oa Unlocking the Offshore CCS Monitoring Puzzle: Time-lapse Gravity and Seafloor Deformation Surveys

Monitoring strategies for offshore CCS cannot simply replicate those used in the oil and gas industry. Regulations mandate monitoring throughout the project’s entire lifespan, including decades post-closure. CCS calls for cost-effective solutions, making expensive high-resolution methods like seismic less viable. Moreover, environmental regulations pose challenges to the intensive use of seismic methods, that feature large CO2 footprints and active sources. Operationally, the presence of infrastructure like wind farms, hinders the systematic surveying patterns required for seismic monitoring. Last but not least, 4D seismic signal detectability can be low when CO2 is injected into depleted gas reservoirs due to the weak contrast with the preexisting residual gas. Time-lapse gravity and seafloor deformation monitoring is a mature commercial technology used for 25 years in Norway to fully replace or complement 4D seismic in gas-producing reservoirs. Recent modeling for CCS projects like Morecambe in the UK demonstrates very significant and robust time-lapse gravity signals after one year of injection, indicating an excellent capability to delineate both the CO2 and the pressure plumes. This offers a viable path forward for monitoring CO2 storage sites sustainably and cost-effectively, while overcoming the operational limitations of 4D seismic. This can be crucial for the long-term success and economic viability of CCS projects.