What gravity monitoring may reveal about CO₂ storage: Insights from the Sleipner site (Norway)

• 4D gravity data reveals CO₂ plume shape and changes in stored mass. • Multiscale gravity methods effectively model complex plume geometries. • Multi-method analysis enhances understanding of Sleipner gravity data. This study explores the use of 4D surface gravity data to monitor CO₂ stored in geological formations. We show that gravity method can provide useful and independent information for tracking CO₂ plume development and estimating stored mass, thereby supporting the safety assessment of carbon storage. Our objective is to delineate the plume spatial extent, evaluate its depth, and estimate net CO₂ mass changes over time. We focus on the Sleipner storage site, where 4D gravity surveys have been used to monitor CO₂ migration. We computed the time-lapse gravity field from multi-physics simulations based on the Utsira benchmark model and defined a workflow to process the data. We first performed the boundary analysis of the gravity field to evaluate its capability to resolve the plume shape within the limits in spatial resolution. Then we showed that, despite the plume complex geometry, the field behaves as a homogeneous field at large altitudes. This allowed the DEXP transformation of the field to yield an accurate depth and mass estimation of the stored CO 2 also confirmed by constrained inverse modeling. This workflow was then applied to the gravity dataset collected as part of the Sleipner monitoring program. Despite limitations in spatial coverage and data sampling, the inferred CO₂ mass and plume extent from the Sleipner gravity data are consistent with injected volumes and the seismic plume boundaries, supporting the robustness of gravity-based monitoring.