A Calibrated Percolation Model for the Utsira CO2 Plume
Andrew J. Cavanagh1,2 and R. Stuart Haszeldine2
1The Permedia Research Group, Ottawa, Canada
2University of Edinburgh, Edinburgh, Scotland
The Utsira experiment, Viking Graben, offshore Norway, is the world's first and largest saline aquifer storage site for carbon sequestration. More than 10 million tonnes of injected CO2 has ascended and ponded beneath a number of flow barriers within the reservoir over the decade 1996-2006. The observed plume remains trapped beneath the caprock of Nordland Group mudstones and 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. The observed ascent rate favours invasive percolation as the probable migration mechanism. However, the vertical flow and apparent breaching of multiple, laterally extensive mudstone barriers, resulting in the 'pancake stack' of thin CO2 layers, has not previously been satisfactorily explained. We present the first accurate 3D representation and calibrated flow simulation of the Utsira plume, using standard petroleum systems modeling software. The model demonstrates the sensitivity of the plume mass balance to layer thickness, and accurately replicates the pattern and approximate thickness of the observed layers. Extremely low buoyancy pressures (40 kPa) are required to breach the barriers vertically and calibrate the model to the observed plume distribution. The unusually low threshold pressure behaviour of the intra-formational barriers is thought to differ from the mudstones of the overlying Nordland Group caprock where published laboratory tests indicate much higher threshold pressures (4 MPa). We therefore infer that CO2 loss by seepage through the caprock has not occurred. We suggest that a network of microfractures (2-3 microns) span the thin mudstone barriers as a result of the novel mechanism of rapid pore-pressure changes during several deglaciations within the last 2 Ma.
AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.