of this multi-barrier system enables the harmful effects of the radioactivity on humans and biota in the surface environment to be reduced to safe levels. Bentonite is an important buffer material used as a component of a multi-barrier disposal system. For example, compacted bentonite rings and blocks are used to protect the copper container, used for the disposal of spent fuel, in the KBS-3 disposal system. As the bentonite saturates, through contact with groundwater from the host rock, it swells and provides a low hydraulic conductivity barrier, enabling the container to be protected from deleterious processes, such as corrosion. The characteristic swelling behaviour of bentonite is due to the presence of significant quantities of sodium montmorillonite. Recently, there have been detailed in situ experiments designed to understand how bentonite performs under natural conditions. One such experiment is the Buffer–Rock Interaction Experiment (BRIE), performed at the Äspö Hard Rock Laboratory near Oskarshamn in the SE of Sweden. This experiment is designed to further understand the wetting of bentonite from the groundwater flow in a fractured granite host rock. In this paper, the observations from the BRIE are explained using an integrated model that is able to describe the saturation of bentonite emplaced in a heterogeneous fractured rock. It provides a framework to understand the key processes in both the rock and bentonite. The predictive capability of these models was investigated within the context of uncertainties in the data and the consequence for predictions of the wetting of emplaced bentonite. For example, to predict the wetting of emplaced bentonite requires an understanding of the distribution of fracture transmissivity intersecting the bentonite. A consequence of these findings is that the characterization of the fractured rock local to the bentonite is critical to understanding the subsequent wetting profiles. In particular, prediction of the time taken to achieve full saturation of bentonite using a simplified equivalent homogeneous description of the fractured host rock will tend to be too short.

ABSTRACT In recent years, outcrop analogue studies have become a powerful tool in sedimentology for the assessment of reservoirs, both in hydrocarbon and aquifer studies. Data from exploratory drilling campaigns can be augmented significantly by observations on the outcrop of the corresponding stratigraphical interval with the objective to validate the borehole information through direct observation. In addition, through the physical separation of the outcrop area and the subsurface, the increased spatial coverage of a reservoir and its equivalents provides additional information about facies and their changes and thus on reservoir properties. This chapter presents results of a study on the Cretaceous sedimentary aquifers in Saudi Arabia (Wasia–Biyadh–Aruma) in order to better assess the storage volume of fossil ground-water, which is of fundamental importance for the hyper-arid kingdom. Besides the regional 3-D stratigraphic framework, the focus was on measurements of porosity and permeability of approximately 150 samples and the interpretation of reservoir quality in terms of sedimentary facies and its diagenetic overprint. In general, both porosity and permeability are varying on a high level (Biyadh: 1–36% / 2100–6500 mD; Wasia: 3–42% / 2100–6500 mD; Aruma: 1–38% / 10 –6 –0.15 Darcy). Apparently, the storage volume and hydraulics of these regional aquifers are controlled not only by their fracturing but also by their matrix porosity. Permeability varies by about an order of magnitude among samples or between vertical and horizontal permeability within some samples. This variation can be well explained by heterogeneity due to sedimentary facies, for example, cross-bedding and bioturbation. In some areas, the kind of cementation and its intensity have a large effect on the permeability. The data obtained enhance the quality of the hydraulic interpretations of this aquifer system. Spectral gamma-ray logs proved to be useful for a regional correlation and the correlation of aquifers and aquicludes. This is based on the recognition of the major unconformities in the logs but also on the identification of various paleosol horizons, which regularly show high emissions of U and Th radionuclides. Intensive weathering during the Cretaceous is responsible for dominantly kaolinitic clay mineralogy and consequently negligible K emissions.

Abstract Research in Cretaceous shales from West Africa has demonstrated that significant permeability can develop within shales at shallow depths (<100 m), equivalent to a permeability of >1 m day −1 . Much of the variation in permeability is related to the degree of burial metamorphism, with shales that have been altered and that approach the anchizone having the highest permeability and those that are largely unaltered (early diagenetic zone) having the lowest permeability. However, further research targeting largely unaltered shales dominated by smectite clay has shown that the presence of small igneous intrusions can radically alter the hydrogeology. Twenty-four exploratory boreholes were drilled into smectite-dominated shale and nine of these boreholes were targeted to small dolerite intrusions within the shale. The dolerite was intensely fractured at the intrusion edge, with significant zeolite growth along the fracture surfaces. The permeability in the fractured dolerite was the highest measured in any shale borehole, with transmissivities of up to 60 m 2 day −1 measured from pumping tests. Fracturing was less where dolerite was intruded into sandstones, however, and the measured transmissivity was lower (<0.5 m 2 day −1 ). We postulate that the low permeability and high water content of the shale enabled high pressures to develop during intrusion, facilitating the development of fractures along the intrusion contact zone.

Abstract Conceptual models of the fracture networks in shale were evaluated at a site contaminated with chlorinated solvents. Prior borehole testing in eight holes under open hole ambient and pumping conditions identified 14 flow zones (140 m bedrock interval) with zero to five zones per hole. Cross-hole testing showed only a few cross-connections between transmissive fractures. The initial conceptual model thus featured a sparse fracture network with few dominant fractures. Detailed profiles (hydraulic head, rock core volatile organic compounds, groundwater volatile organic compounds from packer and multi-level sampling, cross-hole multi-level monitoring of permanganate injections) were collected from several holes and indicated a well-connected fracture network with many hydraulically active fractures not influenced by open hole cross-connection. This contrasting conceptual model contained numerous well-connected horizontal and vertical fractures that allowed chlorinated solvents to penetrate the upper 50–60 m of bedrock as dense non-aqueous phase liquids, followed by diffusion-driven mass transfer from fractures into the porous rock matrix, such that nearly all the contaminant mass resided as dissolved and sorbed phases, measurable in rock core without cross-contamination during drilling. The difference in the two conceptual models has important implications for source zone and plume attenuation.

Abstract The success of translating a conceptual site model for a karst site into a numerical groundwater model will depend on both the experience of the user and the capabilities and limitations of the selected computer program. Despite its numerous advantages, even MODFLOW – probably the most widely used, tested and verified modelling program currently available – has conceptual limitations that many karst hydrogeologists have to deal with on a routine basis while searching for an equivalent porous medium approach that may work. This includes assigning very high values of hydraulic conductivity to those model cells known, or suspected, to contain highly transmissive conduits, or assigning an unreasonable, very low effective porosity to the model cells with virtual ‘conduits’ to simulate high groundwater velocities. A new version of MODFLOW called MODFLOW-USG (UnStructured Grid) has been developed and released to the public domain. This new version retains full compatibility with previous versions of MODFLOW while taking advantage of unstructured grids and finite volume numerical solutions. It enables hydrogeologists to accurately translate even the most complex conceptual site models in karst into a numerical environment, thus eliminating the need for various surrogate modelling solutions based on an equivalent porous medium approach.

Abstract It is common practice to incorporate deterministic transmissibility multipliers into simulation models of siliciclastic reservoirs to take into account the impact of faults on fluid flow, but this is not common practice in carbonate reservoirs due to the lack of data on fault permeability. Calculation of fault transmissibilities in carbonates is also complicated by the variety of mechanisms active during faulting, associated with their high heterogeneity and increased tendency to react with fluids. Analysis of the main controls on fault-rock formation and permeability from several carbonate-hosted fault zones is used to enhance our ability to predict fault transmissibility. Lithological heterogeneity in a faulted carbonate succession leads to a variety of deformation and/or diagenetic mechanisms, generating several fault-rock types. Although each fault-rock type has widely varying permeabilities, trends can be observed dependent on host lithofacies, juxtaposition and displacement. These trends can be used as preliminary predictive tools when considering fluid flow across carbonate fault zones. Fewer mechanisms occur at lower displacements (<30 m), creating limited fault-rock types with a narrow range of low permeabilities regardless of lithofacies juxtaposition. At increased displacements, more fault-rock types are produced at juxtaposition of different lithofacies, with a wide range of permeabilities.

Abstract: Many siliciclastic reservoirs contain millimetre-scale diagenetic and structural phenomena affecting fluid flow. We identified three major types of small-scale flow barriers in a clastic Rotliegend hydrocarbon reservoir: cataclastic deformation bands; dissolution seams; and bedding-parallel cementation. Deformation bands of various orientations were analysed on resistivity image logs and in core material. They are mainly conjugates, and can be used to validate seismically observable faults and infer subseismic faults. Bedding-parallel dissolution seams are related to compaction and post-date at least one set of deformation bands. Bedding-parallel cementation is accumulated in coarser-grained layers and depends on the amount of clay coatings. Apparent permeability data related to petrographical image interpretation visualizes the impact of flow barriers on reservoir heterogeneity. Transmissibility multiplier calculations indicate the small efficiency of the studied deformation bands on flow properties in the reservoir. Deformation bands reduce the host-rock permeability by a maximum of two orders of magnitude. However, host-rock anisotropies are inferred to reduce the permeability by a maximum of four orders of magnitude. The relative timing of these flow barriers, as well as the assessment of reservoir heterogeneities, are the basis for state-of-the-art reservoir prediction modelling.