Ula Field Miscible WAG Flood Assessment

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Ula Field Miscible WAG Flood Assessment

• - Core Log Experience from behind a Maturing
  WAG Front

Simon Thomas, Jon Duncan, Ula Subsurface Team, BP
Norway
FORCE Mature Field Life Extension
Workshop Realising value from existing data, data
acquisition planning and modelling 2nd October
2007
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Ula Field Introduction

• Discovered 1976 First Oil 1986
• Water depth 67m
• 100m thick shallow marine reservoir
• Moderately deep and hot
• Depth 3350-3800mtvdss
• Temperature 150ºC
• Volumes
• In-place 1 billion barrels
• Produced 420 million barrels
• Late field-life initiatives
• Infill drilling Unit 1 horizontal wells
• WAG Injection EOR

ULA
Drilling
Production
Living Q
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Ula Field Reservoir
Reservoir Layering
Ula Sandstone Bioturbated fine-medium
grained sandstone with nodular calcite cement
stringers Depositional Setting Storm dominated
shallow marine shelf
Depositional Environment
Stratigraphy
Retrogradational Shelf
Unit 1
Upper Jurassic Ula Sst Member (Farsund Fm)
Progradational Shelf
Unit 2A1-2
Progradational Shelf
Unit 2A3-6
Aggradational Shelf
Unit 2B
Unit 3A-B
Progradational Shelf
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Ula Field Producing Wells

• 8 Crestal Oil Producers
• 3xUnit 2-3 WAG
• 5xUnit 1 horizontal
• 7 Flank Injectors
• 3xUnit2-3 water injectors
• 4xUnit 2-3 WAG injectors

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Ula Field WAG Surveillence

• Wellhead P T

• Monthly well testing

ULA
Drilling
Production
Living Q
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A15-A3A WAG Panel
A-15 Oil Producer
1500m
A-9A Observation Well
500m
A-3A WAG Inector
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Ula WAG Observation Well Objectives

• Is current Ula Field WAG working?
• Halted production decline increasing GOR
  support pilot success
• Where and how big is the ultimate WAG prize?
• Recent SCAL data indicates high potential prize
  Sorm ? 5 (Sorw ? 30) Sgt ? 35
• How efficient is the WAG process?
• Which intervals of the reservoir are being
  contacted by the WAG injection?
• Fundamentally Sorw, Sorm, Sg, kh kv
• What do we do next?
• How do we optimally design the future WAG
  injector/producer drilling program to access to
  remaining oil in Ula?

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Data Acquisition Challenges

• Wireline Data
• complicated logging environment

• Core Data
• 3-phase fluid mobility system
• Miscible gas injection process

Deep Invasion 3-phase mobility
Sample Invasion Coring Plugging
Gas Presence gas injection
Sample Losses Gas-expansion drive
Mixed Waters formation seawater
Oil Properties density, formation factor
Reservoir Cooling water injection
Water Properties density (salinity)
COST!! Justified by the large size (gt100mmbbl)
but large uncertainty of the potential WAG prize
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Ula A9A Wireline Logging Program

• Baker Atlas Wireline Logging Contract
• Gamma-ray, density, neutron, sonic laterolog
  resistivity
• 57 Pressures 7 fluid samples
• Nuclear Magentic Resonance Gas Non-RT based
  saturation profiles
• Openhole Pulsed Neutron Gas saturation profiles
• Carbon/Oxygen Non RT based saturation profiles
• Electrical resistivity images Small-scale
  heterogeneities
• Directional induction resistivity fluid related
  anisotropy

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Ula A9A Coring Program

• Low invasion, high ROP coring system
• Low invasion water-based (NaCOOH) mud with
  Deuterium tracer
• Assumed that the dominant mobile fluid phase
  would be water
• Slow, staged tripping to avoid gas drive
• Assumed that both free gas and high GOR oil would
  be present

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Ula A9A Core Analysis Program

• Core plugged offshore under oil brine and
  onshore under nitrogen
• Multiple measurements including
• Porosity, vertical, horizontal permeability
  probe permeametry
• Dean Stark Water oil saturations
• Spun Water Resistivities - for calibrating Archie
  water saturation model
• Gas Chromatography Analysis for determining
  degree of WAG contact

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Plug Sampling Program
100m of core 800plugs gt2000 petrophysical
measurements
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Reservoir Quality Layering
Unit 1
Unit 2A
Unit 2B
Unit 3A
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Reservoir Pressure

• 3 layer pressure system
• 2 key pressure barrier/baffles
• Unit 1A shale barrier (1200psi)
• Unit 2A6 baffle (40psi)
• Producing from 1A 2A with injection into Units
  1A2A2B

Pressure
Unit 1A Barrier
1200psi
Calcite
Calcite
Depletion
Unit 2A6 Baffle
Calcite
Calcite
Calcite
Unit 1A Barrier
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Oil Water Saturation Profiles

• Offshore onshore samples
• Oil, water nitrogen cutting
• Sw range between 40-60
• 0-50 sample contamination
• So range between 10-70
• 1.36 FVF used to covert from stock-tank to
  reservoir conditions
• Core So more robust than Sw
• Mobile water may have been lost
• Sw from from logs used to check

No gas or fluid loss
Gas or fluid loss?
Minor Gas or fluid loss?
In-situ So
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Log-based Water Saturation

• Archie Model
• Rw profiles determined from
• 100 spun-water samples from core
• 2 downhole fluid samples
• wellhead produced water samples
• historical water resistivity data
• n-exponent determined from
• 20 oriented electrical plugs

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Fluid Saturation Uncertainty Reduction

• Log Sw reconciles well with core based CSo CSw
• Core-log comparison highlights
• Intervals of remaining gas
• Intervals of lost core water
• Core-log comparison does not illustrate WAG
  contact
• An independent method was required to assess gas
  contact

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Oil Composition Analysis
Original Oil

• Compositional oil analysis key to understanding
  WAG performance
• Toluene extracted oil samples
• Gas chromatography analysis
• C50/Cn ratio analysis
• Identified compositional profiles
• Reduction in lighter end components unambiguously
  identifies WAG contact

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Oil Composition Analysis Results
CSw
CSo
Normalised GC Dataset The upper lt1A3 lower gt3B
intervals are compositionally unchanged The
2A3-2A6 3A intervals appear to have minor
compositional stripping The 1B-2A2 2B1-2B3
intervals have undergone extreme stripping
Raw GC Dataset prior to normalisation. Note the
correspondence between the low remaining oil
saturations combined with gas indications and the
reduced low carbon components.
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Sector Modelling
1A
1B
2A
2B
3A
A-9A OBS
3B
A-15 OP
A-3A INJ
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Summary

• Innovative integration of core log data has
  been key to reducing saturation profile
  uncertainty
• fluid property profiles (resistivity
  composition) are key descriptors
• Gas-flood has contacted only 40 of the reservoir
  within 2 layers
• Sorms reach down to 10-20, consistent with SCAL
  Sorms
• Gas-flood by-passed intervals make up 60 of the
  reservoir
• Water-flood sweep has been variably effective
• Target intervals for future WAG confirmed with
  Sorws of 30-75

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Acknowledgements

• BP Exploration Production Technical Group
  (Sunbury, UK)
• ICCS (UK) Reslab (Norway) Core Analysis
  Laboratories
• Baker Hughes (Wireline Coring Operations)
• Andrew Spence (Independent Core Analysis
  Consultant)

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Questions or Comments?