Spatial and temporal evolution of injected CO2 at the Sleipner Field, North Sea

• Published in: Journal of Geophysical Research: Solid Earth Vol. 117; no. B3
• Main Authors: Boait, F. C., White, N. J., Bickle, M. J., Chadwick, R. A., Neufeld, J. A., Huppert, H. E.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01.03.2012; American Geophysical Union
• Subjects: carbon dioxide; Carbon dioxide fixation; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geophysics; Permeability; Reservoirs; Sandstone; Seismic surveys; Seismology; sequestration; Sleipner; time lapse; Northeast Atlantic; North Atlantic; North Sea; Atlantic Ocean; attenuation; time variations; plumes; Carbon dioxide; reservoirs; impedance; sandstone; seismic surveys; greenhouse gas; brines; amplitude; clastic rocks; models; mudstone; Gravity current; accumulation; topography; eccentricity; CO2 sequestration; cartography; velocity; sedimentary rocks; Effective permeability; migration; growth; three-dimensional seismics; Reflectance
• ISSN: 0148-0227; 2169-9313; 2156-2202; 2169-9356
• DOI: 10.1029/2011JB008603

Time‐lapse, three‐dimensional (3D) seismic surveys have imaged an accumulation of injected CO2 adjacent to the Sleipner field in the North Sea basin. The changing pattern of reflectivity suggests that CO2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone. Nine reflective horizons within the reservoir have been mapped on six surveys acquired between 1999 and 2008. These horizons have roughly elliptical planforms with eccentricities ranging between two and four. In the top half of the reservoir, horizon areas grow linearly with time. In the bottom half, horizon areas initially grow linearly for about eight years and then progressively shrink. The central portions of deeper reflective horizons dim with time. Amplitude analysis of horizons above, within, and below the reservoir show that this dimming is not solely caused by acoustic attenuation. Instead, it is partly attributable to CO2 migration and/or CO2 dissemination, which reduce the impedance contrast between sandstone and mudstone layers. Growth characteristics and permeability constraints suggest that each horizon grows by lateral spreading of a gravity current. This model is corroborated by the temporal pattern of horizon velocity pushdown beneath the reservoir. Horizon shrinkage may occur if the distal edge of a CO2‐filled layer penetrates the overlying mudstone, if the buoyant plume draws CO2 upward, or if the effective permeability of deeper mudstone layers increases once interstitial brine has been expelled. Topographic control is evident at later times and produces elliptical planforms, especially toward the top of the reservoir. Our results show that quantitative mapping and analysis of time‐lapse seismic surveys yield fluid dynamical insights which are testable, shedding light on the general problem of CO2 sequestration. Key Points We have mapped time‐lapse surveys and generated a set of plan form maps The maps show how CO2 has accumulated at different levels within the reservoir Rates of accumulation have then been used to assess different flow models