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Devex 7 th May 2014 Ensuring Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions Colin McPhee Gill Daniels Philip McCurdy

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Page 1: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Ensuring Fit for Purpose Geomechanical Models:

Common Pitfalls and Solutions

Colin McPhee

Gill Daniels

Philip McCurdy

Page 2: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Geomechanics – why bother?

• High cost and reputational risk

• “Wellbore instability is estimated to cost the industry US$8 billion every year” – Harts E&P

• “70% of the world oil and gas reserves are contained in sandstone reservoirs where sand production is likely to become a problem...“ SPE

• well integrity (erosion of upper completion, facilities issues)

• H&SE

• loss of containment

• loss of well control

Page 3: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

The petrophysics elephant in the geomechanics room

• A problem or uncertainty that few want to discuss

• Around 90% of data to build geomechanical models comes from “traditional” core and log petrophysics

• Misinterpretation and poor practice impact on geomechanics modelling

• sand failure analysis

• wellbore stability

• Integrated “forensic” petrophysics will

• improve geomechanics data quality

• reduce uncertainty

• minimise data redundancy

Page 4: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

The geomechanical model

• Components

In Situ

Stress

Rock

Strength

Pore

Pressure

Depletion

Model

Well

Trajectory

Field Geomechanical Model

Sand

Failure

Wellbore

Stability

Compaction

Subsidence

Fault

Reactivation

Fracture

studies

In Situ

Stress

Rock

Strength

Pore

Pressure

Depletion

Model

Well

Trajectory

Field Geomechanical Model

Sand

Failure

Wellbore

Stability

Compaction

Subsidence

Fault

Reactivation

Fracture

studies

• Applications

Page 5: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Strength model: data sources

• Core petrophysics• Core acquisition• Scratch testing• Equotip testing• Rock mechanics tests

• UCS• Triaxial (Mohr-Coulomb)• TWC tests

• Log petrophysics• Logs sensitive to strength

• density• sonic• interpreted porosity

• Strength model• calibrate log to core data• need confidence in both!

Page 6: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Forensic petrophysics: core damage!

• RTFR - read the coring and RCA reports

• Coring• WBM contact in sensitive sands and

shales• weak sand WOB and pump rates

(erosion)• barrel length (core crushing)

• POOH• check trip rates

• Core handling• cut into 1 m sections• preserve shales immediately• stabilise• beware freezing!

• Sidewalls• avoid sidewall cores for strength

tests!

Tripping too fast – differential

core pressure builds up and

exceeds tensile strength.

Causes tensile fracturing

Shear damage (transportation?)

Page 7: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

“bad” strength models

Rotary sidewall cores are taken from the most damaged

part of the formation!

Core below 3058 m: frozen-thawed-frozen

3740

3750

3760

3770

3780

3790

3800

3810

0 1000 2000 3000 4000 5000

Predicted UCS Strength (psi)

Co

re D

ep

th (

m)

UCS SWC

UCS (Conventional)

Page 8: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Forensic petrophysics: core test data!

Low friction angle (4° for sand!)

Undrained pore pressure

1-Pp = ’1

Loaded too rapidly

Low TWC due to eccentric

internal hole

Premature failure along shale lens

Page 9: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Forensic petrophysics: check the CPI!

• Core UCS vs Core Porosity• Good correlation

• Core UCS vs log (CPI) PHIT• No correlation!

• PHIT not calibrated to core

Page 10: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Forensic petrophysics: check the mineralogy

• Blue and green data are from the same sand in different wells

• Why are the strength relationships different?

• Green Sand• 15% to 32% clays

• illite > kaolinite > chlorite > illite/smectite

• no siderite cement

• no quartz overgrowths

• Blue Sand• 12% to 24% clays

• kaolinite > illite/smectite > illite

• no chlorite

• 2% to 22% siderite cement

• Quartz overgrowths

• Chlorite inhibits cementation

• Always test strength of shaly sands in formation water!

Green Sand SEMGreen Sand disintegrates on

contact with FW

Page 11: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Strength models from logs: caveats

• Shear and compressional sonic• e.g. DSI

• logs may not be run in development wells

• base model on logs that will/can be run

• Density and sonic in gas wells• require fluid substitution if gas

effect is large• often creates more uncertainty• PHI interpretation should include

gas correction • Beware generic strength

models• Calibrate/tune against core!!!

))1(0045.00087.0(10087.0 6

0 shshb VVEKxC

Eskdale-2 Interval: 2808.6 - 2843.6 m Eskdale Sand

2805

2810

2815

2820

2825

2830

2835

2840

2845

2850

0 1000 2000 3000 4000 5000

UCS (psi)

Dep

th (

m R

KB

)

Vernick 1

Vernick 2

Formel DT

Global Gdyn Model

Offset Ec Model

Core UCS

GOC

Eskdale-2 Interval: 2808.6 - 2843.6 m Eskdale Sand

2805

2810

2815

2820

2825

2830

2835

2840

2845

2850

0 1000 2000 3000 4000 5000

UCS (psi)

Dep

th (

m R

KB

)

Vernick 1

Vernick 2

Formel DT

Global Gdyn Model

Offset Ec Model

Core UCS

GOC

Page 12: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Successful strength models........

• Are calibrated against QC core data• Use logs that will be run in the

development wells• Can be used to determine probability

of encountering weak rock in development wells

P10: 10% probability of encountering sand with

strength less than 4300 psi TWC

P10

Page 13: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Stress model data sources

• Vertical stress• Density and sonic logs

• Minimum horizontal stress• Elastic modelling (strain-corrected)

• dynamic elastic moduli calibrated to core

• LOT, XLOT, mini-frac

• Maximum horizontal stress• Modelling of wellbore failure

• break out• drilling induced tensile fractures

• Fast and slow shear sonic

• Stress orientation• Break out orientation• DITF orientation

• Pore pressure• Sonic, resistivity and density trends

Page 14: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Forensic petrophysics: vertical stress

• Ensure good vertical coverage

• Extrapolating to mud line can be uncertain• model overestimates stress gradient and does

not honour density data

• Use sonic if available in shallow intervals• e.g. Gardner model

Filter: Range: All of Well

Well: 44_11-2 44_12-1 44_12-2 REFERENCE.TVDSS vs. XWIRE.RHOB Crossplot

Wells: 44_11-2 44_12-1 44_12-2

1.0

01

.00

1.2

51

.25

1.5

01

.50

1.7

51

.75

2.0

02

.00

2.2

52

.25

2.5

02

.50

2.7

52

.75

3.0

03

.00

3.2

53

.25

3.5

03

.50

0 0

2000 2000

4000 4000

6000 6000

8000 8000

10000 10000

12000 12000

14000 14000

16000 16000

18000 18000

20000 20000

RE

FE

RE

NC

E.T

VD

SS

(F

EE

T)

XWIRE.RHOB (G/C3)

16861

162050

0

20

63625.0

23.0 pb V

Page 15: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Shear wave anisotropy

• Anisotropy in fast and slow shear sonic response from aligned fractures, layering, formation damage or from unequal stresses

• DTCO and DTSslow,fast translated into anisotropic elastic rock properties

• h and H Based on 3 shear moduli: C44, C55 and C66 and shear radial variation profiling

• Analysis is normally limited to a clean, medium-high porosity sand and near-vertical wellbore

• Stresses are functions of vertical stress and pore pressure

Page 16: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Rescaling - example

• Interpretation used inappropriate vertical stress and pore pressure gradients

• Rescaling based on constant effective stress ratio Ki

• Rescale h and H using K1 and K2 and appropriate v and Pp models

SSV

SSSSh

Pp

PpK

SS

1

SSV

SSSSH

Pp

PpK

SS

2

elelelvrescaledh PpPpK modmodmod1

elelelvrescaledH PpPpK modmodmod2

Shifted h

Page 17: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Wellbore failure - image logs

Presence and extent of BOs and DITFs used to determine maximum horizontal

stress magnitude and orientation in vertical wells

Requires unambiguous interpretation of well failure features

Do not confuse with drilling-induced borehole features

Page 18: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Mis-identifying breakout

Mud Cake

One or both

diameters smaller

than gauge

Both diameters

larger than gauge

Washout

Breakout

One diameter

larger than gauge

Key Seat

One diameter

larger than gauge

Page 19: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Wellbore failure features - breakout

• Incipient breakout – where failed material has not spalled into the borehole can be confused with DITF

• incorrect assessment of the in-situ stress orientation

• Incipient breakout should occur as two paired narrow vertical features

• Breakout rotations:

• Local pertubations in stress field (faults)

• Variations in rock strength

Incipient breakout can

be mistaken as DITF

Breakout

Incipient breakout

Page 20: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Wellbore failure features - DITF

• Distinguishing inclined DITF and natural fractures is not straightforward in microresistivity images that do not image the entire borehole

• Misinterpretation can lead to errors in assessment of the in-situ stress orientation and magnitudes

• Drilling-enhanced (natural) and drilling-induced tensile fractures can both form systematic sets with similar characteristics and can be mistaken for each other

• not able to fit a sine wave to the inclined DITF features as they are non-planar

Drilling enhanced or drilling

induced fractures?

DITF in

acoustic image

Page 21: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Image log identification confidence

• Breakouts• Paired strips set 180° apart.

• Breakouts in images may not correspond with caliper response

• DITFs • Often subtle; thin and discontinuous

around the wellbore..

• DIFs should be seen as continuous conductive/resistive fractures

• Borehole-parallel traces (tram-lines) set 180° apart (and 90°from BO)

• En-echelon limb segments of a sine-curve centred about point of inflexion of the limbs.

• Assign confidence level• A – high: E – low

• Use only high confidence data

Task Geoscience

Page 22: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Conclusions and benefits

• Key inputs in the geomechanical model come from petrophysical analysis of log and core data.

• Importance of a rigorous and consistent petrophysical interpretation is often overlooked by well construction and production engineers

• Pitfalls have a massive impact on stress and strength modelling

• But....• uncertainties are recognisable and manageable• best practice and robust workflows ensure that core and

petrophysics data are fit for purpose prior to geomechanical analysis.

• a forensic, integrated data quality assessment can eliminate data redundancy and reduce uncertainty in geomechanical models

Page 23: Ensuring Fit for Purpose Geomechanical Models: … Fit for Purpose Geomechanical Models: Common Pitfalls and Solutions ... •“Wellbore instability is estimated ... • wellbore

Devex 7th May 2014

Thanks

• Graham Aplin, Chris Reed, Frans Mulders (Senergy)

• Senergy management

• Devex