Carbon capture and geological storage (CCS) is identified within the portfolio of mitigation options for climate change. Each value chain activity of large scale integrated projects (capture, transport, injection and storage) includes uncertainties and hence potential risks with respect to both environmental and human health protection. With a focus on injection and storage, a structured elicitation of international experts provides quantified judgements and uncertainties and understanding of relative risk of CCS activities. In the 0-50 year, 51-499 year and >500 year time periods, the expert panel suggested an almost equal likelihood of storage leakage occurring, with a marked decrease from minor to major to catastrophic leakage (approximately >1 in 30; 1 in 103; 1 in 104, respectively); for the same time periods, the judgement of likelihood for major leakage that would result in measurable negative effects on human health or the environment was the same (approximately 1 in 103). Insights could stimulate further scientific deliberations about the reliable and effective deployment of this complex and interdisciplinary technological process. A companion paper discusses complementary findings for issues in CCS risk management.

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With a focus on risk management (RM) in injection and storage for carbon capture and geological sequestration (CCS), an expert elicitation of scientific judgements quantified collective uncertainty ranges for a number of difficult environmental and human health risk challenges. Results suggest similarities and differences in opinions, an outcome that may be reflective of both the newness and the complexity of this technology. A suitable monitoring period was estimated at about a century; however, uncertainty was three orders of magnitude, with an upper (5th percentile) value of almost 1,000 years. For selected low probability high impact georisks, only site selection and monitoring were considered 'very' effective RM options. Monitoring, well integrity studies, emergency response plan, automatic emergency shut down system and training were considered 'very' or 'extremely' effective in managing two risks more directly related to human health. Experts responded with a wide uncertainty spread for a regulated threshold of minor, major and catastrophic leakage. A companion paper discusses elicitation findings for issues related to risk assessment.

Carbon capture and storage (CCS) risks depend upon the site geology, potential CO 2 -caprock reactions, anthropogenic pathways (legacy wellbores), and well construction and operation. Herein, we assess the major risks, termed 'georisks', acknowledging that quantitative description must be site-specific, although pathway impact generalisations are possible. We discuss geological and pathway issues to guide general site selection practices to reduce georisks. Events that trigger hazards and the consequences are presented for leakage, low storage capacity/injectivity, the release of hazardous gases and materials, surface uplift, and Induced seismicity. A supplementary literature-sourced hazard tabulation was developed with focus on four largescale North American CCS projects (Quest Project, Weyburn Project, Project Pioneer and FutureGen). Each hazard is classified based on the project phase and trigger activity. The risks of CO 2 , brine, or other fluid leakage through wells (injection, monitoring, decommissioned legacy wells) remain uncertain, but legacy well gas leakage is common, rather than exceptional, despite modern cementing and completion practices.

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