Time-lapse gravity surveying as a monitoring tool for CO2 storage

Time-lapse gravity surveys are a potential low cost method for detecting CO2 migration from a storage site, particularly where accumulation within an overlying aquifer is predicted. The modelled storage system consists of a storage reservoir (1000 m crestal depth) and an overlying aquifer at variable depths (50–750 m crest), within a simple dome structure. In leakage scenarios, these are connected by a single vertical permeable pathway. CO2 leakage was simulated using the Permedia® CO2 simulator, and a gravity model calculated to compare a leakage and a non-leakage scenario. Time-lapse gravity surveys are likely to be able to detect CO2 leakage with CO2 accumulation within an aquifer to depths of at least 750 m, at least within an actively subsiding sedimentary basin where sandstones are expected to have high porosities at shallow burial depths. For a high relief structure in which the CO2 accumulates, the change in gravity cannot be used to detect the location of the leakage pathway as the measured gravity anomaly is centred on the geological structure. The first detection of leakage is possible after 11–15 years of leakage, though a maximum of only c. 1% of injected CO2 will have leaked at this time.