Seismic monitoring of a unique CO2 injection site at Sleipner Ola Eiken, Statoil

Increasing atmospheric CO2 causes global warming Increasing atmospheric CO2 causes global warming. The Kyoto agreement put limits on emissions. A CO2 free vision:

CO2 physical properties Measured at HARC 2000

Sleipner A

CO2 injection well in Utsira Fm.

Sleipner CO2 injection CO2 is injected into a thick sandstone layer at 800-1100 m depth below sealevel 97.5-98 % of the injected gas is CO2 The sandstones have porosities of 35-40 % and 1-8 D permeability seismic survey

Seismic data acquisition and processing 1994 1999 Shooting direction N-S Shotpoint interval (flip-flop) 18.75 m 12.5 m Total airgun volume 3400 in3 3542 in3 No. of streamers 5 4 Crossline nominal spacing 25 m Source towing depth 6 m Cable towing depth 8 m Reprocessing of base survey together with processing of monitor survey. Comprehensive processing sequence with aim of maximum repeatability Deterministic zero-phasing using supplied far-field signatures from 1994 and 1999. Tidal correction based on model Swath consistent static correction Global frequency-amplitude match between surveys New velocity analyses in CO2 injection area

1994 october 1999 difference after injecting ~ 2 mill. tons CO2 since 1996 no change above this level top Utsira Fm. 100 ms injection point 1000 m

. crossline 1122 west east west east 1994 vintage 200 m west east west east horizon A0 horizon A 100 ms horizon A1 horizon B horizon C horizon D B D C . A A1 1994 vintage 1999 - 1994 difference

. southwest northeast 1994 vintage southwest northeast 500 m 100 ms B D C . A A1 1994 vintage southwest northeast 1999 - 1994 difference horizon 0A horizon A horizon A1 horizon B horizon C horizon D

amplitude Horizon B time . . 16000 8000 amplitude 929 949 969 ms

Model of reflection from top and bottom of a thin CO2-filled layer (with velocity 1450 m/s). Bottom of layer is at constant depth. Maximum amplitude Pulse 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Layer thickness in meters

Estimated mass based on seismic time-delay Gas column height: Uncertainty distribution assessed in each factor. Monte Carlo simulation.

Possible causes of discrepancies in mass calculations: CO2 is partly accumulated in thin layers and partly distributed at low saturations between the layers. The reservoir is warmer and CO2 less dense than assumed ? The Gassman theory is not valid ?

Fundamental limitations on seismically based mass estimates: The non-linear relation between Vp and Sg. Causes gas saturations between 0.1 and 1.0 to be ~ unresolvable Lack of information in the 2-10 Hz frequency band. Saturation gradients are not discernible 0 0.5 1.0 Gas saturation 1300 1700 2100 P-wave velocity (m/s) From Claerbout (1985)

Conclusions Injected CO2 is well imaged by time-lapse seismic data. The geometry of the CO2 bubble is well defined. There is no evidence of leakage through the seal. A portion of the injected CO2 is trapped at high saturations beneath thin shales. The observed time-delays is not sufficiently explained by high-saturation CO2 layers. There is a basic limitation in time-lapse seismic data on resolving long-wavelength variations and on quantifying CO2 saturations.

Acknowledgement Rob Arts from TNO, Andy Chadwick from BGS and Erik Lindeberg from SINTEF, who all have contributed a lot in the multi-diciplinary SACS team. Statoil and the Sleipner field partners Exxon, Norsk Hydro and TotalFinaElf. Statoils SACS partners BP-Amoco, Norsk Hydro, Exxon, TotalFinaElf, Vattenfall, British Geological Survey, Bureau de Recherches Gèologiques et Minières, Geological Survey of Denmark and Greenland, Institut Francais du Pètrole, Netherlands Institute of Applied Geoscience, Sintef Petroleum Research. WesternGeco, who acquired and processed the seismic data. The European Union R&D programme Thermie.