the alba field: improved reservoir … alba field: improved reservoir characterisation using 4d...

© Chevron 2005

Elaine CampbellOliver HermannSteve DobbsAndrew WarnockJohn Hampson

The Alba Field: Improved Reservoir Characterisation using 4D Seismic Data

© Chevron 2005 2

Alba Field Location

Block 16/26, UK Central North Sea

130 miles NE of Aberdeen

Discovered 1984

1st Oil January 1994

Equity:

Chevron 23% (operator)

ConocoPhillips 23%

StatoilHydro 17%

BP 13%

Total 13%

Cieco 8%

Endeavour 2%

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Alba Geology

Eocene, high density turbidite

Amalgamated channelised sands deposited in a preexisting scour

Massive, homogeneous unconsolidated sandstone & intrareservoir shales

Sand remobilisation, causing injectites and “wing” structures

High porosity (35%) & perms (2.54 D)

High vertical permeability

Reservoir thickness ~ 250 ft

G EN E R A L ISED A LB AST R A T I G R APH Y

LATE

EO

CENE

MID

DLE

EOCE

NE

AGE

ALBA

FO

RMAT

ION IV

III

II

I

O L IG O C E N E

UNIT FM.

Figure 2.2.1

Alba Schematic G eological C ross Section

Main Reservoir

WingsWings

Alba Schematic G eological C ross Section

Main Reservoir

WingsWings

Surlyk et al. 2007

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Alba

Horizontal attic producers drilled as close as possible to top reservoir, aim to optimise sand length & standoff to OWC

22 Appraisal wells (incl. sidetracks) 74 Development wells 34 Active producers 7 active water injectors, full voidage replacement Heavy oil gravity 19 API, no gas cap Estimated >950 MMBOE OIP Cumulative production: >350 MMBOE

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Chronology of Seismic Surveys

1989 Acquired Alba streamer survey

1991 Acquired Britannia streamer survey

1994 Production started

1998 Acquired Alba OBC survey; not designed for 4D

2002 Veritas Q15 Spec streamer survey acquired; not designed for 4D, but similar orientation as Britannia 1991

July 2008 Acquired dedicated Alba 4D designed to repeat Vertitas 2002 streamer survey

Oct 2008 Delivery of fast track coprocessing of 2002 and 2008 surveys

Jan 2009 Delivery of full coprocessed 1991, 2002 and 2008 surveys

© Chevron 2005

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PWave versus SWave Seismic Surveys

SW NE

PP91 Data

Line 2285

12 Area

Alba Extreme South

SW NELine 2285

Si99 Data Shear Impedance Inversion of S‐Wave (for Lithology)

PWave (PP) Seismic Data shows strong fluid effect but no clear image of the reservoir

SWave (PS) Seismic Data shows lithological changes, injected sands & large intra shale geobodies

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Key Geological Uncertainties

Rugose/injected character of the top reservoir

Nature of field edges, Alba “wings”

Scale & location of intrareservoir shale geobodies

Orientation & nature of fault networks

Understanding fluid transmissibility across faults has become increasingly important over time

15 AREANORTH AREA

12 AREA

CENTRALAREA

EXTREAME SOUTH

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Result : 20002006 modelling effort

Earth Model based on PS interpretation (lithology)

Reasonable history match but problem areas contained geologically unrealistic iterations

Accurate for well planning but often required several sidetracks to place wells optimally for good sand length high in the structure

No hierarchy to fault transmissibility character (baffling)

Previous Reservoir Modelling Effort

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Si99 Shear Impedance Data Line 1382

42


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© Chevron 2005 9

2008 Reservoir Modelling Effort

Reinterpret the 1991, 2002 & newly acquired 2008 PP Seismic & build a new Earth Model which encompassed the optimal learnings from the PS & PP interpretations

Early & continuous collaborative work between the earth scientist & reservoir engineer to validate static & dynamic options

Dynamic simulations to validate the preferred static grid design

Static guidance on fault transmissibility multipliers & pore volume multipliers

Achieve a history match that maintains geological integrity of the Earth model

Customised workflowbased modelling, iterative aspect of the modelling project required the ability to rapidly update the static Earth model

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2008 4D Acquisition

Key Geological Requirements

Interpret & understand the nature of the PP 2002 & 2008 OWC’s across semivertical faults previously interpreted from PS Seismic

Interpret the polygonal fault network & observe relationship with injected sands

Detailed reinterpretation of top & base reservoir, injected sands & Alba “wings” using combination of PP & PS Seismic

Interpret the nature & scale of intrareservoir shale geobodies

© Chevron 200511

N

Time display, vertical exaggeration 2:1

Interpretation Polygonal Fault Network

Results: Polygonal Faults

Mainly visible on PP datasets

Cut the top of the reservoir interval & delineate

Commonly seen at the field edges

Generation mechanism dewatering of shale packages

PP08

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Results: Vertical Faults

More visible on PS data, dominantly at reservoir level

Pronounced topographical feature in the central main field

Fault trend offsetting reservoir, downthrown on the eastern flank

Generation mechanism unknown most likely associated with compaction & sand remobilisation

Impact on sand & fluid distribution recognised

PP08PS99

Interpretation Vertical Fault Network

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Interpretation 2002 & 2008 OWC’s

Results

Significant offsets of OWC’s observed across semi vertical faults

• Steps in contact surface causedby vertical faults• Baffling character (direct input into reservoir model)

• Pronounced production cones

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OWC from 4D seismic OWC from simulation

OWC from 4D seismic Cross section of PP08 seismic showing contrast between simulated & 2008 OWC

Faults act as baffles & in places aid coning beneath producers

Results: SeisSim Comparison

Line 2285

2008 OWC

Simulated 2008 OWC

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Results: SeisSim Comparison

Away from fault zones coning under wells was steeper & sharper than simulated saturation surfaces

Line 2468

40% Swsurfaces 2008 extracted from PSmodel

2008 OWC shows sharp cone development under A23Z producer

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Results

Larger intrareservoir shale geobodies masked the OWC’s

Si99 Data Shear Impedance Inversion of S‐Wave (for Lithology)

PS99 Base Reservoir

Cross section PS99 Earth Model OWC response broken,

no faults interpreted, shear impedance indicates intrareservoir shale geobody

PP08 Data

?

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Interpretation of Top & Base Reservoir

Results

Top & base reservoir interpreted using PP Seismic dataset

Injected sands & field edges interpreted using a combination of PP & PS Seismic dataset

Injectite Horizons

Top Reservoir Horizon

Base Reservoir Horizon

Alba North & 15 Area

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Interpretation of Top & Base Reservoir

Results

Injectites difficult to interpret on PP data

PS Shear Impedance is the best guide

si99Si99 Data Shear Impedance Inversion S‐Wave (for Lithology) Line 1382

Alba North & 15 Area

PS Top Injectite

Line 1382PP08 Data

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Ways to evaluate the 4D signal

2008

1991

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Ways to evaluate the 4D signal

…or just look at the differences

Survey 1 – Survey 2 + 90 Phase shift = Quadrature difference

Only areas of change (where water has replaced the oil)are showing, the rest is noise. The quality depends onhow closely the monitor survey replicates the baseline survey (repeatability).

Bright red areas represent the flushed zone

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Ways to evaluate the data

With the difference data the flushed zones (red on previous slide) can be isolated using volume rendering and geobody detection.

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Current Earth Model

Results:

Reservoir model is considered a realistic representation of the subsurface

Increased team confidence to optimise placement of future wells & better predict well performance

Significant timesaving benefit due to continuous collaboration between the earth scientist and the reservoir engineer & use of workflowbased modelling tools

PP08 Data Line 1382


PP08 PP08

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PS99 PS99

the alba field: improved reservoir … alba field: improved reservoir characterisation using 4d...

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