ensuring fit for purpose geomechanical models: … fit for purpose geomechanical models: common...
TRANSCRIPT
Devex 7th May 2014
Ensuring Fit for Purpose Geomechanical Models:
Common Pitfalls and Solutions
Colin McPhee
Gill Daniels
Philip McCurdy
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
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
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
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!
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?)
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)
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Devex 7th May 2014
Thanks
• Graham Aplin, Chris Reed, Frans Mulders (Senergy)
• Senergy management
• Devex
Devex 7th May 2014
Questions?