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Rezonation of the Åre Formation Heidrun Field, Norwegian Sea

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. Oil Production (mill Sm 3 ). *. Heidrun Field.

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Rezonation of the Åre Formation Heidrun Field, Norwegian Sea

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  1. Rezonation of the Åre FormationHeidrun 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. Oil Production (mill Sm3) * 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 Production profile Ca. 140 drilled wells Active wells: • 34 oil producers • 14 active water injectors, 1 gas injector * Production by July 2007

  3. Reservoirs and hydrocarbon volumes • 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 (From Dalland et al., 1988) *) Produced as of 1st October 2006 STOOIP, reserves, recovery factor and produced per reservoir by 1st October 2006

  4. 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. Biostratigraphic study Proposed re-zonation of Åre Fm. Implementation Sedimentological study 2003 2004 2005 2006 2007

  5. 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.

  6. 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! New Zonation Sedimentological study - Old framework

  7. 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.

  8. Channel system in Australia (W.Nemec) 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.

  9. 1m Example of fluvial deposits Channel sandstone Flood plain mudstone Coal Flood plain mudstone Channel sandstone

  10. Depositional environment Åre 3 - 5 • 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.

  11. 1 km Coastal plain with fluvial channels Depositional environment Åre 3 - 5 Wave-influenced delta Tidally-influenced distributary channel • Modern analogue : Ganges River Delta. • Close interaction of coastal plain elements and marginal-marine, lower delta plain sub-environments Shallow brackish-water embayment Conceptual model Åre Fm. 3-5 Wave-influenced delta Tidally-influenced distributary channel Shallow brackish-water embayment Coastal plain with fluvial channels 3 km Possible modern analogue: Ganges River Delta

  12. Example of bay-fill deposits • Heterolithic upward coarsening/cleaning units. • Challenging in terms of recovery. Bayfloor mudstone Top heteroliths Bayfloor mudstone Baymagin sandstone Base

  13. Example of key stratigraphic marker: Top Åre 3 flooding surface Well B Well C Well A 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.

  14. GR RHOB/NPHI 5 cm Diplocraterion Tidally-influenced distributary channel complex 7.1 Top Åre 6 surface 6.2 Shoreface/shallow brackish-water embayment Example of modern estuary 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). Tidally-influenced distributary channel complex Tidally-influenced distributary channel complex Shoreface/shallow brackish-water embayment Shoreface/shallow brackish-water embayment

  15. Depositional environment Åre 7 • 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.) Tidally-influenced distributary channel complex Shoreface/shallow brackish-water embayment Example of modern estuary

  16. GR RES RHOB/NPHI GR RES RHOB/NPHI RT RT FS 4.2 4.2 4.1 4.1 FS 3.3 3.3 Example of old problem and new solution Producer Injector • 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. Red lines indicate old correlation. (FS = flooding surface)

  17. 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.

  18. Acknowledgement • Partners: • ConocoPhillips • Eni Norge • Petoro

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