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

Abstract:

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.