• Energy Research

Abstract We present a novel approach to detecting and quantifying a subsea release of CO2 from within North Sea sediments, which mimicked a leak from a subsea CO2 reservoir. Autonomous lab-on-chip sensors performed in situ measurements of pH at two heights above the seafloor. During the 11 day experiment the rate of CO2 release was gradually increased. Whenever the currents carried the CO2-enriched water towards the sensors, the sensors measured a decrease in pH, with a strong vertical gradient within a metre of the seafloor. At the highest release rate, a decrease of over 0.6 pH units was observed 17 cm above the seafloor compared to background measurements. The sensor data was combined with hydrodynamic measurements to quantify the amount of CO2 escaping the sediments using an advective mass transport model. On average, we directly detected 43 ± 8% of the released CO2 in the water column. Accounting for the incomplete carbonate equilibration process increases this estimate to up to 61 ± 10%. This technique can provide long-term in situ monitoring of offshore CO2 reservoirs and hence provides a tool to support climate change mitigation activities. It could also be applied to characterising plumes and quantifying other natural or anthropogenic fluxes of dissolved solutes.

Abstract Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (∼3 m below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt.

Abstract We report the first successful use of chemical sensors integrated on to an underwater vehicle to locate, map and estimate flux from a controlled sub-seabed CO2 release, analogous to a leak from a Carbon Capture and Storage (CCS) reservoir. This has global implications for the efficacy and cost of monitoring of offshore CCS sites and hence public and regulatory confidence as this tool for addressing climate change is considered and rolled out. A remotely operated vehicle (ROV) equipped with three different pH sensors was deployed to determine the spatial extent of the controlled release. The sensors each operated on a different principle (spectrophotometric, fluorescence, and electrochemical) and the strengths and weaknesses of each sensor are discussed. The sensor data demonstrated that evidence of the plume was limited to within 3 m of the seafloor, as predicted by previous modelling work. The data were then utilised to develop a model of the plume, to extend the spatial coverage of the data. This comparison of the three sensors and the insight into plume dynamics provided by the model would assist in the planning of future plume surveys to ensure the sensor and vehicle combination can resolve the plume of interest.