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Thermal energy from groundwater in abandoned, flooded, coal mines has the potential to make a significant contribution to decarbonisation of heat and Net-Zero carbon emissions. In Glasgow, UK, a subsurface observatory has been constructed for mine water heat and heat storage research. We synthesise geological and mine water resource findings from a four-year period of borehole planning, drilling, logging and testing. The heterogenous bedrock is typical of the Scottish Coal Measures Group, whereas superficial deposits are more sand- and gravel-dominated than prognosed. Mine water boreholes encountered workings in the Glasgow Upper, Glasgow Ell and Glasgow Main coal seams, proving water-filled voids, mine waste, fractured rock mass and intact coal pillars with high yields on initial hydrogeological testing. Whilst the depth and extent of mine workings delineated on mine abandonment plans proved accurate, metre-scale variability was expected and proved in the boreholes. A mine water reservoir classification established from the observatory boreholes highlights the resource potential in areas of total extraction, stowage, and stoop and room workings. Since their spatial extent is more extensive across the UK than shafts or roadways, increasing the mine water energy evidence base and reducing exploration risk in these types of legacy workings is important. Supplementary material Borehole reports and other datasets are available at https://ukgeos.ac.uk/data-downloads (mixture of over 20 DOI datasets and reports/data packs published openly on https://nora.nerc.ac.uk, all material is deposited in the National Geoscience Data Centre)

Ground gas and gas flux testing has been undertaken in a variety of situations including detecting volcanic and geothermal activity and monitoring landfill and carbon storage. However, there are no documented studies of its application to shallow geothermal investigations, particularly where the water in disused mines will be used as the conduit for heat and the overlying ground has a complex industrial heritage. Background ground gas and gas flux measurements from three separate campaigns at a mine water heat research site in Glasgow, UK did not reveal any concerns regarding mine gas or other potentially harmful gases from previous land uses. The detected CO2 was found to be predominantly of biological origin and reflected the expected quantities based upon land use, seasonal and weather fluctuations and was consistent with other UK sites. One location is recommended for further investigation due to higher-than-expected nitrogen and lower oxygen measurements. Some hydrogen gas was detected, albeit well below explosive limits, which may be present as a result of past industrial site uses, highlighting the need for more investigation into the presence of hydrogen at ex-industrial sites. Apart from this there was very little evidence of the industrial site history in the gas characterisation. A process-based analysis, based upon the stoichiometric relationship of CO2, CH4, O2 and N2, was applied to the results. This complemented, but was not a substitute for, the background survey. There were limitations with the process-based approach when there was not a clear anomalous CO2 signal or where potentially more than one process was occurring simultaneously.

Summary The Bunter Sandstone Formation in the Southern North Sea is an important potential CO2 storage reservoir and is likely to form an integral part of the UK’s carbon capture and storage ambition for industrial clusters in northeast England. In this study, a geological model is developed for the Bunter Sandstone Connected Aquifer in the Southern North Sea. This region is structurally-bound by large faults and salt features that are thought to compartmentalise it from surrounding Bunter Sandstone aquifer(s). Notable features of the Bunter Sandstone Connected Aquifer include a seismic polarity reversal in the top Bunter Sandstone horizon, and the ‘seabed outcrop’, a location at which the Bunter Sandstone subcrops a thin Quaternary sequence. Several storage sites have been identified within the Bunter Sandstone Connected Aquifer, and this model will provide an opportunity to assess regional pressurization, geomechanical modelling and estimates of CO2 storage capacity in the context of injection at multiple locations.

The existence of an extensive, prospective Visean–Namurian Carboniferous petroleum system in and around the hitherto under-explored Mid North Sea High is documented. Evidence is drawn from integrated analysis of over 50,000 line kilometres of seismic data and well data, together with regional source rock screening, reservoir analysis and basin modelling. Visean-Namurian marine and non-marine mudstones and coal source rocks are interbedded within fluvial and marine reservoir sandstones within a stacked succession up to 5 km thick. Source rocks are dominantly gas-prone with oil-prone intervals, and have reached oil to gas maturity levels dependent on location. Burial/thermal history modelling indicates a kitchen area on the southern margin of the Mid North Sea High with northwest and northeastwards migration of gas and oil during Mesozoic and Cenozoic times. A variety of structural and stratigraphic traps are possible, including intraformational Carboniferous traps, with a regional seal at Permian (Zechstein) level. Synthesis of many previously unpublished datasets demonstrates the Visean–Namurian play south of the Mid North Sea High as part of an extensive petroleum system from the East Irish Sea, across onshore to offshore UK and into The Netherlands sector of the North Sea. The purpose of this synthesis is to highlight future exploration opportunities beneath and northwards of the productive Westphalian Southern North Sea gas basin, and to begin to de-risk the petroleum systems that are exemplified by the lower Carboniferous Breagh Field in a frontier area of the mature North Sea province.

Storing CO2 in depleted or depleting oil and gas fields has now been proven at a number of sites worldwide. Key risks have been overcome, for example, relating to site design for dealing with reduced reservoir pressure, re-using infrastructure and managing wellbore integrity risks. Despite this, large-scale “pure” CO2 storage in depleted fields remains to be tested and closure of a large-scale CO2-EOR site has not yet occurred. It is hoped that dissemination of key learnings from these sites will encourage and inform discussions on CO2 storage in depleting fields elsewhere, to allow the early consideration necessary for exploitation of existing facilities (prior to decommissioning), and ultimately fill the short-term CO2 storage gap for quick (and relatively cheap) climate abatement returns until large-scale storage in saline aquifers can be fully implemented.

Large-scale geological heterogeneities and their potential to enhance fluid flow or hinder migration in the subsurface are reviewed. Evidence for flow or fluid containment by faults and fractures, fluid flow features, submarine mass movement deposits, buried tunnel valleys, hydrofractures and glacitectonic features is presented. Wellbore-related flow paths and well integrity studies are summarised; the risk of wellbore leakage may be best understood by simulations of predicted long-term behaviour of well barriers. Rates of fluid flux at natural seepage sites and controlled experimental releases of carbon dioxide (CO2) are compiled and compared, and lessons learnt for monitoring of CO2 storage sites identified. The overburden sequences to five operational or planned CO2 storage sites are compared and those characteristics in common that promote subsurface containment of fluids are highlighted.

Legislation for offshore storage has been developing over the last decade or so and is currently most developed in Europe. Although the large-scale operating sites in Europe were started prior to the regulations coming into force, any planned sites will need to meet these regulatory requirements. Our review of monitoring experiences from both the operating sites and research at experimental injection sites and in areas of natural CO2 seepage suggest that broadly, the technical and regulatory challenges of offshore monitoring can be met. A full report reviewing offshore monitoring including tool capabilities, practicalities and costs is available from IEAGHG (released Q1 2016).

The Bunter Sandstone Formation in the UK Southern North Sea has the potential to become an important CO2 storage unit if carbon dioxide capture and storage becomes a widely deployed option for the mitigation of greenhouse gases. A detailed geological model of a region of the Bunter Sandstone consisting of four domed structural closures was created using existing seismic, well log and core data. Compositional simulation of CO2 injection was performed to estimate the storage capacity of domes within the system. The injection was constrained by both pressure and CO2 migration criteria, and the storage efficiencies of the domes (volume of stored CO2 divided by the pore volume of the dome) were calculated when injection ceased. A sensitivity study evaluated the effect of varying the total aquifer volume, reservoir heterogeneity and injection well location. A wide range of storage efficiency values were obtained across the different simulation cases, ranging from 4% (closed dome) to 33% (homogeneous model). Intra-reservoir heterogeneity, specifically in the form of continuous low permeability layers has an important effect on storage capacity in dome-like structures, because it increases the tendency for CO2 to migrate laterally from the storage complex via structural spill points.

The Bunter Sandstone Formation in the Southern North Sea is a regional saline aquifer that extends across the median line between UK and Netherlands territorial waters. Numerical simulations of CO2 injection into a brine-saturated structural closure located in the UK sector have modelled the temporal development of an injection-induced pressure footprint, together with the potential role of faults in brine migration and pressure dissipation in the aquifer. The modelled pressure footprint extends into the Netherlands Sector and if the faults are considered migration pathways, brine expulsion rates of the order of 50 m3/day/km2 could be expected along the fault zones. This is equivalent to just over 105 Ml, during a 50 year injection period, of which approximately 40% is expelled along a fault beneath Netherlands territorial waters. The simulations have shown that brine displacement will facilitate CO2 injection into the Bunter Sandstone by alleviating pressure build-up, but an understanding of potential brine migration pathways, rates and environmental impacts must be demonstrated to regulators prior to injection.

Carbon dioxide storage sites are selected and designed not to leak. However, the variety of storage scenarios and different trapping mechanisms have a range of associated risks, including potential adverse effects on the environment and other subsurface resources. Monitoring of sites through the deployment of suitable technologies can provide assurance that the site is performing as expected and lend confidence to predictions of future (long-term) behaviour. Monitoring can act as an ‘early warning system’ to trigger appropriate changes in injection strategy and also to monitor any subsequent remediation, should it be required. Analysis of results from a selection of active storage sites shows that storage containment risks can be successfully monitored through the deployment of fit-for-purpose technologies.

Risk assessment-led characterisation of a site for the geological storage of CO 2 in the UK northern North Sea was performed for the EU SiteChar research project as one of a portfolio of sites. Implementation and testing of the SiteChar project site characterisation workflow has produced a ‘dry-run’ storage permit application that is compliant with regulatory requirements. A site suitable for commercial-scale storage was characterised, compatible with current and future industrial carbon dioxide (CO 2 ) sources in the northern UK. Pre-characterisation of the site, based on existing information acquired during hydrocarbon exploration and production, has been achieved from publicly available data. The project concept is to store captured CO 2 at a rate of 5 Mt per year for 20 years in the Blake Oil Field and surrounding Captain Sandstone saline aquifer. This commercial-scale storage of 100 Mt CO 2 can be achieved through a storage scenario combining injection of CO 2 into the oil field and concurrent water production down-dip of the field. There would be no encroachment of supercritical phase CO 2 for more than two kilometres beyond the field boundary and no adverse influence on operating hydrocarbon fields provided there is pressure management. Components of a storage permit application for the site are presented, developed as far as possible within a research project. Characterisation and technical investigations were guided by an initial assessment of perceived risks to the prospective site and a need to provide the information required for the storage permit application. The emphasis throughout was to reduce risks and uncertainty on the subsurface containment of stored CO 2 , particularly with respect to site technical performance, monitoring and regulatory issues, and effects on other resources. The results of selected risk assessment-led site characterisation investigations and the subsequent risk reassessments are described together with their implications for the understanding of the site. Additional investigations are identified that could further reduce risks and uncertainties, and enable progress toward a full storage permit application. Permit performance conditions are presented as SiteChar-recommended useful tools for discussion between the competent authority and operator. © M. Akhurst et al.

Selected European studies have illustrated the impacts of elevated CO2 concentrations in shallow soils on pasture. For the first time, general unified conclusions can be made, providing CO2 thresholds where effects on plants and soil microbiology are observed and making recommendations on how this information can be used when planning projects for CO2 storage. The sites include those where CO2 is being naturally released to the atmosphere from deep geological formations; and a non-adapted site, with no previous history of CO2 seepage, where CO2 has been injected into the unsaturated soil horizon. Whilst soil gas concentrations will be influenced by flux rates and other factors, the results suggest that a concentration of between 10% and 15% CO2 soil gas at 20 cm depth, which is within the root zone, is an important threshold level for observing changes in plant coverage. Site-specific plant ‘indicators’ are also observed for CO2 concentrations at ≥35%. Microbiological changes are seen where CO2 soil gas concentrations are between 15% and 40%. As part of site characterisation, an evaluation of the risks of leakage and their potential environmental impacts should be undertaken.

The measurement of gas concentrations and fluxes in the soil and atmosphere is a powerful tool for monitoring geological carbon capture and storage (CCS) sites because the analyses are made directly in the biosphere in which we live. These methods can be used to both find and accurately quantifying leaks, and are visible and tangible data for public and ecosystem safety. To be most reliable and accurate, however, the measurements must be interpreted in the context of natural variations in gas concentration and flux. Such baseline data vary both spatially and temporally due to natural processes, and a clear understanding of their values and distributions is critical for interpreting near-surface gas monitoring techniques. The best example is CO2 itself, as the production of this gas via soil respiration can create a wide range of concentrations and fluxes that must be separated from, and not confused with, CO2 that may leak towards the surface from a storage reservoir. The present article summarizes baseline studies performed by the authors at various sites having different climates and geological settings from both Europe and North America, with focus given to the range of values that can result from near surface processes and how different techniques or data processing approaches can be used to help distinguish a leakage signal from an anomalous, shallow biogenic signal.

The ultimate purpose of monitoring is to confirm safe and permanent storage of CO2 in the reservoir. Other more specific monitoring objectives are also identified in this chapter. A selection of the technologies able to meet these objectives are described using examples from existing CO2 storage sites. Each site will have different characteristics, so the most suitable combination of monitoring technologies will be site specific. Therefore possible methods to help devise suitable monitoring strategies are also suggested. Monitoring results can be used to validate site performance using temporal modelling and these can build confidence in long term predictions of safe storage.

This paper outlines the development of a methodology which can be used to produce key performance indicators for operability and efficiency of a CO2 storage site. The methodology is based on the premise that individual geological formations and their characteristics can be assessed on the basis of their depositional and tectonic setting and more recent reservoir/site history using hydrocarbon exploration and development data. The methodology is illustrated for a candidate storage reservoir in the Rotliegend Leman Sandstone Formation of the UK Southern North Sea.