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

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

3. CO2 physical properties
Measured at HARC 2000

4. Sleipner A

5. CO2 injection well in Utsira Fm.

6. Sleipner CO2 injection
CO2 is injected into a thick sandstone layer at m depth below sealevel
% of the injected gas is CO2
The sandstones have porosities of % and 1-8 D permeability
seismic survey

7. 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

8. 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

9. . 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
difference

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

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

12. 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
Layer thickness in meters

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

14. 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 ?

15. 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
Gas saturation
P-wave velocity (m/s)
From Claerbout (1985)

16. 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.

17. 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.