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 Sm 3 )

3 Reservoirs and hydrocarbon volumes Reservoir STOOIP (MSm 3 ) Reserves (MSm 3 ) Oil recovery (%) Produced (MSm 3 ) *) Fangst ,2 Tilje ,8 Åre ,7 Total ,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)

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.

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! Sedimentological study - Old framework New Zonation

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 Depositional environment Åre 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)

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

10 Depositional environment Åre 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 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 Depositional environment Åre 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

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

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

14 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 cm Top Åre 6 surface RHOB/NPHIGR Tidally-influenced distributary channel complex Shoreface/shallow brackish-water embayment Example of modern estuary Tidally-influenced distributary channel complex Shoreface/shallow brackish-water embayment

15 Example of modern estuary Tidally-influenced distributary channel complex Shoreface/shallow brackish-water embayment 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

16 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) RHOB/NPHI GR RES RHOB/NPHI GR RES FS RT

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