QICS work package 1: migration and trapping of CO2 from a reservoir to the seabed or land surface

Natural CO2 seeps can be used as analogues for studies into surface flux and impact resulting from leaking engineered geological CO2 reservoirs. However their long-lived nature often means that the local environment has either adapted or evolvedaround the seepage site. The ‘Quantifying Impact of carbon storage’ (QICS) experiment provides the solution to this issue by releasing CO2 into an environment previously untouched by CO2. Work Package 1 (WP1) of the QICS project is primarily concerned with the migration of CO2 in the subsurface and how to relate the results of the relatively shallow experiment to a full storage scale setting in the UK North Sea. The main objectives of WP1 are to investigate potential leakage pathways from the reservoir to the surface, determine possible leakage rates and assess the potential volumes of leaked CO2 that can reach the surface environment

What Have We Learnt About CO2 Leakage in the Context of Commercial-Scale CCS?

The viability of Carbon Capture and Storage (CCS) depends on the reliable containment of injected CO2 in the subsurface. Robust and cost-effective approaches to measure monitor and verify CO2 containment are required to demonstrate that CO2 has not breached the reservoir, and to comply with CCS regulations. This includes capability to detect and quantify any potential leakage to surface. It is useful to consider the range of possible leak rates for potential CO2 leak pathways from an intended storage reservoir to surface to inform the design of effective monitoring approaches. However, in the absence of a portfolio of leakage from engineered CO2 stores we must instead learn from industrial and natural analogues, numerical models, and laboratory and field experiments that have intentionally released CO2 into the shallow subsurface to simulate a CO2 leak to surface. We collated a global dataset of measured or estimated CO2 flux (CO2 emission per unit area) and CO2 leak rate from industrial and natural analogues and field experiments. We then examined the dataset to compare emission and flux rates and seep style, and consider the measured emission rates in the context of commercial scale CCS operations. We find that natural and industrial analogues show very wide variation in the scale of CO2 emissions, and tend to be larger than leaks simulated by CO2 release experiments. For all analogue types (natural, industrial, or experiment) the emission rates show greater variation between sites than CO2 flux rates. Quantitation approaches are non-standardized, and that measuring and reporting both the CO2 flux and seep rate is rare as it remains challenging, particularly in marine environments. Finally, we observe that CO2 fluxes tend to be associated with particular emission characteristics (vent, diffuse, or water-associated). We propose that characteristics could inform the design and performance requirements for CO2 leak monitoring approaches tailored to detect specific emission styles

Digging deeper : the influence of historical mining on Glasgow's subsurface thermal state to inform geothermal research

Studies of the former NE England coalfield in Tyneside demonstrated that heat flow perturbations in boreholes were due to the entrainment and lateral dispersion of heat from deeper in the subsurface through flooded mine workings. This work assesses the influence of historical mining on geothermal observations across Greater Glasgow. The regional heat flow for Glasgow is 60 mW m−2 and, after correction for palaeoclimate, is estimated as c. 80 mW m−2. An example of reduced heat flow above mine workings is observed at Hallside (c. 10 km SE of Glasgow), where the heat flow through a 352 m deep borehole is c. 14 mW m−2. Similarly, the heat flow across the 199 m deep GGC01 borehole in the Glasgow Geothermal Energy Research Field Site is c. 44 mW m−2. The differences between these values and the expected regional heat flow suggest a significant component of horizontal heat flow into surrounding flooded mine workings. This deduction also influences the quantification of deeper geothermal resources, as extrapolation of the temperature gradient above mine workings would underestimate the temperature at depth. Future projects should consider the influence of historical mining on heat flow when temperature datasets such as these are used in the design of geothermal developments

Rapid water-rock interactions evidenced by hydrochemical evolution of flowback fluid during hydraulic stimulation of a deep geothermal borehole in granodiorite : Pohang, Korea

Flowback water from the 4215 m deep (True Vertical Depth) PX-1 borehole, following the August 2017 hydraulic stimulation of a granodiorite geothermal reservoir in Pohang, South Korea, was monitored for a suite of physicochemical, chemical and isotopic parameters. The results provide unique insights into mixing processes, fluid evolution and rapid water-rock interaction in a deep geothermal system. Injected water for stimulation was relatively fresh, oxidising surface water, with temperature 29.5 °C and pH c. 6.5. The flowback water showed an increasing content of most solutes, with the evolution conforming to an exponential ‘flushing’ model for conservative solutes such as chloride. Flowback water became progressively Na–Cl dominated, with a circumneutral pH (7.1) and negative oxidation-reduction potential (c. −180 mV). Some solutes (including, Na, K and Si) increased more rapidly than a flushing model would suggest, implying that these had been acquired by the flowback water due to mineral hydrolysis. Stable isotopes of O and H indicate that initially meteoric waters have undergone geothermal oxygen isotope exchange with minerals. Evolution of redox species in recovered water suggests progressively oxidising zonation around the injection borehole in an otherwise reducing reservoir. Rapidly increasing silica concentrations in flowback water suggests extensive quartz dissolution and indicated a reservoir temperature of up to 169 °C. This lends plausible, if equivocal support to the hypothesis that quartz dissolution by injection water may have contributed to triggering movement on the pre-stressed fault associated with the November 2017 Mw 5.5 Pohang earthquake

Sulphur isotopes in deep groundwater reservoirs: evidence from post-stimulation flowback at the Pohang geothermal facility, Korea

A hydraulic stimulation was carried out on a granodiorite reservoir in an enhanced geothermal system in August 2017 in Pohang, Korea. Water injected into the 4.2 km deep PX-1 well contained c. 330−360 mg/L sulphate, with a negative δ34S. The resulting flowback water became more saline with time, with sulphate and chloride concentrations and dissolved sulphate δ34S all increasing. Compared with conservative advective-dispersive and mixing models, the flowback contained surplus sulphate with an elevated δ34S. The PX-1 reservoir fluid is saturated with respect to anhydrite at downhole temperatures and pressures. Dissolution by injected surface water of secondary anhydrite along fracture surfaces, most likely with elevated δ34S reflecting the reservoir fluid, is likely to have resulted in an excess of 34S-enriched sulphate in the flowback fluid. An alternative hypothesis involving oxidation of pyrite is also plausible but is stoichiometrically inadequate to account for the observed sulphate excess, and unlikely from a sulphur isotopic perspective. This analysis thus contributes to the evidence for water-rock reactions during stimulation of the Pohang granodiorite

The occurrence of elevated δ34S in dissolved sulfate in a multi-level coal mine water system, Glasgow, UK

Stable isotopic compositions of δ18O, δ2H and dissolved sulfate δ34S in water from abandoned and flooded coal mines are used to interpret the water's and solutes' origin and interactive history. These isotopic ratios have been determined in mine water from a shallow (<100 m) series of overlapping coal mine workings at the UK Geoenergy Observatories Glasgow Geothermal Energy Research Field Site (UKGEOS GGERFS). Comparison has been made between dissolved sulfate δ34S of water in mined Carboniferous coal-bearing strata, that of water which had interacted with equivalent unmined strata, and δ34S in sulfide-bearing minerals (pyrite) in the host sedimentary rocks and local evaporite (gypsum) of the Lower Carboniferous Ballagan Formation. δ18O and δ2H confirm a meteoric origin for coal mine waters. The δ34S arithmetic mean and standard deviation of the pyrite (+5.0 ± 15.5‰) and water from unmined strata (+0.3 ± 2.1‰) were found to be similar, whereas the mine water exhibited elevated isotopic values (+20.3 ± 1.1‰), plotting closer to that of modern-day seawater (+21.2‰) and Ballagan Formation gypsum (+18.9 ± 0.5‰). Whilst the origin of dissolved sulfate in the mine water remains unclear, it is unlikely to be wholly to simple pyrite oxidation. Influence of evaporite dissolution, fractionation associated with microbial sulfate reduction, and mixing with saline formation waters of marine, evaporitic or of another origin, cannot be ruled out

Public perception of geothermal energy at the local level in the UK

Successful development of a new renewable energy technology does not only rely on the success of pilot projects but also to a significant extent on the development of an adequate public engagement strategy. To be able to develop such a strategy a good understanding is required of the public perception of the new technology. Geothermal energy is still an emerging technology in the UK; in the absence of public debate on this topic, media reporting provides a suitable proxy for its public perception. Therefore, this study has gauged the public perception of geothermal energy in the UK by evaluating local news articles from 1980 to 2018. A coding scheme was developed to derive the main themes and to identify both the perceived advantages and hurdles for geothermal development. We focussed on local newspapers to be able to compare public perception in different regions in the UK. Results show a mainly positive perception of geothermal energy in all geographical regions across the UK. Only few articles mention risks, induced seismicity and environmental pollution. In contrast, advantages and positive aspects of geothermal energy, such as its carbon neutral footprint, the enormous amount of available geothermal heat and the potential contribution of geothermal development to the revival of local economies, are much more frequently mentioned. Perceived hurdles that are mostly described in the articles are: (1) the absence of geothermal legislation and subsidies in the UK; (2) the lack of available funding; and finally (3) technological and geological challenges or uncertainties. Finally, we show that geothermal energy is most often related to electricity generation and granite resources, while the only successful deep geothermal site in the UK is a direct-use heating scheme exploiting a sedimentary aquifer

A review of the performance of minewater heating and cooling systems

As the decarbonisation of heating and cooling becomes a matter of critical importance, it has been shown that flooded mines can provide a reliable source of low-carbon thermal energy production and storage when coupled with appropriate demand via an appropriate heat transfer technology. This paper summarises the potential resource represented by a long legacy of mining operations, the means heat can be extracted from (or rejected to) flooded mine workings, and then considers the risks and challenges faced by minewater geothermal energy (MWG) schemes in the planning, construction, and operational phases. A combination of site visits, interviews and literature reviews has informed concise, updated accounts for many of the minewater geothermal energy systems installed across the world, including accounts of hitherto unpublished systems. The paper has found that a number of previously reported MWG schemes are now non-operational. Key risks encountered by MWG schemes (which in some cases have led to decommissioning) include clogging of system components with mineral precipitates (e.g., ochre), uncertainty in targeting open mine voids and their hydraulic behaviour, uncertainty regarding longevity of access to minewater resource, and accumulated ongoing monitoring and maintenance burdens

What have we learnt about CO2 leakage in the context of commercial-scale CCS?

The viability of Carbon Capture and Storage (CCS) depends on the reliable containment of injected CO2 in the subsurface. Robust and cost-effective approaches to measure monitor and verify CO2 containment are required to demonstrate that CO2 has not breached the reservoir, and to comply with CCS regulations. This includes capability to detect and quantify any potential leakage to surface. It is useful to consider the range of possible leak rates for potential CO2 leak pathways from an intended storage reservoir to surface to inform the design of effective monitoring approaches. However, in the absence of a portfolio of leakage from engineered CO2 stores we must instead learn from industrial and natural analogues, numerical models, and laboratory and field experiments that have intentionally released CO2 into the shallow subsurface to simulate a CO2 leak to surface. We collated a global dataset of measured or estimated CO2 flux (CO2 emission per unit area) and CO2 leak rate from industrial and natural analogues and field experiments. We then examined the dataset to compare emission and flux rates and seep style, and consider the measured emission rates in the context of commercial scale CCS operations. We find that natural and industrial analogues show very wide variation in the scale of CO2 emissions, and tend to be larger than leaks simulated by CO2 release experiments. For all analogue types (natural, industrial, or experiment) the emission rates show greater variation between sites than CO2 flux rates. Quantitation approaches are non-standardized, and that measuring and reporting both the CO2 flux and seep rate is rare as it remains challenging, particularly in marine environments. Finally, we observe that CO2 fluxes tend to be associated with particular emission characteristics (vent, diffuse, or water-associated). We propose that characteristics could inform the design and performance requirements for CO2 leak monitoring approaches tailored to detect specific emission styles