Appraising a late-middle-aged Brent Group field

Transcription

1 Appraising a late-middle-aged Brent Group field Reservoir learnings after nearly 20 years of Pelican field production Jill Marriott

2 The Pelican Field The Pelican oil field is nearly 20 years old It is a subsea oil development tied back to the Cormorant Alpha Platform (now >35 years old) Located in SW corner of Quad 211 in the NNS Brent Group heartland Pelican and its neighbours (NW Hutton & Lyell) are deep Brent fields with low Recovery Factors (up to 15 %)

3 Pelican Field Depth The Pelican field crest lies at a depth of 10,500 ft tvdss with a spillpoint at 12,300 ft tvdss This is 2,000 ft deeper than its nearest neighbour to the north, Cormorant Block 1 North CORMORANT Block 1 Block 2 NW HUTTON PELICAN 5 km South 3

4 PELICAN MAIN PELICAN NORTH Pelican Field Key Facts Pelican Main has an active well stock of 8 crestal producers and 3 midflank injectors The first producer in Pelican North was drilled in 2013 CA23S2 CA23S1 1 km The well density is skewed towards shallow depths Classic Brent stratigraphy 350 ft TARBERT NESS ETIVE RANNOCH BROOM UPPER BRENT Mid Ness Shale LOWER BRENT 211/26-4 PUP14 PUP13 & 13S1 PUP19 PUW17 PUP16 PUP16S1 PUP15S1 PUP15 PUW11S1 211/26a-12RE PUP22S1 211/26a-13 PUP21 PUP11 PUP18 & 18S1 PUP20 PUP12 & 12S1 PUP10BS2 PUW12S2 PUP10BS1 PUW12S3 PUP14S1 Cored Wells

5 Pelican Reservoir Quality & Depth Marked reduction of permeability with depth Led to the concept of an effective production floor at 11, 800 ft tvdss Causes: complex diagenesis including illite, kaolinite, burial quartz & compaction , ft Depth ft tvdss PELICAN CORE DATA BY DEPTH 11,800 ft Production floor Established thinking: early oil migration into the structure was able to preferentially inhibit diagenesis in the shallower parts of the field ,300 ft spillpoint Horizontal Permeability (md) 1 md Key to wells 1000 md Permeability md 211/26a /26a /26a-4 PU-P11 PU-P14

6 Broom Rannoch Etive Lower Ness Upper Ness Pelican Reservoir in a Typical Well Reservoir quality within a typical Pelican well shows the characteristic reduction of reservoir quality with depth This results in a top-heavy distribution of permeability throughout the Brent GR API 211/26a-12 Poro v/v Perm md Perm often > 1000 md Perm md Best reservoir quality within Upper Ness sandstones Perm 100s md How has this affected reservoir performance? 50 ft

7 Early Early Development Challenges challenges The early development strategy was to perforate all Brent layers in both producers and injectors Early Field Development Strategy: (perforate in all layers) led to Upper Brent over-ride of water injection This led to water-injection over-ride in the highest permeability Upper Ness layers and bypass of the Lower Brent Approximately 75 % of the STOIIP in the Pelican Main Field is in the Lower Brent Average field water-cut increased rapidly reaching 52 % by mid 2001 (3 years after injection start-up), resulting in associated production impairment due to scaling

8 Later Development Strategy Later field development strategy was to drill dedicated Lower Brent producers TAQA acquired Pelican in late 2008 and continued this strategy Drilled and completed 3 producers and 1 injector in the Main Field, perforating only the Lower Brent Later Field Development Strategy: Zonal isolation with a focus on Lower Brent re-development in Pelican Main Between 2008 & 2011, average production from Pelican Main increased from 2500 to 7500 bopd Average field water-cut went from 50 % to 20 % in the same period TAQA drilled the first development well in Pelican North in 2013 (well PUP22S1)

9 Pelican North Well Result (PUP22S1) The well penetrated a full Brent sequence (350 ft thick) at a depth of 11,350 ft tvdss 270 ft of core was acquired in the Tarbert, Ness & Etive Formations Excellent reservoir properties were encountered in the Upper and Lower Ness BUT, Etive character was different to most Pelican Main wells, twice as thick (>100 ft) and largely nonreservoir (<1 md permeability) Cormorant Alpha Platform Pelican North 1 km CA23S1 & S2 PUP22S1 211/26a-13 PUP13 & 13S1

10 Pelican North Well Result There is an extreme drop-off in permeability at the Ness/Etive boundary GR API PUP22S1 Poro v/v Perm md Why is the Etive effectively nonreservoir (<1 md)? Why is the Etive twice as thick as in other Pelican wells? Etive Lower Ness Upper Ness Perm often > 1000 md Core description indicates that the Ness is dominated by medium-coarse grained fluvial channel sandstones whilst the Etive comprises finemedium grained shoreface sandstones Is facies important? 50 ft Perm md Perm <1 md!

11 Pelican and NW Hutton Core Pelican core dataset has limited coverage Pelican Core (x6) NW Hutton Core (x8) Released NW Hutton core data has been used to supplement it and it extends deeper ft Shallow Some NW Hutton cores extend into the water leg ft ft PUP22S1 Intermediate Deep ft PELICAN OWC Very Deep

12 Lower Brent Character 200 ft Pelican North PUP22S1 Etive Rannoch Broom Pelican Main INCISED VALLEY-FILL (IVF) Lower shoreface Middle shoreface Upper shoreface / tidal channels Lower shoreface / offshore transition Shoreface; tidally influenced South NW Hutton Crest Mid Ness Shale Bay mudstone Significant erosion of the Etive took place prior to deposition of the Lower Ness. The Etive has been locally completely eroded at the crest of NW Hutton and thick Lower Ness lies directly on Rannoch Lower Ness Bay head deltas, local channels SEQUENCE BOUNDARY This erosion generated an irregular incised valley topography onto which the Lower Ness was deposited Thick Etive in PUP22S1 is located in an interfluve position

13 The Significance of Facies This poroperm plot highlights the strong contrast between the Etive shoreface sandstones and high energy fluvial channel sandstones from the Upper Ness Upper Ness Fluvial channel sandstones All other data in grey Facies type clearly has an impact on reservoir quality 1 md Etive shoreface sandstones

14 Etive - Shallow Marine Shoreface The Etive is dominated by fine and fine-medium grained shallow marine middle to upper shoreface sandstones 211/26a-13 Core Description Typically planar laminated or massive (bioturbation / dewatering) Middle to upper shoreface

15 Ness - Fluvial Channel Sandstones The Ness is heterolithic 211/26a-12 Core Description The best reservoir quality is recorded within medium to coarse grained, crossbedded sandstones. These represent high energy fluvial barforms Common in the Upper Ness and some parts of the Lower Ness Multistorey fluvial channel ssts Crevasse splay ssts and floodplain mdsts

16 Permeability-Depth Trends The Etive shoreface sandstones show a steep permeability degradation gradient. They are non-reservoir below a depth of 11,500 ft tvdss The Upper Ness fluvial channel sandstones preserve net reservoir to at least 13,000ft tvdss Etive shoreface sandstones PUP22S1 BUT only if channel sandstones are present (not always) There is no relationship between fluid type (oil or water) and permeability 1 md Upper Ness fluvial channels sandstones

17 Upper Ness - Grain Size & Diagenesis The Upper Ness high energy fluvial channel sandstones are typically medium grained and well sorted with little detrital clay Blue stain = porosity The pore system is open 1000 μm Feldspar has suffered grain dissolution but fluid flushing is thought to have prevented the formation of, or removed, diagenetic kaolinite Quartz overgrowths form rigid bridges between grains that help resist compaction Feldspar grain dissolution pore 200 μm

18 Etive - Grain Size and Diagenesis The Etive shoreface sandstones are finer grained with locally abundant detrital clay matrix and smaller pore throats The pore system is closed Feldspar grain dissolution results in diagenetic kaolinite blocking pores 1000 μm Illitisation of kaolinite and detrital clays Quartz overgrowth development is inhibited where detrital clay matrix coats the grains. Where present this tends to occlude pore throats rather than preserving an open pore system Detrital & authigenic clays block pores 200 μm

19 Conclusions Reservoir quality degrades with depth However, grain size is the dominant control on how In shallower parts of the field any facies has the potential to form net reservoir In deep parts of the field only the coarser grained high energy fluvial channel sandstones can form effective reservoir (limited to the Upper Ness and some parts of the Lower Ness) Ness Formation high energy fluvial channel sandstones have the potential to form net reservoir below 13,000 ft tvdss Etive Formation shoreface sandstones are non-reservoir below 11,500 ft tvdss Potential opportunities, previously overlooked, might exist in deeper parts of the Pelican field but are reliant on encountering suitable channel facies (mostly in the Upper Ness) Early oil migration appears to have little or no impact on reservoir quality. Could this change exploration thinking in the basin?

20 Dundas Consultants Bernard Besly Thank you TAQA Derek Littlejohn Thurza Frenz Come and see for yourself - Pelican core in the Boyd Orr Hall!