Subsurface storage

Geological strata provide vast storage space for energy and materials in various forms.

Mines

Subsurface storage

Geological strata provide opportunities to store both energy and waste products from industrial processes, including from energy generation. Porous, high permeability sedimentary deposits, fractured mudstones and crystalline rocks, dissolved cavities and abandoned mines can be used to store heat, energy and carbon dioxide. Ductile salt and clay deposits and massive crystalline rocks are suitable for nuclear waste disposal. Our expertise in investigating different lithologies and basin structures can help identify the right storage sites and ensure their safe use.

Carbon dioxide

Capturing CO2 and storing it for thousands of years in the subsurface (known as carbon capture and storage – CCS) is likely to be a key element in meeting emissions reduction targets. One storage option is the reuse of depleted hydrocarbon reservoirs, which have the necessary properties for CO2 sequestration: porous and permeable formations with an impermeable cap rock to prevent leakage to the atmosphere or to groundwater. Our many decades of research with the hydrocarbon sector ensures Leeds is well-placed to support CCS projects both in the former oil and gas fields and in similar reservoirs.

Injection of liquid CO2 lowers the temperature of the surrounding rock, changing its physical properties and causing thermal contraction. With our state-of-the-art multiphase flow, petrophysics and geo-mechanical laboratories, Leeds researchers can analyse the mechanical and hydraulic properties of the reservoir rock to take these temperature changes into account, to reduce the risks of CO2 leakage.

Our innovative seismic monitoring research, which uses satellite observations to interpret seismic waves in the subsurface, can help to monitor for potential earthquakes caused by large injections of CO2, that can threaten seal integrity and cause leakages.

Energy

Renewable energy sources, such as offshore wind, are growing rapidly, but their output is intermittent, with electricity often generated at times of low demand. The subsurface offers a sustainable way to store this excess energy, following conversion into hydrogen or compressed air, which can then be accessed to run turbines when demand is high. The most viable storage options are porous and permeable rocks, as for CCS, or lined rock caverns such as disused mine workings or excavated salt formations. Geotechnical engineering expertise at Leeds can also provide the strict geo-mechanical planning and monitoring necessary to avoid damage and leakages when using lined rock caverns.

Heat

Subsurface storage of surplus heat from industry can be used to manage variations in supply and demand for heating buildings over long periods, including over seasonal changes. Underground thermal energy storage (UTES) uses similar principles and technologies to geothermal heat extraction, with the storage medium ranging from unconsolidated sediments to rock with or without groundwater. At Leeds, our hydrogeology and geochemical research pedigree, along with links to civil engineering, provides the necessary expertise in groundwater flow, chemistry and composition and thermal properties of the rock or soil, to identify suitable reservoirs with favourable characteristics and dimensions. Close ties to the Priestley International Centre for Climate allow the research to be integrated into whole-system solutions such as district heating schemes.

Nuclear

Many countries, including the UK, retain nuclear waste created during decades of nuclear power generation and nuclear fuel reprocessing. Most plan to store this waste in sealed containers within deep geological disposal facilities (GDFs) over very long timescales, until it decays and becomes less harmful. GDFs need to meet strict criteria, including low permeability of the host rock, protection against groundwater contamination, and long-term mechanical, physical and chemical stability. Suitable sites are usually within granite/gneiss, salt and shale formations.

Decaying heat from the radioactive waste will affect temperatures within and around a GDF, depending on the type of waste involved, the container and backfill material, and the host rock properties. Chemical changes in acidity can be caused by cementitious backfill materials used for sealing of the nuclear waste package and tunnels, which, if not monitored, can cause a-build up of pressure leading to fractures in the GDF.

Detailed site investigations must include descriptive models of the initial state of variables such as geology, hydrogeology, hydro-geochemical features, and mechanical and thermal gas transportation factors. The combined expertise of geochemical and geo-mechanical researchers at Leeds can help identify suitable sites for GDFs, and help to understand and monitor possible thermal and chemical changes over the longer term.

Impact case studies

Several years ago, Sweden’s national nuclear energy company, SKB, identified a suitable site for the country’s repository for spent nuclear waste fuel at Forsmark, 200 km north of Stockholm.
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When contaminants leak out from waste disposal sites or industrial processes, it’s vital to know how far and how fast they will spread. This requires understanding of the materials themselves, but also of the subsurface geology and the chemical conditions through which they will travel.
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The depleted North Sea oil and gas fields are commonly put forward as the best sites in the UK for carbon capture and storage (CCS). Another – often overlooked – option is the layer of Triassic sandstone, saturated with salt water, that sits above layers of salt under the North Sea.
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Recent research projects

Coupling InSAR and geomechanical modelling to monitor spatial and temporal characteristics of CO2 Injection

For on-shore carbon sequestration, there has been a significant drive to integrate satellite interferometric synthetic aperture radar (InSAR) with geomechanical modelling to link surface deformation with the movement and storage of injected CO2. The project analyses the surface deformation resulting from CO2 storage at InSalah site, Algeria, in order to provide constraints on the temporal and spatial evolution of CO2 within the reservoir. This study provides the first practical assessment of CO2 storage and demonstrates that it is possible to develop independent, fast and cost-effective assessments of future schemes in the absence of ground-based surveys. The ability to scrutinize CCS sites globally based on limited input data and cost effectively is crucial for implementation of international monitoring of CO2 sequestration agreements. Lead researcher: Prof Andrew Shepherd.

Reducing uncertainty in predicting the geological storage of CO2 - improved geomechanical models and calibration using seismic data

The objective of this project is to address the fundamental uncertainty related to reservoir stress as a response to the geological storage of carbon dioxide. The research helps develop and advance current approaches in building complex hydro-mechanical models using seismic data, and develop methods to calibrate state-of-the-art hydro-mechanical modelling tools using seismic and surface deformation data. Lead researcher: Prof Quentin Fisher.

REMIS - Reliable Earthquake Magnitudes for Induced Seismicity

Utilisation of subsurface resources for mining, carbon storage and hydrofracturing has the potential to induce earthquakes. Regulators use a “traffic light” system to manage this risk. However, existing methods used to characterise earthquakes do not account for the possible range of magnitudes, meaning that there will be cases where operations are incorrectly permitted to continue (or are halted) based on random variation or bias in the earthquake parameter estimates. The main objective of this project is to develop a new method to better image the Earth and enable the creating of specific, testable hypotheses of Earth processes and structure. This will lead to new recommendations to improve monitoring and high-value decision-making for the future of induced seismicity in the UK and worldwide and as a consequence our ability to utilise the subsurface for future decarbonisation. Lead researcher: Dr Andy Nowacki.

Geological characterisation of fluvial and aeolian sediments

The principal aim of the The Fluvial and Eolian Research Group is to conduct cutting-edge research into the application of fluvial and aeolian sedimentology for developing a better understanding of issues relating to environmental geology, hydrocarbon systems, mining and mineral exploration, appraisal of groundwater aquifers and carbon sequestration. This applied-facing research group seeks to devise new methodologies in subsurface geological characterization. The group operates as a Joint Industry Project. Lead researcher: Prof Nigel Mountney.

Stability analysis of shafts used for minewater heat recovery

Traditional heating using non-renewable energy resources contributes up to 50% of current carbon emission level. The water in abandoned mines provides an alternative source of thermal energy that can be extracted through newly drilled boreholes or existing mineshafts. Surplus heat from power plants, industrial processes, or from installed solar plants can also be stored in these mines. Heat recovery through this mechanism may affect the structural stability of the mineshafts, therefore this project aims to ensure successful and sustainable operation through numerical sensitivity analyses on: (a) water level, (b) temperature fluctuations. Lead researcher: Dr Chrysothemis Paraskevopoulou, Rock Mechanics, Engineering Geology and Hydrogeology Group.

Flow pathways in chalk aquifers; implications for heat storage

This project investigates the sources and sinks of nitrate contamination in chalk groundwater, focusing on the Yorkshire Wolds. Dual stable isotope analysis of nitrate is used to identify and constrain possible sources and processes. Nitrate is of specific concern as heavy use of artificial fertilisers in previous decades has led to increased concentrations in chalk groundwater, and these anthropogenic impacts on water quality in chalk aquifers are complex, due to dual porosity response from the interplay between conductive fractures and less permeable elements. Understanding flow pathways in chalk aquifers is very important because they contain fractured zones and karsts which may be suitable for heat storage. Lead researcher: Dr Jared West, Rock Mechanics, Engineering Geology and Hydrogeology Group.

Hydromechanical and biogeochemical processes in fractured rock masses in the vicinity of a geological disposal facility for radioactive waste

The main aims of this project are to create a set of new and/or improved methodologies for estimating the repository-scale hydraulic conductivity of a fractured rock mass based on geologically realistic fracture network geometries; evaluate suitable seismic monitoring strategies and develop data processing techniques for the characterisation of potential repository sites; and examine the key seismic attributes for identifying fracture properties that play a critical role in repository performance. These developed methods and tools are sufficiently flexible and generic to be used in any fractured geological formation that might be investigated as a potential location of a geological repository in the UK. Lead researcher: Prof Graham Stuart.

Mechanisms of contaminant migration from buried concrete structures

The focus of this project are the mechanisms of alkaline plume generation at near-surface conditions from inactive and potentially radioactive concrete structures and cementitious rubble and soil from UK nuclear sites. The interaction of this high pH leachate with adjacent soils and the mechanisms of alkalinity attenuation (e.g., carbonation, mineral dissolution) are investigated, in addition to the effects of the imposed pH conditions on the leaching and mobility of contaminant metals and radionuclides present. Lead researcher: Dr Ian Burke, Cohen Geochemistry Group.