Amoco - Wellbore Stability

Lost CirculationPoor Hole Cleaning Hole Caving /Collapse WELLBORE STABILITY WELLBORE STABILITY Formation Mud Tensile Tectonics Shear Failure Pore Pressure... MW Reaming Trip Speed Salt S

Lost Circulation

Poor Hole Cleaning

Hole Caving /Collapse

WELLBORE STABILITY

WELLBORE STABILITY

Formation

Mud Tensile

Tectonics Shear

Failure

Pore Pressure

1.0 INTRODUCTION

2.0 BEFORE THE WELLBORE

3.0 AFTER THE WELLBORE

4.0 PROVIDING A STABLE WELLBORE

APPENDIX

4.2 Warning Signs/Corrective Actions 3

A-1 Leak-off Tests

A-2 Lithology Factor (k)

A-3 Wellbore Stress Equations

A-4 Nomenclature

In Situ

In Situ

CONTENTS

MW Reaming

Trip Speed

Salt Shale

Strike Slip

Tensile Failure Hole

Time

Exposed

Wellbore Stability

-Maintaining the Balance of

Rock Stress and Rock Strength

1.1 Wellbore Stability Mission

1.2 Drilling Handbook Objectives

1.0 INTRODUCTION

Wellbore stability

600 million to 1 billion dollars

is the prevention of brittle failure or plastic deformation

of 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 balanced

or 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 squeezing mobile formations produce tight hole problems and stuck pipe.Cavings from failing formation makes hole cleaning more difficult andincreases mud and cementing costs

Hole-Estimated cost to the drilling industry for hole stability problems range

Stuck Pipe

Hole Problems Loss Of

Circulation Well

Control

Relative Costs Of Unscheduled Events

Caused By Wellbore Stability Problems

1.1 Wellbore Stability Mission

The mission of the Wellbore Stability Team is twofold.

Minimize the "learning

curve" when developing

new reservoirs so that

optimal well costs are

obtained early on.

Identify potential drilling problems during the well planning stage so that prevention and operational planning can be developed

to minimize costs associated with wellbore stability problems.

Tensile Shear

Fractures

Loss of Circulation

Cavings Tight Hole Stuck Pipe

Geopressured Hydro-Pressured Unconsolidated Fractured Tectonics

Failure Mechanisms

Wellbore Stability Problems

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

Poor Drilling Practices

Poor

W ell Plan

Reactive shale

Reactive shale

ExcessiveWellborePressure

ExcessiveWellborePressureExcessive RockStress

Excessive RockStress

Hole

Cleaning Hole Cleaning

Hole Enlar gement

Hole Enlar gement

Hole Collapse

Hole Collapse

Well Control

Lost Circulation

Poor logs

Cementing Problems Cementing Problems

Identify and define wellbore stability problems

Suggest consistent terminology

Associate warning signs with stability problem

Suggest corrective actions

Provide the background for preventive planning

Before The Wellbore

2 0 BEFORE THE WELLBORE

2.1 In Situ Conditions

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

Shale

Fluid Filled

Pores

Rock Matrix

0 5 10 15 20 25

Porosity (%)

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 fluid

connection 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

0 5 10 15 20 25

Sandstone Shale

8,000'

3720 psi

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

is less than normal for the vertical depth

of 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 depth

Sub normalPressure

Normal TrendLine

Formation 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 abnormalpore pressure

∆c

STRESS OVER

BURDEN

STRESS

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 depths

somewhat 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

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

Overburden Stress v

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, the

horizontal stresses will differ and are described as a minimum (s ) and amaximum (s )

The minimum horizontal stress (s ) is normally determined from leak-offtests It is difficult to determine the maximum horizontal stress from fieldmeasurements Its value can be estimated using rock mechanics

equations

Horizontal Stress - s sh , H

h H

is 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 car

tire supports the weight of the

car, fluid pressure in the pore

supports a portion of the

overburden load

The remaining portion of

over-burden stress is the load

effectively stressing the rock

matrix

ROCK MATRIX

ROCK MATRIX

5000 PSI Pore Pressure

EffectiveOBSEffectiveOBS 40004000psipsi

9000 PSIOVERBURDEN9000 PSIOVERBURDEN

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

k k

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, theeffective horizontal stresses are described by a maximum and minimumvalue

In extreme tectonic environments, may be sufficient to make the

horizontal stress higher than the vertical stress

fluids like water

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

1000

1

Axial Load

Elastic Limit

Elastic Deformation

Plastic Deformation

Ultimate Failure Ultimate Strength

Strain (% of Deformation)

0 10 20 30 40 50

Axial

Load

(Compressive

Stress) ConfiningPressure

Axial Load (psi)

Confining Pressure

Shear Plane ShearFailure

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 shear stress and possible shear failure.

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 Strength Bonded Grains (Cement)

Horizontal

Stress (s ) h

Horizontal Stress (s ) H

Mean Effective Stress = σv+σh+σH

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 ( ,σ σv h , orσH )

Greatest Effective Stress ( ,σ σv h , orσH ) Least Effective

Stress ( ,σ σv h , orσH )

(Intermediate stress acts perpendicular to the figure)

Failure Shear Stress From Test 1

Failure Shear Stress From Test 2 & 3

< = φ

< = φ

Stress-State 1 Stress-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

= tanThe cohesive strength (S ) and the angle of internal friction ( ) areobtained from conducting compression tests on core samples (or

estimated from logs) from the field Several tests on cores are necessary

to determine these values

The shaded area shown below indicates the "stress-state" of one suchcore sample at failure The compression stress ( ) that fails the coresample (greatest stress) is plotted on the horzontal axis along with theconfining pressure ( ) used for that test (least stress)

shear strength

angle of internal friction ( )φ

φ

σσ

0

f

c

The shear stress that fails the rock must overcome the

(bonding together of the grains), and the frictional resistance betweenthe grains ( The frictional resistance between the grains is the product

of the and the effective compressive stress ( )

cohesive strength, S

The , the

necessary to fail the sample Several tests at increasing confining

pressures produce successive stress-states of increasing shear strength.The " " is approximated by the line giving the best fit

to 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 consideringthe 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

σ σ σ0

v h H

Shear

Stress,τ

Stability Envelope

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 (tensile

stress) Rocks exhibit very low tensile strength

Tensile Stress

Tensile Stress Exceeds the Tensile Strength and the Rock Fails

After The Wellbore

3.1 Near Wellbore Stress-State

3.2 Mechanical Stability

3.3 Chemical Stability

BURDEN

STRESS OVER

BURDEN

STRESS

STRESSES

STRESSES HORIZONT

STATIC

PRESSURE HYDRO

STATIC

PRESSURE

Radial Stress - σ r

Radial Stress - σ r

Axial Stress - σ z

Axial Stress - σ z

These stresses are perpendicular to each other and for mathematical

convenience, are used as a borehole coordinate system

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

Hoop Stress - σθ

Hoop stress is dependent upon wellbore pressure ( ), stressmagnitude and orientation, pore pressure, and hole inclination and

direction 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

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 Hole Additional Components

Of Stress From Overburden

And Horizontal Stresses

Minimum Hoop Stress

Maximum Hoop Stress

= [ & well parameters]

-For a vertical well with equal horizontal stress (s = s ), axial and verticalstress are the same Axial stress in a deviated well is the resolution of theoverburden and horizontal stresses

Axial Stress is The Resolution of Overburden and Horizontal

Stresses

Deviated Well Equal or Unequal Horizontal

-Radial Stress = Wellbore Pressure - Pore Pressure

σr p pw

3.2 Mechanical Stability

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

stresses 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

Mechanical stability management

Axial -σzHoop -σθ

Radial -σr

Hoop

Radial

Shear Stress

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 least

of 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

ShearStrength

Line

Stability Envelope Failure

Least

Stress

Greatest Stress

Shear

Stress

Effective Compressive Stress Stress-State