HR: 11:50h
AN: H12D-07    [Abstracts]
TI: Monotoring of CO2 Sequestration at Sleipner Using Full Waveform Inversion in Time-lapse Mode.
AU: Gosselet, A
EM: gosselet@ipgp.jussieu.fr
AF: Instut de Physique du Globe de Paris, 4 Place Jussieu, Paris cedex 05, 75252, France
AU: * Singh, S C
EM: singh@ipgp.jussieu.fr
AF: Instut de Physique du Globe de Paris, 4 Place Jussieu, Paris cedex 05, 75252, France
AB: It is now widely admitted that recent increase of CO2 in the atmosphere is due to human activities. The consecutive greenhouse effect is a major ecological concern. Geological storage is one proposed way to reduce atmosphere CO2 emissions. The Sleipner methane field, North Sea, is the very first site where CO2 has been injected back into a deep saline aquifer. In 1996, the Norwegian company Statoil and its partners began the production of the methane. The extracted methane contains a relatively high ratio of CO2, between 4% and 9%, that has to be reduced below 2.5% before delivering into the pipeline. An environmental tax introduced in Norway as early as 1991 prompted the company to store the separated CO2 instead of releasing it into the atmosphere as usually done. The CO2 is injected at the base of the Utsira sands. This water bearing formation lies at a depth between 800 and 1000m and is sealed by a thick shale layer. Seismic monitoring is a key tool in this strategy from a security standpoint and for sequestration optimization itself. Consequently, 3D seismic data were acquired before injection in 1994 and after injection in 1999, 2001, 2002, 2004 and 2006. Well-log revealed that the reservoir is crossed by thin shale layers that are 1 to 10m thick. CO2 rises up and is confined vertically by the shale layers, favouring horizontal gas migration and creating gas bearing thin beds. Seismic imaging of the gas pockets is therefore a challenging problem because large velocity variations occur on very short distance. Classical processing of time-lapse data consists in subtracting repeated survey seismic traces from the pre- injection baseline traces to exhibit changes within the reservoir. This approach remains qualitative, providing only the shape and extent of the gas cloud. Instead, we propose to compare elastic models of the subsurface computed through 2D full wave form inversion, an advanced seismic imaging technique. This method is based on the wave equation numerical simulation and can account for complex propagation effects as encountered in the Sleipner time-lapse data. This makes possible quantitative estimation of P and S-wave velocities on the meter scale. We applied the technique to 2D lines from the 1994, 1999 and 2006 vintages. The resulting post- injection models were subtracted to the pre-injection model to determine both the geometry and the velocity structure of the gas bearing areas which will be used to quantify the amount of CO2 in different forms (free versus dissolved).
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0900 EXPLORATION GEOPHYSICS
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1800 HYDROLOGY
DE: 8430 Volcanic gases
SC: Hydrology [H]
MN: 2007 Fall Meeting