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PublicationInformation
(/File%3AVol2DECover.png
PetroleumEngineeringHandbook
Larry W. Lake,Editor-in-Chief
Volume II - DrillingEngineering
Robert F. Mitchell,Editor
Copyright 2006,Society of PetroleumEngineers
Chapter 10 - DrillingProblems andSolutions
By J.J. Azar,University of TulsaPgs. 433-454
ISBN978-1-55563-114-7Get permission for
PEH:Drilling Problems and SolutionsIntroductionIt is almost
certain that problems will occur while drilling a well, even in
very carefullyplanned wells. For example, in areas in which similar
drilling practices are used, holeproblems may have been reported
where no such problems existed previously becauseformations are
nonhomogeneous. Therefore, two wells near each other may
havetotally different geological conditions.In well planning, the
key to achieving objectives successfully is to design
drillingprograms on the basis of anticipation of potential hole
problems rather than on cautionand containment. Drilling problems
can be very costly. The most prevalent drillingproblems include
pipe sticking, lost circulation, hole deviation, pipe failures,
boreholeinstability, mud contamination, formation damage, hole
cleaning, H2S-bearingformation and shallow gas, and equipment and
personnel-related problems.Understanding and anticipating drilling
problems, understanding their causes, andplanning solutions are
necessary for overall-well-cost control and for
successfullyreaching the target zone. This chapter addresses these
problems, possible solutions,and, in some cases, preventive
measures.
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Contents1 Pipe Sticking
1.1 Differential-Pressure Pipe Sticking1.2 Mechanical Pipe
Sticking
2 Loss of Circulation2.1 Definition2.2 Lost-Circulation Zones
and Causes2.3 Prevention of Lost Circulation2.4 Remedial
Measures
3 Hole Deviation3.1 Definition3.2 Causes
4 Drillpipe Failures4.1 Twistoff4.2 Parting4.3 Collapse and
Burst4.4 Fatigue4.5 Pipe-Failure Prevention
5 Borehole Instability5.1 Definition and Causes5.2 Types and
Associated Problems5.3 Principles of Borehole Instability5.4
Mechanical Rock-Failure Mechanisms5.5 Shale Instability5.6
Wellbore-Stability Analysis5.7 Borehole-Instability Prevention
6 Mud Contamination6.1 Definition6.2 Common Contaminants,
Sources, and Treatments
7 Producing Formation Damage7.1 Introduction7.2 Borehole
Fluids7.3 Damage Mechanisms
8 Hole Cleaning8.1 Introduction8.2 Factors in Hole Cleaning
9 Hydrogen-Sulfide-Bearing Zones and Shallow Gas10 Equipment and
Personnel-Related Problems
10.1 Equipment10.2 Personnel
11 Nomenclature12 References13 General References14 SI Metric
Conversion Factors
Pipe StickingDuring drilling operations, a pipe is considered
stuck if it cannot be freed and pulled out of the hole
withoutdamaging the pipe and without exceeding the drilling rigs
maximum allowed hook load. Differential pressure
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pipe sticking and mechanical pipe sticking are addressed in this
section.
Differential-Pressure Pipe StickingDifferential-pressure pipe
sticking occurs when a portion of the drillstring becomes embedded
in a mudcake (animpermeable film of fine solids) that forms on the
wall of a permeable formation during drilling. If the mudpressure,
pm , which acts on the outside wall of the pipe, is greater than
the formation-fluid pressure, pff , whichgenerally is the case
(with the exception of underbalanced drilling), then the pipe is
said to be differentiallystuck (see Fig. 10.1). The differential
pressure acting on the portion of the drillpipe that is embedded in
themudcake can be expressed as
(/File%3ADevol2_1102final_Page_434_Image_0001.png)
Fig. 10.1Differential-pressure sticking.
(/File%3AVol2_page_0434_eq_001.png)....................(10.1)
The pull force, Fp, required to free the stuck pipe is a
function of the differential pressure, p; the coefficient
offriction, f; and the area of contact, Ac, between the pipe and
mudcake surfaces.
(/File%3AVol2_page_0434_eq_002.png)....................(10.2)
From Bourgoyne[1],
(/File%3AVol2_page_0434_eq_003.png)....................(10.3)where
(/File%3AVol2_page_0434_eq_004.png)....................(10.4)
In this formula, Lep is the length of the permeable zone, Dop is
the outside diameter of the pipe, Dh is thediameter of the hole,
and hmc is the mudcake thickness. The dimensionless coefficient of
friction, f, can varyfrom less than 0.04 for oil-based mud to as
much as 0.35 for weighted water-based mud with no
addedlubricants.
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Eqs. 10.2 and 10.3 show controllable parameters that will cause
higher pipe-sticking force and the potentialinability of freeing
the stuck pipe. These parameters are unnecessarily high
differential pressure, thick mudcake(high continuous fluid loss to
formation), low-lubricity mudcake (high coefficient of friction),
and excessiveembedded pipe length in mudcake (delay of time in
freeing operations).Although hole and pipe diameters and hole angle
play a role in the pipe-sticking force, they are
uncontrollablevariables once they are selected to meet well design
objectives. However, the shape of drill collars, such assquare, or
the use of drill collars with spiral grooves and external-upset
tool joints can minimize the stickingforce.Some of the indicators
of differential-pressure-stuck pipe while drilling permeable zones
or known depleted-pressure zones are an increase in torque and
drag; an inability to reciprocate the drillstring and, in some
cases,to rotate it; and uninterrupted drilling-fluid circulation.
Differential-pressure pipe sticking can be prevented orits
occurrence mitigated if some or all of the following precautions
are taken:
Maintain the lowest continuous fluid loss adhering to the
project economic objectives.Maintain the lowest level of drilled
solids in the mud system, or, if economical, remove all drilled
solids.Use the lowest differential pressure with allowance for swab
and surge pressures during trippingoperations.Select a mud system
that will yield smooth mudcake (low coefficient of
friction).Maintain drillstring rotation at all times, if
possible.
Differential-pressure-pipe-sticking problems may not be totally
prevented. If sticking does occur, common fieldpractices for
freeing the stuck pipe include mud-hydrostatic-pressure reduction
in the annulus, oil spottingaround the stuck portion of the
drillstring, and washing over the stuck pipe. Some of the methods
used to reducethe hydrostatic pressure in the annulus include
reducing mud weight by dilution, reducing mud weight bygasifying
with nitrogen, and placing a packer in the hole above the stuck
point.
Mechanical Pipe StickingThe causes of mechanical pipe sticking
are inadequate removal of drilled cuttings from the annulus;
boreholeinstabilities, such as hole caving, sloughing, or collapse;
plastic shale or salt sections squeezing (creeping); andkey
seating.Drilled Cuttings. Excessive drilled-cuttings accumulation
in the annular space caused by improper cleaning ofthe hole can
cause mechanical pipe sticking, particularly in directional-well
drilling. The settling of a largeamount of suspended cuttings to
the bottom when the pump is shut down or the downward sliding of
astationary-formed cuttings bed on the low side of a directional
well can pack a bottomhole assembly (BHA),which causes pipe
sticking. In directional-well drilling, a stationary cuttings bed
may form on the low side of theborehole (see Fig. 10.2). If this
condition exists while tripping out, it is very likely that pipe
sticking will occur.This is why it is a common field practice to
circulate bottom up several times with the drill bit off bottom
toflush out any cuttings bed that may be present before making a
trip. Increases in torque/drag and sometimes incirculating
drillpipe pressure are indications of large accumulations of
cuttings in the annulus and of potentialpipe-sticking problems.
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(/File%3ADevol2_1102final_Page_436_Image_0001.png)
Fig. 10.2Mechanical pipe sticking caused bydrilled cuttings: (a)
cuttings bed during drilling,and (b) cuttings jamming the drill bit
duringtripping out.
Borehole Instability. This topic is addressed in Sec. 10.6;
however, it is important to mention briefly thepipe-sticking issues
associated with the borehole-instability problems. The most
troublesome issue is that ofdrilling shale. Depending on mud
composition and mud weight, shale can slough in or plastically flow
inward,which causes mechanical pipe sticking. In all formation
types, the use of a mud that is too low in weight canlead to the
collapse of the hole, which can cause mechanical pipe sticking.
Also, when drilling through salt thatexhibits plastic behavior
under overburden pressure, if mud weight is not high enough, the
salt has the tendencyof flowing inward, which causes mechanical
pipe sticking. Indications of a potential pipe-sticking
problemcaused by borehole instability are a rise in circulating
drillpipe pressure, an increase in torque, and, in somecases, no
fluid return to surface. Fig. 10.3 illustrates pipe sticking caused
by wellbore instability.
(/File%3ADevol2_1102final_Page_437_Image_0001.png)
Fig. 10.3Pipe sticking caused by wellboreinstability.
Key Seating. Key seating is a major cause of mechanical pipe
sticking. The mechanics of key seating involvewearing a small hole
(groove) into the side of a full-gauge hole. This groove is caused
by the drillstring rotationwith side force acting on it. Fig. 10.4
illustrates pipe sticking caused by key seating. This condition is
created
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either in doglegs or in undetected ledges near washouts. The
lateral force that tends to push the pipe against thewall, which
causes mechanical erosion and thus creates a key seat, is given
by
(/File%3ADevol2_1102final_Page_438_Image_0001.png)
Fig. 10.4Pipe sticking caused by key seat.
(/File%3AVol2_page_0436_eq_001.png)....................(10.5)
where Fl is the lateral force, T is the tension in the
drillstring just above the key-seat area, and dl is the
abruptchange in hole angle (commonly referred to as dogleg
angle).Generally, long bit runs can cause key seats; therefore, it
is common practice to make wiper trips. Also, the useof stiffer
BHAs tends to minimize severe dogleg occurrences. During tripping
out of hole, a key-seatpipe-sticking problem is indicated when
several stands of pipe have been pulled out, and then, all of a
sudden,the pipe is stuck.Freeing mechanically stuck pipe can be
undertaken in a number of ways, depending on what caused
thesticking. For example, if cuttings accumulation or hole
sloughing is the suspected cause, then rotating andreciprocating
the drillstring and increasing flow rate without exceeding the
maximum allowed equivalentcirculating density (ECD) is a possible
remedy for freeing the pipe. If hole narrowing as a result of
plastic shaleis the cause, then an increase in mud weight may free
the pipe. If hole narrowing as a result of salt is the cause,then
circulating fresh water can free the pipe. If the pipe is stuck in
a key-seat area, the most likely successfulsolution is backing off
below the key seat and going back into the hole with an opener to
drill out the keysection. This will lead to a fishing operation to
retrieve the fish. The decision on how long to continueattempting
to free stuck pipe vs. back off, plug back, and then sidetrack is
an economic issue that generally isaddressed by the operating
company.
Loss of CirculationDefinitionLost circulation is defined as the
uncontrolled flow of whole mud into a formation, sometimes referred
to asthief zone. Fig. 10.5 shows partial and total lost-circulation
zones. In partial lost circulation, mud continues toflow to surface
with some loss to the formation. Total lost circulation, however,
occurs when all the mud flowsinto a formation with no return to
surface. If drilling continues during total lost circulation, it is
referred to asblind drilling. This is not a common practice in the
field unless the formation above the thief zone is
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mechanically stable, there is no production, and the fluid is
clear water. Blind drilling also may continue if it iseconomically
feasible and safe.
(/File%3ADevol2_1102final_Page_439_Image_0001.png)
Fig. 10.5Lost-circulation zones.
Lost-Circulation Zones and CausesFormations that are inherently
fractured, cavernous, or have high permeability are potential zones
of lostcirculation. In addition, under certain improper drilling
conditions, induced fractures can become potentialzones of lost
circulation. The major causes of induced fractures are excessive
downhole pressures and settingintermediate casing, especially in
the transition zone, too high.Induced or inherent fractures may be
horizontal at shallow depth or vertical at depths greater
thanapproximately 2,500 ft. Excessive wellbore pressures are caused
by high flow rates (high annular-frictionpressure loss) or tripping
in too fast (high surge pressure), which can lead to mud ECD. In
addition, improperannular hole cleaning, excessive mud weight, or
shutting in a well in high-pressure shallow gas can
inducefractures, which can cause lost circulation. Eqs. 10.6 and
10.7 show the conditions that must be maintained toavoid fracturing
the formation during drilling and tripping in, respectively.
(/File%3AVol2_page_0483_eq_001.png)....................(10.6)
(/File%3AVol2_page_0438_eq_002.png)....................(10.7)
where mh = static mud weight, af = additional mud weight caused
by friction pressure loss in annulus, s =additional mud caused by
surge pressure, frac = formation-pressure fracture gradient in
equivalent mud weight,and eq = equivalent circulating density of
mud.Cavernous formations are often limestones with large caverns.
This type of lost circulation is quick, total, andthe most
difficult to seal. High-permeability formations that are potential
lost-circulation zones are those ofshallow sand with permeability
in excess of 10 darcies. Generally, deep sand has low permeability
and presentsno loss-of-circulation problems. In noncavernous thief
zones, mud level in mud tanks decreases gradually and, ifdrilling
continues, total loss of circulation may occur.
Prevention of Lost CirculationThe complete prevention of lost
circulation is impossible because some formations, such as
inherently fractured,cavernous, or high-permeability zones, are not
avoidable if the target zone is to be reached. However,
limiting
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circulation loss is possible if certain precautions are taken,
especially those related to induced fractures. Theseprecautions
include maintaining proper mud weight, minimizing annular-friction
pressure losses during drillingand tripping in, adequate hole
cleaning, avoiding restrictions in the annular space, setting
casing to protect upperweaker formations within a transition zone,
and updating formation pore pressure and fracture gradients
forbetter accuracy with log and drilling data. If lost-circulation
zones are anticipated, preventive measures shouldbe taken by
treating the mud with lost-circulation materials (LCMs).
Remedial MeasuresWhen lost circulation occurs, sealing the zone
is necessary unless the geological conditions allow blind
drilling,which is unlikely in most cases. The common LCMs that
generally are mixed with the mud to seal loss zonesmay be grouped
as fibrous, flaked, granular, and a combination of fibrous, flaked,
and granular materials.These materials are available in course,
medium, and fine grades for an attempt to seal
low-to-moderatelost-circulation zones. In the case of severe lost
circulations, the use of various plugs to seal the zone
becomesmandatory. It is important, however, to know the location of
the lost-circulation zone before setting a plug.Various types of
plugs used throughout the industry include bentonite/diesel-oil
squeeze, cement/bentonite/diesel-oil squeeze, cement, and barite.
Squeeze refers to forcing fluid into the lost-circulation zone.
Hole DeviationDefinitionHole deviation is the unintentional
departure of the drill bit from a preselected borehole trajectory.
Whetherdrilling a straight or curved-hole section, the tendency of
the bit to walk away from the desired path can lead tohigher
drilling costs and lease-boundary legal problems. Fig. 10.6
provides examples of hole deviations.
(/File%3ADevol2_1102final_Page_440_Image_0001.png)
Fig. 10.6Example of hole deviations.
CausesIt is not exactly known what causes a drill bit to deviate
from its intended path. It is, however, generally agreedthat one or
a combination of several of the following factors may be
responsible for the deviation:
Heterogeneous nature of formation and dip angle.Drillstring
characteristics, specifically the BHA makeup.Stabilizers (location,
number, and clearances).
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Applied weight on bit (WOB).Hole-inclination angle from
vertical.Drill-bit type and its basic mechanical design.Hydraulics
at the bit.Improper hole cleaning.
It is known that some resultant force acting on a drill bit
causes hole deviation to occur. The mechanics of thisresultant
force is complex and is governed mainly by the mechanics of the
BHA, rock/bit interaction, bitoperating conditions, and, to some
lesser extent, by the drilling-fluid hydraulics. The forces
imparted to the drillbit because of the BHA are directly related to
the makeup of the BHA (i.e., stiffness, stabilizers, and
reamers).The BHA is a flexible, elastic structural member that can
buckle under compressive loads. The buckled shape ofa given
designed BHA depends on the amount of applied WOB. The significance
of the BHA buckling is that itcauses the axis of the drill bit to
misalign with the axis of the intended hole path, thus causing the
deviation.Pipe stiffness and length and the number of stabilizers
(their location and clearances from the wall of thewellbore) are
two major parameters that govern BHA buckling behavior. Actions
that can minimize the bucklingtendency of the BHA include reducing
WOB and using stabilizers with outside diameters that are almost
ingauge with the wall of the borehole.The contribution of the
rock/bit interaction to bit deviating forces is governed by rock
properties (cohesivestrength, bedding or dip angle, internal
friction angle); drill-bit design features (tooth angle, bit size,
bit type, bitoffset in case of roller-cone bits, teeth location and
number, bit profile, bit hydraulic features); and
drillingparameters (tooth penetration into the rock and its cutting
mechanism). The mechanics of rock/bit interaction isa very complex
subject and is the least understood in regard to hole-deviation
problems. Fortunately, the adventof downhole
measurement-while-drilling tools that allow monitoring the advance
of the drill bit along thedesired path makes our lack of
understanding of the mechanics of hole deviation more
acceptable.
Drillpipe FailuresDrillpipe failures can be put into one of the
following categories: twistoff caused by excessive torque;
partingbecause of excessive tension; burst or collapse because of
excessive internal pressure or external pressure,respectively; or
fatigue as a result of mechanical cyclic loads with or without
corrosion.
TwistoffPipe failure as a result of twistoff occurs when the
induced shearing stress caused by high torque exceeds
thepipe-material ultimate shear stress. In vertical-well drilling,
excessive torques are not generally encounteredunder normal
drilling practices. In directional and extended-reach drilling,
however, torques in excess of 80,000lbf-ft are common and easily
can cause twistoff to improperly selected drillstring
components.
PartingPipe-parting failure occurs when the induced tensile
stress exceeds the pipe-material ultimate tensile stress.
Thiscondition may arise when pipe sticking occurs, and an overpull
is applied in addition to the effective weight ofsuspended pipe in
the hole above the stuck point.
Collapse and BurstPipe failure as a result of collapse or burst
is rare; however, under extreme conditions of high mud weight
andcomplete loss of circulation, pipe burst may occur.
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FatigueFatigue is a dynamic phenomenon that may be defined as
the initiation of microcracks and their propagationinto macrocracks
as a result of repeated applications of stresses. It is a process
of localized progressivestructural fractures in material under the
action of dynamic stresses. It is well established that a
structuralmember that may not fail under a single application of
static load may very easily fail under the same load if it
isapplied repeatedly. Failure under cyclic (repeated) loads is
called fatigue failure.Drillstring fatigue failure is the most
common and costly type of failure in oil/gas and geothermal
drillingoperations. The combined action of cyclic stresses and
corrosion can shorten the life expectancy of a drillpipeby thousand
folds. Cyclic stresses are induced by dynamic loads caused by
drillstring vibrations andbending-load reversals in curved sections
of hole and doglegs caused by rotation. Pipe corrosion occurs
duringthe presence of O2, CO2, chlorides, and/or H2S. H2S is the
most severely corrosive element to steel pipe, and itis deadly to
humans. Regardless of what may have caused pipe failure, the cost
of fishing operations and thesometimes unsuccessful attempts to
retrieve the fish out of the hole can lead to the loss of millions
of dollars inrig downtime, loss of expensive tools downhole, or
abandonment of the already-drilled section below the fish.In spite
of the vast amount of work that has been dedicated to pipe fatigue
failure, it is still the least understood.This lack of
understanding is attributed to the wide variations of statistical
data in determining type of serviceand environment of the
drillstring, magnitude of operating loads and frequency of
occurrence (load history),accuracy of methods in determining the
stresses, quality control during manufacturing, and the
applicability ofmaterial fatigue data.
Pipe-Failure PreventionAlthough pipe failure cannot be
eliminated totally, there are certain measures that can be taken to
minimize it.Fatigue failures can be mitigated by minimizing induced
cyclic stresses and insuring a noncorrosive environmentduring the
drilling operations. Cyclic stresses can be minimized by
controlling dogleg severity and drillstringvibrations. Corrosion
can be mitigated by corrosive scavengers and controlling the mud pH
in the presence ofH2S. The proper handling and inspection of the
drillstring on a routine basis are the best measures to
preventfailures.
Borehole InstabilityDefinition and CausesBorehole instability is
the undesirable condition of an openhole interval that does not
maintain its gauge sizeand shape and/or its structural integrity.
The causes can be grouped into the following categories:
mechanicalfailure caused by in-situ stresses, erosion caused by
fluid circulation, and chemical caused by interaction ofborehole
fluid with the formation.
Types and Associated ProblemsThere are four different types of
borehole instabilities: hole closure or narrowing, hole enlargement
or washouts,fracturing, and collapse. Fig. 10.7 illustrates
hole-instability problems.
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(/File%3ADevol2_1102final_Page_442_Image_0001.png)
Fig. 10.7Types of hole instability problems.
Hole Closure. Hole closure is a narrowing time-dependent process
of borehole instability. It sometimes isreferred to as creep under
the overburden pressure, and it generally occurs in plastic-flowing
shale and saltsections. Problems associated with hole closure are
an increase in torque and drag, an increase in potential
pipesticking, and an increase in the difficulty of casings
landing.Hole Enlargement. Hole enlargements are commonly called
washouts because the hole becomes undesirablylarger than intended.
Hole enlargements are generally caused by hydraulic erosion,
mechanical abrasion causedby drillstring, and inherently sloughing
shale. The problems associated with hole enlargement are an
increase incementing difficulty, an increase in potential hole
deviation, an increase in hydraulic requirements for effectivehole
cleaning, and an increase in potential problems during logging
operations.Fracturing. Fracturing occurs when the wellbore
drilling-fluid pressure exceeds the formation-fracturepressure. The
associated problems are lost circulation and possible kick
occurrence.Collapse. Borehole collapse occurs when the
drilling-fluid pressure is too low to maintain the structural
integrityof the drilled hole. The associated problems are pipe
sticking and possible loss of well.
Principles of Borehole InstabilityBefore drilling, the rock
strength at some depth is in equilibrium with the in-situ rock
stresses (effectiveoverburden stress, effective horizontal
confining stresses). While a hole is being drilled, however, the
balancebetween the rock strength and the in-situ stresses is
disturbed. In addition, foreign fluids are introduced, and
aninteraction process begins between the formation and borehole
fluids. The result is a potential hole-instabilityproblem. Although
a vast amount of research has resulted in many borehole-stability
simulation models, allshare the same shortcoming of uncertainty in
the input data needed to run the analysis. Such data include
in-situstresses, pore pressure, rock mechanical properties, and, in
the case of shale, formation and drilling-fluidschemistry.
Mechanical Rock-Failure MechanismsMechanical borehole failure
occurs when the stresses acting on the rock exceed the compressive
or the tensilestrength of the rock. Compressive failure is caused
by shear stresses as a result of low mud weight, while
tensilefailure is caused by normal stresses as a result of
excessive mud weight.The failure criteria that are used to predict
hole-instability problems are the maximum-normal-stress
criterionfor tensile failure and the maximum strain energy of
distortion criterion for compressive failure. In the
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maximum-normal-stress criterion, failure is said to occur when,
under the action of combined stresses, one ofthe acting principal
stresses reaches the failure value of the rock tensile strength. In
the maximum of energy ofdistortion criterion, failure is said to
occur when, under the action of combined stresses, the energy of
distortionreaches the same energy of failure of the rock under pure
tension.
Shale InstabilityMore than 75% of drilled formations worldwide
are shale formations. The drilling cost attributed to
shale-instability problems is reported to be in excess of one-half
billion U.S dollars per year. The cause of shaleinstability is
two-fold: mechanical (stress change vs. shale strength environment)
and chemical (shale/fluidinteractioncapillary pressure, osmotic
pressure, pressure diffusion, borehole-fluid invasion into
shale).Mechanical Instability. As stated previously, mechanical
rock instability can occur because the in-situ stressstate of
equilibrium has been disturbed after drilling. The mud in use with
a certain density may not bring thealtered stresses to the original
state; therefore, shale may become mechanically unstable.Chemical
Instability. Chemical-induced shale instability is caused by the
drilling-fluid/shale interaction, whichalters shale mechanical
strength as well as the shale pore pressure in the vicinity of the
borehole walls. Themechanisms that contribute to this problem
include capillary pressure, osmotic pressure, pressure diffusion
inthe vicinity of the borehole walls, and borehole-fluid invasion
into the shale when drilling overbalanced.Capillary Pressure.
During drilling, the mud in the borehole contacts the native pore
fluid in the shale throughthe pore-throat interface. This results
in the development of capillary pressure, pcap , which is expressed
as
(/File%3AVol2_page_0443_eq_001.png)....................(10.8)
where is the interfacial tension, is the contact angle between
the two fluids, and r is the pore-throat radius.To prevent borehole
fluids from entering the shale and stabilizing it, an increase in
capillary pressure is required,which can be achieved with oil-based
or other organic low-polar mud systems.Osmotic Pressure. When the
energy level or activity in shale pore fluid, as , is different
from the activity indrilling mud, am , water movement can occur in
either direction across a semipermeable membrane as a result ofthe
development of osmotic pressure, pos , or chemical potential, c .
To prevent or reduce water movementacross this semipermeable
membrane that has certain efficiency, Em, the activities need to be
equalized or, atleast, their differentials minimized. If am is
lower than as , it is suggested to increase Em and vice versa.
Themud activity can be reduced by adding electrolytes that can be
brought about through the use of mud systemssuch as seawater,
saturated-salt/polymer, KCl/NaCl/polymer, and lime/gypsum.Pressure
Diffusion. Pressure diffusion is a phenomenon of pressure change
near the borehole walls that occursover time. This pressure change
is caused by the compression of the native pore fluid by the
borehole-fluidpressure, pwfl, and the osmotic pressure,
pos.Borehole Fluid Invasion into Shale. In conventional drilling, a
positive differential pressure (the differencebetween the
borehole-fluid pressure and the pore-fluid pressure) is always
maintained. As a result, boreholefluid is forced to flow into the
formation (fluid-loss phenomenon), which may cause chemical
interaction thatcan lead to shale instabilities. To mitigate this
problem, an increase of mud viscosity or, in extreme
cases,gilsonite is used to seal off microfractures.
Wellbore-Stability AnalysisSeveral models in the literature
address wellbore-stability analysis.[2] These include very-simple
tovery-complex models such as linear elastic, nonlinear,
elastoplastic, purely mechanical, and physicochemical.Regardless of
the model, the data needed include rock properties (Poisson ratio,
strength, modulus of elasticity);
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in-situ stresses (overburden, horizontal); pore-fluid pressure
and chemistry; and mud properties and chemistry.Other than the mud
data, the data are often compounded with problems of availability
and/or uncertainties.However, sensitivity analysis can be conducted
by assuming data for the many variables to establish safetywindows
for mud selection and design.
Borehole-Instability PreventionTotal prevention of borehole
instability is unrealistic because restoring the physical and
chemical in-situconditions of the rock is impossible. However, the
drilling engineer can mitigate the problems of
boreholeinstabilities by adhering to good field practices. These
practices include proper mud-weight selection andmaintenance, the
use of proper hydraulics to control the ECD, proper hole-trajectory
selection, and the use ofborehole fluid compatible with the
formation being drilled. Additional field practices that should be
followed areminimizing time spent in open hole; using offset-well
data (use of the learning curve); monitoring trend changes(torque,
circulating pressure, drag, fill-in during tripping); and
collaborating and sharing information.
Mud ContaminationDefinitionA mud is said to be contaminated when
a foreign material enters the mud system and causes
undesirablechanges in mud properties, such as density, viscosity,
and filtration. Generally, water-based mud systems are themost
susceptible to contamination. Mud contamination can result from
overtreatment of the mud system withadditives or from material
entering the mud during drilling.
Common Contaminants, Sources, and TreatmentsThe most common
contaminants to water-based mud systems are solids (added, drilled,
active, inert);gypsum/anhydrite (Ca++); cement/lime (Ca++); makeup
water (Ca++, Mg++); soluble bicarbonates andcarbonates (HCO3, CO3);
soluble sulfides (HS, S); and salt/salt water flow (Na+, Cl).Solids
Contamination. Solids are materials that are added to make up a mud
system (bentonite, barite) andmaterials that are drilled (active
and inert). Excess solids of any type are the most undesirable
contaminant todrilling fluids. They affect all mud properties. It
has been shown that fine solids, micron and submicron sized,are the
most detrimental to the overall drilling efficiency and must be
removed if they are not a necessary partof the mud makeup. The
removal of drilled solids is achieved through the use of mechanical
separatingequipment (shakers, desanders, desilters, and
centrifuges). Shakers remove solids in the size of
cuttings(approximately 140 or larger). Desanders remove solids in
the size of sand (down to 50). Desilters removesolids in the size
of silt (down to 20). When solids become smaller than the cutoff
point of desilters,centrifuges may have to be used. Chemical
flocculants are sometimes used to flocculate fine solids into a
biggersize so that they can be removed by solids-removal equipment.
Total flocculants do not discriminate betweenvarious types of
solids, while selective flocculants will flocculate drilled solids
but not the added barite solids.As a last resort, dilution is
sometimes used to lower solids concentration.Calcium-Ions
Contamination. The sources of calcium ions are gypsum, anhydrite,
cement, lime, seawater, andhard/brackish makeup water. The calcium
ion is a major contaminant to freshwater-based sodium-clay
treatedmud systems. The calcium ion tends to replace the sodium
ions on the clay surface through a base exchange,thus causing
undesirable changes in mud properties such as rheology and
filtration. It also causes added thinnersto the mud system to
become ineffective. The treatment depends on the source of the
calcium ion. For example,sodium carbonate (soda ash) is used if the
source is gypsum or anhydrite. Sodium bicarbonate is the
preferredtreatment if the calcium ion is from lime or cement. If
treatment becomes economically unacceptable, breakover to a mud
system, such as gypsum mud or lime mud, that can tolerate the
contaminant.
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Biocarbonate and Carbonate Contamination. The contaminant ions
(CO3, HCO3) are from drilling aCO2-bearing formation, thermal
degradation of organics in mud, or over treatment with soda ash
andbicarbonate. These contaminants cause the mud to have high yield
and gel strength and a decrease in pH.Treating the mud system with
gypsum or lime is recommended.
Hydrogen Sulfide Contamination. The contaminant ions (HS, S)
generally are from drilling an H2S-bearingformation. Hydrogen
sulfide is the most deadly ion to humans and is extremely corrosive
to steel used duringdrilling operations. (It causes severe
embrittlement to drillpipe.) Scavenging of H2S is done by use of
zinc,copper, or iron.Salt/Saltwater Flows. The ions, Na + Cl , that
enter the mud system as a result of drilling salt sections or
fromformation saltwater flow cause a mud to have high yield
strength, high fluid loss, and pH decrease. Someactions for
treatment are dilution with fresh water, the use of dispersants and
fluid-loss chemicals, or conversionto a mud that tolerates the
problem if the cost of treatment becomes excessive.
Producing Formation DamageIntroductionProducing formation damage
has been defined as the impairment of the unseen by the inevitable,
causing anunknown reduction in the unquantifiable. In a different
context, formation damage is defined as the impairmentto reservoir
(reduced production) caused by wellbore fluids used during
drilling/completion and workoveroperations. It is a zone of reduced
permeability within the vicinity of the wellbore (skin) as a result
offoreign-fluid invasion into the reservoir rock. Fig. 10.8
illustrates formation skin damage.
(/File%3ADevol2_1102final_Page_446_Image_0001.png)
Fig. 10.8Formation skin damage.
Borehole FluidsBorehole fluids are classified as drilling
fluids, completion fluids, or workover fluids. Drilling fluids
arecategorized as mud, gas, or gasified mud. There are two types of
mud: water-based (pure polymer, purebentonite, bentonite/polymer)
and oil-based (invert emulsion, oil). Completion and workover
fluids are mostlybrines and are solids free.
Damage Mechanisms
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Formation damage is a combination of several mechanisms
including solids plugging, clay-particle swelling ordispersion,
saturation changes, wettability reversal, emulsion blockage,
aqueous-filtrate blockage, and mutualprecipitation of soluble salts
in wellbore-fluid filtrate and formation water.Solids Plugging.
Fig. 10.9 shows that the plugging of the reservoir-rock pore spaces
can be caused by the finesolids in the mud filtrate or solids
dislodged by the filtrate within the rock matrix. To minimize this
form ofdamage, minimize the amount of fine solids in the mud system
and fluid loss.
(/File%3ADevol2_1102final_Page_446_Image_0002.png)
Fig. 10.9Formation damage caused by solidsplugging.
Clay-Particle Swelling. This is an inherent problem in sandstone
that contains water-sensitive clays. When afresh-water filtrate
invades the reservoir rock, it will cause the clay to swell and
thus reduce or totally block thethroat areas.Saturation Change.
Production is predicated on the amount of saturation within the
reservoir rock. When amud-system filtrate enters the reservoir, it
will cause some change in water saturation and, therefore,
potentialreduction in production. Fig. 10.10 shows that high fluid
loss causes water saturation to increase, which resultsin a
decrease of rock relative permeability. See the chapter on
transport properties in the General Engineeringvolume of this
Handbook for additional information.
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(/File%3ADevol2_1102final_Page_447_Image_0001.png)
Fig. 10.10Formation damage caused bysaturation.
Wettability Reversal. Reservoir rocks are water-wet in nature.
It has been demonstrated that while drilling withoil-based mud
systems, excess surfactants in the mud filtrate that enter the rock
can cause wettability reversal.It has been reported from field
experience and demonstrated in laboratory tests that as much as 90%
inproduction loss can be caused by this mechanism. Therefore, to
guard against this problem, the amount ofexcess surfactants used in
oil-based mud systems should be kept at a minimum.Emulsion
Blockage. Inherent in oil-based mud systems is the use of excess
surfactants. These surfactants enterthe rock and can form an
emulsion within the pore spaces, which hinders production through
emulsionblockage.Aqueous-Filtrate Blockage. While drilling with
water-based mud, the aqueous filtrate that enters the reservoircan
cause some blockage that will reduce the production potential of
the reservoir.Precipitation of Soluble Salts. Any precipitation of
soluble salts, whether from the use of salt mud systems orfrom
formation water or both, can cause solids blockage and hinder
production. For more information, see theFormation Damage chapter
in the Production Operations Engineering volume of this
Handbook.
Hole CleaningIntroductionThroughout the last decade, many
studies have been conducted to gain understanding on hole cleaning
indirectional-well drilling. Laboratory work has demonstrated that
drilling at an inclination angle greater thanapproximately 30 from
vertical poses problems in cuttings removal that are not
encountered in vertical wells.Fig. 10.11 illustrates that the
formation of a moving or stationary cuttings bed becomes an
apparent problem ifthe flow rate for a given mud rheology is below
a certain critical value.
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(/File%3ADevol2_1102final_Page_448_Image_0001.png)
Fig. 10.11Cuttings-bed buildup in directionalwells.
Inadequate hole cleaning can lead to costly drilling problems
such as mechanical pipe sticking, premature bitwear, slow drilling,
formation fracturing, excessive torque and drag on drillstring,
difficulties in logging andcementing, and difficulties in casings
landing. The most prevalent problem is excessive torque and drag,
whichoften leads to the inability of reaching the target in
high-angle/extended-reach drilling.
Factors in Hole CleaningAnnular-Fluid Velocity. Flow rate is the
dominant factor in cuttings removal while drilling directional
wells. Anincrease in flow rate will result in more efficient
cuttings removal under all conditions. However, how high aflow rate
can be increased may be limited by the maximum allowed ECD, the
susceptibility of the openholesection to hydraulic erosion, and the
availability of rig hydraulic power.Hole Inclination Angle.
Laboratory work has demonstrated that when hole angle increases
from zero toapproximately 67 from vertical, hole cleaning becomes
more difficult, and therefore, flow-rate requirementincreases. The
flow-rate requirements reach a maximum at approximately 65 to 67
and then slightly decreasetoward the horizontal. Also, it has been
shown that at 25 to approximately 45, a sudden pump shutdown
cancause cuttings sloughing to bottom and may result in a
mechanical pipe-sticking problem. Although, holeinclination can
lead to cleaning problems, it is mandated by the needs of drilling
inaccessible reservoir, offshoredrilling, avoiding troublesome
formations, and side tracking and to drill horizontally into the
reservoir.Objectives in total field development (primary and
secondary production), environmental concerns, andeconomics are
some of the factors that intervene in hole angle
selection.Drillstring Rotation. Laboratory studies have shown and
field cases have reported that drillstring rotation hasmoderate to
significant effects in enhancing hole cleaning. The level of
enhancement is a combined effect ofpipe rotation, mud rheology,
cuttings size, flow rate, and, very importantly, the string dynamic
behavior. It hasbeen proved that the whirling motion of the string
around the wall of the borehole when it rotates is the
majorcontributor to hole cleaning enhancement. Also, mechanical
agitation of the cuttings bed on the low side of thehole and
exposing the cuttings to higher fluid velocities when the pipe
moves to the high side of the hole areresults of pipe whirling
action.Although there is a definite gain in hole cleaning caused by
pipe rotation, there are certain limitations to itsimplementation.
For example, during angle building with a downhole motor (sliding
mode), rotation cannot beinduced. With the new steering rotary
systems, this is no longer a problem. However, pipe rotation can
causecyclic stresses that can accelerate pipe failures due to
fatigue, casing wear, and, in some cases, mechanicaldestruction to
openhole sections. In slimhole drilling, high pipe rotation can
cause high ECDs due to the high
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annular-friction pressure losses.Hole/Pipe Eccentricity. In the
inclined section of the hole, the pipe has the tendency to rest on
the low side ofthe borehole because of gravity. This creates a very
narrow gap in the annulus section below the pipe, whichcauses fluid
velocity to be extremely low and, therefore, the inability to
transport cuttings to surface. As Fig.10.12 illustrates, when
eccentricity increases, particle/fluid velocities decrease in the
narrow gap, especially forhigh-viscosity fluid. However, because
eccentricity is governed by the selected well trajectory, its
adverseimpact on hole cleaning may be unavoidable.
(/File%3ADevol2_1102final_Page_450_Image_0001.png)
Fig. 10.12Fluid velocity profile in eccentricannulus (after
Hzouz et al.[3]).
Rate of Penetration. Under similar conditions, an increase in
the drilling rate always results in an increase in theamount of
cuttings in the annulus. To ensure good hole cleaning during
high-rate-of-penetration (ROP) drilling,the flow rate and/or pipe
rotation have to be adjusted. If the limits of these two variables
are exceeded, the onlyalternative is to reduce the ROP. Although a
decrease in ROP may have a detrimental impact on drilling costs,the
benefit of avoiding other drilling problems, such as mechanical
pipe sticking or excessive torque and drag,can outweigh the loss in
ROP.Mud Properties. The functions of drilling fluids are many and
can have unique competing influences. The twomud properties that
have direct impact on hole cleaning are viscosity and density. The
main functions of densityare mechanical borehole stabilization and
the prevention of formation-fluid intrusion into the annulus.
Anyunnecessary increase in mud density beyond fulfilling these
functions will have an adverse effect on the ROPand, under the
given in-situ stresses, may cause fracturing of the formation. Mud
density should not be used as acriterion to enhance hole
cleaning.Viscosity, on the other hand, has the primary function of
the suspension of added desired weighting materialssuch as barite.
Only in vertical-well drilling and high-viscosity pill sweep is
viscosity used as a remedy in holecleaning.Cuttings
Characteristics. The size, distribution, shape, and specific
gravity of cuttings affect their dynamicbehavior in a flowing
media. The specific gravity of most rocks is approximately 2.6;
therefore, specific gravitycan be considered a nonvarying factor in
cuttings transport. The cuttings size and shape are functions of
the bittypes (roller cone, polycrystalline-diamond compact, diamond
matrix), the regrinding that takes place after theyare generated,
and the breakage by their own bombardment and with the rotating
drillstring. It is impossible tocontrol their size and shape even
if a specific bit group has been selected to generate them. Smaller
cuttings aremore difficult to transport in directional-well
drilling; however, with some viscosity increase and pipe
rotation,
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fine particles seem to stay in suspension and, therefore, are
easier to transport.
Hydrogen-Sulfide-Bearing Zones and Shallow GasDrilling
H2S-bearing formations poses one of the most difficult and
dangerous problems to humans andequipment. If it is known or
anticipated, there are very specific requirements to abide by in
accordance withIntl. Assn. of Drilling Contractors rules and
regulations. Shallow gas may be encountered at any time in
anyregion of the world. The only way to combat this problem is to
never shut in the well; divert the gas flowthrough a diverter
system instead. High-pressure shallow gas can be encountered at
depths as low as a fewhundred feet where the formation-fracture
gradient is very low. The danger is that if the well is shut
in,formation fracturing is more likely to occur, which will result
in the most severe blowout problem, undergroundblow.
Equipment and Personnel-Related ProblemsEquipmentThe integrity
of drilling equipment and its maintenance are major factors in
minimizing drilling problems. Properrig hydraulics (pump power) for
efficient bottom and annular hole cleaning, proper hoisting power
for efficienttripping out, proper derrick design loads and drilling
line tension load to allow safe overpull in case of a
stickingproblem, and well-control systems (ram preventers, annular
preventers, internal preventers) that allow kickcontrol under any
kick situation are all necessary for reducing drilling problems.
Proper monitoring andrecording systems that monitor trend changes
in all drilling parameters and can retrieve drilling data at a
laterdate, proper tubular hardware specifically suited to
accommodate all anticipated drilling conditions, andeffective
mud-handling and maintenance equipment that will ensure that the
mud properties are designed fortheir intended functions are also
necessary.
PersonnelGiven equal conditions during drilling/completion
operations, personnel are the key to the success or failure ofthose
operations. Overall well costs as a result of any
drilling/completion problem can be extremely high;therefore,
continuing education and training for personnel directly or
indirectly involved is essential tosuccessful drilling/completion
practices.
Nomenclature
m = activity in drilling mud, dimensionlesss = activity in shale
pore fluid, dimensionlessAc = area of contact, L2 , in.2Dh =
diameter of the hole, L, in.Dop = outside diameter of the pipe, L,
in.Em = efficiency, dimensionlessf = coefficient of friction,
dimensionlessFl = lateral force, F, lbfFp = pull force, F, lbf
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hmc = mudcake thickness, L, in.Lep = length of the permeable
zone, L, in.pcap = capillary pressure, F/L2, psipff =
formation-fluid pressure, F/L2, psipm = mud pressure, F/L2, psipos
= osmotic pressure, F/L2, psir = pore-throat radius, L, in.T =
tension in the drillstring just above the key-seat area, F, lbfp =
differential pressure, F/L2 , psif = additional mud weight caused
by friction pressure loss in annulus,F/L3, lbm/gals = additional
mud weight caused by surge pressure, F/L3, lbm/gal = contact angle
between the two fluids, degreesdl = abrupt change in hole angle,
degreeseq = equivalent mud circulating density, F/L3, lbm/galfrac =
formation-pressure fracture gradient in equivalent mud weight,
F/L3,lbm/galmh = static mud weight, F/L3, lbm/galc = chemical
potential, dimensionless = interfacial tension, F/L, lbf/in.
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SI Metric Conversion Factors
ft 3.048* E 01 = mgal 3.785 412 E 03 = m3in. 2.54* E + 00 =
cm
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in.2 6.451 6* E + 00 = cm2lbf 4.448 222 E + 00 = Nlbm 4.535 924
E 01 = kg= kPa*Conversion factor is exact.Category
(/Special%3ACategories): PEH (/Category%3APEH)
(http://www.addthis.com/bookmark.php?v=300&pubid=ra-52d6c17f4a5b0215)
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