First controlled sub-seabed CO2 release experiment: qualitative and quantitative analysis of high-resolution seismic reflection data

Abstract

Carbon Capture and Storage is a promising climate change mitigation technology which allows the reduction of carbon dioxide emissions into the atmosphere. To assure a safe and permanent CO2 storage, it is vital to adapt efficient monitoring technologies allowing to better understand the fate of the injected CO2 within the subsurface, including its impact on sediment acoustic properties and migration into the overlying layers. The first-controlled sub-seabed CO2 release experiment, Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS), was completed in Ardmucknish Bay, Oban, where 4.2 tonnes of CO2 were injected into unconsolidated shallow marine sediments over 37 days. High-resolution seismic reflection data acquired syn-release reveal many CO2-related acoustic anomalies including seismic chimneys and enhanced reflectivity within the overburden, and bubbles within the water column. CO2 migration is interpreted to be controlled by sediment stratigraphy in the early stages of the experiment, whereas CO2 injection rate/ total injected volume overrode the stratigraphic control towards the end of gas release. Post-release seismic reflection data reveal that injected CO2 was mostly trapped below an erosional unconformity, Horizon 2, where a dip of 3.5° was found to significantly control the up-dip migration of the gaseous CO2 after the cessation of injection. The in situ CO2 content above Horizon 2 is also determined using the syn-release seismic reflection data combined with the Anderson and Hampton geaocoustics model, confirming that most of the injected CO2 was trapped below Horizon 2, or dissolved, during the QICS experiment