North Sea Chalk - A Challenging Reservoir
Mike Millar
LPS Seminar - Challenging Reservoirs
Thursday 21st September 2017
North Sea Chalk - A Challenging Reservoir
Talk Outline
• The reservoir and the issues - why is Chalk a challenging reservoir ?
• Upper Cretaceous to Lower Palaeocene Carbonate Micrite
• high porosity but low permeability
• fractures and re-sedimentation can be important influence on reservoir quality
• potential for tilted Free water level (FWL)
• potential for compaction and subsidence
• What was done
• Collected enough data to understand as much of reservoir as possible
• in-place volumes and potential reserves, we should be able to make money from this field
• platform off-set from structure, avoid subsidence issues
• well placement and completion design, built-in flexibility
• Carry on collecting data during field life to aid reservoir management.
• Results and lessons
• Cased-hole logs show sweep efficiency and
• identified by-passed oil and opportunity for work-over
• Cumulative production exceeds sanction case estimates
• Summary
3
Why is North Sea Chalk a Challenging Reservoir?
• Upper Cretaceous to Lower Paleocene Carbonate
• Micrite - made from coccolithophores
• Very different grain size to sandstone reservoirs, which has a big impact on Chalk reservoir quality.
Chalk
Sandstone
Chalk
4
Why is Chalk a Challenging Reservoir?
• Generally good to high porosity, but low matrix permeability
• Generally very ‘clean’ limestone, but can have marls, flints and 'hardgrounds'
0.5
m2
m
Marl
Flint layers
Why is Chalk a Challenging Reservoir?
• Chalk Saturation Profile
• Chalk has very fine capillaries
• leads to long entry-pressure height,
• so FWL does not equal OWC
• and a long transition zone, generally much longer than Sandstones with similar porosity
• can lead to uncertainty in populating saturations in Geological and Dynamic models
Saturation vs. height above Free water level
6
Why is Chalk a Challenging Reservoir?
• Resedimentation, may enhance porosity
• Hardgrounds, reduce porosity
• Flints can make it difficult to drill
• Structure and fractures may enhance permeabilityFlint layers
7
Why is Chalk a Challenging Reservoir?
• Possibility of tilted Free water levels (FWL)
• Mapping Overpressure indicates pressure gradient in the aquifer which can cause tilted
FWL in Chalk fields
• Uncertainty for in-place volumes and planning well locations
• Dennis et al 2005; Tilted Oil-water Contacts, in Dore and Vining, Proc of 6th Petroleum Geology
Conference.
• Water Saturation and analysis of a tilted free water level in the Judy Joanne chalk field. Goldsmith and
Harris, LPS December 2001 Seminar
Joanne Chalk
8
Why is Chalk a Challenging Reservoir?
• Seismically Obscured Areas
• Shallow gas causes attenuation of seismic response at the Chalk horizons
• Common in North Sea Chalk Oil Fields
• Uncertainty for building the Geological model and thus in-place volumes and planning
well locations
• Compaction and subsidence in Chalk once production starts
• high porosity and relatively weak rock
• water injection might maintain pressure, but it makes Chalk weaker
• Platforms had to be jacked-up at the Ekofisk complex
• Compaction of North-Sea Chalk by Pore-Failure and Pressure Solution in a Producing
Reservoir. Daniel Keszthelyi et al. Front. Phys., 16 February 2016
9
What was done ?
• Collect enough data to understand the resource;
• Plan data acquisition in a team with all the sub-surface disciplines and facilities
engineers
• Mud log and gas log, especially mud losses
• Electric logs, including image logs and FPT
• Core (although recovery might not be very good)
• Well tests
• top reservoir pick to reduce mapping uncertainty
• full petrophysical evaluation integrating all log and core data to reduce
uncertainty on in-place volumes
• well test to reduce uncertainty on production potential and influence of fractures
• Understanding the Fractures, key to productivity
• Fractures on Image Log, Core (UV light) and Thin-section
• Varying scale of the fractures
10
What was done ?
• Property Modelling Porosity
• Porosity modeled by reservoir unit
• For each reservoir unit data analysis performed on petrophysical logs to determine
porosity min, max, mean and standard deviation.
• Using the data analysis as input, porosity was modeled stochastically within the
Geological model using truncated sequential Gaussian simulation on a unit by unit
basis.
• Compare the log data, the upscaled logs and the final stochastic model as a quality
control check on the final 3D volume
• Property Modelling Water Saturation
• Log derived Sw was used to build a saturation height function which is then used in
the Geological model.
• Core has limited influence as we only get recovery from generally the poorer quality
rock, but the core suggests high entry pressure height
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What was done ?
• Property Modelling Permeability
• Permeability modeled by reservoir unit
• For each reservoir unit, maximum, minimum and mean permeabilities were estimated
from all the available data (core, well test, NMR, analog)
• Permeability was then stochastically modeled in Geological model using truncated
sequential Gaussian simulation conditioned on porosity as a trend on a unit by unit
basis.
• Field development
• Platform offset from main structure
• Well inventory - 2 development pilot wells, 2 horizontal production wells, and 2 high
angle water injectors
• LWD and Wireline (Pipe conveyed) including Acoustic image logs
• J meter used to monitor losses and help to identify open fractures
12
What was done ?
• Field development
• Completions
• perforated 7" liner, with isolation packers between perforations
• acid wash across perforations to enhanced flow
• sliding screens to give flexibility in production/injection
• downhole gauges installed
• gas lift plumbing installed
• saturation monitoring logging
• cased-hole pulsed neutron tools
• production logging tools
13
Results and lessons
• Cased-hole Pulsed Neutron Saturation Monitoring logs
• Horizontal Oil Producer, fairly even sweep
• Production Logging
• Horizontal Oil Producer, fairly even sweep
• Cased-hole Pulsed Neutron Saturation Monitoring logs
• Originally a water-injector
• PNC highlighted possibility of by-passed oil
• Well successfully turned around into oil producer
• Cumulative production exceeds initial estimates