gravimetric and seafloor subsidence monitoring – a ... · gravimetric and seafloor subsidence...
TRANSCRIPT
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Gravimetric and seafloorsubsidence monitoring –a reservoir management tool
Ola Eiken1 and Torkjell Stenvold2
1 Statoil R&D center, Trondheim, Norway
2 Norwegian Technical University, Trondheim, Norway Force Seminar on Geophysical Monitoring
26th September 2005
Stavanger, Norway
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Offshore gravity and subsidencemonitoring
•Measure changes in the gravity field at theseafloor using relative gravimeters
•Measure seafloor subsidence using relative water pressure
•Reference stations outside the field
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Reservoir compaction cause surface subsidence
• Reservoir compaction is caused by
– Pressure drop around production wells, and finite formation stiffness
– Chemical and mechanical processes, as reaction with water, or sand production
• Compaction can be important for
– Safety of installations
– Well integrity
– Inferring formation stiffness & pressure drop
– Improve reservoir flow models (drive mechanism) and predict future production
• Compaction may be monitored (logged) in wells
– radioactive bullets
• Surface deformation
– Related to reservoir compaction through overburden strain: geomechanics
– Surface measurements may be inverted for reservoir compaction
– Measurement (monitoring) challenge at sea
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Galileo Galilei (1562 - 1642)
Gravity monitoring
•Different densities of gas, oil and water can cause changes in the
gravity attraction, when they flow (mass redistribution)
– Measured at the surface or in boreholes
•Mass withdrawal or addition due to production or injection will
cause changes in the gravity field
•Surface subsidence will cause gravity changes
– Change of observation height
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Applications of surface gravimetric monitoring
•Hydrothermal energy (onshore since the 1950’s)
•Volcanology (magma chambers)
•Various near-surface problems, as sub-soil washout
•CO2 storage
•Hydrocarbon exploitation:– Groningen gas, the Netherlands
– Troll gas, Norway
– Kuparuk / Prudhoe Bay, Alaska
– Izaute gas storage, France
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Modelling gravity changes
5 km
Conta
ctrise
[m
]
One years contact rise from simulation model
1
2
5
35
25
15
25
10
52
1
15
-1
-2-5
-10
-15
-10-5
-2 -1
1
2
510
15
2510
2 15
15
Gra
vity
chan
ge
[μG
al]
Gravity change:total pressure drop water influx
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Gravity response for different reservoir depths
2 km depth 1 km depth .
Gra
vity
chan
ge
[μG
al]
Gra
vity
chan
ge
[μG
al]
5 km
inversion
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Accuracy of different gravity survey techniques
satellite altimeter
airborne
shipborne
seabottom
land
boreholestationary
1 10 100 1000 10 000μGal (one μGal is about 10-9 of the earth’s gravity)
2002 & 2005 2000 1998
Statoil/Scripp’s seafloor development:
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Gravimeter Seafloor benchmark
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A
B
C
208
207
206
205
203
402401
Troll ATroll A
• Seafloor gravity
– 68 stations
• 2D seismic
• 3D seismic
• Seafloor echo-sounding
Surface geophysical monitoring:
10 km
Monitor well beneath Troll A platform:
–Permanent pressure sensors
–Repeated saturation logging
–Reservoir compaction
Troll field reservoir monitoring
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Seafloor water pressure measurementsPr
essu
re[k
Pa]
Recording time 20 minutes
~1cm
26 228 230 232 234 236 238 240 242 244 246Recording time
Pres
sure
[kPa
]~
40cm
2 days
ROVDOG(mobile recordings)
AanderaaWLR 7&8(stationaryrecordings)
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Error budget – subsidence
1.7Observed time-lapse difference scatter
0.5Zero-level (from reference benchmarksoutside the field)
0.6Varying settling of BMs
0.5Calibration error
0.2Water density difference
0.9Intra-survey error
Standard deviation [cm]2002 - 2000
0
1
2
3
4
5
6
7
8
9
10
-3 -2.4 -1.8 -1.2 -0.6 0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 6Bin [cm]
Freq
uenc
y
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Gravity raw data (time series)
From Sasagawa et al. 2003
Time [hours:minutes:seconds]
Gra
vity
[mG
al]
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Troll gravity changes 2002 – 2000after compensating for gas takeout
•Average gravity increase withinTroll East: 8 μGal +/-5 μGal
•Average gas/water contact rise is estimated at 1.3 +/-0.8 m
•Estimated total water influx: 0.7 (+/- 0.4) x 1011 kg
Water
expansion
Water
influx
Pore
compaction
Gas expansionUnderground
withdrawal
Material balance:
700
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Survey achievements:
• 3-4 μGal gravity repeatability (s.d.)
• 5-6 mm seafloor depth repeatability (s.d.)
800
850
900
950
1000
0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
Density [10-3 kg/m3]
Dept
h [m
]
base casehigh T caselow T casepure CO2injection T
Sleipner CO2 injection• 7.7 million tons CO2 injected since 1996
• Temperature / density uncertainty
• Gravity monitoring in 2002 & 2005
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Potential applications:
• Medium to large gas
accumulations with moving
fluid fronts
– Depletion
– Injection
• Pressure depletion reservoirs
– Subsidence / Compaction
– Gas density change
Ormen Lange
Snøhvit
Valhall
Ekofisk
Statfjord
Shtokman (Russia)
North Field (Quatar)
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Frigg field13.09.1977 - 26.10.2004
Rise of gas-liquid contact per 1993:Unexpected movementsReduced value
Laurie Dake: Fundamentals ofreservoir engineering:
”There is more uncertaintiesattached to the subject of water influx than to any other in reservoir engineering”
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Gravity monitoring data can be interpreted quantitatively in terms of mass changes
Field-wide mapping of subsidence with sub-cm accuracy is possible
There is a straightforward link to material balance / flow models
Cost is medium to low
Combined use of gravity, subsidence, seismic and well monitoring data yield complementary information and a more reliable total view
Conclusions
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Acknowledgement
Thanks to Statoil for permission to publish the paper.
Thanks to Scripps Institution of Oceanography for goodand close cooperation througout the development.
Thanks to Troll partners Statoil, Petoro, Shell, Norsk Hydro, ConocoPhillips and Total for permission to show time-lapse results.