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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 Stenvoldoei@statoil.com; hal@statoil.com; torste@statoil.comwww.statoil.com

Thank you

17 - Classification: Internal 2010-06-11Classification: Internal 2010-05-03Classification: Internal 2010-05-03