Reservoir geology of the Utsira Formation at the first industrial-scale underground CO 2 storage site (Sleipner area, North Sea)

Abstract: At the Sleipner fields in the North Sea, CO 2 is being injected into sands of the Miocene-Pliocene Utsira Formation, which is overlain by thick Pliocene shales. The highly porous (35%-40%) and extremely permeable ( pproximately 2 D) Utsira sands are organized into approxi-mately 30m thick packages. These packages are separated by thin (predominantly 1 m thick), low-permeability shale layers, which are assumed to c ntain potential f uid pathways of erosive or deformational origin. A 6 5 m thick shale layer close to the top of the sands separates aneastward thickening sand wedge from the main s nd package below. Migration simulations indicate that he migration pattern of CO 2 below the shale layer would differ strongly from that within the sand wedge above. Time-lapse seismic data acquired prior to the start, and after three years, of injection con-firmed a reservoir model based on these findings and showed that the thin shale layers act as tempo-rary barriers and that the 6.5 m thick shale layer does not fully inhibit upward migration of CO 2. Long-term underground storage of CO 2 generated by point sources has been suggested as a means of reducing the emission of CO 2 into the atmosphere (e.g. Holloway 1996; Herzog et al. 2000). Injection of CO 2 into the Miocene-Pliocene Utsira Formation at the Sleipner fields in the North Sea (Fig. 1) is the first attempt to store large quantities of CO 2 under-ground with the sole objective of preventing it reach-ing the atmosphere (Baklid et al. 1996). Injection started in 1996 at an annual rate of approximately 0.85 Mt. The planned injection rate is 1 Mt per year during the life of the Sleipner Vest field, which is estimated at about 20 years. Under reservoir conditions, the CO 2 is expected to have a lower density than water. Consequently, it should rise to the top of the formation and accumulate beneath the overlying shale package, which is several hundred metres thick. The CO 2 injection activities are accompanied b