Monitoring the Injection of CO2 at the Sleipner Field, North Sea

Wednesday, 17 December 2014
Laurence Robert Cowton1, Jerome A Neufeld1, Nicholas J White1, Michael J Bickle1, Andrew Chadwick2 and James White2, (1)University of Cambridge, Cambridge, United Kingdom, (2)British Geological Survey Keyworth, Nottinghamshire, United Kingdom
Time-lapse three-dimensional seismic surveys have been acquired to monitor the
evolution of CO2 injected into the Utsira formation at the Sleipner field. The changing
pattern of reflectivity shows that CO2 is accumulating beneath nine impermeable shale
horizons. Areal extents of all CO2-filled horizons have been mapped on surveys up
to 2010, revealing a distinctive change in growth pattern eight years after injection
commenced in 1996. Post 2004, five of the nine layers appear to exhibit either no increase
or a decrease in areal extent. Volumetric measurements of thin CO2 layers (i.e. less than
∼15 m thick) are difficult to obtain because of tuning effects. However, a combination of
travel-time pull-up and reflection amplitude permits improved thickness estimates to be
made for the shallowest layer (i.e. layer 9). Travel-time pull-up and amplitude both vary
non-linearly with CO2 layer thickness, yielding non-unique solutions when individually
correlated. However, these attributes have different tuning thicknesses, permitting the
volume of CO2 beneath layer 9 to be calculated without ambiguity when combined. We
have also analyzed the structure of the bounding upper and lower shale units for layer 9
and the thickness of sandstone between them. We found that CO2 in layer 9 can occupy
up to half the depth of this sandstone unit. Numerical modelling of gravity currents in
variably confined aquifers suggests that the flow of CO2 through thin sedimentary units
is affected by the proximity of the lower boundary.