What have we learnt about CO2 leakage from CO2 release field experiments, and what are the gaps for the future?

Abstract Legislation and guidelines developed for Carbon Capture and Storage (CCS) have set performance requirements to minimize leakage risk, and to quantify and remediate any leaks that arise. For compliance it is necessary to have a comprehensive understanding of the possible spread, fate and impacts of any leaked CO2, and the ability to detect and quantify any CO2 seepage into marine or terrestrial environments. Over the past decade, a number of field scale CO2 release experiments have been conducted around the world to address many of the uncertainties regarding the characteristics of near-surface expression of CO2 in terms of the impact and quantitation of CO2 leaks. In these experiments, either free phase or dissolved CO2 was injected and released into the shallow subsurface so as to artificially simulate a CO2 leak into the near-surface environment. The experiments differ in a number of ways, from the geological conditions, surface environments, injection rates and experimental set-up - including the injection and monitoring strategy. These experiments have provided abundant information to aid in the development of our scientific understanding of environmental impacts of CO2 while assessing state of the art monitoring techniques. We collated a global dataset of field-scale shallow (depths