amoco - wellbore stability

Lost Circulation

Poor Hole Cleaning

Hole Caving /Collapse

WELLBORE

STABILITY

WELLBORE

STABILITY

Formation

Mud

TensileFailure Active

Tectonics

ShearFailure

PorePressure

STABLE

UNSTABLE

Rock StrengthRock Stress

Hole Enlargement

Drill String Fatigue

Tight hole /Stuck Pipe

Drilling Handbook

1.0 INTRODUCTION

2.0 BEFORE THE WELLBORE

3.0 AFTER THE WELLBORE

4.0 PROVIDING A STABLE WELLBORE

APPENDIX

1.1 Wellbore Stability Mission 21.2 Drilling Handbook Objectives 3

2.1 Conditions 12.2 Earth Stress 42.3 Effective Stress 52.4 Rock Strength 6

3.1 Near Wellbore Stress-State 13.2 Mechanical Stability 43.3 Chemical Stability 11

4.1 Planning a Stable Wellbore 14.2 Warning Signs/Corrective Actions 3

A-1 Leak-off TestsA-2 Lithology Factor (k)A-3 Wellbore Stress EquationsA-4 Nomenclature

In Situ

In Situ

Wellbore Stability

CONTENTS

StartStart

Introduction

Formation

Mud


Tectonics

ShearFailure

PorePressure

STABLE

UNSTABLE


SandMWHigh

MWLow Reaming

TripSpeed

ECD MobileSalt

Shale StrikeSlip

TensileFailure

HoleCleaning

ReverseFault

TimeExposed

- Wellbore Stability -Maintaining the Balance of

Rock Stress and Rock Strength

SECTION 1

1.1 Wellbore Stability Mission

1.2 Drilling Handbook Objectives

StartStart TOCTOC

Wellbore Stability

Page - 1Section 1

1.0 INTRODUCTION

Wellbore stability

600 million to 1 billion dollars

is the prevention of brittle failure or plastic deformationof the rock surrounding the wellbore due to mechanical stress or chemicalimbalance.

Prior to drilling, the mechanical stresses in the formation are less than thestrength of the rock. The chemical action is also balanced, or occurring at arate relative to geologic time (millions of years). Rocks under this balancedor near-balanced state are stable.

After drilling, the rock surrounding the wellbore undergoes changes intension, compression, and shear loads as the rock forming the core of thehole is removed. Chemical reactions also occur with exposure to thedrilling fluid.

Under these conditions, the rock surrounding the wellbore can becomeunstable, begin to deform, fracture, and cave into the wellbore or dissolveinto the drilling fluid.

Excessive rock stress can collapse the hole resulting in stuck pipe. Hole-squeezing mobile formations produce tight hole problems and stuck pipe.Cavings from failing formation makes hole cleaning more difficult andincreases mud and cementing costs.

Estimated cost to the drilling industry for hole stability problems rangefrom annually.

StuckPipe

HoleProblems

Loss OfCirculation

WellControl

Relative Costs Of Unscheduled EventsCaused By Wellbore Stability Problems

StartStart TOCTOC Section 1TOC

Section 1TOC

Wellbore Stability

Page - 2Section 1

1.1 Wellbore Stability Mission

The mission of the Wellbore Stability Team is twofold.

Minimize the "learningcurve" when developingnew reservoirs so thatoptimal well costs areobtained early on.

Identify potential drillingproblems during the wellplanning stage so thatprevention and operationalplanning can be developedto minimize costs associatedwith wellbore stabilityproblems.

Chemical Instability Mechanical Instability

Tensile Shear

FracturesLoss of Circulation

CavingsTight HoleStuck Pipe

Reactive Shale Overburden StressedGeopressured

Hydro-PressuredUnconsolidated

FracturedTectonics

Failure Mechanisms

Wellbore Stability Problems


Section 1TOC

Wellbore Stability

Page - 3Section 1

Understanding the conditions that cause stability problems provides for:

More effective planning.

Earlier and easier detection of warning signs.

Contingency plans to avoid the progression of the problem.

!

!

!

GENERAL CAUSES OFSTABILITY PROBLEMS

STABILITY PROLEMS

COST TO OPERATION

RESULTING CONDITIONS

IncorrectMud

IncorrectWell Trajectory

Poor Drilling

PracticesPoor W

ellPlan

Reactiveshale

Reactiveshale

ExcessiveW

ellborePressure

ExcessiveW

ellborePressure Excessiv

e

Rock StressExcessiv

e

Rock Stress

Hol

eCl

eani

ng

Hol

eCl

eani

ng

Hole

Enlargement

Hole

Enlargement

Hole

Collapse

Hole

Collapse

Well ControlWell Control

Lost CirculationLost Circulation

Poor logsPoor logs

Cementing Problems

Cementing ProblemsStuck PipeStuck Pipe

Drill String FatigueDrill String Fatigue

1.2 Handbook Objectives

!

!

!

!

!

Identify and define wellbore stability problems.

Suggest consistent terminology.

Associate warning signs with stability problem.

Suggest corrective actions.

Provide the background for preventive planning.


Section 1TOC

SECTION 2

Before The Wellbore

2.1 Conditions

2.2 Earth Stress

2.3 Effective Stress

2.4 Rock Strength

In Situ

In Situ

StartStart TOCTOC

2. 0 BEFORE THE WELLBORE

2.1 ConditionsIn Situ

Porosity

Porosity is the percent of void space within the rock.

The rocks of sedimentary basins always exhibit some porosity. As porosityincreases, the percent of fluid volume increases while the rock matrixvolume decreases. Increasing porosity weakens the rock. Shale, forexample, will change from brittle rock to ductile clay with sufficient watercontent. The figure below shows typical porosity change with depth due tocompaction and cementation.

Wellbore Stability

Page - 1Section 2

Permeability

Permeability is the ability of a rock to flow fluids; measured in units ofdarcies.

Permeability acts to weaken the rock as the loss of water base mud filtratedissolves the grain-to-grain cement bond. Also, hydrostatic overbalanceforces water filtrate to penetrate the pores of the rock; which also weakensthe rock.

Shale

Fluid FilledPores

RockMatrix

0

5

10

15

20

25

10 20 30 40 50 60

Porosity (%)

Dep

th(f

t)


Section 2TOC

Wellbore Stability

Page - 2Section 2

The figure below shows typical permeability changes relative to depth forshale and sandstone. Shales may have high porosity, but have very littlepermeability.

Formation Pore Pressure - p

Formation pore pressure is the pressure of the naturally occurring fluid(s) inthe pores of the rock.

As long as the increase in overburden load from the rate of deposition doesnot exceed the rate at which fluid can escape from the pore, a fluidconnection exists from surface to the depth of interest. Pore pressure is thenequal to the hydrostatic pressure of formation water (normal pressure).

is equal to the hydrostatic pressure offormation water at a vertical depth of interest.Normal formation pressure

Permeability (Darcies)

Sandstone

Fluid FilledPores

ConnectedPorosity

RockMatrix

0

5

10

15

20

25

1 2 3 4

SandstoneShale

Dep

th(f

t)

Transition Shale

Formation WaterMigrating to Surface

Dep

th

Pressure

.465

psi/ft A

verag

e

8,000'

3720 psi


Section 2TOC

Wellbore Stability

Page - 3Section 2

Pore pressure of a permeable formation can be depleted below normal byproduction operations (subnormal pressure).

is less than normal for the vertical depthof interest.Subnormal formation pressure

If the fluid cannot escape the pore, pore pressure begins to increase at afaster-than-normal rate (abnormal pressure).

is greater than normal for the vertical depthof interest.Abnormal formation pressure

Dep

th

Pressure

Transition Shale

Depleted Zone

AbnormalPressure

Sub normalPressure

Normal TrendLineFormation Water

Migrating to Sand

8,000'

3720 psi

Estimating Formation Pore Pressure

Formation pore pressure prediction is a highly specialized process. Prior todrilling, qualitative geophysical methods are available to qualify thepresence of abnormal pressure at an approximate depth. Offset logs alsohelp estimate pore pressure.

Enhancements in geophysical interpretations have recently been made toquantify the value of abnormal pressure prior to spudding the well. Beforedevelopment of this quantitative method, only qualitative information waspossible prior to drilling.

While drilling, several MWD/LWD logs provide real time evaluation offormation pore pressure. "D" exponent plots can also indicate changes inpore pressure.

Higher than normal porosity and sonic travel time ( t ) indicate abnormal

pore pressure.

∆ c


Section 2TOC

Wellbore Stability

Page - 4Section 2

Weight of overlying rocks &water appliesstress to therock layer at avertical depthof interest

STRESSESHORIZONTAL

OVER

BURDENSTRESS

OVER

BURDENSTRESS

Most formations are formed from a sedimentation/compaction geologichistory. Formations may vary significantly from the earth's surface to anydepth of interest. Shallow shales will be more porous and less dense thanshales at great depths.

Typically a value of 1 psi/ft is attributed to the overburden gradient, but atshallow depths the actual value is much less and at greater depthssomewhat higher.

A density log can be used to determine the weight of the overburden. In theabsence of a density log, the overburden stress may be estimated fromalternatives such as Eaton's variable density curve or the Wylie timeaverage equation using sonic travel time, bulk density and porosity.

Estimating Overburden Stress

2.2 Earth Stress

- s

In Situ

Prior to drilling, subsurface rocks are exposed to a balanced or nearbalanced stress environment. The naturally occurring stress in place iscalled the stress. stress is normally compressive due to theweight of the overburden. For this reason, in rock mechanics compressivestress is defined to be positive.

Overburden stress is the pressure exerted on a formation at a given depthdue to the total weight of the rocks and fluids above that depth.

in situ In situ

Overburden Stress v


Section 2TOC

Wellbore Stability

Page - 5Section 2

As the overburden squeezes the rock vertically, it pushes horizontally.Constraint by surrounding rock creates horizontal stress.

In most drilling areas, the horizontal stresses are equal. When drillingnear massive structures such as salt domes or in tectonic areas, thehorizontal stresses will differ and are described as a minimum (s ) and a

maximum (s ).

The minimum horizontal stress (s ) is normally determined from leak-off

tests. It is difficult to determine the maximum horizontal stress from fieldmeasurements. Its value can be estimated using rock mechanicsequations.

Horizontal Stress - s sh , H

h

H

h

Estimating Horizontal Stress

2.3 Effective Stress

The rock matrix does not support the full load of overburden and horizontalstress. Part of the load is supported by the fluid in the pore (pore pressure).The net stress is the effective stress felt by the rock matrix. Effective stressis used in rock mechanics to determine the stability of the wellbore.

The overburden stress that effectively stresses the rock matrix.

= s -

Effective Overburden Stress - σv

Effective Overburden Stress = Total Overburden Stress - Pore Pressure

σv v p

Much like air pressure in a cartire supports the weight of thecar, fluid pressure in the poresupports a portion of theoverburden load.

The remaining portion of over-burden stress is the loadeffectively stressing the rockmatrix.

ROCKMATRIXROCK

MATRIX

5000 PSIPore Pressure

EffectiveOBS

EffectiveOBS

4000psi

4000psi

9000 PSIOVERBURDEN

9000 PSIOVERBURDEN


Section 2TOC

Wellbore Stability

Page - 6Section 2

Effective Horizontal Stress - ,σ σh H

Similarly, the effective horizontal stresses can be determined. Usually thehorizontal stresses are equal and the effective horizontal stress is equal tothe effective overburden stress times a lithology factor, . The lithologyfactor ( ) is equal to 1 for fluids but is less than 1 for more rigid materialsuch as formation rock.

= = x

kk

kσ σ σh H v

2.4 Rock Strength

Rock mechanics mechanical

stress

strain

elastic deformation

is the study of the behavior of subsurfacerocks.

Core samples (removed from conditions) are usually tested incompression with specialized laboratory equipment. To better simulatesubsurface conditions, core samples tested are also subjected to a confiningpressure ( ). The rock responds to the stress by changing in volume orform (deformation) or both. The change in the rock volume or form due tothe applied stress is called .

Rocks subjected to compressive (+) or tensile (-) stress can go through threestages of strain deformation. In , the rock deforms asstress is applied but returns to its original shape as stress is relieved. Inelastic deformation, the strain is proportional to the stress (Hooke's Law).

in situ

In tectonically active areas, the horizontal stresses are not equal. Themaximum horizontal stresses will be higher, or lower depending on tectonicmovements, by the additional tectonic stresses, In these areas, the

effective horizontal stresses are described by a maximum and minimumvalue.

= x + and = x +

In extreme tectonic environments, may be sufficient to make the

horizontal stress higher than the vertical stress.

t and t .

k t k t

t

h H

h H

H

σ σ σ σh v H v

1000 psi 900 psi 500 psi

Water Putty RockNoncompressiblefluids like waterhave a k factor of 1.

Stiffer materialslike putty have alower k factor (.7 -.9 for example.)

Very stiffmaterials likeformation rockhave a muchlower k factor(.37 is commonfor shale.)


Section 2TOC

Wellbore Stability

Page - 7Section 2

When applied stress reaches the elastic limit, the rock begins to exhibit. In plastic deformation, the rock only partially returns

to its original shape as stress is relieved. If continued stress is applied,fractures develop and the rock fails ( ).

Rocks can fail in a brittle manner, usually under low confining stress, or in aductile manner under higher confining stress.

Under compression rocks actually fail in - it is easier to slide rockgrains past each other than to crush them.

plastic deformation

ultimate failure

shear

Shear Strength and Shear Failure

Stress(xpsi)1000

10 2 3 4

AxialLoad

ElasticLimit

ElasticDeformation

PlasticDeformation

Ultimate

Failure

Ultimate Strength

Strain (% of Deformation)

0

10

20

30

40

50

AxialLoad

(CompressiveStress)

ConfiningPressure

Axial Load (psi)

ConfiningPressure

Shear PlaneShearFailure

High confining pressure resists sliding on the shear plane and the rockappears stronger. If the confining pressure and axial load were equal, therewould be no shear stress on the rock and no shear failure.

Equal stresses promote stability and unequal stresses promote shearstress and possible shear failure.


Section 2TOC

Wellbore Stability

Page - 8Section 2

It is not possible to accurately reproduce the effects of pore pressure onrock strength when testing core samples from the field. In actual boreholeconditions, pore pressure exerts a force that tends to push the rock grainsapart. This is why the effective stress is used in rock mechanics whenapplied to wellbore stability studies.

Cohesive StrengthBonded Grains (Cement)

OverburdenStress (s )v

Increased Pore PressureReduces the Effective Stress

PorePressure

HorizontalStress (s )h

HorizontalStress (s )H

Mean Effective Stress =σ σ σv h H+ +

3

Rock mechanics uses failure models to predict wellbore stability. One suchmodel considers all three effective stresses to calculate the resultant shearstress. The "mean" effective stress is used by this model to describe thestress state of the rock.

The failure model used in the illustrations (Mohr-Coulomb) neglects theintermediate stress and considers only the and effectivestress.The greatest shear stress on the rock occurs on the two-dimensionalplane consisting of the greatest and least stress. The greatest/and or leaststress could be any of the three depending on environment and wellconditions.

greatest least

in situ

Greatest Effective

Stress ( , , or )σ σ σv h H

Greatest Effective


Least Effective


(Intermediate stress actsperpendicular to the figure)


Section 2TOC

Wellbore Stability

Page - 9Section 2

Shear

Stress ( )τ

S0

ConfiningPressure ( )σc

CompressionPressure (Stress)That Fails Core

Sample ( )σf

Failure ShearStress From Test 1

Failure ShearStress From Test 2 & 3

< = φ< = φ

Stress-State 1Stress-State 2

Stress-State 3

Effective Compressive Stress (σ)

The is defined as the shear stress that fails the rock. Thecoefficient of friction is also expressed in terms of an

.

= tan

The cohesive strength (S ) and the angle of internal friction ( ) are

obtained from conducting compression tests on core samples (orestimated from logs) from the field. Several tests on cores are necessaryto determine these values.

The shaded area shown below indicates the "stress-state" of one such

core sample at failure. The compression stress ( ) that fails the core

sample (greatest stress) is plotted on the horzontal axis along with the

confining pressure ( ) used for that test (least stress).

shear strengthangle of internal

friction ( )φµ φ

φ

σ

σ

0

f

c

The shear stress that fails the rock must overcome the(bonding together of the grains), and the frictional resistance between

the grains ( The frictional resistance between the grains is the product

of the and the effective compressive stress ( ).

cohesive strength,S

coefficient of friction ( )

0

µσ).µ σ

Shear Stress = Cohesive Strength + Frictional Resistance

τ µσ= S +0


Section 2TOC

Wellbore Stability

Page - 10Section 2

The , thenecessary to fail the sample. Several tests at increasing confiningpressures produce successive stress-states of increasing shear strength.

The " " is approximated by the line giving the best fitto the maximum shear stress points on the failure plane from severalsuch tests. The equation for this line is given below.

= S + tan

A "shear strength line" or failure envelope shown below is producedfrom such core tests (a similar stability chart is used when considering

the mean effective stress, ( + + ) / 3).

The greatest and least effective stress on the wellbore are also calculatedusing stress, pore pressure, hole inclination, etc., and indicated onthe chart. If the stress-state produces a shear stress that falls beneath theshear strength line, the wellbore is stable.

If the shear stress falls outside the stability envelope, the wellbore isunstable and formation failure will occur.

higher the confining pressure greater the compressive stress

in situ

shear strength line

τ σ φ

σ σ σ

0

v h H

Shear

Stress, τStabilityEnvelope

Failure

S0

Effective Compressive Stress, σ

Shear Strength Line

LeastEffective Stress

GreatestEffective Stress

Stress-State


Section 2TOC

Wellbore Stability

Page - 11Section 2

Time

Geological processes have great lengths of time in which to operate.Although geologic time is impossible to duplicate in a laboratory, it ispossible from experiments to make some deductions concerning theinfluence of time.

One analysis of special interest to drilling operations is that of .Creep is a slow continuous deformation of rock with the passage of time,even though the stress may be above or below the elastic limit.

creep

Tensile Failure

Tensile Failure results from stresses that tend to pull the rock apart (tensilestress). Rocks exhibit very low tensile strength.

TensileStress

Tensile Stress Exceedsthe Tensile Strength and the Rock Fails


Section 2TOC

SECTION 3

After The Wellbore

3.1 Near Wellbore Stress-State

3.2 Mechanical Stability

3.3 Chemical Stability

StartStart TOCTOC

Wellbore Stability

Page - 1Section 3

OVER

BURDENSTRESSOVER

BURDENSTRESS

STRESSES

STRESSES

HORIZONTAL

HORIZONTAL

BeforeDrillingBeforeDrilling

AfterDrillingAfterDrilling

HYDRO

STATICPRESSURE

HYDRO

STATICPRESSURE

Radial

Stress - σr

Radial

Stress - σr

Axial

Stress - σz

Axial

Stress - σz

Hoop

Stress - σθ

Hoop

Stress - σθ

HSP

As the hole is drilled, the support provided by the rock is removed andreplaced by hydrostatic pressure. This change alters the stresses. Thestress at any point on or near the wellbore can now be described in terms of:radial stress acting along the radius of the wellbore; hoop stress actingaround the circumference of the wellbore (tangential); axial stress acting

parallel to the well path. These stresses are designated by ( , , ) and the

additional shear stress components designated by ( , , ).

These stresses are perpendicular to each other and for mathematicalconvenience, are used as a borehole coordinate system.

in situ

σ σ σσ σ σ

r z

r rz z

θ

θ θ

3.0 AFTER THE WELLBORE

3.1 Near Wellbore Stress-State

Before drilling, rock stress is described by the stresses; effectiveoverburden stress, effective minimum horizontal stress, and the effectivemaximum horizontal stress. These stresses are designated by ( , , ).

in situ

σ σ σv h H


Section 3TOC

Wellbore Stability

Page - 2Section 3

Hoop Stress - σθ

Hoop stress is dependent upon wellbore pressure ( ), stress

magnitude and orientation, pore pressure, and hole inclination anddirection. Wellbore pressure ( ) is directly related to mud weight/ECD.

= [ & well parameters] - -

For a vertical wellbore with equal horizontal stresses, hoop stress isdependent upon the mud weight and the magnitude of the horizontalstresses and is equally distributed around the wellbore.

p in situ

p

in situ p p

w

w

σθ w

EqualHorizontal

Stresses

A deviated well creates of hoop stress around thewellbore due to the redistribution of the horizontal and vertical stresses.Hoop stress acting on a cross-section of the wellbore is maximum at thesides of the wellbore perpendicular to the maximum stress.

The same is true when drilling a vertical well in an environment ofunequal horizontal stress. Hoop stress is maximum at the side of thewellbore perpendicular to the maximum horizontal stress.

unequal distribution

in situ

Low Side of Hole

High Side of HoleAdditional Components

Of Stress From OverburdenAnd Horizontal Stresses

MinimumHoop Stress

MaximumHoop Stress


Section 3TOC

Wellbore Stability

Page - 3Section 3

Axial Stress - σz

Axial stress is oriented along the wellbore path and can be unequallydistributed around the wellbore. Axial stress is dependent upon;stress magnitude and orientation, pore pressure, and hole inclination anddirection. Axial stress is not directly affected by mud weight.

= [ & well parameters] -

For a vertical well with equal horizontal stress (s = s ), axial and vertical

stress are the same. Axial stress in a deviated well is the resolution of theoverburden and horizontal stresses.

in situ

in situ pσz

h H

Vertical Well -Equal Horizontal

Stresses

Axial Stress isOnly the Overburden

Axial Stress isThe Resolution of

Overburden and HorizontalStresses

Deviated Well -Equal or Unequal

Horizontal

Radial Stress - σr

Radial stress is the difference in wellbore pressure and pore pressure andacts along the radius of the wellbore.

Since wellbore and pore pressures both stem from fluid pressure actingequally in all directions, this pressure difference is acting perpendicular tothe wellbore wall, along the hole radius.

= -

Radial Stress = Wellbore Pressure - Pore Pressure

σr wp p


Section 3TOC

Wellbore Stability

Page - 4Section 3

3.2 Mechanical Stability

Hoop ( ), radial ( ), and axial ( ) stress describe the near wellbore

stress-state of the rock. is the of thesestresses in an effort to prevent shear or tensile rock failure.

Normally the stresses are compressive and create shear stress within therock. The more equal these stresses, the more stable the rock.

σ σ σθ r z

Mechanical stability management

Axial - σz

Hoop - σθ

Radial - σr

Hoop

Radial

ShearStress

As shown by the right side drawing above, the radial stress is resistingshear caused by the hoop stress.

Hoop, axial, and radial stress can be calculated and the greatest and leastof the three indicated by a stress-state semicircle on the stability chart.

Shear failure occurs if the stress-state falls outside of the stability envelop.Tensile failure occurs if the stress-state falls to the left of the shear stressaxis and exceeds the tensile strength of the rock.

S0

Shear Strength Line

StabilityEnvelope

Failure

LeastStress

GreatestStress

ShearStress

Effective Compressive Stress

Stress-State


Section 3TOC

Mechanical stability is achieved by controlling the parameters that affecthoop, axial, and radial stress.

MW/ECDMud Filter CakeWell Path - Inclination and AzimuthDrilling/Tripping Practices

Unfavorable ConditionsAdverse FormationsConstrained Wellbore Trajectory

Mechanical stability of the well is also impacted by drillingfluid/formation interaction. Chemical instability eventually results inmechanical failure of the rock in shear or tension.

is also an important consideration. The longer the formation isexposed to the drilling mud, the more near-wellbore pore pressureincreases. The rock looses support provided by the mud weight.

Controllable parameters:

Uncontrollable parameters:

Time

!

!

!

!

!

!

!

In Situ

Page - 5Section 3

Wellbore Stability

Whenever hoop or radial stress become tensile (negative), the rock isprone to fail in tension. Many unscheduled rig events are due to loss ofcirculation caused by tensile failure.

Tensile Failure Due toNegative Hoop Stress


Section 3TOC

S0

Shear Strength Line

StabilityEnvelope

FailureEnvelope

RadialStress

HoopStress

ShearStress

Stress-State BeforeMW Decrease

Stress-State AfterMW Decrease

MW Decrease

Wellbore Stability

Page - 6Section 3

Effect of Mud Weight/ECD

Mud weight, ECD, and pressure surges on the wellbore directly effecthoop and radial stress. An increase in MW decreases hoop stress andincreases radial stress. Similarly, a decrease in MW increases hoop stressand decreases radial stress. The result on wellbore stability is dependentupon the magnitude of the mud weight increase/decrease.

Shear Strength Line

StabilityEnvelope

Failure

Radial Stress Hoop Stress

ShearStress Stress-State Before

MW Increase

Stress-State AfterMW Increase

Increase in MW

Excessive Increase in MW

Shear Strength Line

StabilityEnvelope

Failure

New Radial StressNew Hoop Stress

ShearStress Stress-State Before

MW Increase

Stress-State AfterMW Increase


Section 3TOC

Wellbore Stability

Page - 7Section 3

Mud Filter Cake and Permeable Formations

The filter cake plays an important role in stabilizing permeable formations.An ideal filter cake isolates the wellbore fluids from the pore fluids next tothe wellbore. This is important for hole stability and helps preventdifferential sticking as well.

PermeableSand

Pores

WellboreWellbore

Wellbore

FilterCake

pw p

Ideal Filter Cake

If there is no filter cake, the pore pressure near the wellbore increases tothe hydrostatic pressure; the effective radial stress is zero. Thesimultaneous decrease in effective hoop stress causes the stress-state tomove left in the stability envelope; decreasing the stability of theformation. An ideal filter cake helps provide for a stable wellbore.

S0

Shear Strength Line

StabilityEnvelope

FailureEnvelope

RadialStress

σr = 0

HoopStress

σθ

ShearStress Stress-State After

Filter Cake Failure

Example of a Poor Filter Cake

Stress-StateWith Good Filter Cake

The chemical composition of the mud and permeability of the formationcontrol the filter cake quality and the time it takes to form.

StrongThin

FlexibleImpermeable



Section 3TOC

Wellbore Stability

Page - 8

S0

Shear Strength Line

StabilityEnvelope

Failure

MinimumHoop Stress

ShearStress

Stress-State in VerticalHole Section

Stress-State in HorizontalHole Section

RadialStress

MaximumHoop Stress

Drilling a Horizontal Well

Section 3

Hole Inclination and Direction

The inclination and direction of the wellbore greatly impacts the stabilityof the well. Unequal distribution of hoop and axial stress around thecircumference of the well tends to make the wellbore less stable.

MinimumHoop Stress

MaximumHoop Stress

Increased Vertical Stressof the Overburden

For Equal Horizontal Stress

Drilling a horizontal well causes thehoop and axial stress distributionaround the wellbore to change.

Before drilling from vertical, thehoop stress is equally distributed. Asangle increases to horizontal, thehoop stress on the high and low sideof the wellbore decreases, but thehoop increases greatly on theperpendicular sides.

The change in the stress-state at the wellbore wall is shown below. Theradial stress remains fixed but the increasing hoop stress increases thestress-state.


Section 3TOC

Wellbore Stability

Page - 9Section 3

Bottom-hole Temperature

High bottom-hole-temperature wells can experience stability problems ashoop stress changes because of temperature differences between the mudand formation.

If the mud is cooler than the formatio, it reduces the hoop stress as theformation is cooled. This reduction in hoop stress can prevent shear failureand stabilize the hole, if the hoop stress were high due to low mud weight.On the other hand, if the mud weight is too high and close to the fracturegradient, excessive cooling can lower the hoop stress and make it tensile.This could cause tensile failure or fracturing as it effectively lowers thefracture gradient.

If the mud is hotter than the formation, exactly the opposite occurs as hoopstress is increased. This could promote spalling or shear failure.

Consider what happens during a typical round-trip on a deep hightemperature well. During the trip, formation temperature returns to itsambient value. This causes the hoop stress to increase. When back onbottom and circulation resumes, the cooler mud traveling down thedrillstring reduces the temperature of the nearby formation, causing hoopstress to decrease.

As the hot bottoms-up mud circulates past formations at shallower depths,hoop stress increases as the mud heats up the formations.

These variations in hoop stress have the same effect as pressure surgesassociated with swabbing and surging and can cause both tensile and shearfailure downhole.

Variations in Hoop Stress in a High Temperature Well

S0

Shear Strength Line

StabilityEnvelope

Failure

ShearStress

RadialStress

Changes in ShearStress on Formation

Increased Hoop Stress While POOH

Hoop Stress Prior to Trip

Decrease in Hoop Stress While Circulating Bottoms-Up


Section 3TOC

Wellbore Stability

Page - 10Section 3

Impact of Mechanical Stability on the Wellbore

Mechanical stability problems directly account for many unscheduled rigevents. Stability problems also effect overall drilling efficiency by alteringthe shape of the hole being drilled.

Severe hole deformation occurs when extreme stress environmentsare penetrated. The drawing below is indicative of such drilling. Thedrawing is only a slice of the actual wellbore. Consider the path of a typicalwell, and consider this deformation over several thousand feet of open hole;it is easy to see the impact of such a wellbore on operations.

in situ

Maximum HorizontalStress Orientation

Shear FailureZone (Breakouts)

Tensile FailureZone

Cavity

OriginalHole Size

Encroachment ofBrittle Sands

Resulting Operational Problems Include:

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Stuck pipe, casing, logging tools, etc.

Ineffective hole cleaning.

Ledges and breakouts.

High torque and severe slip-stick.

Drillstring failures.


Section 3TOC

Wellbore Stability

Page - 11Section 3

3.3 Chemical Stability

is the control of the drilling fluid/rock interaction;usually most problematic when drilling shales.

are fine grain sedimentary rocks with very low permeability andcomposed primarily of clay minerals (gumbo to shaly siltstone).

Chemical stability

Shales

Clay platelets (2 micronsand less) settling to themud line.

Mud at mud line, 60 - 90%porosity. Clay plateletsmaintain a water envelopeafter burial. Mud readilydeforms (much likepudding).

Compaction drives porewater back to the sea.Platelets begin to contactforming pliable clay (muchlike putty).

Further compaction,geologic time, andtemperature cements theclay platelets into shale(less than 20% porosity).

Mud

Clay

Shale

MuddyWater

One factor that distinguishes shale from other rock is it's sensitivity to thewater component of drilling fluids. With time, shale/water interaction willdecrease the strength of the shale; making it more prone to mechanicalstability failure.


Section 3TOC

Wellbore Stability

Page - 12Section 3

As shale is drilled, a sequence of events take place that can lead to thestressing, weakening, and eventual failure of the shale. Several parameters,described below, contribute to the chemical stability of shale.

is the transport of fluid through shale due to pressuredifferential. Typically, wellbore hydrostatic pressure is greater thanformation fluid pressure. When exposed to a permeable formation, theliquid phase of the mud is "pushed" into the pore openings by the pressuredifferential.

In a highly permeable sand, the flow rate of fluid loss is sufficient to form afilter cake that controls fluid loss. With shales, however, the filter cakecannot develop, since the permeability of a typical shale is much less thanthat of any filter cake. Also, the particle size of a typical filter cake is toolarge to plug the pore throats of shale (much like trying to plug a shakerscreen with beach balls).

Advection

Advection

4500 psi

Wellbore

5000psi

Wellbore

5000psi

PoreThroat

PoreFluid

Shale

PoreSpaces

WellboreWellbore

Wellbore

Capillary Effects

Drilling fluid must overcome capillary pressure to enter the pore throats ofshale. Capillary pressure, developed at the drilling fluid /pore fluidinterface, is dependent on several factors; pore throat radius, interfacialtension, and contact angle.

When drilling water-wet shales with water base mud; surface tensionbetween the mud's water phase and the pore fluid is very low. Underfavorable salinity conditions, the water phase enters the pore throat.


Section 3TOC

Wellbore Stability

Page - 13Section 3

Osmosis

Pressure Diffusion

Osmosis is caused by the imbalance of salt concentration between the mud'swater phase and the pore water. The salinity imbalance is separated byshale which acts as a semi-permeable membrane that allows the transport ofwater only. Water moves from low salinity to high salinity until the salinitydifference (chemical activity) is balanced.

If the mud salinity is too low, water moves into the shale increasing the porepressure. As pore pressure increases, it has an adverse effect on stability.

If the mud salinity is too high, pore water flows into the mud systemdehydrating the shale. As pore pressure decreases, effective hoop stressincreases also promoting shear failure.

is the change in near-wellbore pore pressure relative totime. This occurs as overbalance and osmotic pressures drive the pressurefront through the pore throat, increasing pore fluid pressure away from thewall of the hole. This pore pressure penetration leads to a less stablecondition at and near the wellbore wall.

Pressure diffusion

4500 psi

WBM

5000psi

WBM

5000psi

PoreThroat

PoreFluid

4500 psi

OBM

5000psi

OBM

5000psi

PoreThroat

PoreFluid

SurfaceTension

When drilling water-wet shales with oil base mud, the capillary pressure isvery high (i.e., 8000 to 10,000 psi) due to the large interfacial tension andextremely small pore throat radius. The high capillary pressure preventsentry of the oil phase as overbalance pressures are very low in comparison.However, if the salinity of the mud's water phase is not balanced with shalesalinity, water transfer through osmosis can still occur.


Section 3TOC

Wellbore Stability

Page - 14Section 3

As pressure diffusion increases pore pressure near the wellbore, shearstrength of the rock is reduced. The time for pressure diffusion to impactshale may result in failure of a shale section exposed for several days.

Time required for the pressure front to penetrate a given depth dependsprimarily on the permeability of the shale (connectivity of the pores) andthe pressure differential between the wellbore ( ) and pore pressure

( ).

p in situ

pw

Swelling /Hydration

Over geologic time, mud/clay solidifies into shale as overburden stressdrives off the water envelope (dehydration) and cements the platelets withthe minerals left behind after dehydration.

After drilling, water enters the shale by advection and osmosis. Negativelycharged clay ions attract and hold the polar water. The increasing volumeof attached water produces a swelling stress that "wedges" the clay plateletsapart.

Distance From Wellbore (Hole Diameters)

Pre

ssure

p

pw

0 1 2

Day 1

Day 3

Day 2

Fluid Front

Pore Channel

Pressure Front

In Situp

Pressure( )

( )pw

5000 psi

Pore Throat

4500 psi


Section 3TOC

Wellbore Stability

Page - 15Section 3

The swelling pressure and behavior of shales are directly related to thetype and amount of clay minerals contained in the shale. Shales withhigh concentrations of negatively charged ions can produce very highswelling pressure (50,000 psi plus).

Swelling pressure decreases the strength of the shale by destroying thenatural cement bond between the clay platelets. Brittle shale becomesductile and is pushed into the wellbore by the compressive hoop stressand the swelling stress.

OVERBURDEN LOADOVERBURDEN LOAD

PoreWater

NaturalCement

ShaleClay

ClayPlatelets




ShaleWellbore(1 hour)

Adsorbedwater

SwellingStress

Wellbore(1 Day)


Section 3TOC

SECTION 4

Providing A Stable Wellbore

G

4.1 Planning A Stable Wellbore

4.2 Warning Signs /Corrective Action

StartStart TOCTOC

Wellbore Stability

Page - 1Section 4

.

4.0 PROVIDING A STABLE WELLBORE

4.1 Planning A Stable Wellbore

σσσ

1

2

3

= Greatest effective stress

= Intermediate effective stress

= Least effective stress

σ σv 1=

σ σh 3=

σ σH 2=

NormalFault

σ σv = 3

σ σH 1=

σ σh = 2

ReverseFault

1. Potential Stability Indicators

If the answer to any of the questions below is "yes", preventive measuresshould be taken.

σ σv 2=

σ σH 1=

σ σh 3=

Strike-slipFault

Indications of tectonic activity in the area?

Sudden pressure transition zones expected?o

Adverse formations expected (reactive shale, unconsolidated or fracturedformations, abnormal or subnormally pressured zones, plastic formations?

Is wellbore inclination greater than 30 ?o

3. Determine Magnitude of In Situ Condition (s s s )

Obtained from density logs of offset wells.

Estimated by seismic and logs.

Determined by LOT and/or logs.

v , h , H

Overburden - s

Formation Pore Pressure -

Minimum Horizontal Stress - s

v

h

p

2. Identify Stress Regime


Section 4TOC

Wellbore Stability

Page - 2Section 4

..

5. Research Offset Wells for Indications of Stability Problems

Offset well data is invaluable information for identification of stabilityproblems in the field.

Identify hole sections with stability symptoms.List the conditions that caused the stability problem.Identify similar problems in offset wells occurring at the same verticaldepth. Look for similarity in the conditions that caused the problem.List the drilling parameters effecting the problem (i.e., mud type andweight, hole angle, adverse formations, unusual drilling practices).

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CHEMICAL(Hole Enlargement /Hole Cleaning)

CHEMICAL(Hole Enlargement /Hole Cleaning)

MECHANICAL

Reactive ShaleReactive Shale

PLASTICDEFORMATION

(Tight Hole /Casing Collapse)

PLASTICDEFORMATION

(Tight Hole /Casing Collapse)

Mobile Salt

Mobile Shale

Mobile Salt

Mobile Shale

TENSILEFAILURE

(Lost Circulation)

TENSILEFAILURE

(Lost Circulation)

ExcessiveWellborePressure

ExcessiveWellborePressure

COMPRESSIONALFAILURE

(Hole Caving /Collapse)

COMPRESSIONALFAILURE

(Hole Caving /Collapse)

Overburden Stress

Tectonic Stress

Geo-Pressured Shale

Unconsolidated Formation

Fractured Formation

Overburden Stress

Tectonic Stress

Geo-Pressured Shale

Unconsolidated Formation

Fractured Formation

WELLBORE STABILITY PROBLEMSWELLBORE STABILITY PROBLEMS

4. Use Core Tests or Logs to Determine Formation Rock Strength

Core Tests

ShearStress

StabilityEnvelope

Unstableor Failure

S0


Logs

Rock strength is estimated through correlations with sonic density logssince slow sonic velocity and high porosity generally relate to lower rockstrength.


Section 4TOC

Wellbore Stability

Page - 3Section 4

4.2 Warning Signs and Corrective Action

7. Avoiding Stability Problems

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Select an inhibitive mud for reactive formations.Casing points should allow for mud weight windows determinedfrom stability analysis.Maintain mud weight/ECD in stability window. Use down holeECD monitoring tools in critical wells.Optimize well trajectory based on drilling days vs. stability.Plan for effective hole cleaning and stuck pipe prevention.Follow defensive drilling practices. Control ROP, surge pressures.Train drilling team members..

No single action can prevent stability problems. Wellbore stability must bemanaged by the controllable parameters.

Mud type, composition and density.Drilling practices (minimize ECD, swab /surge pressures).Wellbore angle and direction.

Chemical stability problems occur when reactive shales are drilled with anon-inhibitive drilling fluid. Chemical stability is time dependent anddifficult to quantify. The drilling fluid interaction results in shale hydrationand swelling which leads to shale falling into the wellbore causing holeenlargement and tight hole conditions.

BHA balling and slow drilling, flow line plugging, soft mushycuttings on shaker.Smooth increases in torque/dragOverpull off slips, pump pressure increasing.Increases in mud parameters (mud weight, plastic viscosity, yieldpoint, cation exchange capacity (CEC), and low gravity solids).

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Chemical Stability

Warning Signs of Chemical Stability Problems

6. Select Mud System and Determine Mud Weight Window

Stability spreadsheets and analysis tools are used to determine the mudweight window for each hole section.


Section 4TOC

Wellbore Stability

Page - 4Section 4

Preventive /Corrective Measures

Chemical stability problems are prevented by selecting proper mud type andcomposition. Initial corrective measures are to use suitable mud additives. Ifthe problem persists, replacing the existing mud with a more inhibitive mudmay be necessary.

Addition of various salts (K, Na, Ca) to balance water activity.Addition of glycol to reduce chemical attraction of water to shale.Addition of various "coating" polymers (PHPA, etc.) to reducewater contact with shale.Use of oil base or synthetic oil base mud to exclude water contactand entry into shale.Minimize the open hole exposure time.Plan regular wiper trips.Minimize surge/swab pressures.Ensure adequate hydraulics for bit and hole cleaning.Maintain required mud properties.Use minimum mud weight, if possible.

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Mechanical Stability

Mechanical instability is related to incorrect mud weight /ECD and/orwell trajectory. Too low mud weight can cause hole cavings or collapseresulting in stuck pipe. Too high mud weight /ECD can cause excessivefluid losses to the formation or total loss of returns.

Large size and volume of cavings over shakers.Erratic increase in torque/drag.Hole fill on connections or trips.Stuck pipe by hole pack-off /bridging.Restricted circulation /increases in pump pressure.Loss of circulation.Loss/gain due to ballooning shales.

Two indicators of mechanical stability problems are loss of circulationand increased volume of cavings. Partial or total loss of circulation maybe due to pressure induced or naturally existing fractures. The reducedhydrostatic associated with loss of circulation may cause formation cavingand collapse.

Warning Signs of Mechanical Stability Problems

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Section 4TOC

Wellbore Stability

Page - 5Section 4

Preventing Mechanical Stability Problems

The constraints on wellbore pressure are dictated by formation pressure onthe low end and fracture strength on the high end. Hydraulics planningmust also consider minimizing the shock load imposed to the wellbore.

Measures to prevent/correct mechanical stability problems include:

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Increase the mud weight (if possible). The mud weight valuesshould be determined using a stability analysis model and pastexperience if drilling in a known field.If drilling fractured formations, it is not recommended to increaseMW. Increase the low end rheology (< 3 RPM Fann reading).Improve hole cleaning measures. Maintain 3-rpm Fann readinggreater than 10. GPM for high-angle wells equal to 60 times thehole diameter in inches and half this value for hole angle of less

than 35 .Circulate on each connection. Use back reaming and wiper tripsonly if hole conditions dictate.Minimize surge/swab pressures.Monitor torque/drag and the size and amount of cuttings onshakers.

0

PorePress

PorePressShear

EnvelopeCollapse Caving

Partial TotalLoss Loss

HydrostaticPressureHydrostaticPressure

Break Circ

Swab

Press

Surge

Press

Circ

Press

Solids

Loading

TENSILE FAILURETENSILE FAILURESTABLESHEAR FAILURESHEAR FAILURE

Wellbore Pressure Shock

FracPressFracPress

Wel

lD

epth


Section 4TOC

Wellbore Stability

Page - 6Section 4

Controlling Stability Problems

The entire rig team is responsible for detecting stability problems. Oncedetected, there are many controls to consider that can provide for a stablewellbore. The drilling supervisor, with input from rig team members mustbe aware of the parameters that restore the balance between rock stress androck strength.

Formation

Mud


Tectonics

ShearFailure

PorePressure

STABLE

UNSTABLE


SandMWHigh

MWLow Reaming

TripSpeed

ECD MobileSalt

Shale StrikeSlip

TensileFailure

HoleCleaning

ReverseFault

TimeExposed

- Wellbore Stability -Maintaining the Balance of

Rock Stress and Rock Strength

The drilling team must recognize the warning signs of an unstablewellbore and adjust the drilling program accordingly to thebalance of rock stress and rock strength.

maintain


Section 4TOC

APPENDIX

A - 1 Leak-Off Tests

A - 2 Lithology Factor (k)

A - 3 Wellbore Stress Equations

A - 4 Nomenclature

τmax

σ1

σ2

StartStart TOCTOC

Wellbore Stability

Appendix A-1

LOT data is necessary to determine the maximum mud weight for wellcontrol and hole stability and has a direct influence on casing design. LOTfield data is also helpful for planning future field drilling and productionoperations because it measures the minimum horizontal stress (s ). Theminimum horizontal stress is important for wellbore stability analysis.

Consistency in LOT procedure, accuracy in reading test pressures andproper data reporting all have a direct impact on the quality of thisinformation. Refer to document F96-P-24, Standardization of Leak-off TestProcedure for more detail.

Preparation is a key factor in achieving good quality LOT data. Beforetesting begins:

Check offset well leak-off data for expected leak-off test pressure, pumprates, test problems or any unusual conditions.

Check logs for exposed sands to anticipate straight or curved linepressure plot.

Check for hole washouts to anticipate problems with the cement job.

Perform a casing integrity test (CIT). Test pressure at any point not toexceed 80% of casing burst.

Construct a LOT chart.

1. Drill out the shoe, rathole and 10 to 15 feet of new hole.

2. Circulate the hole clean and condition the mud to a consistent density.

3. Pull the drillstring +/-10 feet above the shoe.

4. Rig up the cement pump on the drillstring and pressure test system.

5. Close the annular BOP and begin the leak-off test.

6. Maintain a constant pump rate during test (1/4 to 1 bbl/min maximum).

7. Plot the pressure every 1/4 barrel pumped.

h

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LOT Procedure

A - 1 Leak-off Tests

StartStart TOCTOC AppendixTOC

AppendixTOC

Wellbore Stability

Appendix A-1

10 to 15Feet

Testpsi

The initial volume pumpedresults in fluid compressionand expansion of thewellbore. After this initialphase, pressure increaseslinearly with barrels pumped.

Pres

sure

LinearIncrease

Shut-in Time(Minutes)

0

0 10 20

1 2 3 4

FluidCompression

Stop Pump

Initial Shut-in Pressure (ISIP)

Min. Horizontal Stress (s )h

Record every min for 20 minutesor until pressure stabilizes

Leak off

Barrels

Leak-off pressure (LOP)

initial shut inpressure (ISIP)

minimum horizontal stress (s )

is the first point where there is a permanentdecrease in the slope (usually equal to or greater than the minimum stresspressure). When the pump is stopped, pressure falls to the

due to the loss of friction pressure.

The is the first point after a permanent

decrease in the slope (usually equal to or less than LOP). Retest to confirmminimum stress measurement.

As the formation is either fractured naturally or fractured during the drillingoperation, leak-off test pressure should range between 1 to 1.1 times theminimum horizontal stress.

h


AppendixTOC

Wellbore Stability

Appendix A-2

k =s -h p

s -v pPr

essu

re

Shut-in Time(Minutes)

0

0 10 20

1 2 3 4

Stop Pump

Initial Shut-in Pressure (ISIP)

Min HorizontalStress - sh

Leak off

Barrels

The Lithology Factor ( ) Calculated from LOT Datak

A - 2 Lithology Factors

Using Poisson's Ratio ( to Calculate the Lithology Factorν)

k =ν

1 - ν

Clay .17very wet .50

Conglomerate .2

Dolomite .21

Limestone:fine, medium .28medium, calcarenitic .31porous .20stylolitic .27fossiliferous .09bedded fossils .17shaley .17

Sandstone:coarse .05coarse, cemented .10fine .03medium .06poorly sorted, clayey .24fossiliferous .01

Shale:calcereous .14dolomitic .28siliceous .12silty .17sandy .12kerogenaceous .25

Siltstone .08

Slate .13

From, Weurker H. G.:"Annotated Tables of Strength and Elastic Properties of Rocks", Drilling, reprint Series SPEDallas (1963)

Poisson's Ratio Poisson's Ratio


AppendixTOC

Wellbore Stability

Appendix A-3

A - 3 Wellbore Stress Equations

Stress transformation from global to wellbore coordinates:

Where is the horizontal angle (azimuth) between s and the wellbore

and is the wellbore inclination.

λα

h

For equal horizontal stresses ( s = s )h H

and for a vertical well with = = 0:λ α

Effective radial, hoop, and axial stresses at the wellbore wall:

s = ( s cos + s sin ) cos + s sinx h H v

2 2 2 2λ λ α α

s = ( s cos + s sin ) sin + s cosz h H v

2 2 2 2λ λ α α

s = s sin + s cosy h H

2 2λ λ

s = sin cos cos (s - s )xy H hλ λ α

s = sin cos sin (s - s )yz H hλ λ α

s = sin cos ( s cos + s sin - s )xz h H vα α λ λ2 2

(1)

(2)

(3)

(4)

(5)

(6)

s = s cos + s sinx h v

2 2λ α

s = s sin + s cosz h v

2 2α α

s = sy h s = sin cos ( s - s )xz h vα α

s = s = 0xy yz(7)

(8)

(9)

(10)

(11)

s = sx h s = sy h s = sz v s = s = s = 0xy yz xz(12) (13) (15)(14)

σr = -p pw

σ θ θθ = ( s + s ) - 2 ( s - s ) cos 2 - 4 s sin 2 -x y x y xy p - pw

σ ν θ θz z x y xy= s - ( 2 ( s - s ) cos 2 + 4 s sin 2 ) - p

σ σr rzθ = = 0

σ θ θθz yz xz= 2 ( s cos - s sin ) - p

(16)

(17)

(18)

(19)

(20)


AppendixTOC

Wellbore Stability

Appendix A- 4

A - 4 Nomenclature

pp

k

t

formation pore pressurewellbore pressure (hydrostatic/ECD)

s total overburden stress

s minimum horizontal stress

s maximum horizontal stress

effective overburden stress

effective minimum horizontal stress

effective maximum horizontal stress

lithology factor

Poisson's ratio

tectonic stress

angle of internal friction

coefficient of friction

S cohesive strength

effective radial stress

effective hoop stress

effective axial stress

greatest effective stress

intermediate effective stress

least effective stress

w

v

h

H

v

h

H

0

r

z

1

2

3

σσσ

ν

φµ

σσσ

σσσ

θ


AppendixTOC

amoco - wellbore stability

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