Assessing the CO2 stored mass at the Sleipner storage site from time lapse gravity data

This study shows that multiscale imaging methods applied to time-lapse gravity data can be effective to estimate the subsurface stored mass of CO₂. Differently from previous studies based on simplified plume shapes, we show that a multiscale analysis of gravity data is particularly useful to properly estimate the excess mass from gravity anomalies associated with complex plume geometries and characterized by multi-homogeneity properties. With a multiscale approach, in fact, we can exploit the scaling behavior of the potential fields and assess the variation in the degree of homogeneity and, consequently, the estimation of the structural index of the source. 

The simulated gravity dataset and the estimated homogeneity degree values at different altitudes showed that, as the distance from the source increases, the gravity field associated with the CO₂ plume becomes progressively smooth and can be approximated as homogeneous. Moreover, the multiscale analysis effectively reduces the noise effect, that is particularly advantageous for CO₂ storage monitoring, where low signal-to-noise ratios are expected. The excess mass inferred using our approach results closely equal to the true value with accuracy higher than 99%. Our multiscale analysis was also successfully applied to the real time-lapse gravity dataset acquired at the Sleipner site.

This study presents a useful approach for developing new monitoring strategies for CCS purposes. Time-lapse gravity surveying has again proven to be an effective tool for inferring key reservoir properties, complementing seismic monitoring techniques.