Experiences on S

1. Experiences on Seafloor Gravimetric
and Subsidence Monitoring Above
Producing Reservoirs
Håvard Alnes, Torkjell Stenvold & Ola Eiken (Statoil ASA)
72nd EAGE Conference, Barcelona 2010
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2. Outline
• Theory of gravity
• Seafloor gravity and subsidence
measurements
• Gravity monitoring at Troll East
• Gravity monitoring at Sleipner
CO2 storage
• Summary
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3. Theory of time-lapse gravity
• Gravity depends only on density
• Changes in gravity depends only on
(reservoir) density changes
• Largest density change occurs with
a moving gas-liquid contact
• Typical reservoir parameters:
1 m rise of GWC ≈ 2-3 µGal Gas
(1 µGal = 10-8 m/s2 ≈ 10-9 g) Water
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4. Gravity measurement precision
satellite
altimeter
Statoil/Scripps seafloor development:
airborne
shipborne
2009 & 2002 &
2006
2000 1998 seafloor
2007 2005
land
borehole
stationary
1 10 100 1000 10 000
Gal (one Gal is about 10-9 of the earth’s gravity)
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5. Offshore gravity and subsidence
monitoring
• Permanently deployed concrete benchmarks on the
seafloor
• Mobile instrument carried by ROV, measuring ~20 minutes
at each site
• Measure changes in the gravity field at the seafloor using
relative gravimeters (+/- 3 µGal)
• Measure changes in seafloor depth (subsidence) using
water pressure gauges (+/- 5 mm)
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6. Ongoing projects
Field No. of year year year year year
stations
Troll 80 1998 2000 2002 2005 2009
Sleipner CO2 40 2002 2005 2009
Midgard 50 2006 2009
Mikkel 20 2006
Snøhvit + 76 2007
Albatross
Ormen Lange 8 2007 2009
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7. Reservoir monitoring at Troll East
Troll Field
C
C
B
A
• Monitor well
: Well
10 km
• Gravity monitoring
TOGI
• Time-lapse 2D seismic
• Time-lapse 3D seismic
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8. Seafloor subsidence 2002 - 2009
Pore compressibility:
Troll West PDO (1991): ~ 80·10-5/bar
Revised core data (2000): ~ 9·10-5/bar
Subsidence data: ~ 3·10-5/bar
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9. Gravity change 2002-2009
corrected for gas takeout
Oil production in Troll West
(gas-oil contact going down)
Gas production in Troll East
(rise of gas-water contact)
These results have been used to
update aquifer strengths in the
reservoir simulation model
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10. Gravimetric monitoring of CO2 storage at
Sleipner East
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11. Survey layout
• Designed to monitor CO2 injected into
Utsira fm.
• 30 benchmarks in two lines across CO2
injection point.
• Additional contribution from Ty fm. due to
gas/condensate production.
• Surveyed in 2002, 2005 and 2009.
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12. Observed gravity change
(2002-2009)
• Observed gravity change of
more than 50 µGal in 7 years
• Caused by water influx to Ty fm.
and CO2 injection to Utsira fm.
• The two contributions can be
distinguished due to separation
both laterally and in depth
• Water influx to Ty fm. is also
confirmed by well observations
and 4D seismic
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13. Gravity change from CO2 injection
Gravity change
(compensated for production in 4D seismic CO2 density at top
underlying gas reservoirs) Utsira
In-situ CO2 density:
690 – 760 kg/m3
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14. Summary
• Gravity and subsidence monitoring can provide field-wide quantitative
information on water influx and reservoir compaction.
• Interpretation is straightforward since the results can easily be compared with
reservoir simulation models.
• Complementary information to well observations and 4D seismic – combined
they give a more complete picture.
• Patented technology – method is unique to Statoil (offshore).
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15. Acknowledgement
Thanks to:
Mark Zumberge and Glenn Sasagawa at Scripps Institution of Oceanography for
thirteen years of close cooperation
The Troll license partners Statoil, Shell, Total, Petoro and ConocoPhillips for
continuous support and permission to show time-lapse results.
The CO2ReMoVe project for permission to show time-lapse results.
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16. Thank you
Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs
Ola Eiken, Håvard Alnes and Torkjell Stenvold
oei@statoil.com; hal@statoil.com; torste@statoil.com
www.statoil.com
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17. 17 - Classification: Internal 2010-05-03
2010-06-11Classification: Internal 2010-05-03