Quantitative Monitoring of the CO2 injection plume at Sleipner, North Sea: meeting the regulatory requirement. (Invited)
Abstract
CO2 produced at the Sleipner gas field is being injected into the Utsira Sand, a regional saline aquifer beneath the northern North Sea. Injection started in 1996 with some 11 Mt of CO2 now injected. Time-lapse seismic surveys have been acquired in 1999, 2001, 2002, 2004, 2006 and 2008 in order to monitor the growth of the plume. The plume is imaged as a sequence of high amplitude sub-horizontal reflectors within the aquifer; the reflections are thought to represent mostly tuned responses from thin layers of CO2 trapped beneath intra-reservoir mudstone baffles. Time-lapse imaging of the overburden shows no evidence of CO2 migration from the reservoir. Quantitative interpretation analysing reflection amplitudes and velocity pushdown was able to obtain a satisfactory, though non-unique, mass verification for the 1999 plume, but since then, increasing signal attenuation in the deeper part of the plume have rendered this progressively more difficult. The topmost layer of the plume is perhaps the most tractable in terms of detailed analysis. It is very clearly imaged, with detectability down to about 1 metre, and does not suffer from the effects of progressive signal attenuation. The manner in which the layer is spreading laterally beneath the reservoir topseal is providing clear insights into how the plume will behave into the future. History-matched flow simulations of the topmost layer indicate lateral migration within a very high permeability (> 3 Darcy) sand, largely controlled by buoyant ponding beneath a rough caprock topography. Perfect history-matching of the layer spread is challenging however, likely due to small uncertainties in the depth-converted base topseal surface. Although full quantitative verification of injected mass is not a European regulatory requirement, the regulations do require that robust understanding of current storage behaviour can be demonstrated and that predictions of future performance are reliable. In particular the EU Storage Directive requires the following key criteria to be met for site closure: 1. The actual behaviour of the injected CO2 conforms with the modelled behaviour. 2. There is no detectable leakage. 3. The storage site is evolving towards a situation of long-term stability. By combining high quality monitoring datasets such as at Sleipner with long-term predictive simulations and key observations from pilot-scale studies, particularly those in the post-injection phase such as at Nagaoka, it is proposed that storage sites can be closed under the developing (European) regulatory regime. The 2006 time-lapse seismic cube showing the plume and the topmost CO2 layer
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.U12B..05C
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS / Seismic methods