The Role of 4D Seismic Interpretation and Dynamic Modelling in the Successful Drilling of a Late Stage Development Well in the

West Brae Field, North Sea

F. Bacciotti

K. D’Amore

J. Seguin

Introduction

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Generalised tectonic framework for North West Europe (from Millennium Atlas)

West Brae field:

Started production in 1997

4D seismic survey acquired in 2007 to aid the identification of potential late stage development targets

Late drilling campaign in 2010-2011

Presentation focus:

Interpretation of the 4D seismic response

Integration with the dynamic field behaviour for the Flugga reservoir

Comparison of pre-drill models with well results

West Brae

Presentation Outline

Introduction

West Brae Overview

4D seismic acquisition and interpretation

Integrating the 4D results with dynamic modelling of the Flugga reservoir

2010 drilling campaign

Conclusion

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West Brae

4

Top Balder reservoir map

183Z

3

2

4

4Z

V134Z

34Y

34

31W8Z W6Z

W7Z2

32

W7

W2

W2Z

W3

W5

W4

W8

W1Z

W1

16/6a

16/7a

W6

Two stacked turbiditicreservoirs:

Balder sandstone

Flugga sandstone

Amalgamated units of massive unconsolidated sandstones:

90% net-to-gross

30% porosity

Darcys permeability

Strong aquifer support

Field production (2010):

Cumulative: 87 MMboe

Recovered: 45%

Water cut: 78%

Balder reservoir

Flugga reservoir

4D Seismic Acquisition and Processing

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West Brae

4D seismic survey

West Brae 4D seismic survey

Seismic acquisition:

1993 Baseline and 2007 Monitor 3D surveys (streamers)

Same geometry, parallel processing

4D Signal Interpretation

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Pressure changes have little influence in West Brae

Mostly fluid response:

Water replacing oil produces a hardening (e.g. water coning)

Gas replacing oil generate a softening (e.g. gas coming out of solution – an effect of reservoir depletion – or gas coning)

4D seismic difference volume (pseudo-acoustic impedance)N S

Flugga water coning

Balder water coning

Balder gas

coning

4D Seismic Interpretation of the Flugga Reservoir

In the Flugga reservoir, main 4D response is water rise caused by producing wells

No water rise in the NW of the field

Linear weak feature along the edge of the reservoir (“NW rise”)

Uncertainties Detection/resolution limits (tuning

of thin water layers or low water saturations sections)

Existence of NW rise as it fails to stand out

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Flugga reservoir

4D water response

NW rise

4D seismic maximum amplitude map

-H

ardening +

W8Z

W8

Water signal

4D Seismic Interpretation

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No water rise in the NW of the field whereas:

W8Z well producing since 2005

Good reservoir properties in surrounding wells and along well path

N S

W8Z

W8

34Y

Cross section along W8Z on 4D seismic difference volume

Original oil-water contact

Moved oil-water contact

Absence of 4D signal

Supporting Evidence of Compartmentalisation

W8 pilot hole

Good sand

No water rise in 2005

16/7a-34Y appraisal

No gas above field GOC level

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Flugga reservoir

4D water response

34Y(1999)W8Z

(2005)

W8(2005)

W1Z(1997)

W6(1999)

Data Integration to the Dynamic Modelling

History matching process indicated extensive barriers to fluid flow /pressure transmission were required

To be effective in high net-to-gross, high permeability reservoir

Barrier types:

Faults: West Brae lies over a horst and major graben fault

Shale: shale drape or overbank shale of turbiditic complex

Combination of both

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West-east seismic full stack reflectivity section

West Brae

Graben fault system

W E

BasementWest Brae

Data Integration to the Dynamic Modelling

Numerous iterations of history matching

Determine geometries to reproduce 4D signal shape, while honouring production/pressure data

Possible elements of scenarios:

Under seismic resolution rise or water saturations

NW rise feature is noise

W8Z toe not contributing

Faults or shales creating protection from water encroachment

Etc…

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Examples of history matched models

Water saturation

Base Case Model

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3D view of West Brae pre-drill model

Flugga reservoir outline

Interpreted oil-water contact

Isolated compartment

W8Z

W8

north

Objective of the Drilling Campaign

Targets un-swept oil in the Flugga reservoir

Pilot hole objectives:

Determine presence of gas

Determine water encroachment, test 4D data and interpretation

Understand compartmentalisation (pressure data)

Confirm reservoir quality

3 well path options depending on pilot results

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Drilling plan

34Y

W8Z

Pilot hole Well path options: shallow, mid, deep

Planned well trajectories

Pilot hole

Water cone

Aquifer

Gas cap

Pilot Results

Outcome of the pilot holes

Gas cap

Good reservoir properties

Water encroachment

No significant aquifer rise

Unswept compartment

Upper section depleted

Decision to drill the horizontal in un-swept compartment

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Pressure 1 Pressure 2

W8Z

W9 (2010) W9Z (2010)

Target

W8Z

GOC

Original OWC

Water encroachment

Horizontal Results

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Drilled through expected swept compartment and NW rise

Penetrated un-swept compartment as planned

Completed un-swept sections with sand screens equipped with Inflow Control Device, which strength against the flow diminishes towards the toe

Wells confirmed base case scenario

0.320.32 0.8

Swept section

Production section

W9X wellS N

Final Model

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Interpretation of the well results

300mVertical exaggeration 10

S N

Conclusion

Wells confirmed a complex model based on 4D image

Importance of integrating all subsurface disciplines tightly

4D key tool for West Brae reservoir management

4D changed how reservoir was viewed

Targets would not have been identified/drilled without 4D

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W9X

W9W9Z

Acknowledgment

We thank Marathon’s management and our partners for the permission to show the data and their support throughout the project.

The authors would like to thank their colleagues: Lisa Ashman, Ross Cameron, David Eickhoff, Tor Ellis, Simon Freeman, Alicia McGeer, Falene Petrik, Anna Vitali.

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