Case studies of time-lapse gravity and seafloor deformation monitoring and prospects for carbon storage

Abstract Time-lapse gravity and seafloor deformation monitoring is a geophysical method that simultaneously measures gravity changes at the seabed and vertical seabed displacement through repeated surveys. These measurements provide direct sensitivity to mass redistribution and pressure variations within subsurface reservoirs respectively. This integrated approach has a proven track record as a powerful tool for surveillance and management of gas fields, either complementing 4D seismic or serving as an effective alternative in cases where seismic is limited by operational constraints, cost, or inherently low sensitivity. For carbon capture and storage (CCS) applications, time-lapse gravity and seabed deformation monitoring can play a crucial role in monitoring plans, reducing reliance on costly 4D seismic and providing an alternative in scenarios with low seismic sensitivity, such as CO2 injection into formations containing residual gas or where shallow gas is present. This article presents an overview of recent field deployments at Troll, Snøhvit, Mikkel, and Ormen Lange, highlighting how history matching of the data informed reservoir management decisions. Subsequently, feasibility studies from CCS projects in the UK, Malaysia, Australia, and Norway are reviewed to show that time-lapse gravity can delineate CO2 plumes with the precision needed to ensure conformance and detect leakage to shallower formations. Furthermore, the studies show that seabed uplift measurements can map the pressure plume within the target formation, thus mitigating leakage risks through abandoned wells and providing early warning of potential impacts on neighboring producing reservoirs. Finally, to complement these depleted-gas-reservoir cases, we include a simple modeling exercise that illustrates expected gravity signal magnitudes for CO2 injection into a saline aquifer, demonstrating how key sensitivities extend to this class of storage formation.