Developments since 2005 in understanding potential environmental impacts of CO2 leakage from geological storage
Introduction
One of the concerns regarding Carbon Capture and Storage (CCS) is that CO2 might leak out of the storage reservoir towards the ground surface, with possible adverse impacts on underground drinking water supplies or ecosystems either onshore or offshore.
The IPCC Special Report on Carbon Dioxide Capture and Storage (IPCC, 2005) concluded that, with appropriate controls in place, ‘the local health, safety and environment risks of geological storage would be comparable to the risks of current activities such as natural gas storage, EOR, and deep underground disposal of acid gas’. They envisaged two possible leakage scenarios that might give rise to environmental impacts: (1) abrupt leakage, through injection well failure or leakage up an abandoned well, and (2) gradual leakage, through undetected faults, fractures or wells. These remain the two most likely leakage scenarios (e.g. Paulley et al., 2013, Paulley et al., 2013, RISCS, 2014).
In terms of possible onshore environmental impacts, the IPCC considered that elevated CO2 concentrations in the shallow subsurface could include lethal effects on plants and subsoil animals and the contamination of groundwater, while high fluxes in conjunction with stable atmospheric conditions could lead to local high CO2 concentrations in the air that could harm animals or people. However, the IPCC report said little about offshore impacts of CCS, dealing more with the possible impacts of ocean storage of CO2, which is currently excluded from consideration under both the London Protocol/Convention and the Convention for the protection of the marine environment of the North–East Atlantic (OSPAR). A significant proportion of storage capacity is offshore, particularly in Europe (Geocapacity, 2009).
With regard to soil, the impacts could be the result of increased CO2 and/or the displacement of oxygen by CO2. The IPCC recognised that high quality baseline data would be needed to detect low rates of leakage. The IPCC 2005 report mentioned effects on microbes at depth but said little about soil microbes. With regard to plants it stated that: ‘While elevated CO2 concentrations in ambient air can accelerate plant growth, such fertilization will generally be overwhelmed by the detrimental effects of elevated CO2 in soils, because CO2 fluxes large enough to significantly increase concentrations in the free air will typically be associated with much higher CO2 concentrations in soils’. ‘The effects of elevated CO2 concentrations would be mediated by several factors: the type and density of vegetation; the exposure to other environmental stresses; the prevailing environmental conditions like wind speed and rainfall; the presence of low-lying areas; and the density of nearby animal populations’. The most obvious characteristic of long-term elevated CO2 zones at the surface is the lack of vegetation; ‘New CO2 releases into vegetated areas cause noticeable die-off. In those areas where significant impacts to vegetation have occurred, CO2 makes up about 20–95% of the soil gas, whereas normal soil gas usually contains about 0.2–4% CO2. Carbon dioxide concentrations above 5% may be dangerous for vegetation and as concentration approach 20%, CO2 becomes phytotoxic’.
According to the IPCC report, groundwater impacts could be directly from CO2 or indirectly through displaced brine as a result of pressure increases attendant on CO2 injection. If CO2 were to leak into an overlying potable aquifer it would dissolve into the water to form carbonic acid, which in turn could react with the aquifer mineral phases and potentially mobilize in situ (toxic) metals, such as Pb or As, SO42− or Cl−. Instead, if brine associated with the storage reservoir were to migrate into an aquifer it may increase the groundwater's salinity, or add toxic elements, that may otherwise exist in low concentrations in the aquifer mineral phases. Although it was known that plume migration and element mobility in either case will be a complex function of interacting chemical reactions (adsorption–desorption, dissolution–precipitation) and site specific characteristics (e.g. aquifer mineralogy, groundwater versus leakage flow rates, redox conditions) that could greatly influence any potential impact, almost no research focussed on these issues had been published when the IPCC report was originally written.
Gaps that were recognised in the IPCC report included:
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The temporal variability and spatial distribution of leaks that might arise from inadequate storage sites.
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Microbial impacts in the deep subsurface.
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Environmental impact of CO2 on the marine seafloor.
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Methods to conduct end-to-end quantitative assessment of risks to human health and the local environment.
They considered that research was required on:
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Further knowledge of the history of natural accumulations of CO2.
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Effective and demonstrated protocols for achieving desirable storage duration and local safety.
There have been quite significant developments in research into potential environmental effects of leakage from CO2 storage since the IPCC report in 2005. Onshore impacts have been studied in the last 10 years through a wide range of projects. Experimental injection sites have been set up to study both soil/plant/microbial and aquifer impacts on four continents from depths of less than a metre to several hundred metres. Research has also continued on natural CO2 release sites and laboratory experiments have been conducted to assess effects under more closely controlled conditions with modelling to further understanding. Research into potential near surface (essentially soil) impacts of CO2 leakage is described further in Section 2, while drinking water aquifer impacts are considered in Section 3. The lack of studies of seafloor impacts has been addressed through a number of recent projects, especially in Europe and Japan, including the world's first offshore injection experiment off the coast of Scotland for the QICS project. Mesocosm experiments have considered offshore impacts on individual species, whole communities and ecosystem level processes at a variety of scales and observations have been made at offshore natural CO2 sites. Experiments have also been conducted in situ through the use of benthic chamber lander systems. These developments are addressed in Section 4.
Effects on man and other terrestrial animals directly from atmospheric CO2 are well understood and have not been a major focus of research by the CO2 storage community. There have, however, been some studies into fatalities from the large number of natural CO2 occurrences in Italy, which concluded that the risk of accidental death from 286 documented seepage sites was significantly lower than many socially accepted risks (Roberts et al., 2011). Their modelling suggested that seepage from storage would be less than that of the natural seeps. This aspect of impacts work will not be considered further here, as we concentrate on impacts to other parts of the ecosystem.
CCS regulations have developed significantly since 2005 (Dixon et al., 2015 this volume). Legislation to allow sub-seabed storage has been enshrined in the London Protocol and Convention. Offshore storage requirements have been produced for the NE Atlantic through OSPAR (Dixon et al., 2009, OSPAR, 2007) and much of the OSPAR regulations taken forward into the EU Directive on geological storage of CO2 (European Union, 2009), which also covers onshore storage. Regulation has also been developed outside Europe, most notably in the US, Canada, Australia, and Japan. Of particular relevance to this account are the requirements for CO2 injection Class VI wells in the USA (United States Environmental Protection Agency, 2012), as these set out to protect underground sources of drinking water.
OSPAR requires characterisation of site specific risks to the marine environment and collection of baseline data for monitoring. The site operator must consider the risk of adverse impacts and assess possible effects of leakage on the marine ecosystem, including human health and impacts on legitimate users of the marine environment. Site selection should consider the risk of adverse impacts on sensitive, or endangered, habitats and species and natural resources. This includes possible effects from the CO2 itself, impurities within the injected CO2 and any fluids that might be displaced as a result of CO2 injection. Monitoring should be linked to the risk assessment and ‘impact hypothesis’ which includes evaluation of potential ecosystem impacts.
OSPAR (2007) also recognised the need for further research into the effects of CO2 on marine ecosystems to consider more species at different life stages, and include microbial communities, experimental field studies and models.
Similarly, marine environmental protection is enshrined in Japanese legislation through an amendment to the marine pollution prevention act (Carbon Dioxide Capture and Storage (CCS) Study Group, 2009) based on the London Protocol. Guidance on the safe operation of a demonstration storage site includes a consideration of the potential environmental impacts (both onshore and offshore) and collection of baseline data against which to compare monitoring results gathered during CO2 injection. Storage must not harm the conservation of the marine environment and the pollution status of the site must be monitored. Baseline environmental surveys have been carried out at the Tomakomai CCS demonstration site (Tanaka et al., 2014). Offshore regulations in Australia also follow the London Protocol and require a plan that demonstrates that environmental impacts and risks will be at an acceptable level (Office of Parliamentary Counsel, 2014). The Australian state of Victoria is the only one to currently have both onshore and offshore regulations and storage there must not cause significant risk to the environment or human health (State of Victoria, 2008, State of Victoria, 2010).
Whilst the requirement for Environmental Impact Assessments for CCS projects is still under review in the Canadian province of Alberta, the federal government did require them for projects such as QUEST (Alberta Energy, 2013).
The EU Directive on CCS (European Union, 2009) and associated guidance reflects the OSPAR FRAM requirements but also includes onshore concerns. Thus one of the purposes of monitoring is for ‘detecting significant adverse effects for the surrounding environment, including in particular on drinking water, for human populations, or for users of the surrounding biosphere’. Guidance following the Directive (European Commission, 2011) states that ‘valuable natural resources in proximity to a potential storage complex have to be documented and the risk linked to the exposure to CO2 leakage has to be carefully assessed’. This includes consideration of conservation areas, and potable groundwaters in risk assessments and environmental monitoring as part of baseline surveys.
US regulation, in particular the designation of Class VI wells for CO2 injection for storage (United States Environmental Protection Agency, 2012) stresses the protection of underground sources of drinking water (USDWs). It requires the monitoring of water quality in an agreed network of monitoring wells, above the primary seal on the reservoir, within a defined area of review that is likely to expand as injection proceeds and the CO2 plume spreads in the subsurface. Water quality has to be tested prior to, during and after injection, initially on a quarterly basis. QA/QC requirements are stipulated as are a minimum list of likely analytes with options to broaden that list. Data are to be reported every 6 months. The regulators may also insist on surface gas monitoring to further evaluate any threat to USDWs or to meet additional state or federal legislation.
Any environmental impacts from CCS need to be considered relative to the benefits to be gained from reduction in greenhouse gas emissions and in the context of the natural variability of the ecosystem and other environmental impacts, including the effects of global climate change (e.g. changing weather patterns, increased frequency of extreme events, ocean acidification), industrial contamination and trawling.
Something of a geographical split has developed between onshore and offshore storage. In North America emphasis is on onshore projects (e.g. Decatur, Cranfield, Weyburn, Quest, Aquistore), although offshore sites are being considered off the Gulf Coast. In contrast, most current (Sleipner, Snøhvit) and proposed (ROAD, Peterhead-Goldeneye and White Rose) sites in Europe are offshore, where much of the storage capacity is (e.g. Geocapacity, 2009), and onshore projects have faltered, largely because of public opposition. The Tomakomai project in Japan involves offshore storage. Developments in China (e.g. Shenhua Ordos) and Australia (Gorgon, Otway) are, so far, onshore whilst proposals in South Korea may involve a mix of onshore and offshore sites.
Comparatively little quantified data has been published describing possible leakage scenarios due to high degrees of uncertainty, especially in predicting geological flow mechanisms and rates. Whilst there are some analogues in the form of well blowouts, when drilling into natural CO2 occurrences, that may provide upper limits, there is no direct evidence of significant leakage from existing storage sites. Given this, risk assessments have tended to investigate a range of theoretical leakage scenarios, from a minimum inconsequential leak up to a plausible maximum, invoking operational or geological mechanisms of leakage in each case. Upper limits to possible leakage from transportation can be easily constrained as pipeline flow rates are known (e.g., ∼3 ktonnes d−1 at the Sleipner field), and it is assumed that such leaks could be stemmed in a matter of hours to days. Leakage from pipelines at lower rates, and from storage, is more speculative, with fluxes estimated, for example, from <1 tonne d−1, to 10–100 tonnes d−1, to >1 ktonne d−1 being associated with seepage, abandoned wells/geological discontinuities and catastrophic operational failures, respectively (IEA, 2008, Klusman, 2003). A similarly wide range of possible flux rates was also felt to be plausible by a more recent assessment (Paulley et al., 2012) with areas of emission varying from the diameter of a borehole (e.g. 0.03 m2) up to several point sources in an area of 50,000 m2 for fault-related leakage.
Section snippets
Conclusions
Since the publication of the IPCC Special Report in 2005 there has been a significant focus worldwide on the potential impacts of leakage from geological storage of CO2. Projects have examined possible effects on near surface ecosystems, both onshore and offshore, and on underground drinking water supplies. Much of this work has arisen from developing regulations for CCS. However, relevant information has also come from other avenues of research, most notably from investigation of other impacts
Acknowledgements
This paper has drawn on the results of many studies. We would like to acknowledge in particular our involvement in a number of these and the many colleagues who contributed. These include the FP6 European Network of Excellence CO2GeoNet; Research into Impacts and Safety in CO2 Storage (RISCS), funded by the EC 7th Framework Programme (Project No. 240837) and by Industry Partners ENEL, I&I, Statoil, Vattenfall AB, E.ON and RWE; ECO2 – Sub-seabed CO2 Storage: Impact on Marine Ecosystems, also
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