Geoenergy - Current Issue

The Lower Triassic Bunter Sandstone Formation is a major prospective reservoir for carbon capture, utilization and storage in the UK Southern North Sea, and is likely to play a pivotal role in the UK reaching mid-century Net Zero targets. A knowledge gap in reservoir quality exists between previous detailed, but highly focused front-end engineering and development projects, and large-scale regional analysis. This study integrates a regional approach with locally derived reservoir characterization, offering a holistic analysis of the prospectivity of the Bunter Sandstone Formation for subsurface CO2 storage. Petrophysical analysis of ninety-six wells across the UK Southern North Sea is coupled with seismic interpretation to understand spatial variations of reservoir thickness, facies and quality that underpin theoretical CO2 storage capacity models. Electrofacies classification is employed to identify and correlate baffles and barriers to permeability over areas currently licensed for geological carbon storage. Our findings point to variable, but broadly favourable reservoir conditions, though identification and correlation of laterally extensive intraformational mudstones and halite-cemented horizons will likely present challenges to CO2 injection. Within carbon storage license blocks CS001, CS006 and CS007, the Bunter Sandstone Formation has the potential to store 5700 MCO2t, the equivalent of seventy-nine years of the UK's 2022 business and industrial CO2 emissions. A further 434 MCO2t is offered by Triassic closures within license CS005, with many neighbouring moderate (100–1000 MCO2t) and small (<100 MCO2t) closures forming part of newly awarded carbon storage licenses that will likely form part of the UK SNS CCUS portfolio in the future.

Supplementary material: well-correlation panels and tabulated velocity and storage capacity modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.7027450

With the global drive for net-zero emissions, it has never been more important to find clean energy sources. There are thousands of abandoned oilfields worldwide with the potential to be reactivated to produce clean energy with air injection and subsequent waste fluid sequestration. Air injection, and the development of a fire-front, may be used with enhanced geothermal systems by taking advantage of the inherent increase in heat and pressure. Conventionally used as an enhanced oil recovery technique, air injection has gained the reputation of being a high-risk intervention due to the many failures in its history. Knowledge of how petrophysical rock properties and oil physical and chemical properties control the consequences of air injection is key to optimzing the selection of late-life, or even abandoned oilfields for use in such systems. Here we use one-dimensional modelling to test the effect of varying porosity, permeability, oil viscosity and API gravity on the success of air injection. Modelling shows that the most important factor controlling temperature is the porosity of the reservoir, followed by the API gravity and then the viscosity of the oil. The most important factors controlling velocity of the fire-front are API gravity followed by oil viscosity. We show that reservoirs with high porosity and low permeability with high viscosity and low API gravity oil reach the highest fire-front temperatures. The significance of this work is that it provides several geoscience-related criteria to rank possible candidate reservoirs for reactivation and clean energy generation via air injection: the best candidates will have the highest total porosity, relatively low permeability, highest oil viscosity and lowest API gravity, such fields can then move on to bespoke and more complex simulations.

Irrespective of the host rock and the repository concept, bentonite plays a central role in the geological disposal of radioactive waste by serving as the main sealing element in most repository concepts. Due to its favourable chemical, hydraulic and mechanical properties, bentonite is of paramount importance in the overall safety of a repository for radioactive waste. This work gives an overview of the use of bentonite in the German repository concepts in different host rocks. Starting with the current repository safety regulations in force in Germany, the principles for the design of the engineered barrier system are described. Among them, the principle of diverse redundancy has been established as the main design principle to fulfil the requirements of the Repository Safety Ordinance. This principle allows the arrangement of different geomaterials in the design of the geotechnical barriers where bentonite plays a key role. Bentonite is used in the German repository concepts in different host rocks as either buffer, backfill and/or sealing material. In clay and in crystalline rock, highly compacted bentonite blocks are considered for the sealing of disposal boreholes and for the construction of drift seals. Processed excavated material is mixed with bentonite to backfill the remaining voids inside the repository. For the closure of shafts, bentonite as binary mixture of briquettes and powder or as equipotential sand-bentonite-segments are considered. The present work also brings to light how bentonite is used in German repository concepts. We also discuss the differences between German and international geological disposal programmes in regards to the applications of bentonite. The innovative aspect of the German approach is demonstrated through the adoption of a dual bentonite/asphalt sealing system and the implementation of equipotential segments comprising layers of sand and bentonite.

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Active heating of fibre-optic (FO) cable combined with distributed temperature sensing has been applied as a quality control tool for emplacing granulated bentonite buffer. For calibrating the tool, the FO cable is installed in specimens with known dry densities. Layer-by-layer compaction is often employed to prepare the specimens. Although the average dry density is guaranteed, some studies report possible variations in the dry density within a layer. In this study, heat transfer from a heated FO cable installed in granulated bentonite with different compaction-induced variabilities was numerically simulated. The reference case used homogeneous bentonite with an average dry density of 1.5 g cm−3. For three other cases the granulated bentonite was filled in two, three and four layers, respectively, each of which had a distinct dry density gradient ranging from 1.3 to 1.7 g cm−3. For each case, the FO cable placed at the centre of the container was numerically heated, and the thermal conductivity was calculated using temperature changes. In the cases where the FO cable was placed on the layer interface at which the dry density was discontinuous, the calculated apparent thermal conductivity was found to have been underestimated with an error of as much as 4%. In the other two cases, the thermal conductivities obtained were nearly identical to those corresponding to the average dry density. The influence of the compaction-induced variability in the dry density was found to be insignificant and the estimated thermal conductivity was essentially that of the bentonite represented by its average dry density.

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

There is significant international consensus that the appropriate approach for long-term management of radioactive waste is disposal in an underground geological disposal facility. The gaps between waste containers and the host rock may be filled with bentonite, which swells to fill the gaps as it absorbs groundwater. Bentonite is soft, absorbing shear displacements in the host rock, and has low permeability, which restricts groundwater flow and hence corrosion of the waste containers. Many experiments have studied localized inflows of water into a region of bentonite pellets. Upward flow against gravity has often been observed. This study attempted to build understanding of this by modelling one experiment. A systematic approach was adopted in which models of increasing complexity were used, with the parameters at earlier stages used as a guide for later stages. Initially, only water flow was considered. Then water absorption and swelling, which lead to stresses and strains, were addressed. On the basis of the understanding developed, a conceptual model that represents the observed behaviour is proposed.

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Current methods for tracing decades-old groundwaters rely on isotope geochemistry to determine groundwater age and altitude at the point of infiltration. Temporal and spatial variability in atmospheric conditions, and water–rock interactions, can make the interpretation of isotopes uncertain. Here, we propose a new method of groundwater tracing based on the fingerprinting of natural dissolved organics. We present our initial findings from the Grimsel Test Site in Switzerland, located within a fractured granite. Using 2D gas chromatography, we derive detailed organic fingerprints from surface soils at several locations and show that different soils produce distinctly different dissolved organic signatures. We then compare the soils with groundwater and lake water using a non-targeted approach employing principal component analysis and hierarchical cluster analysis. Our analysis finds three statistically significant clusters. Most groundwaters are clustered with the lake-water samples but two are clustered with soil from the highest altitude surface sampling location. We hypothesize that for samples to form a significant cluster, they must have been derived from a common environment, with a unique combination of organic compounds. For groundwaters to cluster with soil samples or lake water, we theorize there must be a hydraulic connection between the type of infiltration environment and the groundwater sampling locations within each cluster. Our research demonstrates that organic molecules derived from the surface environment can be used to discriminate near-surface environment(s) through which meteoric groundwater has infiltrated. Organic fingerprinting could prove a powerful tool for improved understanding of groundwater flow systems, particularly when combined with other complementary techniques.

Supplementary material : Compound alignment data set and supplementary tables are available at https://doi.org/10.6084/m9.figshare.c.7129987

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

The successful implementation of plans to develop a repository for radioactive waste in the United Kingdom will require an assessment of the ability of the facility to meet appropriate regulatory safety criteria at all stages during its construction, operation and after closure. As part of this assessment, an environmental safety case (ESC) will be presented and this will contain a collection of claims, arguments and evidence which collectively demonstrate that long-term safety can be achieved and maintained. Natural analogues (NA) can be helpful in demonstrating understanding of aspects of repository performance by, for example, providing evidence that certain materials can survive for long periods. Appropriate NAs can be critical to providing long-term practical demonstrations to support the theoretical and mathematical arguments of the ESC and they may have a significant role in the overall process understanding. As part of the ongoing repository development programme in the UK, an updated version of an existing catalogue of NA studies (focussed on the requirements of the ESC) has been produced and the background to NA studies in general, and to the catalogue in particular, are presented here.

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Bentonite is used in various geological repository designs for highly radioactive and low- and intermediate-level waste. Stability of such materials has been evaluated in long-term safety assessments, with recent emphasis on potential chemical erosion of these clays in fresh groundwater conditions. The fracture-filling mineralogy at two investigation sites, Kivetty and Romuvaara, in Finland shows the existence of different types of smectites characterized by their respective dominant elements, such as Mg-, Fe- and Na–Ca-rich smectites. Of these, Ca- and Na-smectites are likely to be montmorillonites that provide potential natural analogues for the bentonite buffers and their long-term performance in fractured bedrock environments. The smectite minerals observed reflect the local parent rock mineral composition, suggesting in situ formation during hydrothermal alteration, well predating the current freshwater conditions in the fractured bedrock. These observations support the stability of smectites, including Na- and Ca-montmorillonites, in the fractured bedrock of glaciated terrain with fresh continental groundwater extending to repository-relevant depths. Further, no erosion or sedimentation processes have been observed to have taken place in the fracture systems studied. However, uncertainties remain related to detailed exchangeable cation compositions of the smectite. Current groundwater systems at both sites would alter the composition of montmorillonite towards the Ca-rich form, and more detailed investigations are required to assess the stability of Na-montmorillonite in connection to open fractures. In general, bentonite alteration in fractures to Ca-montmorillonite would be beneficial in repository scenarios that consider potential chemical erosion by dilute groundwater.

Supplementary material: Supplementary material 1 (XCT data for fracture smectite samples from Kivetty and Romuvaara) are available at https://doi.org/10.23729/1ce75faa-d52a-43cd-af40-3961b3081f70 [last accessed 5 July 2023] and Supplementary material 2 (XRD diffractogram data) and Supplementary material 3 (SEM-EDS data) are available at https://doi.org/10.6084/m9.figshare.c.7126477

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive