A calibrated model for the Utsira CO2 plume - ice sheet loading and hydraulic fracturing?
Dr. Andrew Cavanagh
Senior Geologist & CO2 Specialist
The Permedia Research Group Inc.
Time: Monday, October 27, 2008, 10:30
Room: Building A43, lecture room
The Sleipner experiment, offshore Norway, is the world's first and largest saline aquifer storage site. More than 10 Mt of CO2 has ascended and ponded beneath a number of flow barriers since the injection of the dense-phase CO2 commenced in 1996. The plume remains within the Utsira sandstone aquifer, approximately 800 meters below sea level. Five seismic reflection surveys over the last decade ('99, '01, '02, '04, '06) clearly portray the 'footprint' emergence and spreading of the nine principal layers. However, a mass balance estimate for the plume has eluded observers due to the resolution limits of remote geophysical observations with respect to layer thickness. We demonstrate the sensitivity of the mass balance to layer thickness, and present the first accurate 3D fluid flow model for the Utsira plume The model replicates the layering pattern and approximate thickness of the observed layers. To calibrate CO2 flow to the observed layer distribution, it is necessary to simulate thin mudrock barriers within the Utsira sandstone with low threshold pressures (40 kPa) that are able to transmit CO2 vertically while retaining the observed thin layers of CO2. The unusual behaviour of the intra-formational barriers is thought to differ from the mudrocks of the overlying Nordland Group caprock, where published laboratory tests indicate much higher threshold pressures (2-4 MPa). We therefore infer that CO2 loss by seepage through the caprock will probably not occur. We suggest that a network of transmissive microfractures (2-3 microns) was induced in the thin mudrock barriers by the novel mechanism of rapid pore-pressure changes during several deglaciations within the last 1 Ma. Gas fields within the region retain glacial overpressure today, demonstrating robust seals for thick mudrocks.