slp - wellborestability
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Basic understanding of drilling terms and procedures Stuck Pipe Self Learning Package.TRANSCRIPT
Wellbore Stability Self Learning Package Sugar Land Learning Center
SUGAR LANDLEARNING CENTER
Wellbore StabilitySELF-LEARNING COURSE
USEFUL PRE-REQUISITES
Basic understanding of drilling terms and proceduresStuck Pipe Self Learning Package
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Table of Contents
OBJECTIVES………………………………………………………………………….. 3
THE STRESS IN THE EARTH BEFORE WE DRILL A BOREHOLE……………...…………….……4THE STRESS IN THE EARTH AFTER WE DRILL A BOREHOLE….…………………….…...……..8ROCK FAILURE……………………………..………………………………………………..……...….10REVIEW QUESTIONS I…………………………………………………………………………..……..14WELLBORE STABILITY PLANNING AND PREVENTATION…………………………….………..15REVIEW QUESTIONS II…………………………………………………………………………….…..29ANSWERS TO REVIEW QUESTIONS …………………………………………………………….…..30
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Objectives
Upon completion of this training module you should be able to:
� Describe the stresses in the earth before we drill a borehole� Describe the stresses in the earth after we drill a borehole� Describe the different types of rock failure� Describe the characteristics of a mini-frac� Describe the 2 main outputs of wellbore stability planning� Understand the differences between Tabular, Angular and Splintered Cavings� Describe the common wellbore monitoring techniques and the 4 most common wellbore
instability mechanisms� Describe remedial actions that are taken to fix a failed / failing wellbore
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The Stress in the Earth before we Drill a Borehole
Before we drill a borehole the rock in the earth is in a state of equilibrium. This state is called the“Initial State”.
In the earth, there are 3 stresses that are perpendicular to each other:
σv Principal Stress in vertical axisσh Principal Stress in horizontal axisσH Principal Stress in horizontal axis
σH is the maximum of the 2 horizontal stresses and σh is the minimum.(ie σH > σh )
In Rock Mechanics we also describe earth stresses in order of magnitude:
σσσσ1 Maximum Earth Stressσσσσ2 Intermediate Earth Stressσσσσ3 Minimum Earth Stress
These can be ordered in any way: for example σ1 could be the vertical stress or one of thehorizontal stresses, depending on the sedimentary basin in which we are drilling.
σσσσhσσσσH
σσσσV
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Slip Fault Regime
σH = σ1
σv = σ2
σh = σ3
Thrust (Reverse) FaultRegime
Gentle sloping
σH = σ1
σh = σ2
σv = σ3
Normal FaultRegime
Steep sloping
σv = σ1
σh = σ3σH = σ2
Figure 1: Tectonic dependence on earth stresses
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The earth’s stresses are related to a number of different variables including:
Tectonic Setting, Pore pressure, Depth, Lithology, Temperature, Structure
The tectonic setting can affect the relationship of the earth’s stresses. Consider figure 1.1
a) In a Normal Fault Regime, the vertical stress (σv) is the maximum principal stress (σ1): σv > σH > σh
b) In a tectonically stressed regime, horizontal stress (σH) is the maximum principal stress (σ1):σH > σh > σv
c) Slip fault regime, the horizontal stress (σH) is the maximum principal stress (σ1):σH > σv > σh
Pore Pressure supports a portion of the total applied stress in a rock.
In general:
Total stress (in given direction) = Effective Stress of Rock Grains (given direction) + Pore Pressure
If a formation is “normally pressured” the pore pressure mechanism can be described asfollowing:
Sediment burial →→→→ full pore fluid escape →→→→ porosity decreases →→→→ effective rock stress increases→→→→ pore pressures are hydrostatic (normal)
If a formation is “over-pressured” the pressure in the formation is greater than the pressureexerted by a column of water at that same depth.
There are 2 main mechanisms causing overpressure:
a) Loading mechanisms:Sediment burial →→→→ pore fluid escape fully restricted →→→→ porosity & effective stress are bothconstant →→→→ pore pressures increases at the same rate as the overburden (ie overpressure)
b) Unloading mechanisms7:(i) Aquathermal expansion or hydrocarbon generation or mineral dehydration (smectite→illite) or osmosis → sealed formation → fluid-volume increase can result in rapid pore pressure increases that unload the rock grain matrix.
(ii) Uplift / Erosion → unloading rock grain matrix → sealed formation → formation has same pore pressure as before but due to closed system is abnormally pressured compared with neighbor formations at same depth.
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Figure 2: The 3 Wellbore Stresses
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The Stress in the Earth after we Drill a Borehole
Before a wellbore is drilled the rock is in a state of equilibrium. This state is called the “InitialState”.
The stresses in the earth under this condition are known as the Far Field Stresses (σσσσh , σσσσH , σσσσv )or in-situ stresses.
When a well is drilled it introduces a perturbation in the initial stress field. The perturbationcauses a ‘new’ set of stresses known as wellbore stresses that act on the formation at thewellbore wall.
There are 3 wellbore stresses. These are:
• Radial Stress• Tangential Stress• Axial Stress
Figure 2 shows these 3 wellbore stresses.
The wellbore stresses depend on 2 different things:
a) The mud weight usedb) The magnitude of the far field stresses (σv , σH and σh)
If we know what these wellbore stresses are then we will have a better idea of whether aborehole will fail when we drill it.
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Figure 3: The 2 different ways a rock can fail
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Rock Failure
Generally a rock can fail in 2 different ways:
a) Shear Failure:
This is caused by 2 perpendicular stresses that are different in magnitude.
b) Tensile Failure:
This is caused by one stress exceeding the tensile strength of the rock.
Figure 3 shows schematically a shear failure and a tensile failure.
Both of these failures can cause wellbore instability.
When a rock fails by either shear or tensile failure, 2 things can happen depending on the type ofshear/tensile failure:
a) Loss circulation can occur (due to mud losses in the cracks of the rock)b) Stuck pipe can occur (pack off due to the borehole collapsing)
We need to prevent these failures from occurring (if we can) to minimize the amount of NonProductive Time (NPT)
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Leak off Pressure
Formation Breakdown Pressure pbdpw
Time
Pumping stops
Fracture Closure Pressure = σh
Tensile Strength To
Fracture opening pressure
Figure 4: A Mini-Frac Test
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Figure 4 shows an example of a mini-frac. The y-axis shows the wellbore pressure (ie the mudweight).
The formation is basically broken down and the pressure trace is examined – from this we candetermine certain properties of the rock and this will give us geomechanical information that willultimately help us manage wellbore stability.
It can be seen that there is a linear trend (the elastic region) until The Leak Off Pressure.At this point (the Leak off Pressure) the plot deviates from the straight line; the formation grainsstart to move apart and take mud. The formation is on the threshold of moving from an elasticstate to a plastic state.
The Formation Breakdown Pressure pbd represents the “maximum strength” of the rock before itbreaks.
This will be equivalent to the pressure exerted by the mud in the borehole. The tensile strengthTo of the rock is the corresponding Tangential Stress at this mud weight. (For simplicity of thisSLP we will neglect Axial and Radial Stress).
Therefore, the condition for tensile failure is when the tangential stress is equal to the tensilestrength of the rock.
Figure 5 shows some examples of borehole failure from RAB images.swbo, ssko and shae are examples of shear failurestver is an example of a tensile failure (a vertical fracture in this case)
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Figure 5: Borehole Failure in RAB images
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Review Questions I
1)What is the relationship between the earth stresses while drilling in a tectonically active region?
2) What are the 2 main mechanisms that cause a formation to be overpressured ?
3) What are Wellbore Stresses and what do they depend on ?
4) Describe the 2 ways that a rock can fail
5) What is the difference between the Leak off Pressure and Formation Breakdown Pressure ?
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Wellbore Stability Planning and Preventation
Wellbore instability / Rock Failure is undesirable because it can lead to Non Productive Time(NPT) such as:
• Pack offs (formation failure leading to excess of cuttings)• Excessive trip and reaming time• Mud losses• Stuck Pipe and BHAs → Loss of equipment / Fishing / Sidetracks• Inability to land casing, casing collapse• Poor logging and cementing conditions
These can be caused by the following:
breakouts, sloughing, natural fractures/weak planes, drilling induced fractures, faulting, undergauge hole, interbedded sequence, overpressured formation, unconsolidated formation, mobile formation, permeable formation, chemical activity.
Even relatively minor wellbore stability problems in tectonically passive settings can beextremely expensive ($100,000 to $250,000 per day offshore).
The key to effective reduction of NPT is planning for wellbore stability.
One process used to reduce the NPT is the Mechanical Earth Model.
This integrates all geomechanical data available from a field/basin into one “database” which isthen used to predict wellbore stability problems that are likely to occur in an upcoming well.
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Safe Mud WeightWindow
σσσσhPorePressure
snbo
swbo
shae
sdko
slae ssko tcyl
tver
Increasing MudWeight
Figure 6: Designing a Mud Weight Window
Figure 7: A typical Mud Weight Window (North Sea)
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Horizontalwell
MudWeight(g/cc)
Vertical well
Increase the mudweight or increasethe risk of shearfailure
Sh
Figure 8: Trajectory Analysis for Anisotropic Stress Field, Relaxed Basin (σv is max)
S
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Two of the most important outputs that emerge from wellbore stability planning are thedetermination of a safe mud weight window and the safest direction to drill, especially for highlydeviated wells.
Figure 6 shows that it is often desirable to drill with a mud weight between swbo (a shear failurecondition) and σh (the minimum horizontal stress).
Figure 7 shows an example from the North Sea where the safe mud weight window shouldbetween the black dashed line (Minimum Borehole Stability or shear failure) and the formationpropagation pressure (or the minimum horizontal stress).
Figure 8 shows that in a relaxed basin it is often safer to drill the well in the direction of theminimum horizontal stress (σh). Also it can be seen that the safe mud weight window narrows aswell deviation increases (ie you need to increase the mud weight to keep the wellbore stable butbe careful because the maximum mud weight before borehole instability occurs will now belower).
The open hole section of a wellbore must be maintained in a condition that is good enough toallow drilling and casing to be run. This does not mean that it is necessary to eliminate allformation failure.
Indeed the wellbore can remain stable even after a period of prolonged formation failure.
An example of this is the Cuisiana field, Colombia where the wellbore has remained stablebecause the cavings from borehole failures can be cleaned out of the hole.In this example the wellbore instability was managed (or contained) rather than prevented.
In these cases it becomes difficult to find a solution that will completely prevent the instabilityfrom occurring in the first place and wellbore stability management is required: for example,loss circulation might be avoided at all costs, and techniques to manage the shear failure areimplemented such as good hole cleaning practices.
Real time Wellbore stability management (control) is a twofold process involving:
a) Continuous monitoring – ie downhole/surface signatures to diagnose onset of a problem.
b) Remedial actions – ie drilling parameters to fix a failed or failing wellbore.
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a) Continuous monitoring
Real Time Wellbore Stability Control relies on an integration of all data available at theWellsite:
Surface signatures:Cavings analysis – Wellbore Failure,Cuttings volume – Hole Cleaning,Pit volumes – Gains (overpressured zone), losses,Surface Drilling Parameters
MWD data:Downhole Drilling ParametersDWOB, DTORQ – Friction / DragECD behaviour – Hole Cleaning, pack off
LWD data:Gamma Ray, Resistivity – Identify zones of potential instability from MEMSonic – Pore pressure prediction while drillingCaliper measurements – if pattern is forming in some intervals, can identify unstable formations
A reliable diagnosis of the instability mechanism requires use of all available data.
If tabular cavings due to natural fracturing are observed then the resistivity log should bechecked for evidence of mud invasion into fractures and the mud records require examining forlosses.
Similarly, if splintered cavings due to over-pressured formations are seen then high gas levels,kicks or mud gains may also be present.
The observation of angular cavings due to breakouts requires the debris levels in the hole to bediscerned. In all cases, the cavings volume should be compared to the ECD and the degrees oftight hole and restricted circulation to discern the effectiveness of the hole cleaning and theseverity of instability.
(see cavings analysis on the following pages)
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Figure 9: Tabular Cavings
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Cavings Analysis:
An analysis of cavings can provide a signal that the borehole is failing and indicates both thenature of the instability and the troublesome formations.
Cavings dimensions range from a few millimetres to 10 cm or more, with larger examples risingto the surface while lodged in the BHA.
There are four main types of caving:
Tabular,Angular,SplinteredThose which cannot be characterized.
Tabular cavings are the result of natural fractures or weak planes.In the case of natural fractures, the fluid pressure in the annulus exceeds the minimum horizontalstress, resulting in mud invasion of fracture networks surrounding the wellbore.
This can result in severe destabilization of the near wellbore region, due to the movement ofblocks of rock, leading rapidly to high cavings rates, lost returns and stuck pipe.
The blocks of rock are bounded by natural fractures planes and, therefore, have flat, parallel,faces.Figure 9 shows examples of tabular cavings due to natural fractures.The other characteristic is that bedding, if any, will not be parallel to the faces of the caving.
In the case of weak planes, the combination of low mud weight and a borehole axis that is withinapproximately 15 degrees of the bedding direction can induce massive failure along the planes ofweakness, leading to the symptoms described above.Cavings that are the result of weak planes are characterized by having flat, parallel, faces. Thebedding direction is also parallel to the faces.
Angular cavings are a consequence of breakouts. These cavings are characterized by curvedfaces with a rough surface structure. The surfaces intersect at acute angles (much less than 90degrees). Figure 10 shows Angular Cavings.
Splintered cavings have two nearly-parallel faces with plume structures. This type of caving isdue to tensile failure occurring parallel to the borehole wall and commonly occurs inoverpressured zones drilled with a small overbalance. Figure 11 shows Splintered Cavings.
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22
The higher the cavings rate the more severe the failure for a given hole cleaninThe dominant caving should be noted not the proportion of different cavings.
The cavings rate is measured by the time required to fill a bucket placed undernThe cavings volume is then proportional to the amount of cavings in the buckeCARE MUST BE TAKEN – IF HOLE CLEANING IS POOR THERE WILL BE FEW
Figure 10:Angular Cavings
Figure 11: SplinteredCavings
g efficiency.
eath the shakers.t.
ER CUTTINGS
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Chemical Wellbore Instability:
Wellbore instability can be classified as either mechanical (for example, failure of the rockaround the hole because of high stresses, low rock strength, or inappropriate drilling practice) orchemical.
Chemical Wellbore Instability arises from damaging interactions between the rock, generallyshale, and the drilling fluid.The integration of understandings of chemical and mechanical damage remains problematical.
In wellbore stability monitoring, it is important to determine whether a particular drillingproblem is mechanical or chemical in origin.
Figure 12 describes how to diagnose the 4 most important wellbore stability mechanisms.3 of these are mechanical and 1 of these is chemical in origin. The 3 tables that follow showexamples of wellbore stability from surface, downhole and miscellaneous signatures.
Figure 12: Diagnosing the 4 most common wellbore instability mechanisms
Wellb
Mec
Permform
Intersoft/sFaultactivSloug
OverformUnde
UncoformMobi
Chem
Brea
DrillfractClosenaturweak
ore Stability Self Learning Package Sugar Land Learning Center
hanism Lost Time WellboreTrajectory
In-situstresses
FormationStength
PorePressure
Geology
eableation Stuck pipe
Lowcompared tomud pressure
beddedtrong rocks Stuck pipe Tortuous
Frequentchanges
Thick sections collapse more
slip/ation
Stuck pipe,excessive reaming
High stressdeviation
Faults present
hingHole fill after trips
Weak Proximity to salt dome or faults,tectonically active
pressuredation Hole fill after trips High Recently crossed faultrgauge hole Excessive slack
off while trippingHigh meanstress
Low yieldstrength
nsolidatedation
Restricted pipemovement
Large sand or fractured section
le formationHole fill after trips
Highoverburden
Proximity to salt dome, evaporatesequence
ical activityProblems worsenwith time, slightflow
Low
koutsStuck Pipe High
stress/stengthratio
ing induceduresly spacedal fracs / planes
Stuck Pipe, holefill after trips
Planes ofweakness
Mudpressure>porepressure
Figure 13: Wellbore instability – Miscellaneous signatures
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raltm25
Mechanism ROP DWOB DTOR Caliper γγγγ-ray
Permeableformation
Decreases Thick filter cake GAPI ≤60
Interbeddedsoft/strong rocks
Frequent & rapidchanges
Frequent & rapidchanges
GAPI>60,& GAPI ≤60 often
Fault slip/activation Decrease
Local boreholeelongation
Sloughing Decreases Low Boreholeenlargement
GAPI >60
Overpressuredformation
High, given rockstrength
Low Boreholeenlargement
Undergauge holeLow
Low Diameter lessthan gauge
Unconsolidatedformation High Decreases
Boreholeenlargement
GAPI <60
Mobile formation Decreases
Chemical activity Decreases DecreasesHole tightenswith time, ordissolves
GAPI >60
Breakouts Decreases LowBoreholeenlargement
Drilling inducedfractures Low
GAPI >60
Close spacedfracs/weak planes Decreases
Low Boreholeenlargement
GAPI >60
Figure 14: Wellbore instability – MWD, LWD & Wireline
Resistivity UBI
Frequent &rapid changes
Frequent welldiameter changes
Detected.Rotation ofbreakouts
High dip (>60°)
BoreholeenlargementBoreholeenlargementDiameter lessthan gaugeBoreholeenlargement
Swelling detected
Orientation &span detected
Diametricallyopposed &long
Possibledetection
Fracture &bedding planeorientation
Boreholegeometry
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raltman@26
Mechanism Pump pressure Circulation Mud Cuttings andcavings
Hookload
Permeableformation
Gradual decrease Flowdecreases
Water loss,high solids
Increases
Interbeddedsoft/strong rocks Spikes Flow erratic
Volume ratechangesfrequently
High
Fault slip/activation Spikes Flow erratic Loss HighSloughing Increase Flow
decreasesLarge & flat High when
pumps offOverpressuredformation Increase
Pit levelincrease
Backgroundgas high
Large, brittle,fissile, concave
Large overpulat connections
Undergauge holeSpikes
Flow erratic Abrasive &hard
High
Unconsolidatedformation
Increase Flowdecreases
Unconsolidated& uncemented
Large overpulat connections
Mobile formationIncrease Flow
decreasesSalt present,rise in Cl
Salt grains Large overpulat connections
Chemical activity IncreaseFlowdecreases
MW & solidsincrease
Soft,watersoluble.Gumbo
Large overpulat connections
Breakouts Spikes Flow erraticApparent loss High volume High
Drilling inducedfractures Decrease Flow
decreases Loss
Close spacedfracs/weak planes Decrease
Flowdecreases
Loss at similarweights acrossfield
Squarish, highvolume
High
Figure 15: Wellbore instability – surface signatures
slb.com
SurfaceTorque
Drillstring
Higher
Erratic Packed off
Increase Diametrical wearHigh Packed off
l Increase
High &erratic
UndergaugeBHA
l Erratic
l Erratic Packed off
l Increases
HighPacked off
High
Wellbore Stability
FrequentWiperTrips
Breakouts 1
Sloughing 0NaturalFractures /Weak Planes
0
DrillingInducedFractures
0
FaultActivation
0
UndergaugeHole
1
InterbeddedSequence
1
OverpressuredFormation
0
UnconsolidatedFormation
0
MobileFormation
0
PermeableFormation
0
ChemicalActivity
0
Figure 16: Actions
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InfrequentWiperTrips
DecreaseROP
IncreaseMud GelStrength
IncreaseMudCirculationRate
LimitOD size /DrillCollars
IncreaseMudWeight
DecreaseMudWeight
UseMinimumOverbalance(200 psi)
EnsureOverbalanceexceeds 200psi
AddFluidLossAgents
UseInhibitiveMud
Minimizeswab andsurgeaffects
0 1 1 1 0 1 0 0 0 0 0 1
1 0 1 0 0 0 0 0 0 0 0 01 1 1 0 0 0 0 1 0 1 0 1
0 0 1 0 0 0 1 0 0 1 0 1
0 0 0 0 0 0 0 1 0 1 0 1
0 0 0 0 1 1 0 0 0 0 0 1
0 0 1 1 0 1 0 0 0 0 0 1
0 0 0 0 0 0 0 0 1 0 0 1
1 0 1 1 0 1 0 0 0 0 0 1
1 0 0 1 0 1 0 0 0 0 0 1
1 0 0 0 0 0 0 1 0 1 0 1
1 0 0 0 0 0 0 0 0 0 1 0
inhibiting the instability mechanisms. A "1" indicates that the action suppresses the instability.A "0" indicates that the action has no influence on instability or makes it worse.
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b) Remedial Actions
If wellbore instability becomes severe as detected from a) continuous monitoring, and holecleaning cannot remove cavings from the wellbore then the wellbore would be unstable.
The ability to deal effectively with the consequences of the unstable wellbore depend on theinstability mechanism and its severity.Remedial action generally involves the control of surface parameters (e.g. ROP, RPM, flow rate,mud weight/rheology).
For example, if mud losses are currently occurring, but a mud weight decrease is not possibledue to conditions that will be encountered while drilling through formations below the currenthole bottom (cavings generation), then decreasing the ROP will reduce cuttings loading andtherefore the ECD. This may be sufficient to eliminate mud losses and also reduce cuttingsloading in deeper intervals
The emphasis when considering remedial actions, which either suppress instabilities or minimizetheir consequences, should be the entire open hole interval, rather than focusing on problemfixing at the bit.
The ROP and hole cleaning efficiency form the key links between wellbore instability andoperations. Rock debris in the annulus, resulting from drilling and/or wall failure, will increase ifhole cleaning is inadequate, raising the risk of pack-offs and stuck pipe. The ability to clean thehole is also related to the ROP.
Figure 16 outlines the various actions that are recommended for various given wellbore stabilitymechanisms. It can be seen that minimizing swabbing and surging affects helps to suppress moreinstability mechanisms than any other drilling practice.
Also it can be seen that drilling practices such as wiper trips that are often considered as routineare sometimes detrimental to wellbore stability. Minimising wiper trips can help suppressactions that are sensitive to mechanical agitation of the formation such as mobile formations /sloughing shales, weak planes.
Increasing mud weight is not necessarily the answer to wellbore stability problems. Whilst thispractice can help suppress breakouts, it can cause drilling induced fractures or activate naturalfracture networks by drilling above the minimum horizontal stress. However, whereoverpressure occurs, it is desirable to drill with an overbalance that exceeds 200 psi. In all cases,calculations are required prior to drilling to determine optimal parameters.
This problem becomes amplified in deviated and especially horizontal wells where the mudwindow between shear and tensile failure becomes so small that sometimes there is no stablemud weight window.
Good drilling practices such as Circulation, Rotation, Reciprocation, of the drillstring toremove excess cuttings in highly deviated wells, and close examination of shale shakers toexamine volume of cuttings and their geometry is desirable to manage (suppress) unstablewellbores.
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Review Questions II
6) When planning a mud program, how is the mud weight often determined ?
7) In a relaxed basin, as well inclination changes from vertical to horizontal, what happens to the “Mud Weight Window” ?
8) Describe the 3 main types of cavings found on the shale shakers ?
9) For the following wellbore instability problems, what drilling practices would you use to surpress or control the problem ?
a) Borehole Breakoutsb) Natural Fractures / Weak Planesc) Unconsolidated formations
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Answers to Review Questions
1) σH > σh > σv
2) Loading mechanisms – where the pore fluid cannot escape as quickly as the rock compaction rate, and the pore fluid gets squeezed and pressured because it can’t escape.
Unloading mechanisms – where a formation rises to a shallower depth, and the pore fluids cannot escape, then the formation is overpressured compared to surrounding (shallower) formations (because the pore fluids still have the same pressure as before the formation rose). Hydrocarbon generation where the pore fluids are trapped is another example.
3) When we drill a hole in the rock, we replace the rock with a cylinder of mud and a set ofstresses are created in the region of the wellbore wall. These stresses are known as
“Wellbore Stresses”. They depend on the mud weight used, and the far field stresses σH , σh and σv
4) Tensile failure – occurs when the rock grains are held in tension and are pulled apart.Shear failure – occurs when the rock grains are under a state of compression by 2 stressesthat are acting perpendicular to each other and their magnitudes are very different.
5) Leak off Pressure – the wellbore pressure at which the rock begins to yield and the formation grains begin to move apart and take mud. Formation Breakdown Pressure – the wellbore pressure at which the rock physically breaks down.
6) Often (but not always) between the condition for shear failure and the minimum horizontal stress σh
7) It generally becomes more narrow (ie you have to less of a margin in which to drill safely )
8) Tabular – from natural fractures (where the cavings will have flat, parallel faces with bedding not parallel to the parallel faces of the caving). or from weak planes (the same as natural fractures but the bedding is parallel to the faces of the cavings). Angular – from borehole breakouts (they have curved faces with rough surface structure) Splintered – from overpressured zones (concave flat, thin, planar structures)
9 a) Perform frequent wiper trips, ensure hole is kept clean by: increasing mud gel strength,increasing mud circulation rate, increase mud weight. Also minimize swabbing / surging tostop borehole breakouts from getting worse, circulate / rotate / reciprocate (in extendedreach or highly deviated wells).
b) Minimise wiper trips otherwise might make situation worse (especially if problem is due to weak planes), increase mud gel strength to help decrease fluid mobility, add fluid loss agents to help control loss circulation, ensure drilling with minimum overbalance, minimize swab/surge.
c) Minimise wiper trips, increase mud weight, minimise swab/surge, ensure hole cleaning.