Monitoring tools and best practice guidelines for attribution and quantification of potential CO2 leakages into deep waters at ocean crust CCS sites

Carbon dioxide (CO2) removal (CDR) methodologies have been proposed as essential measures to limit global warming and mitigate climate change. Carbon mineralisation, a natural process that involves the reaction of CO2 in the fluid form with reactive geologic formations to produce water-insoluble carbonates, when accelerated can permanently remove and sequester CO2 from the atmosphere at a scale of gigatons per year. The estimated storage capacity along mid-ocean ridges exceeds anthropogenic CO2 emissions by orders of magnitude but to date no offshore carbon capture storage (CCS) at ocean crust is performed. For offshore CCS, there are challenges in implementing accurate monitoring strategies and in developing robust and cost-effective technologies to support these strategies. Here we compare the sensitivities to leakage attribution and quantification of different approaches considered suitable for monitoring offshore CCS sites. As a proof of concept, we limited our target environment to the water column, specifically ocean waters below 1000 m. The effectiveness of the considered monitoring techniques and technologies was compared in terms of spatial and temporal coverage, reliability, costs and technology readiness level. A theoretical monitoring system design was proposed in order to provide guidelines for prompt detection of CO2 leakages into deep ocean waters. Appropriate monitoring tools and solid protocols for the assessment of potential environmental impacts will enhance public confidence and support efficient management of CCS operations in deep water ocean crust.