Sedimentary rocks with high natural CO 2 concentrations provide invaluable analogues for the long-term engineered storage of CO 2 . Some previous studies have reported high trace metal concentrations in sandstone aquifers exposed to CO 2 , a cause for concern should stored CO 2 leak into underground sources of drinking water. However, the intensively studied Jurassic sandstone aquifer in the San Rafael – Green River (Utah, USA) area has trace metal concentrations that are within USA Environmental Protection Agency’s limits for drinking water. Exceptions are As which is plausibly introduced into the aquifer by saline brines external to the aquifer, and salinity which largely is. This shows that CO 2 in aquifers does not inevitably cause trace metal contamination. CO 2 - water-rock batch experiments elucidated the controls on the trace metal concentrations. After the addition of CO 2 , the experiments reproduce well Cu, Cd, Hg, Ni and Zn, with less good agreement for Cr and Pb although these are still low compared to drinking water standards. Major cations used as fingerprints for mobilisation mechanisms suggest that the trace metals are largely derived by desorption, possibly from grain-coating Fe-oxides, rather than by the dissolution of mineral phases. Possible exceptions are Pb and Ni, plus As which is derived from saline brines.

To control reservoir pressure during CO2 injection for Carbon Capture and Storage, it may be necessary to produce native porewaters to the surface. These porewaters could contain potentially toxic metals mobilised from the reservoir rock by the injected CO2, which would then be discharged into the ocean if offshore, or treated if onshore. To evaluate the risk, both chip and grain samples from a UK North Sea sandstone that is a candidate for CO2 storage were exposed to CO2-saturated water in 30 day leaching experiments, and the metal load of the porewaters was analysed. Only Pb and Zn were convincingly mobilised (median 30 vs 2 mu g/L for Pb; 130 vs 25 mu g/L for Zn), and these elements have been previously reported to be more easily mobilised in experiments than during in-situ CO2 injection. Hence, in this case, the risk of releasing toxic metals into the environment is assessed as small, and comparable to existing hydrocarbon operations. Results are significantly variable within a single sandstone reservoir, suggesting that experiments with multiple samples are required to make a realistic assessment of leaching potential. An assessment of other potential chemical data for assessing trace metal leaching suggested that only the comparatively lengthy leaching experiments generated useful data.

This report shows that accelerating deployment of CCS can enable CO2- EOR in the UKCS. Part of the CO2 that would otherwise need to go directly to dedicated storage in CCS projects can be used to drive CO2-EOR. That gives significant benefits to the wider UK economy - extending the producing life of the North Sea, reducing imports of oil, maintaining employment, developing new capability to drive exports, and additional direct and indirect taxation revenues. At a national level this synergy between CCS and CO2-EOR could provide the overall most cost effective way to accelerate this energy transition between 2018 and 2030, to meet Committee on Climate Change de- carbonisation pathways. This CO2-EOR route also achieves two desirable UK objectives. A business demand is created, which drives sequential construction of CO2 capture, which develops learning and reduces costs of CO2 supply, which enables cheaper low-carbon electricity. CCS by this route, with secure CO2 storage already proven, develops more rapidly to protect the onshore UK economy and industry from increasing carbon prices.

Offshore storage of CO2 in the UK North Sea may lead to the production of reservoir fluids which have the potential to contain significant concentrations of contaminant metals, which could be of harm to the environment. Laboratory batch leaching experiments with CO2 at elevated temperatures, and sequential extraction analysis of reservoir materials have determined that metal concentrations in a selection of North Sea reservoir sandstones are low (parts per billion – parts per million), and that their mobility under weak acid leaching is also low. For the metals investigated, concentrations in waters produced as a consequence of CO2 storage are unlikely to exceed concentrations from current UK offshore oil and gas activities.