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Experiences on Seafloor Gravimetric
and Subsidence Monitoring Above
Håvard Alnes, Torkjell Stenvold & Ola Eiken (Statoil ASA)
72nd EAGE Conference, Barcelona 2010
• Theory of gravity
• Seafloor gravity and subsidence
• Gravity monitoring at Troll East
• Gravity monitoring at Sleipner
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
Gravity measurement precision
Statoil/Scripps seafloor development:
2009 & 2002 &
2000 1998 seafloor
1 10 100 1000 10 000
Gal (one Gal is about 10-9 of the earth’s gravity)
Offshore gravity and subsidence
• Permanently deployed concrete benchmarks on the
• 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)
Field No. of year year year year year
Troll 80 1998 2000 2002 2005 2009
Sleipner CO2 40 2002 2005 2009
Midgard 50 2006 2009
Mikkel 20 2006
Snøhvit + 76 2007
Ormen Lange 8 2007 2009
Reservoir monitoring at Troll East
• Monitor well
• Gravity monitoring
• Time-lapse 2D seismic
• Time-lapse 3D seismic
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
Gravimetric monitoring of CO2 storage at
• Designed to monitor CO2 injected into
• 30 benchmarks in two lines across CO2
• Additional contribution from Ty fm. due to
• Surveyed in 2002, 2005 and 2009.
Observed gravity change
• 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
Gravity change from CO2 injection
(compensated for production in 4D seismic CO2 density at top
underlying gas reservoirs) Utsira
In-situ CO2 density:
690 – 760 kg/m3
• 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).
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.
Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs
Ola Eiken, Håvard Alnes and Torkjell Stenvold
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