First controlled sub-seabed CO2 release experiment: Insights into gas migration pathways and impacts on sediment physical properties

Carbon Capture and Storage (CCS) is a key technology to potentially mitigate global warming by reducing the amount of carbon dioxide (CO2) from industrial facilities and power generation that escapes into the atmosphere. In order to broaden the usage of geological storage as a safe and reliable climate change mitigation option, it is vital to understand CO2 behaviour after its injection within a storage reservoir, including its migration through overlying sediments, as well as its biogeochemical and ecological impacts in the event of leakage at the seafloor. To address these issues, the first controlled CO2 release experiment, entitled 'Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS)', took place in Ardmucknish Bay, Oban, in May-July 2012. This experiment involved the injection of CO2 of known flux under shallow unconsolidated marine sediments over 36 days and repeated monitoring using geophysical and geochemical techniques. High resolution seismic reflection data (chirp and boomer), covering both pre-release and release stages, allows the detection of various CO2-related anomalies including seismic chimneys, enhanced reflectors within the sediment overburden and bubbles into the overlying water column. CO2 migration pattern is predominantly controlled by the stratigraphy in the early stages of the experiment. However, the increasing flow rate becomes the dominant factor determining CO2 migration, towards the end of the experiment. In addition, analysis of reflection coefficients and seismic attenuation indicates the effect of CO2 on sediment physical properties.