plumbing the depths of the pelican field - devex conference · plumbing the depths of the pelican...

Plumbing the Depths of the Pelican FieldThurza Frenz

Pelican

Talk Objectives

• To discuss the development of the northern area of thePelican Field, in particular:

• The relationship between reservoir quality, facies anddepth and how this impacted the most recent wellplacement in the northern area

• The early performance of the PUP22S1/PUW23S1producer/injector pair and some interestingobservations about how these wells interact

May 18th 2016 DEVEX 2

The Pelican Field: Introduction

DEVEX 3

Location: East Shetland Basin, NNS

Discovered: 1975 (211/26-4)

Type: Subsea oil development tied back tothe Cormorant Alpha Platform

STOIIPs and Recovery:TOTAL: 486 MMstbRecovery to date: 74 MMstb (15 % RF)

PELICAN MAIN: 369 MMstbRecovery to date: 71 MMstb (19 % RF)

PELICAN NORTH: 116 MMstbRecovery to date: 3.31 MMstb (3 % RF)

Depth: 10,500 - 12,300 ft TVDSS

Contact: OWC not penetrated, estimatedat 12,300 ft

The Pelican Field: Introduction

DEVEX 4

Location: East Shetland Basin, NNS

Discovered: 1975 (211/26-4)

Type: Subsea oil development tied back tothe Cormorant Alpha Platform

STOIIPs and Recovery:TOTAL: 486 MMstbRecovery to date: 74 MMstb (15 % RF)

PELICAN MAIN: 369 MMstbRecovery to date: 71 MMstb (19 % RF)

PELICAN NORTH: 116 MMstbRecovery to date: 3.31 MMstb (3 % RF)

Depth: 10,500 - 12,300 ft TVDSS

Contact: OWC not penetrated, estimatedat 12,300 ft

The Pelican Field: Reservoir in a Typical Well

• Middle Jurassic Brent Group

• Field average: 350 ft thick

• Crestal erosion of Tarbert and Upper Ness

• Well shows reduction of reservoir quality withdepth

• Results in top-heavy distribution of permeabilitythroughout the Brent

• Best quality reservoir in Upper Ness sands

• Leads to water cycling through Upper Nessinterval - potential bypass of Lower Brentreserves

• 65 % of field STOIIP is within Lower Brent

May 18th 2016

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Pelican North

• Appraisal wells CA23S1 & CA23S2 drilledinto head area of the Pelican Field in 1998

• Found 350 ft thick Brent interval

• Significantly less sand in Upper and LowerNess

• Etive - tight, low resistivity sands

• Broom - water bearing

• CA23S2 perforated but failed to producedue to low Productivity Index

• In 2013 TAQA drilled first successfulproducer into the North - PUP22S1

DEVEX

Pelican North








DEVEX 7

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Pelican North








DEVEX

Pelican North: PUP22S1 Well Results

• Encountered 350 ft thick Brent sequence at11,350 ft TVDSS

• Excellent reservoir properties in both Upperand Lower Ness intervals

• Significant drop-off in permeability belowLower Ness

• Reservoir more similar to flank NW Hutton Fieldthan Pelican Main - presence of Etive Interfluve

DEVEX 9

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DEVEX

• Significant erosion of the Etive took place prior toLower Ness deposition

• Erosion created an irregular incised valley topographyonto which the Lower Ness was deposited

• Thick Etive encountered at PUP22S1 is located in aninterfluve position where less erosion took place thanthe main field and nearby NW Hutton

Adapted from Marriott, 2015


• Encountered 350 ft thick Brent sequence at11,350 ft TVDSS

• Excellent reservoir properties in both Upperand Lower Ness intervals

• Significant drop-off in permeability belowLower Ness

• Reservoir more similar to flank NW Hutton Fieldthan Pelican Main - presence of Etive Interfluve

• Pressure tests showed northern area to be atvirgin pressure

• Produced 3.31 MMstb since August 2013 (toFebruary 2015)

• Injector needed to maximise recovery andmaintain pressures for future drilling

DEVEX 11

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The Significance of Facies

• Reservoir quality is severely affectedby depth

• PUP22S1 results prompted re-look atreservoir degradation with depth

• Use of analogue data from NWHutton Field was key

• Core data from both fields re-assessed

• Key link found between facies (grainsize) and reservoir quality


Etive Shoreface Sandstones

Upper Ness Fluvial Channels

Adapted from Marriott & Besley, 2015

Facies & Depth

• Coarse grained fluvial channels retainbest reservoir properties at depth

• Found exclusively within Ness interval

• Net reservoir preserved at depths inexcess of 12,500 ft TVDSS

• Finer grained facies such asmudstones and lower shoreface showthe most degradation with depth

• Become non-net below a depth of11,800 ft TVDSS

• Primarily affects permeability butslight degradation of porosity alsoseen




PU

P2

2S1


The Pelican Field: Well Depth Comparison

May 18th 2016 DEVEX Cored Wells

De

pth

(TV

DSS

ft)

PU

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3S1

PUW23S1: Key Risks & Uncertainties

Facies

• Presence and quantity of good qualityfluvial channel facies

• Presence of Etive Interfluve

Connectivity

• Sealing/ baffling faults between injectorand producer

Pressure

• Potentially large pressure differentialacross Mid Ness Shale due to productionat PUP22S1 leading to drillability issues

DEVEX 15

PUW23S1: Pre-Drill Pore Pressure Prediction

• Pre drill estimates of depletion ~ 4400psi in Upper Ness with virgin pressurein Lower Brent units

• No safe mud window

• Mud Weight > Fracture Gradient inUpper Ness

• Borehole strengthening material used

• Estimated additional strength of 400 psi

• No losses during drilling of reservoirsection

• 4000 psi depletion measured in UpperNess

DEVEX

PUW23S1 PPFG Prognosis

PUW23S1: Pre-Drill Pore Pressure Prediction

• Pre drill estimates of depletion ~ 4400psi in Upper Ness with virgin pressurein Lower Brent units

• No safe mud window

• Mud Weight > Fracture Gradient inUpper Ness

• Borehole strengthening material used

• Estimated additional strength of 400 psi

• No losses during drilling of reservoirsection

• 4000 psi depletion measured in UpperNess

DEVEX

PUW23S1 PPFG Prognosis

Addition of Wellbore Strengthening

Material

PUW23S1: Results

• Encountered top reservoir at 11,797 ftTVDSS

• ‘Top heavy’ reservoir qualitydistribution

• Good quality Upper and Lower Nessinterval with good permeabilities

• Poor quality Lower Brent similar tothat seen at PUP22S1

• Etive Interfluve encountered as atPUP22S1


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PUW23S1: Results

• Electro-facies show some good qualityFluvial Channel facies (yellow) withinthe Upper Ness

• Pressures taken throughout the UpperNess show depletion of 4,000 psi -consistent with predicted pressures

• Shows good communication withPUP22S1 producer

• Unable to get pressures throughoutLower Brent due to very low mobility


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PUW23S1: Results

May 18th 2016

• Facies, in particular grain size, plays asignificant role in preserving reservoirquality at depth

• Coarse grained fluvial channelspreserve much better permeabilitiesthan the finer grained sands at thesame depth

• Fluvial channels approx. 60 mD

• Finer grained sands approx. 2 mD



PU

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PUW23S1: Early Injection Performance

May 18th 2016

• Fractured injection of>10,000 bpd

• Oil benefit >1,000 bopd

• Water breakthrough after 3months

• Oil rates stabilised aboveprevious declining trend

PUP22S1 & PUW23S1 Pressures13 psi/day BHP

increase 3.0 MMbbls

STOIIP

Very rapid response between PUW23S1

and PUP22S1

3.0 psi/day BHP increase 40 MMbbls

STOIIP

Unexplained drop in

PUP22S1 BHP

Pelican Field Oil Production

Sep/2015 Dec/2015 April/2015

Observations


• Depositional facies plays an important role in preserving reservoir quality atdepth:

• Diagenesis strongly influenced by original grain size

• Coarse grained fluvial channel facies retain good reservoir quality below 12,500 ft

• Increased confidence and opportunities available to Pelican development at depth

• North of Pelican still challenged due to structure and distance from manifold

• Coarse grained fluvial channel facies presence far less certain than main field

• Assessment of reservoir drilling risks was robust and use of wellborestrengthening material allowed drilling risk to be mitigated

• Injectivity shows excellent connection between producer/injector pair withuplift in production justifying injector well

Acknowledgements


• Derek Littlejohn

• Andrew Bostock

PUW23S1: Injection Related Hydraulic Fracturing

• Hydraulic fracturing took longer than expected based on experience from PUW12S3 injector:

• Investigation underway to compare injection and understand differences:

• Wellbore strengthening material increasing near field stresses

• Well angle, length and orientation in relation to stress field orientation and injection water cooling

• Uncertainty in rock properties in relation to elastic and thermal properties

• Sand exposure and distribution relating to restriction of fracture growth vertically

PUW23S1 Injection - Temperature and Pressure Effects on Fracture Breakdown Pressure – Expected scenario

Heather Caprock

Upper Ness Sands

SAN

DFA

CE

INJE

CTI

ON

PR

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Top Brent

Top Heather

PUW23S1 Injection - Temperature and Pressure Effects on Fracture Breakdown Pressure – Possible problematic case

Heather Caprock

Upper Ness Sands

SAN

DFA

CE

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CTI

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Top Brent

Top Heather

Relationship of diagenetic style to depositional environment

Besley, 2015

Upper Ness - Grain Size & Diagenesis

• The Upper Ness high energy fluvial channelsandstones are typically medium grainedand well sorted with little detrital clay

• The pore system is “open”

• Feldspar has suffered grain dissolution butfluid flushing is thought to have preventedthe formation of, or removed, diagenetickaolinite

• Quartz overgrowths form rigid bridgesbetween grains that help resist compaction Feldspar grain

dissolution pore

Blue stain = porosity

1000 μm

200 μm

Marriott, 2015

Etive- Grain Size & Diagenesis

• The Etive shoreface sandstones are finergrained with locally abundant detrital claymatrix and smaller pore throats

• The pore system is “closed”

• Feldspar grain dissolution results indiagenetic kaolinite blocking pores

• Illitisation of kaolinite and detrital clays

• Quartz overgrowth development isinhibited where detrital clay matrix coatsthe grains. Where present this tends toocclude pore throats rather thanpreserving an open pore system

Detrital & authigenicclays block

pores

1000 μm

200 μm

Marriott, 2015

plumbing the depths of the pelican field - devex conference · plumbing the depths of the pelican...

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