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Experiences on Seafloor Gravimetric
     and Subsidence Monitoring Above
     Producing Reservoirs


     Håvard Alnes, To...
Outline

     • Theory of gravity
     • Seafloor gravity and subsidence
       measurements
     • Gravity monitoring at ...
Theory of time-lapse gravity

     • Gravity depends only on density
     • Changes in gravity depends only on
       (res...
Gravity measurement precision
                                                                            satellite
      ...
Offshore gravity and subsidence
      monitoring
     • Permanently deployed concrete benchmarks on the
       seafloor

 ...
Ongoing projects
     Field          No. of     year   year   year   year   year
                    stations

     Troll ...
Reservoir monitoring at Troll East
                              Troll Field

               C
               C




      ...
Seafloor subsidence 2002 - 2009




                         Pore compressibility:
                         Troll West PDO...
Gravity change 2002-2009
     corrected for gas takeout




                                       Oil production in Troll...
Gravimetric monitoring of CO2 storage at
       Sleipner East




10 -
Survey layout

       • Designed to monitor CO2 injected into
         Utsira fm.
       • 30 benchmarks in two lines acro...
Observed gravity change
       (2002-2009)

       • Observed gravity change of
         more than 50 µGal in 7 years
    ...
Gravity change from CO2 injection
        Gravity change
        (compensated for production in   4D seismic    CO2 densit...
Summary

       • Gravity and subsidence monitoring can provide field-wide quantitative
         information on water infl...
Acknowledgement
       Thanks to:

       Mark Zumberge and Glenn Sasagawa at Scripps Institution of Oceanography for
    ...
Thank you
       Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs

       Ola Eike...
17 - Classification: Internal   2010-05-03
                                2010-06-11Classification: Internal   2010-05-03
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Alnes et al gravity and subsidence monitoring

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Alnes et al gravity and subsidence monitoring

  1. 1. Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs Håvard Alnes, Torkjell Stenvold & Ola Eiken (Statoil ASA) 72nd EAGE Conference, Barcelona 2010 1-
  2. 2. Outline • Theory of gravity • Seafloor gravity and subsidence measurements • Gravity monitoring at Troll East • Gravity monitoring at Sleipner CO2 storage • Summary 2-
  3. 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 3-
  4. 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) 4-
  5. 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) 5-
  6. 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 6-
  7. 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 7-
  8. 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 8-
  9. 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 9-
  10. 10. Gravimetric monitoring of CO2 storage at Sleipner East 10 -
  11. 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. 11 -
  12. 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 12 -
  13. 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 13 -
  14. 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). 14 -
  15. 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. 15 -
  16. 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 16 -
  17. 17. 17 - Classification: Internal 2010-05-03 2010-06-11Classification: Internal 2010-05-03

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