Among the measures to reduce the amount of greenhouse gases in the atmosphere, the geological storage of carbon dioxide (CO2) is one of the most promising. Since 1996, more than 11 million tons of CO2 have been injected in a saline aquifer, the Utsira sand, of the Sleipner field (North Sea). An extensive seismic monitoring programme over the CO2 injection area have been carried out. Using more or less sophisticated rock physics models, seismic images of the injected zone were previously interpreted quantitatively in terms of the total volume and of the total mass of CO2 in place. In this study, starting from the cubes of the prestack-inverted seismic impedances, we propose a method for computing the cube of CO2 saturation, and for estimating quantitatively the CO2 plume (volume and mass) without using any petroelastic model. In contrast we exploit some characteristics of the particular flow regime associated with CO2 injection in a saline aquifer, namely the first drainage regime. To our knowledge, such a detailed CO2-saturation cube has not been published before. Summation over the whole cube provides an acceptable match for the known injected CO2 mass.
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