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Geomechanics of Salt and Geomechanics of Salt and Petroleum EngineeringPetroleum Engineering
Maurice Dusseault
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tStraits of HormuzStraits of Hormuz
Salt-Cored Anticlines (dry climate)
Salt-Cored Domes
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tSummarySummary
� Salt is a viscous, slowly flowing material
� Creep rates are sensitive to T and σ (depth)
� Borehole squeeze is the issue in salt drilling
� Stresses around salt structures can be extremely complex, rapid changes in σhmin
� Rubble zones, open fractures exist around salt
� The salt / rock interface is a critical region for drilling and also casing (casing collapse)
� Salt behavior may affect reservoir response
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tSalt and HC EntrapmentSalt and HC Entrapment
saltdome
gasoil
sulphur
Halokinesis over time periods of106 - 108 years helps create oil
and gas structural traps
gasoil
salt strata mother salt
Oil is found trapped against flanks of salt domes, above salt-cored anticlinal structures, trapped under salt canopies or beds
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tCharacteristics of SaltCharacteristics of Salt
� Salt is highly soluble in water� Salt is geochemically (ionically) active with
respect to drilling mud additives�Suppresses polymer behavior�Flocculates fresh water muds�Suppresses clay hydration & shrinks shale
� …and, salt is a viscoplastic substance�Creeps continuously under shear stress�Thermally activated creep rate
ε = ƒ(T, σ)·
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Salt
Drilling Problems in Salt Rock Drilling Problems in Salt Rock
Ledgesand
blocks
Largewashout
Saltpinch
Limestoneor
dolomitebit
BHA
Drillpipe
Squeeze Washouts Ledges & Blocks
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tProblems in Drilling SaltProblems in Drilling Salt
� Salt deteriorates WBM functions (cake quality, clay hydration, polymer action…)
� Salt squeezes rapidly into the hole
�BHA stuck in hole during POOH
�Can’t get to TD during RIH
� Salt is excessively dissolved
�Poor mud velocity and hole cleaning
�Mud rings, etc.
� + Casing and cementing problems
� + Associated effects (LC, unusual σ, high T)
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tSalt in the GoMSalt in the GoM
J. Couvillion, Chevron
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tOn the GoM Continental SlopeOn the GoM Continental Slope
From 2000 m to 6000 m salt canopyWater depth from 1500 to 3000 m
J. Couvillion, Chevron
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tSqueeze and Trip ProblemsSqueeze and Trip Problems
� BHA or bit get stuck in squeezed zone�Back ream out of hole�Dissolve the salt by diluting the aqueous
phase� Squeezed section in RIH
�Drill to bottom, re-examine mud strategy� Adjust mud properties accordingly
�Raise the weight to counteract squeeze�Drill with slightly non-saturated aqueous
phase
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tTransOcean Cajun ExpressTransOcean Cajun Express……
Can drill to 11 km depth in 3 km of water through
thick salt sequences
J. Couvillion, Chevron
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tOther Drilling ProblemsOther Drilling Problems
� Large washouts in salt can lead to poor hole cleaning and excessive mud-rings
� Hitting ledges associated with insolubles in the salt sequence (bedded salts)
�Particularly deviated wells
� Blocks of rock break off, wedge BHA
� Massive LC near salt dome flanks & top
� Exceptionally high T
�Salt is an excellent thermal conductor
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tTriaxial Tests on RockTriaxial Tests on Rock
UCS, σ´3
= 0 εa- strain- %0
10%
20
40
60
80
σ´3
= 2.0 MPa
σ´3
= 15.0 MPa
σ´3
= 7.0 MPa
Triaxial Test Results
Dev
iato
ric s
tres
s
σ1
– σ3, MPa
εa- strain- %
10%
Vol
ume
chan
ge +ve
-ve
σ´3
= 2.0
σ´3
= 7.0 MPa
σ´3
= 15.0 MPa
Strain-weakening behavior
Brittle behavior
Elastoplastic behavior
σ3= (σ2) = σr
σ1= σ´ - axial stressa
Stress Conditions
Brittle behavior: crystal debonding and axial extension fractures
Strain-weakening: single or several narrow shearing surfaces
Elastoplastic: bulging with slip distortion along many small shear planes
Failure Modes
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tMM--C Yield Criterion for SaltC Yield Criterion for Salt
To~
-2 MPaUCS ~25 MPa
50 MPa
50
σ′3
σ′1
Y
Mohr stress circles at yield
cohesive -elastoplastic behavior
frictio
nal-cohesiv
e behavior
brittle yield
Shear stress - τ, MPa
Normal stress - σ’, MPa
σ 3 = ( σ 2) = σ r
σ1 = σ ´ - axial stressa
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tSalt Strength or Creep?Salt Strength or Creep?
� Salt strength is almost never the issue in petroleum geomechanics (but it is in mines)
� It is the salt creep rate that is important
� Salt is highly soluble in H2O; + there is usually 0.5-2% brine-filled intercrystalline porosity
� When a differential stress is applied, mass transfertakes place in the brine phase, called…
� FADC – Fluid-Assisted Diffusional Creep
� Salt is not the only material that creeps, but the most important one for petroleum development
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tCreep Testing of SaltCreep Testing of Salt
σa
σr
Time
Axial strain –∆L/L
Creep test-apply confining stress σr-let stay for a day or two-increase σa suddenly-measure instantaneous strain, -primary (decelerating) creep, -secondary creep – steady-state
Steady-state strain rate
ε·
σa
σr
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tCreep Creep –– Strain (Strain (εε) ) With TimeWith Time
Time - t
Steady-state creep (secondary or
stationary creep)Rupture
Instantaneous elastic strain = +∆σ/E
Transient or primary creep
Str
ain
-ε
Tertiary creep
T1
T2
(> T1)
Rupture
εT1.
εT2
.
Elastic strain recovery = -∆σ/E
Creep recovery
Permanent (irrecoverable) strain
(Actual salt behavior)
(Red lines are the “classical”model, blue lines represent salt)
*It is important to always be aware that creep of salt in situ is different than creep of metals, plastics, and other materials that cannot display FADC
More typical of ductile
shales
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tCreep of SaltCreep of Salt
� Occurs under any deviatoric stress (σ1 - σ3)
� The higher the σ1 - σ3, the faster the creep
� The higher the T, the faster the creep
� Thus, deep salt (>4000 m) flows like butter
� Creep occurs with “no damage” (i.e. micro-fissures heal faster than they are formed)
� Solution-precipitation processes – FADC – are important in salt creep, and they also lead to the healing (or annealing) process
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tSteadySteady--State Creep FactorsState Creep Factors
� We often use a simple Norton Creep Law
� εss is the steady-state deformation rate (usually in s-1)� A is a laboratory parameter (fabric-dependent)� σ1 - σ3 is the plastic stress, σo normalizes it, in MPa� n is a mechanism-dependent exponent� Q is activation energy, V activation volume, p mean stress
� Stress and temperature both activate creep� If you double the depth (stress), the creep rate
increases usually be a factor of 8
RTQn
o
31ss eA
−
⋅
σσ−σ=ε
.
.
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tSalt and Tectonic StressingSalt and Tectonic Stressing
UC sand
shale
salt
sandstone
stresslithotype stiffness
limestone
E
0.5E
0.75E
1.5E
loading
unloading
salt isviscoplastic
mud
depthassumedinitial σh
stresses areisotropic
In the virgin condition at depth, salt stresses are all equalAlso, the effective stress concept does not apply - no po
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tCharacteristics of Salt StrataCharacteristics of Salt Strata
� Salt is low density – 2.16 – buoyancy = domes!� Stress state in situ is isotropic (σ1 = σ2 = σ3)� Generally, in the salt σsalt ~ σv (vertical)� Exceptionally high thermal conductivity� Impermeable (k < 10-12 Darcy for pure salt)� Salt strata may have thick insoluble layers (e.g.
anhydrites, carbonates in bedded salts)� Structural complexity and major stress
alterations in strata near salt diapirism (stresses, fracturing, …)
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tStructural ComplexityStructural Complexity
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tCreep Rates & TCreep Rates & T
ln εss
1/T
-Qa/R
-Qb/R
constant stress
creep mechanismshave different
activation energies
Creep is also thermally activated. Approximately, for pure halite (NaCl), the creep rate is doubled for each 15°C
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tSimple Rheological ModelsSimple Rheological Models
Eη
viscoelastic, Maxwell
viscoelastic, Maxwellplus Kelvin-Voight
viscoelastic, Maxwellplus viscoplastic
E1
E2
E
η1
η1
η2
η2
K
Simple models help us to understand the behavior of salt
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tSimple Creep ModelsSimple Creep Models
time
strain
E1
2
1
2
∆σ
∆σ
Here, we see a very simple instantaneous strain + steady-state creep model (2), and a more realistic model (1)
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tSimple AxiSimple Axi--symmetric Creep Model symmetric Creep Model
� Axisymmetric, homogeneous model
� Assume constant T throughout
� Allows instantaneous calculation of creep rate as ƒ(MW, T, and stress –σ)
Temperature - T Uniform far-field stress σ/z
Internal p = MW
This simple model can be calibrated in real cases and
used to estimate the beneficial effects of more MW or cooling the mud
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tNorth Sea Salt DrillingNorth Sea Salt Drilling
� Zechstein Fmn. salts, offshore, oil below salt
� Not much structure (flat-lying)
� The halite (salt) creeps normally
� Also, zones with carnallite + bischofite, which creep faster than NaCl!
� To simulate halite, we used published data for GoM salt (well-tested)
� We also simulated a “fast” and a “slow” salt
� …and studied closure rate vs. depth, T (cooling)
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Salt and Drilling
Required Spreadsheet InputsDrilling Mudweight 14.6 lbs/gal
Borehole Radius 8.5 in
Depth of Interest 11,000 ft
Temperature Gradient (Within Salt Unit) 1.5 o F/100 ft
Temperature (Top of Salt Unit) 195 oF
Stratigraphy Unit ThicknessUnit From To (ft) Density
Sea Water 0 60 60 1.979 slugs/ft3
Soft Sediments 60 10,500 10,440 4.610 slugs/ft3
Salt 10,500 11,500 1,000 4.280 slugs/ft3
N/A 11,500 12,000 500 5.000 slugs/ft3
N/A 0 0 0 0.000 slugs/ft3
Depth (ft)
Case History, North Sea, 11000Case History, North Sea, 11000′′′′′′′′
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Borehole Closure Rate (at specified depth)Mudweight 14.6 lbs/gal
Depth of Interest 11,000 ft
Overburden Stress 11,259 psi
Mud Stress 8,343 psi
Slow Creeping Salt Fast Creeping SaltBorehole Closure 2.13 %/day Borehole Closure 21.34 %/day
Closure Rate 9.07E-02 in/day Closure Rate 9.07E-01 in/day
Case History, North Sea, 11000Case History, North Sea, 11000′′′′′′′′
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Borehole Closure Rate (at specified depth)Mudweight 16.7 lbs/gal
Depth of Interest 11,000 ft
Overburden Stress 11,259 psi
Mud Stress 9,543 psi
Slow Creeping Salt Fast Creeping Salt
Borehole Closure 0.44 %/day Borehole Closure 4.35 %/day
Closure Rate 1.85E-02 in/day Closure Rate 1.85E-01 in/day
Case History, North Sea, 11000Case History, North Sea, 11000′′′′′′′′
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-5
0
5
10
15
20
25
14 15 16 17 18 19 20
Mud Weight - #/gal
Clo
sure
rat
e, %
/day
Conditions:11,000’ depthT @ 11000 ~ 95°CStress in salt at 11000’ = 19.7#/gal MWSalt type: Fast-creeping salt (high
interstitial H2O content)Hole size: 8.5“
overburden
MW vs. Closure RateMW vs. Closure Rate
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t 0
2
4
6
8
10
12
14
-35 -30 -25 -20 -15 -10 -5 0 5 10
Cooling Amount (deg C)
Clo
sure
Rat
e (%
/day
)
Conditions:11,000’ depth, MW is 16 #/galBase case (x = 0) is at 95°C temp.Stress in salt at 11000’ = 19.7#/gal MWSalt type: Fast-creeping salt (high
interstitial H2O content)Hole size: 8.5“
cooling heating
MW vs. Closure Rate + CoolingMW vs. Closure Rate + Cooling
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10,000
10,500
11,000
11,500
12,000
0.000 0.500 1.000 1.500 2.000 2.500 3.000
Borehole Closure (%/day)
Dep
th (
ft)
Slow Creeping Salt Fast Creeping Salt
Case of MW of 2 #/gal less than overburden stress in the salt. For fast salt, the closure rate approaches 2% per day
Closure vs. DepthClosure vs. Depth
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10,000
10,500
11,000
11,500
12,000
0.000 0.100 0.200 0.300 0.400 0.500Borehole Closure (%/day)
Dep
th (
ft)
Slow Creeping Salt Fast Creeping Salt
Case of MW of 1 #/gal less than the overburden stress in the salt. For the fast salt, the closure rate approaches 0.3%/day
Closure Rate vs. DepthClosure Rate vs. Depth
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tSo, the Geomechanics of SaltSo, the Geomechanics of Salt……
� Creep effects in boreholes can be simulated…
� …as can salt overburden response…
� Models can be calibrated through field data or
� …lab data may be used if feasible…
� Complications…�Salt may be impure, bedded with shale, etc…
�Other salts may be present
�Data may be lacking
� Nevertheless, we can account for salt’s behavior and reduce unexpected risks
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Some Extra Slides on Stress Conditions Some Extra Slides on Stress Conditions around Salt Domes and Drillingaround Salt Domes and Drilling
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tClassical Salt DomeClassical Salt Dome
σv
Dome crest
Neck or stock
Mother salt
overhang
syncline
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tAvery Island Dome SchematicAvery Island Dome Schematic
Normal faulting low σh , gas present in shales…
Anticlinal shapes
Dissolution residuum and brecciated rocks, could be a severe LC zone
Upturned bed traps, beds inclined to well, sheared
Flanking synclines
“Mother” salt
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tSimple Regime ClassificationSimple Regime Classification
strike-slip
thrust
normal
thrust
strike-slip
Normal faults, low stresses, gas in shales and sands
Thrust conditions near the dome shoulders
Brecciated residual zone, often lost circulation
Sheared zone on flank of domes, difficult drilling
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tSalt TonguesSalt Tongues
salt tongue intrusion
sea level
deep-lying “mother” salt
neck or stock
500 to 2200 m thick
10-35 km
somewhat deformed sediments
zone of high σr, low σθ (zone of “push”)
Zone of drag (reduced σh)
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tSalt Tongues (Sheets)Salt Tongues (Sheets)
stress
depth
σv ≈ σh
σv = σ1salt tongue intrusion
sea level
deep-lying “mother” salt
σh
σv
neck or stock
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Salt and Drilling
Stresses Above Salt DomesStresses Above Salt Domes
saltdome
salt ridge
deformedstrata
deep-lying “mother” salt
sheared zone
radialstresses
increased Salt intrusionleads to alteredstresses in thebounding rocks
extensional, σv = σ1
A
A’
B B’
hydrostat
σh
σv
Section A-A’
Generally, low mud weights are absolutely necessary to drill through the sediments above the top of the dome
stress
small σv gain, σh loss
σv
Section B-B’
stressσh
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tStress Trajectory DeflectionStress Trajectory Deflection
traces of σ3direction
Regional stress field
Zone affected (6-8 × D): the local salt dome stress field
Tangential stress (σθ) is σ3 near the dome, and radial stress (σr) is σ1 (at depth near the flanks of
the dome)σHMAX
σhmin
D
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tFracturing Around DomesFracturing Around Domes
saltdome
gasoil
sulphur
fracture
A A´salt
salt dome
Fractures reflect the local stress field, and tend to elongate asymmetrically. The arm pointing to the diapir stock
develops more strongly than the outward-directed fracture arm.
Close wells
More distant wells
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tSalt Dome Flank StressesSalt Dome Flank Stresses
saltdome
gasoil
salt dome
borehole trajectory
mother salt at depth
σv
σHMAX
σhmin
normal fault regime
thrust fault
regime
strike-slip
regime
normal
thrust
strike-slip
fracture
Stresses along wellbore trajectory
po
salt
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tThick Salt SheetsThick Salt Sheets
Dep
th in
feet
0 5000 10,000 15,000 psi
5000
10000
15000
0
Sea water, ρ ~ 1.04
Soft seds, ρ ~ 1.7
Salt, ρ ~ 2.16
Sub-salt sediments
σv
σhmin
σhmin
11 12 13 14 1510Mud weights
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Salt and Drilling
Where to Drill?Where to Drill?
normal
thrust
strike-slip
Borehole TrajectoriesThrough the salt
Through the flanks
From a distance
Which is the Best Borehole Trajectory Near a Salt Diapir?
Critical exit point!
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tSalt and Surrounding StressesSalt and Surrounding Stresses
� Domes are usually found in extensional regime, σh < σv (= σ1)
� Salt domes alter the local stresses (3 - 5 D)
� Locally, around dome in the non-salt rocks, σr (σHMAX ) is larger than σθ (σhmin)
� Stress state is different for NaCl “tongues”
� In tongue regime, σv = σ3, compressional
� Less effect on local stress distributions
� Less fracturing of rocks and folding
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tWhat Can We Do to Control?What Can We Do to Control?
� Only three options are available:
� Use a high mud weight so that the rate of creep is reduced (i.e. lower σ1 - σ3)
� Control the aqueous phase saturation to control the dissolution rate of salt� If OBM, MW = σ to avoid squeeze, no dissolving
� Cool the mud aggressively to reduce creep rate (has other benefits on upper shales)
� Of course, we can drill quickly, watch out for sharp transitions, etc.