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ZWEIGEL, PETER,ANE E. LOTHE, and ERIK LINDEBERG, IKU Petroleum Research, Trondheim, Norway

Abstract: Offshore underground CO2-disposal - 'Reservoir geology' of the Neogene Utsira Formation, Sleipner Field, North Sea

Underground deposition of industrial emitted CO2 in offshore aquifers is currently considered as a large-scale option to reduce the amount of greenhouse gases released to the atmosphere. Disposal of CO2 by oil companies may be necessary to fulfil environmental requirements, e.g., to be allowed to produce gas from CO2-rich reservoirs, as in the case of the Sleipner Field in the North Sea.There, Statoil and license partners started in 1996 to inject CO2 into sands of the Utsira Formation. This project serves as a test case for offshore underground CO2-disposal in general, and for possible future use of the wide-spread Utsira Formation in particular. The Utsira Formation is a sand-unit of Miocene and/or Pliocene age that occurs in the northern North Sea. It is highly permeable, which facilitates injection and migration of CO2. The Utsira Formation is overlain by the shaly Pliocene Nordland Group that is down-lapping onto it. This potential cap-rock is generally considered to be impermeable for CO2. However, it contains silty, and possibly sandy intervals of so far unknown geometry. The Utsira Formation has been affected by faulting, which may have deformed its cap-rock, too. A reliable geological model of the 'reservoir' and the cap rock is crucial to be able to predict the retention time of CO2 by simulations.The sensitivity of how various reservoir parameters affect the migration and distribution of CO2 in the Utsira Formation will be illustrated. These parameters will include permeability of the sand, imperfections, lithology and deformation of the cap rock.

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