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

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

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

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

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

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

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

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:

Gravimeter Seafloor benchmark

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

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)

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

Gravity raw data (time series)

From Sasagawa et al. 2003

Time [hours:minutes:seconds]

Gra

vity

[mG

al]

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

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

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)

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”

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

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.