Savage Earth Associates Ltd

Queens Park, United Kingdom

Savage Earth Associates Ltd

Queens Park, United Kingdom
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Wilson J.C.,Quintessa Ltd. | Benbow S.J.,Quintessa Ltd. | Metcalfe R.,Quintessa Ltd. | Savage D.,Savage Earth Associates Ltd. | And 2 more authors.
Energy Procedia | Year: 2011

Evidence from experimental investigations, natural analogue studies and demonstration projects support the assertion that significant quantities of CO2 will not leak from a properly selected and managed CO2 storage complex. However, site operators must nevertheless justify the expectation of secure CO2 containment to regulators and other stakeholders. Secure containment will depend upon well seal stability inter alia. This paper presents fully-coupled geochemical models of well seal stability, focusing on cement degradation. The models have been developed with the ambition of reducing the simulated chemical system to the simplest possible configuration that is able to reproduce key features from experimental and field observations. Simplifying the models in this way makes them suitable for inclusion in larger scale system-level models of the storage system, where the use of highly-detailed chemical models can be impractical. Two sets of models were constructed in order to simulate: (1) a cement carbonation experiment (9 days in duration); and (2) field observations of cement degradation from the 'SACROC' site (30 years of reaction time). Although some model input parameters are uncertain, the experimental system was successfully simulated and the model was subsequently 'up-scaled' and applied to the SACROC core data. Over longer time-scales (102 - 103 years), the effects of parameter and thermodynamic data uncertainties on model output are more significant than at shorter time-scales. Although further work is required to bound the effects of these uncertainties, this paper illustrates the practicality of constructing simplified fully-coupled models to explore the significance of uncertainties by carrying out sensitivity calculations. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd.

Wilson J.C.,Quintessa Ltd. | Benbow S.,Quintessa Ltd. | Sasamoto H.,Japan Atomic Energy Agency | Savage D.,Savage Earth Associates Ltd. | Watson C.,Quintessa Ltd.
Applied Geochemistry | Year: 2015

Engineered barrier system (EBS) designs for the geological disposal of high-level radioactive waste often include a bentonite buffer, the primary function of which is to protect metal waste containers or overpacks from mechanical shearing. The buffer also acts as a barrier to solute transport. One potentially deleterious process that may occur in the buffer is the alteration of swelling clay to iron-rich minerals, some of which have a limited capacity to swell. There is a dearth of relevant natural analogues of iron-bentonite interactions, and experimental data do not provide an unequivocal indication of the conditions that will promote non-swelling clay minerals (such as berthierine) to form rather than swelling clays (such as iron-rich saponite). In addition, many of the previously-published reactive transport models of iron-bentonite or iron-claystone interfaces have not considered how evolution of mineral-fluid equilibria in the bentonite buffer could affect the nature and rate of steel corrosion. In this study, new thermodynamic models of iron-rich clay minerals are presented which suggest that the activities of major ions, especially Fe2+, Fe3+, Al3+, H+ and SiO2(aq), act as key controls on the relative stabilities of iron-rich clay minerals. In particular, they suggest that Fe-saponite is stabilised under low fO2(g) conditions when SiO2(aq) activities are buffered by quartz or more soluble silica polymorphs (e.g. chalcedony). Iron-rich 1:1 clay minerals, such as berthierine, tend to be stabilised in fluids that are quartz under-saturated. The thermodynamic models were used to inform the development of three fully-coupled reactive transport models of a steel-bentonite interface: (1) steel corrosion reaction applied on a boundary directly in contact with bentonite at a fixed rate; (2) steel corrosion reaction applied on a boundary directly in contact with bentonite at a diffusion-limited rate; and (3) a 'corrosion cell' representation with a fixed steel corrosion rate. The extent and nature of the alteration predicted by the models was found to be sensitive to model conceptualisation. The corrosion cell assumption leads to steel corrosion products including magnetite and siderite and the alteration of primary minerals to berthierine (~2. cm thick layer forming over 100. 000. years, with partial loss of montmorillonite to a depth of ~9. cm). In contrast, the boundary corrosion assumption with a fixed steel corrosion rate leads to quicker alteration to iron-rich clay minerals. If the diffusion-limited corrosion rate assumption is made, the steel corrosion rate varies over time as the bentonite porewater composition evolves, and the spatial extent of alteration is much more limited (millimetre scale). Although much progress has been made, a number of uncertainties associated with modelling bentonite evolution remain, especially with regard to ion transport through smectite interlayers and the potential for complex couplings between chemical and physical processes. © 2015 Elsevier Ltd.

Savage D.,Savage Earth Associates Ltd | Liu J.,Swedish Radiation Safety Authority
Applied Clay Science | Year: 2015

The performance of bentonite used in geological repositories for radioactive waste may be impaired by long-term clay transformations to non-swelling minerals. Intrinsic to alteration processes is the role of water/clay ratio, defined in a bentonite-pore fluid system by (the inverse of) porosity. Water/(water+clay) mass ratios are low for both 'total' (≤0.25) and 'free' (≤0.05) porosities in compacted bentonite at the dry density envisaged for waste package buffers (≥1500kgm-3). A survey of laboratory experimental studies of clay alteration has shown that they have tended to focus on systems with dispersed clays at high water/(water+clay) mass ratios (≥0.75) because of experimental practicalities and a desire to accelerate reactions.New thermodynamic calculations have illustrated that the fluid/clay ratio can have an important impact not only upon the magnitude of alteration, but also upon the nature of the reaction path. Reaction of a pure Na-montmorillonite with cement pore fluids, a Fe-rich fluid and a KCl solution to attempt to simulate reaction of clay with cement/concrete, iron/steel, and potassium-rich fluids (to investigate the smectite to illite reaction path), respectively has shown that under fluid-dominated conditions (high water/clay ratio), clay alteration consisted of C-S-H solids, low-Si zeolites, and chlorite. Under clay-dominated conditions (low water/clay ratios), alteration typically consisted of high-Si zeolites, feldspar and Mg-corrensite. Consequently, it is of key importance that the most relevant water/clay ratio ('porosity') is used not only in geochemical calculations, but also in experimental systems. © 2015 Elsevier B.V.

Watson C.,Quintessa Ltd. | Wilson J.,Quintessa Ltd. | Savage D.,Savage Earth Associates Ltd | Benbow S.,Quintessa Ltd. | Norris S.,Radioactive Waste Management Ltd
Applied Clay Science | Year: 2016

Cement and concrete will be used as fracture grouts, shotcretes, tunnel seals, and as matrices for waste encapsulation in many radioactive waste geological disposal facility concepts. Alteration of the disposal facility host rock and/or swelling clay in waste package buffers and tunnel backfills by alkaline solutions leached from cement/concrete may have implications for system performance. The Maqarin natural analogue has received considerable attention over the last twenty-five years as it represents some of the conditions of interest for a cementitious radioactive waste disposal system; as naturally-occurring high pH waters have been transported through fractures passing through clayey marls, causing mineralogical alteration in the wall-rock and sealing of the fractures as minerals precipitate. In this study, reactive-transport simulations were constructed of the mineral-fluid interactions at the Maqarin site with a particular emphasis on mineral formation in fractures, including armouring of surfaces, and how this can be simulated in geological environments that may host a radioactive waste disposal facility. The evolution of fractures is important as they may act as pathways for water and contaminant transport.In a 'base case' reactive-transport model, a pseudo 2-D approach was used to simulate flow along a fracture and diffusion of solutes into the rock matrix. The model predicts that ettringite, thaumasite and C-S-H (jennite and tobermorite), dominate the fracture-filling minerals, whereas alteration products forming at the expense of the primary silicates in the rock matrix include scolecite (Ca-zeolite), ettringite, C-S-H and small amounts of sepiolite, in broad agreement with observations of the rock present at Maqarin. A more realistic conceptual model of mineral precipitation in the fracture has also been developed, whereby minerals precipitated on the fracture walls in an 'armoured' layer rather than uniformly throughout the fracture cells, thus impeding diffusion into the rock matrix. In this model only jennite and calcite precipitated in the fracture, eventually filling it.The modelling carried out here confirms that, where alkaline fluids derived from cement degradation come into contact with rocks saturated with neutral pH, bicarbonate-rich groundwaters, sealing of fractures may occur. However, fracture sealing is complex and is dependent upon a range of factors such as: rock composition (mineralogy, grain size, porosity); groundwater composition and flow rates; and mineral reaction kinetics inter alia. Although the precise timing of fracture sealing at Maqarin is unknown, the consideration of a number of uncertainties in reactive-transport modelling conducted here would suggest that sealing is moderately rapid (of the order of hundreds of years). The geochemical modelling approach used here could be applied to potential fractured host rocks for radioactive waste disposal facilities, in order to elucidate the evolution of water flow and contaminant transport pathways. © 2015 Elsevier B.V.

Savage D.,Quintessa Ltd. | Savage D.,Savage Earth Associates Ltd
Mineralogical Magazine | Year: 2011

Cement and concrete will be used as fracture grouts, shotcrete, tunnel and borehole seals, and as matrices for waste encapsulation inter alia in geological repositories for radioactive wastes. Alteration of the host rock and/or swelling clay in waste package buffers and tunnel backfills by hyperalkaline solutions from cement/concrete may be deleterious to system performance through changes in the physicochemical properties of these barrier materials. Analogue systems (and timescales) relevant to the understanding of the alkaline disturbed zone include: industrial analogues, such as alkaline flooding of hydrocarbon reservoirs (up to 30 y), cement-aggregate reactions (up to 100 y) and the Tournemire tunnel (up to 125 y); and natural analogues, including the hyperalkaline springs at Maqarin, Jordan (more than 100,000 y), saline, alkaline lakes (more than 1,000,000 y) and certain fracture fillings in granites (more than 1,000,000 y). These systems show that alkaline alteration can be observed for thousands of years over distance scales of hundreds of metres under extreme conditions of hydraulic gradients in fractured rocks (Maqarin), but may be limited to a few centimetres over tens to a hundred years in mudstone (Tournemire). Important reaction mechanisms for retardation of alkaline fluids include: fluid mixing (alkaline oil floods, Maqarin), ion exchange (alkaline oil floods, Tournemire) and kinetic mineral dissolution-precipitation reactions (all systems studied). Qualitative and quantitative kinetic data for mineral reactions are available from cement aggregate reactions and the Searles Lake analogue, respectively. Short-term alteration observed in cement-aggregates is characterized by calcium silicate hydrate (C-S-H) minerals and incipient zeolite formation, whereas evidence from the Tournemire tunnel shows the growth of K-feldspar after relatively short time intervals (tens to a hundred years). There is a tendency for alkaline alteration to result in porosity decreases, but locally, porosity may be enhanced (e.g. near-injection well interactions in alkaline oil floods, or at fracture margins at Maqarin, Jordan). Data from industrial and natural analogues may thus supply some key data for bridging time and space scales between laboratory and in situ experiments on one hand and the requirements for safety assessment on the other. © 2011 Mineralogical Society.

Walke R.,Quintessa Ltd. | Metcalfe R.,Quintessa Ltd. | Limer L.,Quintessa Ltd. | Maul P.,Quintessa Ltd. | And 2 more authors.
Energy Procedia | Year: 2011

This paper reviews the application of a freely accessible on-line database of generic Features, Events and Processes (FEPs), designed to support the analysis of geological CO2 storage systems during performance assessments. The Generic CO2 FEP Database was established by Quintessa in 2004 through international collaboration under the auspices of the Weyburn project. Subsequently, development of the database has continued and its use has become widespread, with over 1000 people having registered to access the database. Most commonly, the database has been used as an audit tool to help build confidence that a systems analysis covers all relevant FEPs and to document transparently those FEPs that are not being considered. In other applications the generic FEP database has been screened to identify relevant FEPs that are then used directly to build conceptual models. As a generic resource, the Generic CO2 FEP Database covers the range of FEPs that might be relevant to assessments, from those associated with the storage formation and cap rock to potential impacts on humans and the environment. The range of applications to date demonstrates its use in support of different scales of assessment for different components of the system. Examples include total-systems models, assessments focusing solely on potential loss of containment from the storage formation and natural analogue studies of potential impacts. Over the past five years the use of the Generic CO2 FEP database has helped to build confidence in assessments relating to long-term geological storage. Additionally, the database represents a knowledge base relating to the potential performance and safety of storage systems. The experience gained from application of the database to date helps to inform the way in which it can be applied in future. The database continues to be developed, based on experience gained in its application. Recently references and links have been updated and a targeted review has revised descriptions and FEPs relating to the marine environment. Further targeted reviews and updates are planned. For the database to continue to structure the latest knowledge and understanding relating to geological storage, on-going feedback from its user base is sought. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd.

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