This presentation prepared by Amoco discusses wellbore stability issue

1. Wellbore Stability-Amoco

2. Objectives
❖Identify and define wellbore stability problems.
❖Suggestconsistentterminology.
❖Associate warning signs with stability problem.
❖Suggestcorrective actions.
❖Provide the background for preventive planning.

3. Minimize wellbore instability(avoiding stuck pipe, tight hole, lost circulation,
sidetracksand well collapse)
..Optimize mud weights and drillingpractices
..Minimize casing strings
..Optimize wellbore trajectory
..Optimize surface location
..Optimize salt exit location to minimizeriskof lost circulation
.. Minimize hole collapserisk due to salt creep
Objectives

4. Wellbore Stability
Maintainingthe Balance of Rock Stress and Rock Strength.
Prevention of failure/plasticdeformationof the rock in the vicinity of the wellbore due
to mechanical stress or chemical imbalance.
Post drilling
Mechanical stressesin the formation< rock strength.
Chemical action balanced/ at a rate relative to geologic time.
Rocksstable.
Upon Drilling
Rock surroundingthe wellbore sufferstress changes (tension, compression,and shear)
Chemical reactions occur ( contact with drillingfluid).
rock surroundingthe wellboremay become unstable (deform, fracture, and cave-in ).

5. Wellbore Stability Mission
Identify potential drilling problems in well planning stage to wellbore instability .
• Reduce NPT
• Deduct costs
• Reduce risk
➢ During Exploration
❖ Reduces exploration risk with Fault Leakage Analysis
➢ During Drilling
❖ –Provides more accurate Safe Operating Mud Window
❖ Reduces kicksand lost circulation
❖ Improves wellborestability
❖ Reduces stuck pipe, sidetracks, washingand reaming
❖ Reveals feasibilityof UnderbalancedDrilling
➢ During Production
❖ Improves production fromNatural Fractures
❖ Predicts and manages Sand Production
❖ OptimizesHydraulic Fracturing operation
❖ Reduces CasingShear and Collapse

6. Wellbore Instability
Chemical Instability Mechanical Instability
Reactive Shale • Overburden Stressed
• Geopressure
• Hydro-Pressured
• Unconsolidated
• Fractured
• Tectonics
Failure Mechanisms
Tensile Shear
•Fractures
•Loss of Circulation
•Carvings
•Tight Hole
•Stuck Pipe

7. Wellbore Instability
Chemical Instability
Mechanical Instability
Reactive Shale
• Overburden Stressed
• Geopressure
• Hydro-Pressured
• Unconsolidated
• Fractured
• Tectonics
Compressional
Failure
Tensile
Failure
Plastic
Deformation
•Lost Circulation
•Fracture
•Cavings
•Collapse
•Tight Hole
•Casingcollapse
•Excessive
wellbore
pressure
• Mobile Salt
• Mobile Shale
•Washout
•Hole cleaning

8. Effect of Drilling Direction

9. Geomechanical Events

10. Mud Window

11. Geomechanical Model Components
Sv = Vertical stress
SHmax = Maximum horizontalstress
Shmin = Minimumhorizontal stress
SHmaxAzi = Stress direction
Pp = Pore pressure
UCS = Rock strength
Pp
UCS

12. Geomechanical Model Components Source
Rock Strength (UCS) : Core tests,
logs, cuttings, wellborefailure
analysis
Min HorizontalStress (Shmin)
LOT, XLOT, minifracs, losses
SHmax Orientation Wellbore
failureanalysis (Using image
and/or caliper log), fault analysis,
world stressmap :
Overburden Stress (Sv) :
density and/or sonic logs
Pore Pressure(Pp): MDT/RFT/PWD,
drilling incidents,
sonic/resistivity/density logs &
seismic
Max HorizontalStress (SHmax)
Wellbore failureanalysis (image
and/or caliper log), laboratory
measurements,sonic scanner

13. Causes & Consequences
CAUSES OF STABILITY PROBLEMS
➢ Inadequate well planning
➢ Problematicformation
➢ Inadequate operational practices
➢ Inadequate mud weight
➢ Inadequate well trajectory
STABILITY PROLEMS
Excessive wellborepressure
Excessive Rockstrength
Reactive shale
RESULTING CONDITIONS
Hole cleaning
Hole washout
HOLE COLLAPSE
Stuck pipe
Drillstingfatigue
Cementing problems
Loss circulation
Poor logs
Well control

14. Understand conditions causing stability problems
➢ Effective planning.
➢ Detectionof warning signs.
➢ Contingency plans

15. Post Drilling
I. In situ conditions
II. In-situ stress
III. Effective stress
IV. Rock Strength
Overburden Stress
Horizontal Stress

16. I. In Situ Conditions
➢Sedimentaryrocks have porosity.
• As porosity increases,pore fluid volume increases and rock matrix volume
decreasesweakening the rock.
• Porosity change with depth due to compaction and cementation.
➢Permeabilityweaken the rock (water base mud filtrate attacks grain-to-grain
cement bond).
➢Hydrostatic/dynamicoverbalance forces mud filtrate into pores weakeningthe
rock.
➢FormationPore Pressure
• If increase in overburden load does not exceed pore
fluid drainage rate, pore pressureis equal to the
hydrostaticpressureof formationwater (normal
pressure).
• If pore fluid cannot escape, pore pressureincreases
at a faster-than-normal rate(abnormal pressure).
• Pore pressureof a permeable formationcan lower
than normal pressure(subnormalpressure).

17. II. In Situ Stress
Undisturbedsubsurfacerocksin a balanced or near balanced stressconditions.
Stress in place called In-situ stress.
Stress is normally compressivedue to weight of the overburden .

18. Overburden Stress Sv
Overburden stress:-pressureexerted on a formationat a given depth due to the total
weight of the rocksand fluids above that depth.

19. Horizontal Stress – Sh & SH
In most drilling areas, the horizontalstressesare equal.
Drilling near massive structurese.g. tectonic areas, the horizontalstressesdiffer:
❑Minimum (sh) and
❑Maximum (SH).

20. Law of Effective Stress (Terzaghi)
Effective stress: σΝ’ = (σN – αPf )
• Holds for all normal stresses
• σS’ = σS (shear stressesnot affected)
• α Value depends on pore fluid saturation and compressibility.
• For a rock fully saturated with water: α = 1
• Pressurizedpore fluids acts to weaken rocks.

21. III. Effective Stress
➢Rock matrix supports part of overburden and horizontalstress.
➢The other part is supported by pore fluid (pore pressure).
➢Effective stress:Resultantstress on rock matrix.
➢Effective stressdetermine wellbore’sstability.
Effective Overburden Stress - σv
Overburden stressthat stressesrock matrix.
Effective Overburden Stress(σv)= Total OverburdenStress - Pore Pressure

22. Effective Horizontal Stress - σh, σH
Horizontalstressesare equal and the effective horizontalstressis equal to
the effective overburden stresstimes a lithology factor .
The lithology factor (k ) is less than 1 for more rigid material (rocks).
σh = σH = k x σv
In tectonically active areas, the horizontalstressesare not equal.
The maximumhorizontalstresseswill be higher, or lower depending on tectonic
movements, by the additional tectonic th and tH stresses,In these areas, the effective
horizontalstressesare described by a maximumand minimumvalue.
σh = k x σv + th
σH = k x σv + tH
Noncompressible
fluids like water
have a k factor of 1.
Stiffer materials
like putty have a
lower k factor (.7 -
.9 for example.)
Very stiff materials
like formation rock
have a much lower k
factor (.37 is common
for shale.)

23. Anisotropy
Homogeneous Isotropic Formations:Propertiesthe
same in all directions.
AnisotropicFormations:Propertiesvary
depending on direction.
TIV Media: TransverselyIsotropic with a vertical
axis of symmetry.

24. Stress, Strain and Rock deformation
➢Weight of overlying rock strata: --> LITHOSTATIC
➢Motions of lithosphericplates (convection in the mantle) --> TECTONIC
➢Pressureexerted by pore fluids --> HYDROSTATIC

25. IV. Rock Strength
Rock mechanics: study of the mechanical behavior of subsurfacerocks.
Core samples:tested in compression,subjected to a confiningpressure(stress ), with
specializedlaboratory equipment.
Strain: Rocksrespond to the stressby changing in volume or form (deformation)or
both.
Stages of strain deformation(compressive(+) or tensile (-) stress):
Elastic deformation:rock deformsas stress is applied but returns to its original shape
as stressis relieved; strain is proportionalto the stress (Hooke's Law).
Plastic deformation:When applied stress reaches the elastic limit, the rock
only partially returns to its original shape as stressis relieved.
Ultimate failure:If continued stressis applied, fractures develop and the
rock fails.
Rockscan fail in:
• Brittle manner (under low confining stress).
• Ductile manner (under higher confining stress).

26. Failure Prediction
Failure Criterion: predictwhether rockswill fail or remain intact under any given state of
stress:
• Consider the Criterionfor Frictional Sliding
• Consider the case where a fracture plane (i.e. a fault) already exists in the rock
• Sliding will occur when σs on the plane overcomesthe frictional resistance
• Analogous to the classical problemof a block sliding on an inclined plane
A: Contact area between block and plane
Θ: Critical angle of for which the block will just slide= angle of friction (φ),
Friction Coefficient (μ) ( independent of surface contact areaANDthe normal
force)
Amontons’Law

27. Amontons’ Law represented graphically on a Mohr diagram
Mohr circle is a tangent to the critical sliding line (Amontons’Law)
• Frictional slidingwill occur on the fault in the optimum orientation θ to the σ1
direction
• Sliding will NOT occur on faults with any other orientation
• But in general the fault will NOT necessarilybe at the optimum angle!

28. Shear Strength and Shear Failure
Under compressionrocksactually fail in shear (slide rock grains past each other ).
➢Confining pressureresistsslidingon the shear plane and the rock appears stronger.
➢Confining pressure= axial load are equal: no shear stress on rock (no shear failure).
➢ Equal stressespromote stability.
➢ Unequal stressespromote shear stress(possible shear failure).
Overburden
Stress (s )
Horizontal
Stress (Sh)
Cohesive Strength
Bonded Grains (Cement)
Increased Pore Pressure reduces the Effective Stress

29. Effect of Pore Fluid Pressure on Rock Deformation
• Rocks are generally saturated with fluid:
- mostlywater or an aqueous solution
- occasionallyhydrocarbons (oiland/or gas)
• Applied stressesattempt to close up the pore space in rock
• Fluid in the pore space resiststhe closure
• Hence a pressureis built up in the fluid : Pore Fluid Pressure(Pf)
• Pf (hydrostaticpressure)acts normal to the surface of the grains
• Results to reduce effect of the external applied stress

30. Formation Strength Evaluation
Parameters:
• Uniaxial CompressiveStrength, UCS
• Tick-Wall Cylinder Strength
• Friction Angle
• Cohesion
• Young’s Modulus
• Poisson’sRatio
Sources:
• Rock Mechanics Tests
• Petrophysicallogs
• Cuttings analysis

31. Dynamic Elastic Constants
Required logs RHOB, DTS, DTCO

32. UCS Models
UCS: Uniaxial compressive strength[MPa]
Dt: Compressional sonic transit time [ms/ft]
E: Young’s modulus [GPa]
: Porosity [fraction]
Sandstone
Shale
Carbonates

33. Rock Strength Histograms
Shale Interval
Sand Interval

34. OVERBURDEN STRESS
• Density Logs
• Calculatedpseudo-density from
sonic/seismic (Gardner’s Method: = aVb)
• Regional empiricalrelations
• Cuttings, Cavings, GeologicalReports
• Core densities from whole core or sidewall
coring

35. PORE PRESSURE PREDICTION
Pore pressure or formation pressure is defined as the pressure acting on the fluids in the
pore space of a formation.
Using Overpressure Indicators
• Field Measurement – DST, RFT, MDT
• Normal CompactionTrend-lineMethods
• Effective Stress Methods

36. Normal Compaction Trends
A normal compaction trend is an estimate of the log responsein a normally
pressuredenvironment.

37. Wireline/LWD Response to Overpressure

38. Ratio Method
The differencebetween observed values and the normaltrend-line extrapolated to the
same depth is proportionalto the increasein pressure.
For sonic logs =
For density logs =
For resistivitylogs =
Where : ΔTn = the value of the normal trend-line at a given depth, P = the pressurevalue
to be calculated, Phyd = normal hydrostaticpore pressure,ΔT log = log-value value for
each curve correspondingto the required pressurevalue.

39. Eaton Method
Calculates a pore pressurebased on the relationshipbetween the observed
parameter/normaltrend-line ratio and the overburdengradient
• for resistivity
Eatonexponent
• for sonic
Where : P = formation pressure, S = overburden, Rsh = resistivity of shale, DT = sonic
transit times, log = observed values of the log at the given depth, n = value of normal at
the given depth, hyd = normal hydrostaticpressure

40. Pore Pressure Prediction

41. STRESS ORIENTATION
(http://www-wsm.physik.unikarlsruhe.de/pub/home/index_noflash.html)

42. World Stress Map – Middle East

43. Near Wellbore Stresses
Induced Stresses
• Max hoop stress acts
in the direction of sh
• Min hoop stress acts
perpendicularto sh
direction
• Shear failure expected
along sh if stresses
exceeds rock
compressive strength

44. MAXIMUM HORIZONTAL STRESS MAGNITUDE

45. Failure models
Failure models:predict wellborestability.
Mohr-CoulombModel: uses three effective stressesto calculate the resultant shear
stress.
Mean Effective Stress = (σv + σh + σH )/3
Describes the stress state of the rock.
Mohr-Coulomb failuremodel neglects the intermediatestressand considersonly the
greatest and least effective stress.
The greatest shear stresson the rock :
• Occurs on the two-dimensionalplane consisting of the greatest and least stress.
• Could be any of the three depending on in situ and well conditions

46. Greatest Effective
Stress (σv , σh , or σH)
Least Effective
Stress (σv , σh , or σH )
Mohr-Coulombneglects the intermediate stressand considersonly the greatest and least
effective stresses.
Failure models
Shear Plane

47. Cohesive Strength
Shear stressthat fails the rock > cohesive strength So (bonding together of the grains),
and the frictional resistancebetween the grains (µσ)
Shear Stress = Cohesive Strength + Frictional Resistance
δ= So + µσ
δ : shear stressthat failsthe rock
So: Cohesive strength (bonding together of the grains),
µσ: Frictional resistancebetween the grains
µ: friction coefficient = tang θ
σ: effective compressivestress.
Θ: Angle of internal friction.

48. Criterionfor Shear Failure of Intact Rock: Coulomb
Failure Criterion
consider the situation of a rock mass without a pre-existing fracture (fault) plane.
• Factors which resistthe applied shear stress:
o Normal stressacting acrossthe potential shear plane[doesnot yet exist]
o Cohesive shear strength of the rock [the rock has cohesion because the grains
are bonded together]
• The failurecriterion:
• c = the cohesion, and φI is the angle of internal friction (since no real friction exists
because there is no fault plane until failure)
• Similar to that of Amontons’Law

49. Coulomb Failure Criterion representedona
Mohr diagram:
For most rocks:φI is about 30°
So: θ is also about 30 °
• Failure envelope separatesstable regions (no failure)from unstable regions ( failure).
• Orientation of fracture plane at failure: φI + θ = 90° or θ = (90° –φI)/2

50. Fracture and Failure Modes, pore pressure
Failure modes depend on the magnitude of the confiningpressure(Pc: Overburden
stressdue to depth):
a) Pc = 0 : long axial tension cracks.
b) Moderate Pc: a single shear fracture (fault).
c) Intermediate Pc: a pair of conjugate shear fractures.
d) High Pc: cataclastic flow and barrellingwith no shear fracture.

51. Fracture and Failure Modes, pore pressure
• Normal stressesare reduced by the same amount
• Mohr stresscircle thereforeremainsthe same size
• Circle moved to the left by an amount Pf
• Pf mayinduce failure in rock

52. Magnitude of Pore Fluid Pressure
Overburden (lithostatic) stressincreases with depth
Pc= ρR g h
If pore fluid is water and with interconnectedpore spaces:
Pf = ρw g h = hydrostatic
For rock density = 20.875 ppg, water density ~8.35 ppg
Pore fluid pressureincreaseswith depth at about 0.434 psi/ft
Ratio of pore fluid pressureto lithostatic stressis about 0.4
CAUTION :
Mechanical compaction, digenetic cementation, and expulsion of excess water from
metamorphicdehydration reactions can lead to increasein pore fluid pressureabove
hydrostaticvalues.

53. Limitations : Coulomb Failure Criterion
❖ Not good at predicting tensile failure.
❖ At high values of σ3 failure by ductile flow may occur.
❖ Ductile Flow Stress (DFS) is independent of pressure.

54. Mohr’s Circle.
Shear
Stress
δ
Effective Compressive Stress σ
θ
So
Confining pressure σc
Compressionpressurethat
fails core sample σf
Shear strength line:line giving the best fit to the maximum shear stresspoints
on the failureplane from several stress tests.

55. Mohr’s Circle.
Shear
Stress
δ
Effective Compressive Stress σ
Confining pressure σi Compression pressure that fails
core sample σf
So

56. Tensile Failure
Tensile Failure resultsfrom stressesthat tend to pull the rock apart (tensile stress).
Rocksexhibit very low tensile strength.
Tensile Stress
Failure: Tensile Stress
exceeds rock tensile
strength

57. Drilling The Wellbore
A. Near Wellbore Stress-State
B. Mechanical Stability
C. Chemical Stability
Wellbore Pressure
Horizontal Stress

58. A. Near Wellbore Stress-State
σr : Radial stressacting along the radius of the
wellbore.
σθ : Hoop(tangential) stress acting around the
circumferenceof the wellbore.
σz : Axial stressacting parallel to the well path.
Additional shear stresscomponents designated
by (σrθ , σrz, σθz ).

59. Hoop Stress - σθ
Hoop stressis dependent upon wellbore pressure( Pw), stressmagnitude and
orientation, pore pressure,and hole inclination and direction.
Wellbore pressure(Pw ) is directly related to mud weight/ECD.

60. Axial Stress - σz
Axial stress is oriented along the wellborepath and can be unequally distributed around
the wellbore.
Axial stress is dependent upon; stress magnitude and orientation, pore pressure,and
hole inclination and direction.
Axial stress is not directly affected by mud weight.

61. Radial Stress - σr
Radial stressis the differencein wellbore pressureand pore pressure and acts along
the radius of the wellbore.
Since wellbore and pore pressures bothstem from fluid pressure acting equally in all
directions, this pressuredifferenceis acting perpendicular to the wellbore wall, along
the hole radius.
= -
Radial Stress =Wellbore Pressure - Pore Pressure
Σr = Pw - P

62. Mechanical Stability
Mechanical stability: Management of the near wellbore stress-state(Hoop, radial, and
axial stress)to prevent shear or tensile rock failure.
Mechanical stability is achieved by controlling the parametersthat affect hoop, axial,
and radial stress.
Axial - σz
Radial - σr
Hoop - σθ
Radial - σr Radial - σr
Hoop - σθ
Hoop - σθ
Shear Stress

63. Mohr’s Circle.
Shear
Stress
δ
Effective Compressive Stress σ
Confining pressure σi Compression pressure that fails
core sample σf
So

64. Controllable & UncontrollableParameters
Controllableparameters
•MW/ECD
•Mud Filter Cake
•Well Path - Inclination and Azimuth
•Drilling/TrippingPractices
Uncontrollableparameters
•UnfavorableIn Situ Conditions
•Adverse Formations
•Constrained Wellbore Trajectory

65. Effect of Mud Weight/ECD
Shear
Stress
δ
Effective Compressive Stress σ
Radial Stress Hoop Stress
Increase in MW
Stress-State Before
MW Increase
Stress-State
After
MW Increase
So

66. Effect of Mud Weight/ECD
Shear
Stress
δ
Effective Compressive Stress σ
New Radial Stress
New Hoop Stress
Excessive Increase in MW
Stress-State Before
MW Increase
Stress-State
After
MW Increase
So

67. Effect of Mud Weight/ECD
Shear
Stress
δ
Effective Compressive Stress σ
Radial Stress Hoop Stress
Decrease in MW
Stress-State
Before
MW Decrease
Stress-State
After
MW decrease
So

68. Hole Inclination and Direction
Wellbore inclination and direction impacts wellborestability.
Increased Vertical Stress of the Overburden
Maximum
Hoop Stress
Maximum
Hoop Stress
Minimum
Hoop Stress
Minimum
Hoop Stress

69. Mud Filter Cake and Permeable Formations
Shear
Stress
δ
Effective Compressive Stress σ
Radial Stress σr=0 Hoop Stress δθ
Stress-State
With Good Filter Cake
So

70. Bottom-hole Temperature
Mud cooler than the formation
▪ Reduces the hoop stress as the formationis cooled.
▪ Reduction in hoop stresscan prevent shear failureand stabilize the hole, if the
hoop stresswere high due to low mud weight.
▪ Mud weight is too high and close to the fracture gradient, excessive cooling can
lower the hoop stressand make it tensile, causing tensile failure or fracturing as it
effectively lowers the fracture gradient.
Mud hotter than the formation
Increase in Hoop stress: promotesspalling or shear failure.

71. Variationsin Hoop Stress in a High TemperatureWell
Shear
Stress
δ
Effective Compressive Stress σ
Decrease in Hoop Stress While Circulating Bottoms-Up
Hoop Stress Prior to Trip Increased Hoop Stress While POOH
Changes in Shear Stress on Formation
So
Radial Stress

72. Impact of Mechanical Stability on the Wellbore
Deformationsof the vicinity of a Wellbore occur when in situ rock stressare
redistributed.
Original
Hole Size
Cavity
Encroachment
Shear Failure
Zone (Breakouts)

73. Resulting Operational Problems
➢Stuck pipe, casing, logging tools, etc.
➢Ineffective hole cleaning.
➢Ledges and breakouts.
➢High torque and severe slip-stick.
➢Drillstring failures.

74. Chemical Stability
Chemical stability: control of the drillingfluid/rock interaction; usually most
problematicwhen drilling shales.
➢ Shales: fine grain sedimentaryrocks having very low permeability and composed
primarilyof clay minerals(gumbo to shaly siltstone).
➢ Clay platelets 2 micronsand less.
➢ Settling to the mud line; 60 - 90% porosity.
➢ Clay platelets maintain a water envelope after burial.
➢ Mud readily deforms.
➢ Compaction drains pore water back to the sea.
➢ Platelets begin to contact forming pliable clay.
➢ Further compaction, geologic time, and temperature cements the clay platelets
into shale (less than 20% porosity).
➢ Shale: sensitivity to the water (WBM filtrate).
➢ Shale-water interaction decreasesshale strength ( instable/failure)

75. Advection
Advection: transport of fluid through shale due to pressuredifferential.
➢ Wellbore hydrostaticpressure> formationfluid pressure.
➢ Permeableformation:pressuredifferential“pushes” Mud filtrate into rock
pores.
➢ Highly permeable rock:Mud filtrate flow rate form a filter cake that controls
fluid loss.
➢ Shales: filter cake cannot develop due to very low permeabilityand mud
solids are large to plug off shale pore throats.

76. Capillary Effects
➢Drilling fluid must overcomecapillary pressureto enter the pore throats of shale.
➢Capillary pressure,developed at the drillingfluid /pore fluid interface, dependents
On:
• pore throat radius,
• Interfacial tension, and
• contact angle.
➢Water-wet Shale drilling with WBM: surfacetension between mud filtrate and the
pore fluid is very low.
•Under favorable salinity conditions, mud filtrate invades pore throat.
➢When drillingwater-wet shales with OBM, capillary pressureis very high due to the
large interfacial tension and extremely small pore throat radius.
•High capillarypressureprevents entry of the oil phase (overbalance pressures
low).
•If salinity of the OBM's water phase is not compatible with shale salinity, water
invades by osmosis.

77. Osmosis
➢ Osmosiscaused by chemical imbalance of salt concentration between the mud's
water phase and the pore fluid.
➢ Salinity imbalance separated by shale, acting as a semi-permeablemembrane that
allows the transport of water only.
➢ Water moves from low salinityto high salinity until the salinity difference(chemical
activity) is balanced.
❖ Mud salinity is too low: water moves into the increasing shale pore pressure,
leading to adverse effecton stability.
❖ Mud salinity is too high: shale pore water flows into mud systemdehydrating
the shale.
o As shale pore pressuredecreases, effective hoop stressincreases,
promoting shear failure.

78. Pressure Diffusion
Pressurediffusion:change in near-wellborepore pressurerelative to time.
➢ Occurs as overbalance and osmotic pressuresdrive the pressurefront through
the pore throat, increasingpore fluid pressureawayfrom the wall of the hole.
➢ Pore pressurepenetration leads to a less stable condition at and near the
wellbore wall.
➢ Pressurediffusionincreasespore pressurenear the wellbore, shear strength of
the rock is reduced (may result in failure of a shale section exposed for several
days).

79. Swelling/Hydration
➢ Drilling shale: water entersthe shale by advection and osmosis.
➢ Negatively charged clay ions attract and hold the polar water.
➢ Increasing volume of attracted water produces a swelling (stress that "wedges"the
clay platelets apart).
➢ The swelling pressureand behavior of shales are directly related to the type and
amount of clay mineralscontained in the shale. Shales with high concentrations of
negatively charged ions can produce very high swelling pressure(50,000 psi plus).
➢ Swelling pressuredecreasesthe strength of the shale by destroying the natural
cement bond between the clay platelets. Brittle shale becomes ductile and is pushed
into the wellboreby the compressivehoop stressandthe swelling stress.

80. Providing A Stable Wellbore
A. Planning A Stable Wellbore
B. Warning Signs /Corrective Action

81. Planning A Stable Wellbore
1. Potential Stability Indicators
➢ Indications of tectonic activity in the area
➢ Sudden pressuretransition zones expected
➢ Adverse formationsexpected (reactive shale, unconsolidatedor fractured
formations,abnormalor subnormallypressuredzones, plastic formations)
➢ Wellbore inclination greater than 30 .
2. Identify Stress Regime
3. Determine Magnitude of In Situ Condition
➢ Overburden (Sv) – Obtained from density logs of offset wells.
➢ FormationPore Pressure(p): Estimated by seismicand logs.
➢ Minimum HorizontalStress(Sh): Determined by LOT and/or logs.
4. Use Core Tests or Logs to Determine Formation Rock Strength
5. ResearchOffset Wells for Indications of Stability Problems.
➢ Identify hole sections with stability symptoms.
➢ List the conditions that caused the stability problem.
➢ Identify similarproblemsin offset wells occurringat the same vertical
depth. Look for similarityin the conditions that caused the problem.
➢ List the drillingparameterseffecting the problem (i.e., mud type and weight,
hole angle, adverse formations,unusual drilling practices).

82. Planning A Stable Wellbore
6. Select Mud System and Determine Mud Weight Window
7. Avoiding Stability Problems
➢ Select an inhibitive mud for reactive formations.
➢ Casingpoints should allow for mud weight windows determinedfrom stability
analysis.
➢ Maintain mud weight/ECD in stability window.Use down hole ECD monitoring
tools in critical wells.
➢ Optimize well trajectory based on drillingdays vs. stability.
➢ Plan for effective hole cleaning and stuck pipe prevention.
➢ Follow defensive drillingpractices. Control ROP, surge pressures.
➢ Train drillingteam members..

83. Warning Signs and Corrective Action
➢Mud type, composition and density.
➢Drilling practices (minimize ECD, swab /surge pressures).
➢Wellbore angle and direction.

84. Chemical Stability
Chemical stability problems:Reactive shales are drilled with a non-inhibitive drilling
fluid.
Warning Signs of Chemical Stability Problems
❑BHA balling and slow drilling, flow line plugging, soft mushycuttings on
shaker.
❑Smooth increasesin torque/drag
❑Overpull off slips, pump pressureincreasing.
❑Increases in mud parameters(mud weight, plastic viscosity, yield point,
cation exchange capacity (CEC), and low gravity solids).

85. Preventive /Corrective Measures
Prevention: Select adequate mud type and composition.
Initial corrective measures:suitablemud additives.
Problem persists:replaceexisting mud with a more inhibitive mud.
➢ 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 reduce water
contact with shale.
➢ Use of oil base or synthetic oil base mud to exclude water contact and
entry into shale.
➢ Minimizethe open hole exposuretime.
➢ Plan regular wiper trips.
➢ Minimizesurge/swab pressures.
➢ Ensure adequate hydraulics for bit and hole cleaning.
➢ Maintain required mud properties.
➢ Use minimummud weight, if possible.

86. Mechanical Stability
Mechanical instability:
➢incorrect mud weight /ECD and/or
➢well trajectory.
Too low mud weight can cause hole cavings or collapse resulting in stuck pipe.
Too high mud weight /ECD can cause excessivefluidlosses to the formationor total
loss of returns.
Warning Signs of Mechanical Stability Problems
➢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.
➢Restrictedcirculation /increases in pump pressure.
➢Loss of circulation.
➢Loss/gain due to ballooning shales

87. PreventingMechanical Stability Problems
Constraints on wellborepressuredictated by:
➢ Formationpressure(low end) and
➢ Fracture strength (high end).
Minimizeshock load imposed to the wellbore.
Measures to prevent/correct mechanical stability problems
➢Increase the mud weight (if possible). The mud weight values should be
determined using a stability analysismodel and past experience if drillingin a
known field.
➢If drilling fractured formations,it is not recommendedto increase MW. 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 the hole diameter in inches and half
this value for hole angle of less than 35 .
➢Circulateon each connection. Use back reamingand wiper trips only if hole
conditions dictate.
➢Minimizesurge/swab pressures.
➢Monitor torque/drag and the size and amount of cuttings on shakers.

88. Controlling Stability Problems
The entire rig team is responsiblefordetecting stability problems.Once
detected, there are manycontrols to consider that can provide for a stable
wellbore.The drilling supervisor,with input from rig team membersmust
be aware of the parametersthat restore the balance between rock stressand
rock strength.
The drillingteam must recognizethe warningsigns of an unstable wellbore and adjust
the drillingprogram accordinglyto the balance of rock stressand rock strength.

89. APPENDIX
A - 1 Leak-Off Tests
A - 2 Lithology Factor (k)
A - 3 Wellbore Stress Equations
A - 4 Nomenclature

90. A - 1 Leak-Off Tests
LOT data is necessary to determine the maximummud weight for well control and hole
stability and has a directinfluence on casingdesign.
LOT field data is also helpful for planning future field drillingand production
operations because it measuresthe minimumhorizontalstress(s ).
The minimumhorizontalstress is important for wellbore stability analysis.
Consistencyin LOT procedure, accuracy in reading test pressuresand proper data
reportingall have a directimpact on the quality of this information.
Refer to document F96-P-24, Standardizationof Leak-off Test Procedurefor more
detail.
Preparation is a key factor in achieving good quality LOT data.
Before testing begins:
Check offset well leak-off data for expected leak-off test pressure,pump rates, test
problemsor any unusual conditions.
Check logs for exposed sands to anticipate straight or curved line pressureplot.
Check for hole washoutsto anticipate problemswith the cement job.
Performa casing integrity test (CIT). Test pressureat any point not to exceed 80% of
casing burst.
Constructa LOT chart.

91. LOT Procedure
1. Drill out the shoe, rat hole and 10 to 15 feet of new hole.
2. Circulatethe hole clean and condition the mud to a consistentdensity.
3. Pull the drillstring +/-10 feet above the shoe.
4. Rig up the cement pump on the drillstring and pressuretest 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 pressureevery 1/4 barrel pumped.

92. Leak-off pressure (LOP)

93. A - 2 Lithology Factors
The Lithology Factor (k)
Calculated from LOT Data
Using Poisson'sRatio to
Calculate the Lithology
Factor

94. A - 3 Wellbore Stress Equations
Stress transformation
from global to wellbore
coordinates:
For equal horizontal
stresses( s = s )
Effective radial, hoop,
and axial stressesat
the wellborewall

95. A - 4 Nomenclature

97. Coordinate system for a hole
Assume that the principalst
formation are: σν, the vertic
largest
horizontalstress, and σh, th
horizontal stress.
A coordinatesystem (x', y', z
that x' is
parallelto σH , y' is parallel
parallel to σν
(i.e. z'-axis is vertical; fig.1).
vicinity
of the deviatedhole are mo
described in a
coordinatesystem (x, y, z,) w
parallel to
the hole, y-axis to be horizon
be parallel
to the lowermost radialdire

98. Coordinate transformation
As can be seen in Figure 2, a coordinate
transformation
from system (x', y', z') to system (x, y, z) can be
obtained
by two operations:1) a rotationâ around z'-
axis, and 2) a
rotationî around the y-axis. The angle î
represents the hole
inclinationand the angle â represents the
azimuth angle.

99. Mohr-Coulomb failure criterion
τ = τ0 + σtanφ
Shear and normal stressescan be calculated
σ'1 and σ'3 are maximum and minimumeffective stresseswhich can be calculated:
P0 : pore pressure and α is biot’s coefficient.
Combining the equations above, the failurecondition becomes:
Mohr-Coulombfailure criterion in τ - σ space

100. Wellbore Breakouts
Stress-induced wellbore breakoutsoccur when the compressivestressconcentration
around the boreholewall exceeds the rock strength.
Presence, orientation, and severity of failureare a function of the :
• In situ stressfield,
• Wellbore orientation, and
• Rock strength .
Vertical well (overburden is a principal stress):breakouts may formon opposite sides
of the wellboreat the azimuth of the minimum horizontalfar-fieldcompression
(compressivehoop stressis greatest).
Wellbore is inclined to the principal stresses:location of the breakouts is a complex
function of the:
• Orientation of the wellbore and
• Orientations and magnitudes of the in situ stresses.

101. Wellbore Breakouts
Stresses acting on a vertical wellbore
wall when overburden(Sv) is a principal
stress.
When the circumferentialhoop stress,
σθθ, exceeds the compressivestrength of
the rock, breakouts form90° from the
direction of SHmax. If the hoop stress
becomes tensile and exceeds the tensile
strength of the rock, tensile fractures will
form in the direction of Shmax .

102. Tensile Failure
Drilling-induced tensile fractures occur in the borehole wall where the circumferential
hoop stressis negative and exceeds the tensile strength of the rock.
➢ Fine-scale featuresoccur only in the wall of the borehole (due to the localized
stressconcentration) and do not propagateaway from the hole.
➢ Fractures form either parallel to the borehole axis or, in the case in which the
borehole axis is not parallel to one of the principal stresses(e.g., in deviated
wellbores), in en echelon patterns where the fracture planes are inclined to the
borehole axis.

103. Effect of Mud Weight
Occurrence of drillinginduced tensile wall fractures due to excess mud weight
(causes a component of tensile stressto be added to the hoop stress acting around
the wellbore)
Decrease in hoop stress is simply proportionalto the excess mud weight.

104. Effect of wellbore cooling
Excess mud weight and wellborecooling can influence the occurrence of drilling
induced tensile wall fractures (cause a component of tensile stressto be added to the
hoop stressacting around the wellbore and can play a role in the
formationof tensile fractures.
Thermal stressat the borehole wall proportionalto the amount of cooling and the
physical propertiesof the rock:
where
α is the linear coefficient of thermal expansionand
E is Young’s modulus.
ΔT Temperature differencebetween the wellborefluid and the rock surroundingthe
well

105. Stress can be Compressional, Tensional, or
Shear
Compressionalstress pushes matter (rock
layers) together
Tensional stress pullsmatter (rock and dirt
layers) apart.
Shear stress is rotational,the stress is parallel
to a face of the material

106. Folds and faults: geologic structures
Geologic structure
Folds – a bend
Anticline – arch
upwards
Syncline – arch
downwards
Dome – circular
anticline
Basin – circular syncline

107. Folds and faults: geologic structures
Joints – fracture without motion
Joints decrease with depth
Folds, Faults & plate boundaries
Convergent – compression
Divergent – extension (pulling)
Transform – strike-slip
Overturned Folds

108. Folds and faults: geologic structures
Faults – fracture with displacement
Slip – the distance of motion
Fault zone – numerous, closely spaced fractures
Hanging wall – miner’sreference,the side of a fault “overhead”
in a mine
Footwall – (as above) the fault side underfoot
Fault terms (cont)
Normal fault – pulling apart of landscapecauses hanging wall to
move downward
Graben – a down-dropped, wedge-shaped block
Horst – the blocks between grabens
Reversefault – compressionof landscape forceshanging wall up
relative to footwall
Thrust fault – low angle reverse fault
Strike-slip fault – fracture close to vertical, motion horizontal
(transform)

109. Types of Faults
Normal Fault: One side of the fault has
slipped down in comparisonto the other side.
Normal Faults are caused by layers being
pulled apart, by TENSIONAL stresses.
Gravity causes one side toslipdown.
ReverseFault: One side of the fault has
slipped up in comparisonto the other.
When layers are pushed together, by
COMPRESSIONAL forces,this kind of fault
occurs.
Lateral or Strike-Slip Fault: Two layersof
rock are shifted horizontallyor parallel to
the fault plane.