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t

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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t

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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t

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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t

-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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t

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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t

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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t

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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t

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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t

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.