Deep geological storage of CO2, offshore UK – Supplementary note B: Geological leakage risks

This document provides additional technical information that underpins our assessment of the containment certainty of deep geological storage of CO2 in offshore sites on the UK continental shelf (UKCS). The aim of this document is to present a thorough explanation and justification of the key findings and conclusions of the main report (Daniels et al. 2023), and is aimed at a technically experienced geoscience audience.

Many studies and pilot projects to date have concluded that geological storage of CO2 in suitably selected and assessed sites is safe, and that there is very high certainty that a very high proportion of the injected CO2 will remain within the storage site for the foreseeable future. Since containment certainty is extremely high, its corollary, “risk of leakage” is extremely low; however, it is not zero. This appendix focuses on the geological aspects that contribute to the non-zero risk of CO2 leakage.

Document layout:

• Sections 1 and 2 of this report focus on general aspects of geological storage risk.
• Sections 3 to 6 each describe an overall category of geological leakage pathway in more detail.
• The quantitative results of our assessment of leakage rates and leakage probabilities are given in Sections 7 and 8, respectively.
• Section 9 collates our combined leakage rates and probabilities in comparison with other published and unpublished assessments.