Geophysical methods for CO2 plume imaging: Comparison of performances

Abstract Geophysical methods are adequate for imaging the CO2 plume in order to follow its migration within the reservoir and possible leakages through the caprock formation. In particular, changes in density, seismic velocity or electrical resistivity are associated with changes in the gas saturation and make methods such as gravity, 4D seismic and electrical resistivity tomography (ERT) or CSEM powerful tools to follow up the fate of injected CO2. Respect to the minimum amount of CO2 stored that can be quantified for verification purpose, there is an order of magnitude between 4D seismic (few 100s of Kt) and the other two methods (few Mt). Respect to the detection of leakage at the reservoir level, only 4D seismic could be considered as useful. Using downhole measurements, such as crosshole electric or downhole gravity will increase the resolution of these methods and therefore its ability to detect leakage. In case of CO2 leakage upwards and accumulation within a secondary reservoir located at a few hundreds of meters depth, the resolution of the three methods is increased by several order of magnitude and small amounts of CO2 could be detected, depending whether it is in gaseous phase or dissolved. It is expected that controlled experiments of leaking CO2 at shallow depth will help to define more precisely the conditions of use of the three methods.