Maximize chance of drilling injected sand: Examples from Gryphon area

Guillaume Berthereau

7th May 2014

Agenda

Quad 9: location, history & geological settings

Derisking new targets

Drilling strategy: associated risks & uncertainties

Conclusion

Optimize well path design

Location & history• Block 9/18

• Discovered in 1987

• Production started in 1993

• Injection wing targeted for the first time

in 2001

• Production from injection wings started

in 2004

• STOIIP: 412 MMstb

• Cumulative oil (to date): 121 MMstb

• New 3D survey in 2011

• 3D processed (2011)

• 4D processed (1990, 2002 & 2011)

Regional depositional model

Gryphon

Maclure

Harding

• Early Eocene Balder Formation

• Episodic deposition of basin floor high density offset stacked turbidites sandstone

• Sourced from failure of the Dornoch shelf/delta front to west

• Slowed by the slope of the Crawford Ridge

Tullich

Top Balder (TWT)

From Newman & al., 1993

Geological settings: sand remobilisation

5619.00

’

5622.00

’

5625.00

’

5628.00

’

5628.00

’

5631.00

’

5622.00

’

5625.00

’

DYKE

DYKE

INJECTION

BRECCIA

DYKE

Feeder dyke

system

DYKE

Stepped

discordant

base

• Remobilisation of the turbidites sandstone

in early Frigg times

• Creation of multi scale (from core to

seismic scale) injection wings

• Mainly within Balder sometimes extending

into Frigg age Horda Formation

T Sele

T Balder

Adapted from Hurst and Cartwright

Challenges• Identify and derisk new targets in an

already mature field

• Infill targets

• Near field exploration

• Assess STOIIP:

• Injectite generally tuned and dipping

• Sub-seismic injected sand

• N/G (thin sand / seismic resolution)

• Moved contacts (production /

injection)

• Drilling challenges

Top Balder (Z)

Min

. am

plitu

de –

GO

C/O

WC -

CIf

ullsta

ck

Derisking injectites: parent sand and intrusion path

Lithology volume

CI full stack

Depositional sand

Balder Fm

Top Sele

Top Balder

Injected sand

North West

South East

• Parent sand presence: volume, quality, distance

• Relationship between intrusion and parent sand

Sand

Sand

Amplitude anomaly

Derisking injectites: Reservoir

FaultingVariance map over

CIMG anomaly

Gas capMin. amplitude GOC-200ft / 0ft

Top balder

jackup

Gas cap

GOC

OWC

Injectite

geometry

AVO QC

Bowl

shape

Elongated

shape

Intercept

Gra

die

nt

Derisking injectites: 4D

• 4D observed theoretically only due to fluid movement

(no significant pressure change)

• Fluid movements over Gryphon:

• Gas into oil (due to production): softening

• Water into oil (due to production): hardening

• Oil into gas (water injection): hardening

• Detect and estimate moved contacts

• Integration of seismic and dynamic model: sim2seis

Amplitude anomaly

4D (water coning)

Drilling strategy• 2 strategies:

• 1 single target

• Combination of targets

• Depends on:

• Economics

• Drill ability

• Minimum net sand

Sum –ve amplitude

Fluid vol. (2011)

GOC/OWC

On the risk of drilling along strike…

• Well drilled on top of injected body (Dipping SE)

• Best pay section matching with main amplitude

anomaly

• Otherwise more thin sand corresponding to dimmer

amplitude (poorer N/G)

• Still a good producer though: Does it matter as long

as drilling on top of the injectites?

• Miss the main sand because of:

• Lateral positioning (seismic migration)?

• Depth conversion error?

oGOC

oOWC

Top Balder

Top Sele

Injectite positioning uncertaintyMovement due to migration:

Δx= (V * TWT * sinθ) / 2

• Migration velocity used for depth

conversion

• Calibrated to well data

• All scaler values between 0.99 and 1.01

(1% error)

TWT Balder = 1.65s

% Velocity error: 1%

Vav Balder ~ 6600ft/s

ft

Movement due depth conversion error:

Depth error (De)

θ

Δx= De / Tanθ

• 1 st. deviation error at Top Balder= 27ft

• 1 st. deviation error at Top Sele= 44ft

• Dip of injectites: between 10 and 30°

ft

Optimizing well path: depth conversion

• 3 velocity models proved reliable available

• Selection of the best depth conversion on the target area

based on:

• Statistics of surrounding wells: Balder / Sele / HC

• 4D around targeted area and/or surrounding wells

• Fluid contact assessment: gas cap

Water coning

Water coning

Water coning

Hard

en

ing

So

ften

ing

Gas coning

Gas coning

Gas coning

GOC

OWC

GOC

OWCBrightening stops at GOC

Amplitude

Amplitude

TV

Dss

TV

Dss

Depth conversion 1

Depth conversion 2

Optimizing well path design: Datum

• Dealing with 3 phases:

• Significant gas cap

• Oil phase

• Strong aquifer

• Relatively homogeneous PHIE, Sw (undrained)

• Optimum datum depends on:

• Sand distribution

• Presence of 3 phases

• 4D: consistency between 4D seismic and

simulation model (sim2seis)

Conclusion

• Understanding the distribution of injectites and their characteristics helps

identifying and derisking more challenging infill or near field targets

• Importance of understanding connection with aquifer and gas cap of every

single injectite for simulation model

• Importance of depth conversion assessment in well path optimization

Acknowledgment