• Energy Research

Abstract Geological storage of the greenhouse gas CO 2 has the potential to be a widespread and effective option to mitigate climate change. As any industrial activity, CO 2 storage may lead to adverse impact on human health and the environment in the case of unexpected leakage from the reservoir. These potential impacts should be considered in a risk assessment process. We present an approach to assess the impacts on human health in case of CO 2 leakage emerging in the unsaturated zone under a building. We first focus on the migration of the CO 2 in the unsaturated zone and the foundation through numerical simulation with sensitivity analysis. Our results show that the intrusion of CO 2 into a building is substantially attenuated by the unsaturated zone and the foundation and may lead only under very specific conditions (very low ventilated parts of buildings, high flow rate and/or building situated very close to a leaking pathway) to hazardous CO 2 indoor concentrations. We have then integrated the former results in a global toolbox that provides an efficient and easy-to-use tool for decision support, which enables to assess the impacts on human health of CO 2 leakage from the reservoir to a building.

One of the main objectives of operators and regulators involved in CO2 geological storage activities is to ensure that the injected CO2 will remain safely in the underground for a long period of time. Therefore, in addition to the screening and evaluation of the performance of a potential CO2 storage site, risks of unwanted migration in the subsurface should be addressed and adequately managed. This can include the use of methods to mitigate those risks and ultimately to remediate potential adverse effects. This paper reviews the status of knowledge with regards to the mitigation and remediation technologies, from mature techniques adapted from other fields, such as oil and gas industry and environmental clean-up, to research topics offering potential new possibilities. Several categories can be defined: (1) interventions on operational or decommissioned wells to re-establish their integrity; (2) pressure/fluid management techniques for countering the leakage driving forces and/or removing the leaking fluids; (3) emerging technologies providing new mitigation opportunities for controlling undesired CO2 migration; (4) techniques to remediate the impacts potentially induced by such a migration. This technical state of the art is completed by the actual practices in the emerging field of CO2 geological storage established from the regulatory requirements and guidelines, and from the experience gained in existing storage projects over the world. This article concludes on important best practices stemming from this review and on future challenges in terms of research topics and operational needs.

Abstract Leakage of CO2 or brine coming from CO2 geological storage sites constitutes a risk for overlying fresh groundwater resources. One of the main risks is the potential alteration of groundwater quality by the intrusion of contaminants such as trace elements. This paper reviews studies that address the potential impacts of CO2 geological storage leakage on fresh groundwater quality. Leakage can directly modify the chemical properties of fresh water (pH, redox potential, chemical composition) and, as a result, indirectly modify the effect of biogeochemical processes controlling trace element availability. The ability of a CO2 or brine leak to introduce or mobilize trace elements and potentially degrade the quality of water in an overlying aquifer depends on the composition and quantity of the leaking fluids, the nature of the solid phases making up the aquifer (buffering and scavenging capacity) and the concentrations of undesirable or toxic elements that can be mobilized following any such modification. Furthermore, hydrogeological conditions will control the potential dissemination into groundwater. To date, studies have shown that trace elements can be significantly mobilized without necessarily exceeding quality thresholds. In a few cases where aquifers are naturally rich in trace elements (i.e. whose natural concentrations in groundwater are already high), CO2 is able to mobilize these trace elements (e.g. Fe, Mn, Ni, As, Ba, U) and increase concentrations up to or exceeding threshold values. This literature review provides a return on experience essential for both assessing biogeochemical risks prior to the installation of future CO2 geological storage sites and designing and installing fresh groundwater quality monitoring networks.