Borehole Stability during Drilling

Borehole Stability Problems

Tight hole / Stuck pipe incidents Responsible for 5-10% of drilling time Most frequently occurring in shale Often high pore pressure, and in presence of

swelling clay minerals (e.g. smectite) Often in deviated wells

Lost circulation / Mud losses May lead to kick / blow-out Caused by fluid lost into natural fractures or by

new fractures generated

Tight hole / Stuck pipe

Causes: Mechanical borehole collapse (often by shear

failure) Increased hole size by brittle failure; stuck because of

accumulated cavings (sloughing shale) Reduced hole size by large (plastic) hole deformations

(gumbo shale) Inappropriate hole cleaning Differential sticking (only in permeable zones with

mud cake) Difficult hole trajectory: Key-seat, dog-legs

Tight hole / Stuck pipe

Consequences: Lost time, reaming, side-track (or $) Problems in further well operations (logging,

cementing; continued drilling) Solutions:

Overall well design i.e. Casing programme Mud weight Mud composition Drill somewhere else..

Note: The solution depends on the cause Need for diagnostics

Tight hole / Stuck pipe Diagnosis Hole

collapse Inappropriate hole cleaning

Differential sticking

Drilling environment Shale * * [ (Permeable) reservoir rock * *

Observations during drilling Rotating before stuck * 0 Moving up / down before stuck * 0 Rotating after stuck [ Circulating after stuck * Excessive cuttings & cavings *

Observations after drilling Non-gauge hole diameter from caliper * [ Low density / high porosity / low wave velocities

*

Lost circulation / Mud losses

Consequences: Dangerous situation, a major safety issue Risk of life and equipment

Solutions: Overall well design i.e.

Casing programme Mud weight Lost circulation material (LCM)

Borehole Stress AnalysisStresses at vertical impermeable borehole wall (based onlinearly elastic rock and isotropic horizontal stresses):

0 1 2 3 4 5 6 7 8 9 10

r/R

S

t

r

e

s

s

e

s

z=v

Borehole wall

pw

r

h

pf

2r w

h w

z v

pp

== =

Impermeable wall:

Perfect mud cake

During drillout in shale

Case a: z r > >

Borehole Stress Analysis

Case b: z r > >

0 1 2 3 4 5 6 7 8 9 10

r/R

S

t

r

e

s

s

e

s

r

z=v

hpw

pf

Borehole FailureAnalysis Borehole stresses + Mohr-Coulomb

failure criterion Minimum permitted well pressure to

prevent shear failure at borehole wall(hole collpase)

20( )

,min 2

2 (tan 1)tan 1

h faw

C pp

+ = +2

0( ),min 2

(tan 1)tan

v fbw

C pp

+ =or min minw wp gD=

' ' 21 0 3 tanC = +

Minimum mud weight

Borehole Failure Analysis

Tensile failure at the borehole wall mayoccur at high well pressure:

Tensile failure may also occur at lowwell pressure (in underbalance):

'0T = ,max 02fracw h fp p T= +

,,min 0

rad tensw fp p T= ' 0r T =

The Mud Weight Window

Minimum mud weight Hole collapse in shale (shear failure case a or b) Radial tensile failure in shale Pore pressure (in case underbalanced drilling is

prohibited) Maximum mud weight h (minimum horizontal stress) in case of pre-

existing natural fracturesFracturing of borehole wall

Vertical borehole in anisotropic horizontal stress field

2( )cos 22 ( )cos 2

r w

H h H h w

z v H h

pp

== + =

h HH

h

Boreholes in anisotropic stress fields If hole axis is parallel to a principal stress, then

we can use the borehole stresses for a verticalhole also for horizontal holes, but we need to rotate the coordinate system first.

In general: Holes are most stable towards shearfailure initiation when drilled along a directionwith low stress anisotropy and with low stress level in the plane perpendicular to it.

Deviated holes are usually less stable because ofshear stresses at the borehole wall.

Borehole Stability

so far elastic behaviour + brittle failure

But: Note the following field observations:

Boreholes are often stronger than predictedby elastic+brittle theory

Hole collapse is often time-delayed ( days) with respect to drill-out

Oil-based mud gives better stability thanwater-based mud

Addition of salt (in particular K+) mayimprove stability

Borehole Stability: Plasticity

Stress-strain curves for

( Elastic-brittleElasto-plastic )

material behaviour

Elastic-brittle: No load-bearing capacity after

failure initiation Elasto-plastic: Can still sustain load after failure

initiation Borehole failure criterion can be specified by a critical amount of plastic

strain, or by a critical extent of a plastic zone Using the elastic-brittle equations with an upscaled strength gives

acceptable results

Borehole Stability: Time-delayed borehole failure

Mechanisms:CreepConsolidationCoolingChemistry

time

s

t

r

a

i

nCreepThermally activited process onatomic / molecular level. Rocks may creep to failure.

Time-delayed borehole failure:Consolidation Establishment of pore pressure equilibrium takes

time Time constant is given by shale permeability

nanoDarcy week Borehole stresses from mechanical + fluid flow

equilibrium After steady state has been reached (vertical hole;

isotropic horizontal stresses):

( )( )

0

0

1 21

1 21

2 w f

w f

f

w

h w

z v

w

r

p p

p p

p

p

p

p

+ ==

=

+ =

Time-delayed borehole failure:Consolidation

0 1 2 3 4 5 6 7 8 9 10

Distance from Borehole Centre r/R

B

o

r

e

h

o

l

e

S

t

r

e

s

s

e

s

[

M

P

a

]

Tangential stress

Radial stress

Axial stress

Pore pressure (permeable wall)

Pore pressure (impermeable wall)

Permeable vs. Impermeable wall

Well pressure

0

5

10

15

20

25

0 10 20 30 40 50

Normal Stress [MPa]

S

h

e

a

r

S

t

r

e

s

s

[

M

P

a

]

Mohr-Coulomb Shear Failure criterion

Impermeable borehole wall (t=0)Permeable

borehole wall (t= )

Time-delayed borehole failure:Cooling The drilling fluid is usually (at t=0) colder than the

formation to be drilled. Thermal stresses at borehole wall:

Leads to improved stability wrt collapse, but enhancesrisk of fracturing (and mud losses).

The thermal stress contribution may be very significant! Temperature equlibrates with surroundings over time, so

improved stability is temporary.

( )( )

21

1

T

r w

h w f

z wv T

w

f

E T T

E T

p

T

p

==

+

=

+

Chemistry: Mud composition and Time effects on borehole stability

Water-based drilling fluids: Osmosis: Shale is thought of as a semipermeable

membrane, only permitting water movement Ions do move through shales and interact through

e.g. ionic exchange Oil-based drilling fluids:

Capillary support Osmosis; oil phase acting as a membrane?

Mud Chemistry: Osmosis

Shale

Ionic exchange

ions

water

High salinity drilling fluid

Shale

Ionic exchange

ions

water

High salinity drilling fluid

Osmotic potential acts like an excess pore pressure:

is reduced by membraneefficiency

Mud Chemistry: Osmosis Borehole stresses in presence of an osmotic

potential:

This is a temporary effect: As time goes, equilibrium will be established, and thechemical effect removed.

One conclusion from this: The more salt, thebetter!

1 21

1

2

21

r w

h w

z v

p

p

+ +

=

=

=

Mud Chemistry: Ionic exchange

Overbalance and exposure

-2.0

-1.0

0.0

1.0

2.0

3.0

0 20 40 60 80 100 120 140 160 180 200 220Overbalance and exposure time [h]

A

x

i

a

l

s

t

r

a

i

n

[

m

S

t

r

a

i

n

]

Deionised water

20wt% NaCl

20wt% KCl

16wt% CaCl 2

5wt% KCl

3.5wt% NaCl5wt% CaCl 2

Seawater

We observe: Ion type dependent shrinkage (K>Na>Ca); not explained by osmosis

Expectedosmoticshrinkage:

~ 0.5 milliStrain for allstrongelectrolytes

Tertiary fieldshale

Overbalance and exposure

-2.0

-1.0

0.0

1.0

2.0

3.0

0 20 40 60 80 100 120 140 160 180 200 220Overbalance and exposure time [h]

A

x

i

a

l

s

t

r

a

i

n

[

m

S

t

r

a

i

n

]

Deionised water

20wt% NaCl

20wt% KCl

16wt% CaCl 2

5wt% KCl

3.5wt% NaCl5wt% CaCl 2

Seawater

We observe: Ion type dependent shrinkage (K>Na>Ca); not explained by osmosis

Expectedosmoticshrinkage:

~ 0.5 milliStrain for allstrongelectrolytes

Tertiary fieldshale

From SINTEFs Shale research project

Mud Chemistry: Ionic exchange Experiments indicate that shales when exposed to

KCl: shrink because of ionic exchangeKCl promotes hole

stability up to a limit ( mud cooling) become more plastic KCl promotes hole stability possibly get reduced strength KCl reduces hole

stability become more permeable KCl reduces hole stability

Ionic exchange requires ionic transport into shale; thus a time-delayed effect.

All in all: An optimum salt concentration required.

Mud Support Capillary support by non-wetting fluid (e.g. oil):

: surface tension; oil-water = 5010-3 N/m r: pore size; rshale 10 nm pc ~ 10 MPa

So: An overbalance of 10 MPa is required for oil to penetrate into an intact shale. Since there is always an impermeable membrane, the t=0 stability will prevail.

Oil-based muds are not pure oils (so chemical effectscould play a role; cf. osmosis)!

2cp r

=

Mud Support

Mud additives that could lead to impermeable borehole wall:

Sodium silicate! Long-chained molecules (glycol; polymers

etc) that are thought to build molecuylarfilter cakes on shale surface.

Borehole stress fieldBorehole wall boundaryconditions

Mud cake efficiencyOsmotic membrane efficiency

Mud vs rock temperature

Time dependence

PermeabilityIonic diffusivity & exchange

Thermal diffusivity

Poro-chemo-thermo-elasticity (+plasticity)

Borehole failure criterionShear failure

(/plastic strain limit)/ Tensile failure

Minimum permitted mud weightGuidelines on mud composition & temperature

Maximum permitted mud weightGuidelines on mud composition & temperature

Rock propertiesElastic (& plastic) parameters

Strength parameters

Earth stressesVertical & horizontal stresses

Pore pressure

Well trajectoryHole deviation angle & azimuth

Borehole Stability Analysis

Borehole Stability Challenges

Drilling in tectonic areas (ex: SouthAmerica)

Fractured and faulted rock Depleted reservoirs Deep water

Borehole Stability during DrillingBorehole Stability ProblemsTight hole / Stuck pipeTight hole / Stuck pipeTight hole / Stuck pipe DiagnosisLost circulation / Mud lossesBorehole Stress AnalysisBorehole Stress AnalysisBorehole Failure AnalysisBorehole Failure AnalysisThe Mud Weight WindowVertical borehole in anisotropic horizontal stress fieldBoreholes in anisotropic stress fieldsBorehole StabilityBorehole Stability: PlasticityBorehole Stability: Time-delayed borehole failureTime-delayed borehole failure:ConsolidationTime-delayed borehole failure:ConsolidationTime-delayed borehole failure:CoolingMud Chemistry: OsmosisMud Chemistry: OsmosisMud Chemistry: Ionic exchangeMud Chemistry: Ionic exchangeMud SupportMud SupportBorehole Stability Challenges