Long-Term Monitoring of Relict Wells: the Development of a Real-Time Acoustic-Chemical Lander for Project Greensand

The worldwide development of marine carbon capture and storage complexes necessitates sophisticated monitoring tools capable of detecting real-time changes. In coastal seas, there are growing tensions between wind farm developments and proposed carbon capture and storage complexes which underlie them. 3D seismic reflection surveys are not thought to be possible within the boundaries of wind farms, and while 4D seismic reflection can sometimes track large scale sub-surface gas migration, they struggle to detect small scale leaks and are expensive and environmentally unfriendly.

One of the most likely leakage pathways are relict wells which could be pathways for the rapid ascent of buried CO2 to the seabed. As the location of relict wells in storage sites is well known it is possible to design a Measurement, Monitoring and Verification plan which incorporates a number of landers to continuously monitor these “higher risk zones” throughout a complex’s life. Here we describe a lander developed as part of Project Greensand Phase 2, a large-scale CCS initiative offshore Denmark, and present results from a dockside experiment. The lander comprises chemical sensors to monitor pH, nitrate, alkalinity, local currents, and the salinity; a multibeam echosounder which can detect CO2 bubble streams; battery and communication equipment.