Rezonation of the Åre Formation Heidrun Field, Norwegian Sea

Classification: Statoil Internal Status: Draft

Rezonation of the Åre Formation

Heidrun Field, Norwegian Sea

Arve Næss (1), Camilla Thrana (1), Mali Brekken (1), Simon Leary (3,2), Stuart Gowland (3)

(1) StatoilHydro, HD Petek, (2) StatoilHydro, F&T LPT Geo, (3) Ichron Limited

2

Heidrun Field

• Heidrun Field is located in the Norwegian Sea, 350 km offshore mid-Norway

• Part of StatoilHydro's Operation North business area

• Oil and gas producing field, on stream since 1995

Ca. 140 drilled wells

Active wells:

• 34 oil producers

• 14 active water injectors, 1 gas injector

Production profile

* Production by July 2007

*

Oil Production (mill Sm3)

3

Reservoirs and hydrocarbon volumes

Reservoir STOOIP

(MSm3)

Reserves

(MSm3)

Oil recovery

(%)

Produced

(MSm3) *)

Fangst 145 88 61 73,2

Tilje 117 42 36 26,8

Åre 180 50 27 13,7

Total 443 180 40 113,7

• Heidrun reservoir stratigraphy is composed of Lower to Middle Jurassic formations

• A large fraction of remaining reserves and IOR potential is located within the Åre Fm. reservoir intervals

*) Produced as of 1st October 2006

STOOIP, reserves, recovery factor and produced per reservoir by 1st October 2006

(From Dalland et al., 1988)

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2003 2004 2005 2007

Proposed re-zonation of Åre Fm.

Biostratigraphic study

Sedimentological study

2006

Implementation

The Åre study• Project initiated in 2003 as a consequence of several challenges related to reservoir characterisation:

– Previous zonation based on a limited dataset– Few seismic horizons in the stratigraphic framework– Poor biostratigraphical control– Limited understanding of variability in facies development

• 2006: Sedimentological and biostratigraphical studies completed based on an improved well database.

• New reservoir zonation proposed.

• 2007: Reservoir zonation implemented in all wells.

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Problems with the previous reservoir zonation• 2 main units

― The Åre Formation is geologically not split into 2 distinct elements.

• 32 zones

― Scheme was too complex.

― Complicated the understanding of the basic geological model.

• Difficult to apply

― Poorly documented and partly inconsistent framework.

― Often based on wireline log picks of unknown geological significance.

• Too many uncertainties

― Picks were occasional inconsistent between closely spaced wells.

― Reservoir zones became ‘tram-lined’ using thickness comparisons.

― ‘Ad-hoc’ adjustments were implemented to make reliable correlations.

― Unknown lateral facies variation across the field.

Åre

AARE FM. 2.10 –2.13

AARE FM 1.1-1.4

1.5-1-6

AARE FM. 1.7-1.12

AARE FM. 1.13

AARE FM. 1.14-1.17

1.18-1.19

AARE FM. 2.1-2.4

AARE FM. 2.5-2.9

Tidal influenced marginal marine

channels and tidal flats

Fluvial and marine influenced, stacked

bayfill units

Delta plain channels and floodplain facies

Lower fluvial plain channels


Åre

AARE FM. 2.10 –2.13

AARE FM 1.1-1.4

1.5-1-6

AARE FM. 1.7-1.12

AARE FM. 1.13

AARE FM. 1.14-1.17

1.18-1.19

AARE FM. 2.1-2.4

AARE FM. 2.5-2.9

Tidal influenced marginal marine

channels and tidal flats

Fluvial and marine influenced, stacked

bayfill units


Lower fluvial plain channels


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Developing new reservoir stratigraphy - work flow

• Link the sediments into packages of facies which are genetically related

better understanding of the depositional system easier prediction of facies within each new reservoir

zone

• Constrain a broad stratigraphic framework underpinned by field-wide key surfaces

– Easily recognisable in both core and log expression

• Integrate data from many sources

• Be easy in its application for geologists/geophysicists/reservoir/production and drilling engineers

• Most importantly, be documented!

Sedimentological study -

Old framework

New Zonation

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New reservoir zonation

• 7 major reservoir zones divided into several subzones.

• Candidate flooding surfaces represented

by mudstone intervals seem to be some of

the best correlative markers within the Åre

Fm.

• These key markers display distinct log signatures.

• Reservoir zones bounded by field-wide mudstones should also correspond to flow units.

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Depositional environment Åre 1 - 2

• Åre 1 and 2 were deposited in a wet and vegetated coastal plain.

• The amount of vegetation had an impact on the confinement and isolation of the fluvial channels.

challenges with regards to efficient drainage and pressure support.

Channel system in Australia (W.Nemec)

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Channel sandstone

Flood plain mudstone

1m

Example of fluvial deposits

Flood plain mudstone

Coal

Channel sandstone

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• Coastal plain setting gradually replaced by a marginal-marine environment

interaction of fluvial and marine processes.

• Åre 3 – 5: small brackish water embayments and wave-influenced deltas.

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Shallow brackish-water embayment

Coastal plain with fluvial channels

Wave-influenced

delta

Tidally-influenced distributary channel

Conceptual model Åre Fm. 3-5

Possible modern analogue: Ganges River Delta

3 km


• Modern analogue : Ganges River Delta.

• Close interaction of coastal plain

elements and marginal-marine, lower

delta plain sub-environments

1 km

Tidally-influenced distributary channel

Shallow brackish-water embayment

Coastal plain with fluvial channels

Wave-influenced

delta

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Base

Top

Bayfl

oor m

ud

sto

ne

Baym

ag

in s

an

dsto

ne

Bayfl

oor m

ud

sto

ne

hete

rolith

s

Example of bay-fill deposits

• Heterolithic upward coarsening/cleaning units.

• Challenging in terms of recovery.

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Example of key stratigraphic marker: Top Åre 3 flooding surface

Well A Well B Well C Well D

This boundary is a field-wide flooding surface, proven to be one of the best correlateable surfaces within the Åre Fm.

Core expression: 1-4 m thick bed of intricately laminated, carbonaceous claystone, often cemented.

Log expression: High GR, wide negative separation on NPHI/RHOB logs.

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Tidally-influenced distributary channel complex

Shoreface/shallow brackish-water embayment

Depositional environment Åre 6

• Åre 6: Tidally-dominated channels and flats interacting with brackish water bays.

• The top of Åre 6 represent an important change in depositional style from a marginal-marine to a fully marine setting (Åre 7).

Diplocraterion

6.2

7.1

5 cm

Top Åre 6 surface

RHOB/NPHIGR



Example of modern estuary



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Example of modern estuary



• Åre 7: ”Tilje-type” shallow marine environment.

• Kept as a Åre reservoir zone to avoid confusion in the database and rezonation of the overlying stratigraphy (Tilje Fm.)

Depositional environment Åre 7

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• The identification of key flooding surfaces revealed a former stratigraphic misinterpretation of 40 m within an injector.

• Wrong zonation led to perforation at a deeper level than intended in the injector

• A complete re-interpretation of the stratigraphy improved the understanding of flow between the injecting and producing wells.

• This shows that the old zonation was not robust enough.

Injector Producer

Example of old problem and new solution

Red lines indicate old correlation. (FS = flooding surface)

4.14.1

3.33.3

4.2 4.2

RHOB/NPHIGR RES RHOB/NPHIGR RES

FS

FS

RT RT

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Outcome of the Åre study

•Benefits– Robust and predictable reservoir zonation

– Improved understanding of facies development

– Improved stratigraphic control during drilling operations

– Input to geological and reservoir simulation models

– More precise production forecasts

– More robust drainage strategy

– Better fitted well solutions

Performing such a radical reinterpretation of the reservoir has

consequences for all disciplines and work processes on Heidrun.

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Acknowledgement

•Partners:

– ConocoPhillips

– Eni Norge

– Petoro

Documents

Rezonation of the Åre Formation Heidrun Field, Norwegian Sea