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Experiences on Seafloor Gravimetricand Subsidence Monitoring AboveProducing Reservoirs
Håvard Alnes, Torkjell Stenvold & Ola Eiken (Statoil ASA)
72nd EAGE Conference, Barcelona 2010
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Outline
• Theory of gravity
• Seafloor gravity and subsidence measurements
• Gravity monitoring at Troll East
• Gravity monitoring at SleipnerCO2 storage
• Summary
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Gas
Water
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
(1 µGal = 10-8 m/s2 ≈ 10-9 g)
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Gravity measurement precision
satellite altimeter
airborne
shipborne
seafloor
land
boreholestationary
1 10 100 1000 10 000
Gal (one Gal is about 10-9 of the earth’s gravity)
2002 & 2005 2000 1998
Statoil/Scripps seafloor development:
2006 & 2007
2009
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Offshore gravity and subsidencemonitoring
• Permanently deployed concrete benchmarks on theseafloor
• Mobile instrument carried by ROV, measuring ~20 minutesat each site
• Measure changes in the gravity field at the seafloor usingrelative gravimeters (+/- 3 µGal)
• Measure changes in seafloor depth (subsidence) usingwater pressure gauges (+/- 5 mm)
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Ongoing projects
2009
2009
2009
2009
year
8
76
20
50
40
80
No. ofstations
yearyearyearyearField
2007 Ormen Lange
2007Snøhvit +Albatross
2006Mikkel
2006Midgard
20052002Sleipner CO2
2005200220001998Troll
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Troll Field
A
B
C
10 km
B
C
A
TOGI
: Well
Reservoir monitoring at Troll East
• Monitor well
• Gravity monitoring
• Time-lapse 2D seismic
• Time-lapse 3D seismic
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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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Gravity change 2002-2009corrected 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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Gravimetric monitoring of CO2 storage at Sleipner East
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Survey layout
• Designed to monitor CO2 injected into Utsira fm.
• 30 benchmarks in two lines across CO2injection point.
• Additional contribution from Ty fm. due to gas/condensate production.
• Surveyed in 2002, 2005 and 2009.
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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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Gravity change from CO2 injectionGravity change(compensated for production in underlying gas reservoirs)
In-situ CO2 density: 690 – 760 kg/m3
4D seismic CO2 density at top Utsira
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Summary
• Gravity and subsidence monitoring can provide field-wide quantitativeinformation on water influx and reservoir compaction.
• Interpretation is straightforward since the results can easily be compared withreservoir simulation models.
• Complementary information to well observations and 4D seismic – combinedthey give a more complete picture.
• Patented technology – method is unique to Statoil (offshore).
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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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Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs
Ola Eiken, Håvard Alnes and Torkjell [email protected]; [email protected]; [email protected]
Thank you
17 - Classification: Internal 2010-06-11Classification: Internal 2010-05-03Classification: Internal 2010-05-03