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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.

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.

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.

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.