CO₂ Flow in the Utsira Formation: Inferences Made from 4D Seismic Analyses of the Sleipner Area

Christian Hermanrud, Peter Zweigel, Ola Eiken, Jon Lippard, and Terje Andresen
Statoil ASA, Trondheim, Norway

Nine million tonnes of CO₂ have been separated from the hydrocarbon gases from the Sleipner West middle Jurassic gas field since production started in 1996. This CO₂ has been successfully stored in the Tertiary Utsira formation, which is a 300m thick unit of unconsolidated sands with a set of intraformational mudstones. Most of these mudstones are thin and cannot be correlated between wells, but a 6.5 m thick mudstone of regional extent is present close to the top of the Utsira formation.
When injection started, it was expected that the CO₂ would rise to the nearest mudstone layer. It was then supposed that the CO₂ would flow beneath this layer to its pinchout, and then rise to the next mudstone layer. However, seismic imaging demonstrated that the CO₂ flowed to the top of the Utsira formation within a few years after the injection started. The seismic images revealed the presence of a vertical zone of disturbed seismic signals in the Utsira formation (but not in the overburden rocks) above the injector well.
The rapid vertical flow of CO₂ through the mudstones demonstrates that the low-permeable mudstones did not significantly impede the vertical CO₂ movement. This observation suggests that the CO₂ changed the vertical flow properties of the Utsira formation, and that the continuity of the mudstones was broken by the migrating CO₂. We attribute this effect to the lack of consolidation of the Utsira formation, possibly combined with dissolution of carbonate shell fragments by the CO₂. We also suggest that the vertical zone of disturbed seismic signals reflects the presence of a vertical flowpath of CO₂ in the Utsira formation.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece