Practical Qualitative and Quantitative Models in Carbon Capture Utilization and Storage Risk Assessment

Carbon Capture Utilization and Storage (CCUS) has been recognized as a tool to aid the decarbonisation of carbon heavy industries, with the storage of CO₂ in the subsurface over geological timescales being a key component in the CCUS process chain. Assessing and managing the risks involved in subsurface storage of CO₂ is required so decisions can be made about the design, operation, monitoring and acceptability of potential projects. As with existing industrial activities, a number of approaches exist for assessing risk ranging from purely qualitative approaches through to quantitative approaches.

A structured, proportional risk assessment approach is proposed, aimed at identifying, analysing and evaluating the risk from a candidate subsurface store through a combination of qualitative and quantitative techniques to gain the most benefit. Development of a register of subsurface containment risks allows for the identification of scenarios of concern, which can be coupled with risk matrices to provide an estimation of the risk level together with its acceptability. More in depth qualitative techniques such as bowtie analysis encourage different disciplines to collaborate and enhance communication, both internally and externally, of the controls present for the most significant risks. The combination of bowties with a quantified, event tree-based model can allow for numerical determination of leakage probability and magnitude. Quantitative methods can therefore estimate insurance liabilities and allow comparison of options or with acceptance criteria. However, these assessments are underpinned by the quality of inputs. Given that the CCUS industry is still in its infancy, data surrounding event likelihoods and leak magnitudes, especially for geological leakage pathways, can have large amounts of associated uncertainty; this uncertainty must be taken into account when evaluated the overall acceptability of projects. Whilst both quantitative and qualitative methodologies for risk assessing subsurface CO₂ storage have their advantages and disadvantages, it is when combined as part of a structured CCUS risk assessment approach that the most benefit is gained.