Geoenergy - Volume 2, Issue 1, 2024

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.

Energy from Earth resources (geoenergy) in the form of coal, oil and gas has fuelled the global society since the Industrial Revolution began. Amongst the consequences of fuelling society and associated population growth, is climate change, driven by the emission of greenhouse gases liberated through unabated combustion of fossil fuels.

There is much more to Earth energy systems, however, than just coal oil and gas. The Earth contains, in human terms, an unlimited supply of accessible heat and pressure (differences), as well as copious quantities of storage space, non-hydrocarbon gases and valuable solutes. These resources can be targeted to provide sustainable energy sources with low to zero carbon footprints.

This report does not contain any new radical technologies that will deliver energy free from all environmental impacts but it does show that, when considering geoenergy, society needs to look at the whole system, which combines chemical, thermal, potential, kinetic, gravitational and other energy forms that could be used from individual developments to minimize waste, maximize efficiency and reduce unwanted impacts.

We demonstrate that geoenergy will continue to play a key role in decarbonized energy systems for centuries to come.

The temporal and spatial scale of interest of CO2 storage studies lies in between reservoir and basin models. While reservoir modelling software is best fitted to address some of the multiphysics issues related to the behaviour of CO2 once injected into the subsurface (adsorption, dissolution, near injection wellbore mechanics and temperature, and, in some cases, fluid rock interaction) within a human timescale, basin modelling tools handle better the full basin volume and time-scale heterogeneities that impact the storage potential and risk associated with CO2 injection. This study takes a basin modelling approach to provide an assessment of the influence of geological evolution on CO2 storage capacity, both at the reservoir level, by helping to estimate the amount of CO2 that can be stored in its connected porosity, and at the cap-rock level, by assessing the seal integrity. Our basin model also captures the evolution of the pressure plume induced by the CO2 injection, taking into account the pressure and temperature fields, aquifer connectivity and permeability, and seal integrity, on a much shorter timescale than is usually considered by such a model. The results show the impact of basin evolution on aquifer properties and consequently on the dissipation of the induced pressure plume. They also highlight the large-scale influence of the CO2 on the pressure field both vertically along the stratigraphic column, when the pressure plume reaches shallower aquifers through unconformities, and horizontally, when good aquifer injectivity and connectivity allows the pressure plume to dissipate widely.

Carbon capture and storage (CCS) requires the safe, long-term storage of CO2 in the subsurface. This challenges subsurface practitioners to build on and adapt many of the techniques and processes developed for hydrocarbon exploration and production to create innovative approaches to assessing CO2 storage risk and uncertainty. A wide-ranging and integrated understanding of the processes controlling CO2 behaviour in the subsurface is required to facilitate effective risk identification and characterization.

In this paper we explore the different physical processes acting within storage formations and their evolution throughout the injection project life cycle to illustrate key controls on CO2 plume behaviour and frame risk identification and characterization. An approach applying wide-reaching premortem risk identification coupled with evidence-led uncertainty characterization is described to frame inputs for detailed risk characterization and management activity during early screening of prospects for CO2 storage.

The Northern Lights full-scale commercial CO2 transport and storage project is open to multi-industry capture plants, in a business still under development. The geological sequestration concept of a sloping saline aquifer raised several subsurface challenges related to timeline, data availability and development constrains, which all affected the decision process. The subsurface focus on storage security required the different disciplines to perform adjustments to the established workflows available from the hydrocarbon industry. A risk management-focused approach, front-end loading and early emphasis on containment were paramount. Here we share the experience from the Northern Lights subsurface maturation process and highlight the crucial contribution from the different disciplines in a challenging new industry.

A key aspect of transferrable oil and gas expertise to subsurface CO2 storage (SCS) relates to risk assessment. While initial subsurface risk and volume assessments for SCS projects are similar to oil and gas prospect evaluations, full life-cycle risk assessment requires evaluation of the current knowledge of the storage complex and future potential events that may have impacts over long time frames. It is important to learn from past water, gas, and CO2 injection and storage projects, examples of which are reviewed here. The concepts of aleatory and epistemic risk and uncertainty are discussed, and the use of standard risk matrices for evaluation of long-term, low-frequency but potentially high-impact events is challenged. Drawing on the large volume of previous work, this paper highlights key elements for development of a robust risking framework, including thorough project framing and implementation of a staged approach to project execution. The paper assesses methods for ensuring all potential hazards are captured and addresses the challenges of defining a quantitative risking scale suitable for long-term SCS projects. Example quantitative risk profiles can be used to calculate the timing and duration of ‘Peak Risk’, augment monitoring and mitigation planning for management of risk and capital exposure, and help to ensure successful project outcomes.

Thematic collection: This article is part of the Geoscience workflows for CO2 storage collection available at: https://www.lyellcollection.org/topic/collections/geoscience-workflows-for-CO2-storage

In August 2022, the world's longest running offshore industrial CO2 injection project celebrated its 26-year anniversary. During these years, the Sleipner CO2 injection project has been invaluable in demonstrating that offshore CO2 storage is feasible, safe, and efficient. We will here show how time-lapse seismic monitoring of the CO2 plume development has revealed depositional architecture in the Utsira Formation, and how thin mudstone layers have contributed to distributing the CO2 in a larger rock volume, promoting trapping by dissolution. The relatively shallow depth (800–1000 m) of Utsira Formation in the Sleipner area also makes the Sleipner CO2 injection site a good proxy for understanding the effects of overburden stratigraphy for deeper injection sites, giving important knowledge of detectability of thin, shallow CO2 accumulations. Finally, we will show how the experience from Sleipner CO2 injection has built confidence when planning monitoring programmes for future CO2 injection sites.

Hydrogen is an energy carrier that can balance the divergent variations in seasonal energy demand and energy supply from renewables. Underground hydrogen storage in porous formations, such as depleted gas sandstone reservoirs or saline aquifers, provides the capacities needed for large-scale, long-duration energy balancing. This paper reports on the fundamental behaviour of hydrogen in a model reservoir setup, involving a two-phase (H2, water) system and a two well (injector, producer) setup placed at different depths in the reservoir. We specifically focus on the impact of natural heterogeneities, and associated permeability contrasts, on flow and efficacy of hydrogen injection and production. We found that positioning the wells, both injector and producer, at the top of the reservoir facilitates the highest hydrogen production. We also found that permeability contrasts of three to four orders of magnitude significantly affect hydrogen flow; however, factors affecting the pressure gradient also need to be considered. These factors include compartmentalization, the behaviour of co-existing fluids and the localized pressure gradient created by the hydrogen plume. Our research underlines the need to understand the architecture of the whole reservoir, from seismic to sub-seismic scales, not just the zones surrounding the wells and pathways in-between, as this controls capacity, pressure fluctuations and informs operational management decisions.

Thematic collection: This article is part of the Hydrogen as a future energy source collection available at: https://www.lyellcollection.org/topic/collections/hydrogen

Several sources of natural hydrogen are known or postulated but the process of serpentinization, the action of water on ultramafic rocks, is shown to be the most effective. Studies indicate that the rates and volumes generated by high-temperature serpentinization, (i.e. in the temperature range of 200–320°C), could feed a focused hydrogen system potentially capable of sealing and trapping gas-phase hydrogen in commercially-sized accumulations.

Natural hydrogen is generated by serpentinization wherever ultramafic rocks can be penetrated by aqueous fluids. This includes diverse geotectonic settings ranging from divergent and convergent plate margins to intra-plate orogenic belts and Precambrian cratons.

The ‘hydrogen system’ describes the generation, migration and sealing/trapping of hydrogen. There are two parts to the ‘generic hydrogen system’: the ‘source-generation sub-system’ requires an ultramafic protolith, usually in basement, and a supply of water penetrating basement rocks. In the ‘migration-retention sub-system’ migration, sealing and entrapment of gas-phase hydrogen behaves the same as for hydrocarbon gases.

The hydrogen system by serpentinization is used to develop play models to guide exploration in the accessible and exploitable geotectonic settings of continental cratons, ophiolites and convergent margins.

Thematic collection: This article is part of the Hydrogen as a future energy source collection available at: https://www.lyellcollection.org/topic/collections/hydrogen

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