a guid iding lig light to cost- effective reserv rvoir monitoring - - PowerPoint PPT Presentation

4D Gravity and Subsidence: a guid iding lig light to cost- effective reserv rvoir monitoring

Egypt-Norway Technology Days

Outline

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• About OCTIO Gravitude
• The principles of the technology
• 4D gravity
• Whole-field subsidence monitoring
• Case studies: the value of gravity and subsidence
• Concluding remarks

2013 IP and competence transfer

About Octio Gravitude

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• Value of the technology proven in many fields on the NCS
• Gravitude performed 8 surveys in 6 fields since 2012
• Best results to date
• Safe operations with no HSE incidents
• Ormen Lange field (operated by Shell) surveyed since 2012
• Octio has ten years of operational experience
• Highly skilled technical team with diverse background

2012 GRAVITUDE created within Statoil Late 90’s technology developed by Statoil

The surveys in a nutshell

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ROV Primary measurements: gravity and

pressure at the seafloor

Sensor frame with 3 gravimeters and 3 pressure sensors Concrete platform 20’ per measurement Repeated visits

Gravity and subsidence monitoring on the NCS

Field 1st survey # of surveys Seafloor depth (m) Reservoir depth (m) Area (km2) N. stations Comment Troll 1998 6 320 1400 30 x 50 113 Norway’s largest gas field Mikkel 2006 4 230 2500 3 x 12 21 Smaller, deeper reservoir Sleipner 2002 4 80 800 / 2350 4 x 10 50 Gas production + CO2 injection Ormen Lange 2007 5 295-1130 2000 15 x 50 120 Second largest gas field in Norway Challenging oceanography, Shell-operated Statfjord 2012 2 140-200 2750 5 x 25 53 Oil field, subsidence is the main motivation Midgard 2006 4 240-310 2500 10 x 20 60 Deep reservoir Snøhvit / Albatross 2007 2 250-340 2500 20 x 20 86 Gas production + CO2 injection

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Gravity for reservoir monitoring

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• Sensitive to changes of density in the subsurface
• It allows monitoring movements of fluid interfaces. Example:
• 𝜍𝑥𝑏𝑢𝑓𝑠 > 𝜍𝑕𝑏𝑡 ⇒ Δ𝑕 > 0 observed
• Magnitude proportional to the raise of the contact
• Spatial distribution of Δ𝑕 tells about comparmentalization, permeability, aquifer

strengths

Before production start After production start

gas gas water from aquifers sea sea gravimeter gravimeter

Some applications of gravity

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hydrocarbon water

Unproduced compartments? Acquifer strengths? Volume of gas in place?

Accuracy of gravity at the seafloor

1 µGal means:

• 10-8 m/s2, or 10-9 g, or 100 kg at 0.8 m
• Sub-meter sensitivity in a gas-water contact

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Year 10 000 1 10 100 1000 Satellite altimeter Airborne Shipborne Seafloor Land Borehole Stationary 1998 2002/2005 2006/2007 2009 2013 2015

mGal

Seafloor

Increased recovery

Value of the gravity data

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Infill well planning Well data Planning of production, pipeline use Lateral compart- mentalization Aquifer strength

Gravity

Update actual volume of hydrocarbon reserves Prediction of water breakthrough Reservoir parameters

(e. g. permeability)

Understanding reservoir behavior away from wells Installation of compression facilities Seismic

• At a cost ~15% of that of time-lapse seismic

Subsid idence

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Subsidence

Measured through time-lapse changes in water pressure (after applying tide and environmental corrections)

The value of subsidence data

• Accuracy on subsidence (published values):
• Mikkel 2006-2011 time-lapse: 2.5 - 3.7 mm
• Sleipner 2002-2005 time-lapse: 2.3 mm

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Compressibility, pressure depletion Drilling window for infill wells Geological model Installation safety and integrity Caprock integrity

Reservoir Overburden

Subsidence (or uplift)

Aquifer

Compressibility, permeability Lateral compartmentalization

Survey setup

• Pressure and gravity measured at each of the concrete

platforms (typically 20 minutes)

• Tide gauges deployed during the whole survey at a

subset of stations

• Allow to refer all pressure measurements to normal sea and

atmospheric conditions

• Enables comparison of data from different vintages
• In all:
• No storage issues: < 2 Gb per survey
• Fast turnaround: three months from survey to final report
• Environmental friendly

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Example: Troll field

Field cases

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Troll 2002-2009: subsidence

Pore compressibility:

• Troll West PDO (1991): ~ 80·10-5/bar
• Revised core data (2000): ~ 9·10-5/bar
• Subsidence history-matching: ~ 3·10-5/bar

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Maximum ~1 cm / year

Zero-level stations

http://www.slideshare.net/Statoil/alnes-et-al-gravity-and-subsidence-monitoring

Alnes, H., Stenvold, T. and Eiken, O. [2010] Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs, 72nd EAGE Conf. and Exhib., L010.

Corrected for gas takeout

Troll 2002-2009: 4D gravity

• Significant raise of ~2 m of the gas-water contact
• bserved in some stations already in 2002-2005
• Time-lapse seismic lines shot in 2002 and 2006

showed no evidence of the raise yet

• 5-10 m were required for it to be visible over effect of

pressure drop

• Updated aquifer strengths in the reservoir simulation

model

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Oil production Gas production, movement

• f the water front

http://www.slideshare.net/Statoil/alnes-et-al-gravity-and-subsidence-monitoring

Alnes, H., Stenvold, T. and Eiken, O. [2010] Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs, 72nd EAGE Conf. Exhib, L010.

Mikkel 2006-2011: 4D gravity

• Initial uncertainties on:
• External aquifers
• Total volume of gas
• The system has low tolerance for water production

due to risk of hydrate formation

• Results show lower water influx than expected
• Aquifer volume reduced by factor 4
• Mikkel contains more gas than expected
• This input enabled:
• Better prediction of water breakthrough
• Better long term planning

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From Vevatne, J. N., et al., [2012] Use of Field-wide Seafloor Time-lapse Gravity in History Matching the Mikkel Gas Condensate Field, 74th EAGE Conference & Exhibition, Extended Abstracts, F040

Data – model discrepancy

 Less water than expected  More water than expected

Forward-modelled gravity change (µGal)

Midgard 2006-2012: 4D gravity and subsidence

• Uncertainties in the model:
• Aquifer support and drainage patterns
• Fault distribution and compartmentalization
• Learnings from 4D gravity and subsidence: one

segment underproduced, indicating sealing faults

• Reinterpretation of available seismic data
• Updated reservoir model with the sealing fault as the most

likely realization

• A new well target was identified, which is now

number one producing well in all the Åsgard complex

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Ueland, I., et al., [2015] Modnet fram brønn på utradisjonel vis, Origo Statoil ASA, pp. 40-41 September.

Ormen Lange

• Increased recovery involved decisions on:
• Compression facilities
• Infill wells
• Water break-through and

compartmentalization identified as a significant uncertainties

• Feasibility studies for measuring water influx:
• Nearly impossible with 4D seismics
• Would take > 7 years with EM
• Abstract submitted to EAGE 2017 with

Gravitude and Shell authors

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Van den Beukel, A. et al. [2014], Integrated Reservoir Monitoring of the Ormen Lange field: Time lapse seismic, Time lapse gravity and seafloor deformation monitoring, The Biennial Geophysical Seminar, NPF, Kristiansand 2014

Forward-modelled gravity change (µGal), 2 years time-lapse

Outtake Water influx

Dunn et al., A long-term seafloor deformation monitoring campaign at Ormen Lange gas field, first break volume 34, October 2016

Significantly less subsidence than modelled in the south

Measured 2012-2014

Gravitude’s feasibility studies

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Define alternative scenarios reflecting:

• Uncertainties on reservoir model
• Value of the new information

Environment conditions

• Oceanography
• Seafloor quality

Model the strength of 4D gravity and subsidence signals for each alternative scenario Requirements from installation safety on subsidence precision Feasibility report

• Ability to distinguish reservoir scenarios
• Increased safety from subsidence monitoring
• Survey design and cost

Value of the gravity and subsidence data

Optionally

Conclusions

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• 4D gravity - subsidence surveys provide:
• Key information for reservoir management, e.g.
• Movement of fluid contacts
• Identification of non-producing compartments
• Reservoir compaction
• Information for whole-field – not only producing wells
• Improved safety of the field and installations

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Thank you

hugo.ruiz@octio.com