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The capture and geological storage of CO 2 is essential for net zero. Large volumes of secure subsurface storage will be required, much in unexplored saline aquifers. At the Otway International Test Centre we have executed a series of storage tests, including geophysical surveys, geochemical and petrophysical characterization, drilling and instrumenting six deep wells and conducting two intensively monitored injections each of 15 Kt CO 2 . We summarize the execution and outcomes to draw out the implications for larger-scale storage and for migration trapping of CO 2 plumes. Immobilization of most of the plume is essential for long-term safety. Suitable saline aquifers should have sealing caprocks but need not have structural traps. Buoyant CO 2 could move kilometres beneath the seal, with immobilization occurring by capillary forces and dissolution. Predicting and monitoring plume behaviour with practical methods integrated with risk management will be necessary. We characterized and monitored migration trapping, demonstrated plume immobilization, and tested monitoring methods tailored to large saline aquifers. We developed pressure tomography and continuous seismic monitoring; these provide less detailed information than conventional methods but can be targeted at risks. Passive methods, such as the seismic response to earthquakes, can also act as sentinels for specific events. These methods can bridge the ‘monitoring gap’ between detailed but infrequent methods and more frequent but simpler methods and the results will enable better trade-offs between cost, complexity and risk for a technology crucial for controlling CO 2 levels in the atmosphere. Supplementary material : Detailed descriptions of unpublished modelling are available at https://doi.org/10.6084/m9.figshare.c.7148197
Jenkins et al. (Wed,) studied this question.