www.agr.com www.agr.com

Naturally Fractured Reservoirs in the UKCS

Tim Wynn

AGR TRACS

20 May 2018 1

www.agr.com

UKCS Fractured Reservoir Status

Cretaceous Triassic U Permian Permian U Carboniferous Devonian Silurian Precambrian

UKCS Fractured Reservoirs – Units & Lithologies

20 May 2018 2

Precambriangneiss

3

PermianRotliegend Sandstone

13

(Fields partially fractured)

SilurianGranite

1

DevonianSandstone

4

U CarboniferousSandstone

1

U PermianZechsteinCarbonate

5

TriassicSherwood Sandstone

4

CretaceousChalk

11

Definition:Fields where natural fractures (joints, veins or faults) enhance flow compared with the matrix

470 UKCS fields analysed

38 NFR fields (8% of total). Number of fields up for debate!

Talk focusses on long term field performances related to natural fractures

www.agr.com

Chalk Reservoirs

20 May 2018 3

www.agr.com

Machar – The Classic N Sea Chalk Diapir?

20 May 2018 4

13Z

210

5

1RE

16

12Z

After Ward et al 2014

www.agr.com

Machar

20 May 2018 5

After Foster & Rattey 1993

www.agr.com

Machar – Production History

20 May 2018 6

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

200

400

600

800

1000

1200

1400

1600

Dec-93 Dec-95 Dec-97 Dec-99 Dec-01 Dec-03 Dec-05 Dec-07 Dec-09 Dec-11 Dec-13 Dec-15 Dec-17

Wat

er C

ut

Frac

tio

n

Pro

du

ctio

n 0

00

's b

bl

Date

Machar Field Monthly Production

Ph 2 PWRI & field rejuvenation

119.7 mmbbls

97.8 mmbbls

After Ward et al 2014

Ph 1EWT

STOIIP400-500 mmbbls

Ph 3 & infill

SHmax

www.agr.com

Machar - Viscous Displacement vs Imbibition

In fractured reservoirs viscous displacement often fails:

The high permeabilities of the fractures effectively “short circuits” the matrix blocks, making it difficult to establish sufficiently high pressure gradients to allow effective viscous displacement of the oil in the matrix

A new important effect comes into play -imbibition:

7

DFN 2 Validated

Faults

Background

From McDonald 2005

www.agr.com

Triassic Reservoirs

• Fluvial and lacustrine Sherwood Sandstone

• EISB but possibly also Wytch Farm

20 May 2018 8

www.agr.com

EISB – Normal Faulting of Sandstones

20 May 2018 9From Bentley & Elliot 2008

After Yaliz & McKim 2003

www.agr.com

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

200

400

600

800

1000

1200

1400

1600

1800

Jan-96 Jan-98 Jan-00 Jan-02 Jan-04 Jan-06 Jan-08 Jan-10 Jan-12 Jan-14 Jan-16 Jan-18

Wat

er C

ut F

ract

ion

Pro

du

ctio

n 0

00

's b

bl

Date

Douglas Field Monthly Production

Douglas – Sandstone with Fault Influence

20 May 2018 10

96.8 mmbbls

285.7mmbbls

After Yaliz & McKim 2003

Well 17

Well D1ZSTOIIP

200 mmbbls

www.agr.com

Douglas – Anisotropic Fault Permeability

20 May 2018 11

After Gutmanis et al 1998, Bentley & Elliot 2008

www.agr.com

U Permian Zechstein Carbonate Reservoirs

20 May 2018 12

CNSAuk – Zechstein, Rotliegend reservoirs

Alma – Zechstein, Rotliegend, Devonian reservoirs

SNSHewett – Bunter, Zechstein, Rotliegend reservoirs. Zechstein heavily fractured, some anhydrite cementing.

Wissey – Zechstein. Fractured near main bounding fault to S. Gas production from well near fault but water ingress up intra field fault

www.agr.com

Auk – Zechstein and Rotliegend

20 May 2018 13After Trewin et al 2003 & Talisman 2011 (DEVEX)

From Talisman 2011 (DEVEX)

www.agr.com

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

500

1000

1500

2000

2500

Apr-75 Apr-78 Apr-81 Apr-84 Apr-87 Apr-90 Apr-93 Apr-96 Apr-99 Apr-02 Apr-05 Apr-08 Apr-11 Apr-14 Apr-17

Wat

er C

ut F

ract

ion

Prod

ucti

on 0

00's

bbl

Date

Auk Field Monthly Production

Zechstein & Rot Vert

Auk – Production History

20 May 2018 14

150.5 mmbbls

447.9mmbbls

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

100

200

300

400

500

600

700

Jun-10 Jun-11 Jun-12 Jun-13 Jun-14 Jun-15 Jun-16 Jun-17 Jun-18

Wat

er C

ut F

ract

ion

Pro

du

ctio

n 0

00

's b

bl

Date

Auk North Field Monthly Production

8.5 mmbbls

34.2mmbbls

Rotlieg horizontalZechstein

STOIIP800

mmbbls

www.agr.com

Auk – Production Mechanism?

20 May 2018 15

Modified after Trewin et al 2003

Zechstein pervasivelyfractured

Rotliegend fault with fractures?

Rotliegend dispersed joints?

W E

OWC

Some Rotliegend flowfrom better dune slip face and reworked wind ripple facies

From Talisman 2011 (Devex)

www.agr.com

SNS Gas Rotliegend Sandstone Reservoirs

20 May 2018 16

Sole Pit Area

www.agr.com

Devonian Sandstone and Basement Reservoirs

20 May 2018 17

Lancaster

UtsiraHigh

Cairngorm

AlmaDevonian has some fractures but do they contribute?

www.agr.com

Clair – Devonian & Lewisian Heavy Oil

20 May 2018 18After Ogilvie et al 2015

www.agr.com

Clair – Devonian & Lewisian Heavy Oil

20 May 2018 19After Barr et al 2007

LCG Devonian – Mainly fluvial sands and channel sands. Some aeolian reworking in III, more heterolithics in VI. Fractures important in VI, some fracture influence in V and III which have best matrix

UCG Carboniferous – Fluvial channels and heterolithics. Damp conditions, more clay.

www.agr.com

Clair – Devonian & Lewisian Heavy Oil

20 May 2018 20After Witt et al 2010

www.agr.com

Clair Phase 1 – Devonian Heavy Oil Waterflood

20 May 2018 21

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Jan-05 Jan-06 Jan-07 Jan-08 Jan-09 Jan-10 Jan-11 Jan-12 Jan-13 Jan-14 Jan-15 Jan-16 Jan-17 Jan-18

Wa

ter

Cu

t Fr

act

ion

Prod

ucti

on 0

00's

bbl

Date

Clair Field Monthly Production

Initial depletion to determine connectivity. Injectors placed to optimise pressure support and sweep

Some injectors connected to producers via faults / fracture corridors

Others injectors appear to have distributed joints that get water into matrix where imbibition occurs. Good recovery

115.7 mmbbls

49.9mmbbls

Phase 1STOIIP1500

mmbbls

www.agr.com

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

200

400

600

800

1000

1200

1400

1600

1800

Jan-81 Jan-84 Jan-87 Jan-90 Jan-93 Jan-96 Jan-99 Jan-02 Jan-05 Jan-08 Jan-11 Jan-14 Jan-17

Wat

er C

ut F

ract

ion

Prod

ucti

on 0

00's

bbl

Date

Buchan Field Monthly Production

Buchan – Long Column, Light Oil, Depletion

20 May 2018 22From Wynn & Saundry (In Review)

NE SW

147.5 mmbbls

47.6mmbbls

STOIIP350-600 mmbbls

www.agr.com

Basement Reservoirs - Cairngorm

20 May 2018 23From Enquest 2016

1.5 km

www.agr.com

Cairngorm – Migration Route

20 May 2018 24From Enquest 2016

www.agr.com

Basement Poroperm System Concept

20 May 2018 25

• Identifying the elements – Seismic, logs, core

• Calibration of effective fracture porosity – Very difficult

• Viscous sweep only or imbibition / gas oil gravity drainage possible?

OWC

www.agr.com

Observations & Conclusions

20 May 2018 26

• Non fractured units– Hod Chalk – High clay content and isotropic stresses, no open fractures

– Jurassic. Lithologies too clay rich? Not enough burial / cementation prior to reactivation?

– Triassic Bunter & Skagerrak. Lithologies too clean? Mainly granulation seams?

• Pervasive fracture influence – imbibition / good matrix sweep or rapid water cut– Chalk (Machar, Banff, Curlew C, Affleck, Kyle?, Harrier?) – Imbibition / good matrix sweep?

– Devonian Sandstone (Clair, Buchan, Stirling) – Imbibition / good matrix sweep?

– Zechstein (Auk, Alma, Hewett?, Wissey) – Viscous sweep of pervasive fractures – early water

• Locally important fracture influence – fault conduits / baffles– Triassic Sherwood (Douglas, Lennox, Wytch Farm?). Viscous sweep in localised damage

zones? Early water or gas

– Rotliegend (Auk, West Sole, Newsham, Hoton, Ensign, Clipper, Barque, Leman, Camelot) –Locally increased well rates but early water (e.g. Auk)

• Crystalline basement (Clair, Lancaster, Cairngorm)– Permeability and storage in faults, fault damage zones, weathered intervals and

microfractures? What proportions of these elements occur in each reservoir / discovery?

– How much measured porosity is effective for storage & flow?

– What is the long term (1-2 yr plus) potential for production?

www.agr.com

Managing the Risks

20 May 2018 27

• Identifying the poroperm system elements. Integrate seismic, drilling, logs, core, tests, production data– Non-intuitive open fracture directions– Connectivity of facies to fractures– Multiple scenarios and iterate models

• Permeability architecture– Early understanding of dynamic performance important but well tests /

Early Production Systems don’t always see everything– Phased development optimal?

• Connectivity of well / injectors to fracture system– Skin and drainage. Stimulations required?– Injector placement WRT wells and fracture system– Patchy fractures > more wells?

• Well performance – manage offtake & injection– High rates > More oil but high water cut?– Low rates > Less water but less oil?

www.agr.com

Acknowledgements

20 May 2018 28

• AGR TRACS for permission to give this talk

• All colleagues and clients for projects and discussions on fractured reservoirs

Proterozoic reactivated shear zone in Archaean gneiss, Achmelvich, Sutherland