Mineralogical and porosity characterisation of potential aquifer and seal units for carbon capture and storage methodologies for the CASSEM Project

This report describes the methods used and data collected in mineralogical and petrological characterisation of potential carbon capture and storage (CCS) aquifer and seal units in Yorkshire-Lincolnshire-Nottinghamshire and the Forth area of eastern central Scotland. It forms part of CASSEM work package 1 (WP1)

Results of laboratory carbonation experiments on Nirex Reference Vault Backfill cement

Some repository concepts envisage the use of large quantities of cementitious materials – both for repository construction and as a buffer/backfill. However, some wastes placed within a subsurface repository will contain a significant amount of organic material that may degrade to produce carbon dioxide. This will react with cement buffer/backfill to produce carbonate minerals such as calcite, which will reduce the ability of the buffer/backfill to maintain highly alkaline conditions and as a consequence its ability to limit radionuclide migration. The reaction may also alter the physical properties of the buffer/backfill. The work involved in this study investigates these processes through elevated pressure laboratory experiments conducted at a range of likely future in situ repository conditions. These will provide information on the reactions that occur, with results serving as examples with which to test predictive modelling codes. This report details a series of batch experiments to study carbonation of Nirex Reference Vault Backfil (NRVB) cement. Thirty-two static batch experiments were pressurised with either CO2, or with N2 for ‘nonreacting’ comparison tests at 20°C or 40°C, and 40 or 80 bar. Twenty-six of these were left to react for durations of between 10-40 days, with six more left to react for a year. The aim of them was to help investigate mineralogical and fluid chemical changes due to the diffusional ingress of CO2 into unconfined NRVB samples measuring 2.5 cm in diameter and 5 cm long. All the cement samples showed rapid reaction with CO2, manifested by a colour change from grey to light brown. Petrographic analysis of the reacted cement revealed that this colour change reflected the breakdown and dissolution of primary calcium ferrite and calcium alumina-ferrite (CAF) cement clinker phases (e.g. brownmillerite, Ca2(Al,Fe)2O5 to form calcium carbonates and finely-disseminated free ferric oxide (probably hematite, Fe2O3), as a result of reaction with CO2 to give a ‘rusty’ colour. It should be noted that his is not an oxidation reaction as the iron is present as Fe3+ in the original cement phases. The cement blocks remained intact, even after prolonged exposure to CO2-rich fluids. Carbonation was associated with an increase in weight of up to 8.5% during CO2 uptake, though the samples did not change in overall size. There is potential therefore, for carbonation to immobilise 14CO2 if that were present. Free-phase CO2 gave slightly more reaction than dissolved CO2, possibly because of its higher concentration and greater ability to penetrate the samples. In terms of major reactions during carbonation, these were the breakdown of portlandite, calcium silicate hydrate (CSH) phases, calcium aluminate (or calcium aluminate hydrate) phases, and ettringite-like phases, and the formation of carbonate phases and silica gel. Carbonation also revealed that heterogeneity within the cement samples had a major impact on migration pathways and extent of carbonation. This heterogeneity may have been a result of casting, and was only observed in some of the samples studied. It led to faster carbonation in some areas, and may account for some of the differences observed in the reacted cement samples. Such heterogeneity may be present within a repository, and should be taken into account when assessing repository performance

Inhibition of the formation and stability of inorganic colloids in the alkaline disturbed zone of a cementitious repository

The generation and stability of inorganic colloids have been studied under hyperalkaline conditions. For the generation of colloids, intact cores of Bromsgrove Sandstone were flushed with simulated cement leachates, and the eluates were ultrafiltered sequentially (12 μm, 1 μm, 0.1 μm and 30 kDa) for the separation of any colloids found. No colloid formation was observed during the experiments; however the analysis by ICP-MS of the eluates showed significant increases in Si and Al, indicating silicate mineral dissolution, as well as reduction of the concentration of Ca in the leachates indicating precipitation of secondary Ca-rich phases. Flow experiments with cement leachates spiked with tritiated water showed a noticeable reduction of the porosity of the sandstone as well as changes in the pore distribution. Additional stability experiments were carried out using model silica and Fe2O3 colloids. The experiments indicated that the stability of the colloids was mainly controlled by the concentration of Ca in solution and that both types were unstable under the chemical conditions in the alkaline disturbed zone. The presence of cement additives such as superplasticisers could enhance the stability of the colloids

A simple reactive-transport model of calcite precipitation in soils and other porous media

Calcite formation in soils and other porous media generally occurs around a localised source of reactants, such as a plant root or soil macro-pore, and the rate depends on the transport of reactants to and from the precipitation zone as well as the kinetics of the precipitation reaction itself. However most studies are made in well mixed systems, in which such transport limitations are largely removed. We developed a mathematical model of calcite precipitation near a source of base in soil, allowing for transport limitations and precipitation kinetics. We tested the model against experimentally-determined rates of calcite precipitation and reactant concentration–distance profiles in columns of soil in contact with a layer of HCO3−-saturated exchange resin. The model parameter values were determined independently. The agreement between observed and predicted results was satisfactory given experimental limitations, indicating that the model correctly describes the important processes. A sensitivity analysis showed that all model parameters are important, indicating a simpler treatment would be inadequate. The sensitivity analysis showed that the amount of calcite precipitated and the spread of the precipitation zone were sensitive to parameters controlling rates of reactant transport (soil moisture content, salt content, pH, pH buffer power and CO2 pressure), as well as to the precipitation rate constant. We illustrate practical applications of the model with two examples: pH changes and CaCO3 precipitation in the soil around a plant root, and around a soil macro-pore containing a source of base such as urea

The Cretaceous Continental Intercalaire in central Algeria: subsurface evidence for a fluvial to aeolian transition and implications for the onset of aridity on the Saharan Platform.

The Lower Cretaceous Continental Intercalaire of North Africa is a terrestrial to shallow marine continental wedge deposited along the southern shoreline of the Neotethys Ocean. Today it has a wide distribution across the northern Sahara where it has enormous socio-economic importance as a major freshwater aquifer. During the Early Cretaceous major north-south trending basement structures were reactivated in response to renewed Atlantic rifting and in Algeria, faults along the El Biod-Hassi Messaourd Ridge appear to have been particularly important in controlling thickness patterns of the Lower Cretaceous Continental Intercalaire. Subsurface data from the Krechba gas field in Central Algeria shows that the Lower Cretaceous stratigraphy is subdivided into two clear parts. The lower part (here termed the In Salah Formation) is a 200 m thick succession of alluvial deposits with large meandering channels, clearly shown in 3D seismic, and waterlogged flood basins indicated by lignites and gleyed, pedogenic mudstones. The overlying Krechba Formation is a 500 m thick succession of quartz-dominated sands and sandstones whose microstructure indicates an aeolian origin, confirming earlier observations from outcrop. These interbed with brick red, highly oxidised mudstones representing deposition in temporary lakes or lagoons under an arid climate. The switch from fluvial to aeolian sedimentation at Krechba on the Saharan Platform occurred in the late Aptian and Albian and is thus synchronous to a comparable change observed by previous authors in Lower Cretaceous non-marine deposits of NE Spain. This was probably driven by a combination of sea-level fall and the northward shift of global arid belts into western Neotethys caused by oceanic rifting between Africa and South America

A long-term experimental study of the reactivity of basement rock with highly alkaline cement waters: reactions over the first 15 months

A series of long-term laboratory experiments was started in 1995 to investigate longer-term dissolution/precipitation reactions that may occur in the alkaline disturbed zone surrounding a cementitious repository for radioactive waste. They consist of samples of UK basement rock reacting with either Na-K-Ca-OH water (‘young’ cement porewater) or Ca-OH water (‘evolved’ cement porewater) at 70°C. This paper summarizes results of reactions occurring over the first 15 months. Experiments of both fluid types showed many similar features, though primary mineral dissolution and secondary mineral precipitation were more extensive in the experiments involving Na-K-Ca (younger) cement porefluids compared to more evolved (Ca-rich) cement porefluids. Dissolution of dolomite, and to a lesser extent silicates (probably K-feldspar, but also possibly mica) occurred relatively rapidly at 70°C. Dolomite dissolution may have been a key factor in reducing pH values, and may be a key mineral in controlling the extent of alkaline disturbed zones. Dissolution was followed by precipitation of brucite close to dolomite grains, at least two generations of C-S-H phases (which may have contained variable amounts of K, Al and Mg); overgrowths of calcite; small crystals of hydroxyapophyllite; and elongate crystals of celestite. Though hydroxyapophyllite was observed (a phase commonly associated with zeolites), there was no evidence for the formation of zeolites in the experiments. Fluid chemical changes track the mineralogical changes, with C-S-H phases being a major control on fluid chemistry. In the ‘young’ porewater experiments there were decreases in pH, and K, Ca and Mg concentrations, together with transitory increases in SiO2 concentrations. In the ‘evolved’ porewater experiments there were decreases in pH, Mg, Ca and Sr concentrations, together with small increases in K and SiO2 concentrations. A number of experiments are still running, and will be sampled in coming years

Petrography and diagenesis of the Bunter Sandstone Formation in the UK Southern North Sea

This report describes a regional petrographic study of the Triassic aged Bunter Sandstone Formation (BSF) in the northern part of the UK Southern North Sea. To date, the spatial relationships leading to the prediction of regional reservoir quality in the BSF in terms of its potential for storage of CO2 or hydrogen have not been fully understood. Previous studies have been largely focused on the hydrocarbon fields and these have shown that the presence or absence of diagenetic cements can both create barriers and enhance pathways for fluid flow. More recent studies investigating CO2 storage potential through regional dynamic modelling have demonstrated the importance of physical and chemical property distributions. This applies not only to the immediate injection and containment of the CO2 plume but also to the impacts of the pressure footprints created from injecting a large volume of fluid and the potential interaction between neighbouring sites. However, a consensus allowing an extrapolation of reservoir quality prediction to the more data-poor saline aquifer parts of the BSF has not proved straightforward as evidenced by results of a CO2 storage exploration well drilled in 2010. This has highlighted the importance of improving regional understanding of reservoir storage potential for future CO2 or hydrogen appraisal activities. This report adds to the body of research attempting to discern reservoir rock property distribution in the BSF through detailed petrographic observations to understand their diagenetic histories through the analysis of 83 samples across 12 wells. Diagenetic observations are described and presented as a proposed diagenetic history in the figure below. Detrital and near-surface diagenesis (eodiagenetic) characteristics of the BSF are consistent with terrestrial deposition under arid conditions, principally with fluvial origins and a minor aeolian input. The eodiagenetic phases that are particularly characteristic of arid conditions include widespread and locally abundant nodular grains (carbonate and sulphate) and cements (also carbonate and sulphate) that preserve un-compacted and expanded (displaced) grain frameworks. Additionally, some cements have evaporitic textures (e.g. pseudomorphed ‘desert rose’ forms, enterolithic anhydrite). The carbonate nodules (calcite and dolomite) are an abundant framework grain constituent throughout the BSF. These are characterised by dominantly rounded sand sized forms with concentric structures defined by sequential zones of micritic and/or radial fibrous carbonate, and hematitic clays. Nodule cores comprise a mix of silt-grade silicate grains, mixed micrite and clays, and nodule fragments with evidence of varied degrees of reworking. They are most abundant in wells from the central to eastern parts of the study area (Quadrants 43 and 44). These nodules are not ooids sensu stricto, because evidence that they formed through both surface and shallow sub-surface processes is abundant and widespread. Carbonate nodules are locally concentrated in laminations and, together with associated eodiagenetic cements, form dolocrete and calcrete layers, mostly hosted in finer grained laminations with sub-millimetre to centimetre (plus) scale thicknesses. Variable lateral continuity typically reflects the structure of the hosting sedimentary laminations. These features present partial barriers to larger scale porosity interconnectivity and are of sub-seismic-resolution. Subsequent burial diagenesis (mesodiagenesis) is dominated by the formation of further widespread pore-filling cements, mostly of anhydrite and halite. These cements are typically also partially replacive of eodiagenetic nodules. Several episodes of cementation and some of dissolution, have been identified. These are generally poikilotopic cements, which in the case of anhydrite, differentiates it from eodiagenetic sulphate nodules. Anhydrite largely pre-dates halite. Enclosure of anhydrite by halite is largely passive (i.e. anhydrite crystals have euhedral margins), but there is local evidence for dissolution of anhydrite prior to, or during, halite emplacement. Both of these cement phases are preferentially developed in coarser sandstones. Major halite cement is only observed at and below a current burial depth of ~1400 m. This suggests that the halite distribution must be, in part, controlled by current and / or recent conditions. Locally, halite dissolution has occurred preferentially along coarser grained sandstone laminations in otherwise fully cemented intervals. Diagenetic silicate cements are rare over most of the study area. The exceptions to this are samples from the western edge of the study area (Quadrant 41 wells) where compaction textures are well developed and quartz cement is widespread. Heterogeneity of compactional textures is a key characteristic of the BSF observed across the study area. On a sub-millimetre scale, areas of well compacted framework grains exist next to areas with open and expanded fabrics. The looser textures can only partially be explained by grain replacement and dissolution, and the current distribution of diagenetic cements. We conclude that the samples were partially cemented prior to maximum burial, but the cement distribution has changed subsequently. It is clear that the halite and anhydrite cements in their current distribution, cannot have been the primary control on the degree of compaction currently observed in the BSF. This observed textural heterogeneity is consistent with our proposed diagenetic model, which infers that the sandstones had abundant, but not complete, early cements that preserved shallow framework fabrics. As these cements were partial, compactional fabrics were created in the surrounding less- or un-cemented zones. Subsequent dissolution, replacement and / or mobilisation of some or all of the cement phases, post maximum burial, has resulted in the widely recognised heterogeneous compaction fabric which does not correspond to current cement distributions. Since both anhydrite and halite show evidence for both multiple phases of formation and partial dissolution, these are the primary candidate minerals for dissolution / mobilisation. As these phases have also partially replaced some of the framework carbonate nodules, then their subsequent dissolution / mobilisation could also create an apparently uncompacted fabric. One expected outcome of abundant, pre-maximum-burial cementation, is that BSF porosities should be detached from a simple linear variation with maximum burial depth. This is what is observed for the BSF. It is recognised that the conclusions of this study are constrained by the limited numbers of samples (for the extent of the study area) and the fact that they are all sourced from hydrocarbon-interest boreholes which have targeted potential reservoir structures. Many of these structures are a consequence of site-specific halokinesis, therefore with potentially atypical thermal, fluid and structural conditions. Whilst we have gained significant insight into the diagenetic paragenesis, we are unable to adequately predict porosity, a major interest for CO2 and energy storage interests. This is a consequence of the heterogeneities of the BSF in texture, cement distribution and paragenesis. To improve the remaining knowledge gaps and predictabilities of major reservoir properties, further studies are needed: 1. To obtain a better understanding of the distributions of grain fabrics and diagenetic cements, in order to improve predictability of pore size and connectivity, and porosity distribution at a regional scale: • Extend the study to include more samples for detailed modal analysis and minus cement porosity calculation. • Apply petrographic image analysis to more samples and a wider range of properties to characterise actual porosity, grain size and compactional fabric distributions. 2. Develop a high-resolution diagenetic sequence through isotopic studies of the main cements, tied to their paragenetic sequencing using: • Strontium isotope analysis (87Sr/86Sr) to inform the origins of solutes in the diagenetic fluids, and extent of rock-water interaction (target phases - calcite, dolomite, anhydrite and halite). • Stable (oxygen, carbon, sulphur) isotope analysis (δ13C, δ18O and δ34S). These techniques would further inform the mineralisation temperature, and carbon and sulphur sources (target phases - anhydrite, dolomite and calcite cements). • U-Pb dating to obtain absolute dates for carbonate mineral formation. Using petrographically-guided targeting, this will place the paragenetic sequence in absolute time. A major issue for this will be potential contamination of the carbonate phases by finely-disseminated hematite, which is known to preferentially concentrate U and Pb

Dissolution experiments in halite cores: comparisons in cavity shape and controls between brine and seawater experiments

There is an increasing need for underground storage of natural gas (and potentially hydrogen) to meet the UK’s energy demands and ensure its energy security. In addition, the growth of renewable energy technologies, such as wind power, will be facilitated by the development of grid-scale energy storage facilities to balance grid demand. One solution lies in creating large-scale compressed-air energy storage (CAES) facilities underground. Whilst a number of lithologies offer storage potential, only three operational CAES facilities exist in the UK. They are constructed in specifically designed solution-mined salt (halite) caverns, similar to those currently used for natural gas storage. The influences exerted on salt dissolution by petrology, structure and fabric during cavern construction are not fully understood, with some occurences of caverns with noncircular cross-sections being less than optimum for gas storage and especially CAES

Retention of chlorine-36 by a cementitious backfill

Radial diffusion experiments have been carried out to assess the migration of 36Cl, as chloride, through a cementitious backfill material. Further experiments in the presence of cellulose degradation products were performed to assess the effect of organic ligands on the extent and rate of chloride diffusion. Results show that breakthrough of 36Cl is dependent on chloride concentration: as the carrier concentration increases, both breakthrough time and the quantity retained by the cement matrix decreases. Experiments in the presence of cellulose degradation products also show a decrease in time to initial breakthrough. However, uptake at various carrier concentrations in the presence of organic ligands converges at 45% of the initial concentration as equilibrium is reached. The results are consistent with organic ligands blocking sites on the cement that would otherwise be available for chloride binding, though further work is required to confirm that this is the case. Post-experimental digital autoradiographs of the cement cylinders, and elemental mapping showed evidence of increased 36Cl activity associated with black ash-like particles in the matrix, believed to correspond to partially hydrated glassy calcium-silicate-sulfate-rich clinker

Microbial impacts of CO2 transport in Sherwood Sandstone

Work carried out by BGS and the Japan Atomic Energy Authority (JAEA) has shown that microbial processes can have profound effects on the transport properties of host rock (i.e. the movement of fluids and contaminants through the host material) relevant to radioactive waste disposal. Recent research, performed as part of the BGS Radtran project, has examined Sherwood Sandstone samples in the context of radioactive waste disposal; this particular formation is also a potential reservoir for carbon dioxide storage in the UK. As part of the BGS opportunities fund programme, this project has, for the first time, evaluated interactions between fluids saturated with carbon dioxide/Sherwood Sandstone/microbes (Pseudomonas aeruginosa) in transport experiments, using BGS developed apparatus under pressurised subsurface conditions. This pilot study has highlighted the impacts of differences in the physical characteristics of core Sherwood Sandstone samples collected adjacent to each other in a core sample, and the ability of P. aeruginosa to survive in CO2 saturated artificial groundwater and the potential to form a biofilm in an environment suitable likely to be found at a carbon capture and storage location. These results demonstrate that in this short study, the injection of P. aeruginosa into the biotic experiment does not appear to impact on the physical transport properties of the Sherwood Sandstone, although the presence of CO2 appears to enhance the mobilisation of a number of chemical species. However, in other work which utilised the same organism and rock type but without introduction of CO2 saturated fluid, post-inoculation injection changes were observed. These included short but rapid saw-tooth like changes in the pressure profile (Wragg et al, 2012). These impacts were not observed in the current study which suggests that the CO2 saturated fluid was impacting on the ability of the microbes to alter permeability. This short study has, however, indicated the need to carry out longer term investigations to reproduce these initial findings