Drilling Problems and Solutions

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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

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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.


From Bourgoyne[1],

(/File%3AVol2_page_0434_eq_003.png)....................(10.3)where


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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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.


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


Fig. 10.4Pipe sticking caused by key seat.


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.


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.



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.


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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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


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.


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.


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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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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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.


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.

References

Bourgoyne, A.T., Millheim, K.K., Chenevert , M.E. et al. 1986. Applied Drilling Engineering.Richardson, Texas: Textbook Series, SPE.

1. McLean, M.R. and Addis, M.A. 1990. Wellbore Stability Analysis: A Review of Current Methods ofAnalysis and Their Field Application. Presented at the SPE/IADC Drilling Conference, Houston, Texas,27 February-2 March. SPE-19941-MS. http://dx.doi.org/10.2118/19941-MS (http://dx.doi.org/10.2118/19941-MS).

2.

Azouz, I., Shirazi, S.A., Pilehvari, A. et al. 1993. Numerical Simulation of Laminar Flow of Yield-Power-Law Fluids in Conduits of Arbitrary Cross-Section. Trans. of ASME 115 (4): 710-716.

3.

General ReferencesAadnoy, B.S. 1988. Modeling of the Stability of Highly Inclined Boreholes in Anisotropic Rock Formations(includes associated papers 19213 and 19886 ). SPE Drill Eng 3 (3): 259-268. SPE-16526-PA. http://dx.doi.org/10.2118/16526-PA (http://dx.doi.org/10.2118/16526-PA).Aadnoy, B.S. 1996. Modern Well Design. Rotterdam, The Netherlands: Taylor & Francis.Abrams, A. 1977. Mud Design To Minimize Rock Impairment Due To Particle Invasion. SPE Journal ofPetroleum Technology 29 (5): 586-592. SPE-5713-PA. http://dx.doi.org/10.2118/5713-PA (http://dx.doi.org/10.2118/5713-PA).

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Annis, M.R. and Monaghan, P.H. 1962. Differential Pressure Sticking-Laboratory Studies of Friction BetweenSteel and Mud Filter Cake. J Pet Technol 14 (5): 537-543. http://dx.doi.org/10.2118/151-PA (http://dx.doi.org/10.2118/151-PA).Azar, J.J. and Samuel, G.R. 2007. Drilling Engineering. Tulsa, Oklahoma: PennWell Corporation.Azar, J.J. and Lummus, J.L. 1975. The Effect of Drill Fluid pH on Drill Pipe Corrosion Fatigue Performance.Presented at the Fall Meeting of the Society of Petroleum Engineers of AIME, Dallas, Texas, 28 September-1October. SPE-5516-MS. http://dx.doi.org/10.2118/5516-MS (http://dx.doi.org/10.2118/5516-MS).Azar, J.J. and Sanchez, R.A. 1997. Important Issues in Cuttings Transport for Drilling Directional Wells.Presented at the Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, Brazil, 30August-3 September. SPE-39020-MS. http://dx.doi.org/10.2118/39020-MS (http://dx.doi.org/10.2118/39020-MS).Becker, T.E., Azar, J.J., and Okrajni, S.S. 1991. Correlations of Mud Rheological Properties With Cuttings-Transport Performance in Directional Drilling. SPE Drill Eng 6 (1): 16-24. SPE-19535-PA. http://dx.doi.org/10.2118/19535-PA (http://dx.doi.org/10.2118/19535-PA).Billingston, S.A. 1963. Practical Approach to Circulation Problems. Drilling Contractor.(JulyAugust) 52.Bourgoyne Jr., A.T., Caudle, B.H., and Kimbler, O.K. 1972. The Effect of Interfacial Films on the Displacementof Oil by Water Porous Media. Society of Petroleum Engineers Journal 12 (1): 60-68. SPE-3272-PA.http://dx.doi.org/10.2118/3272-PA (http://dx.doi.org/10.2118/3272-PA).Bradley, W.B. 1974. Deviation Forces from the Wedge Penetration Failure of Anisotropic Rock. ASME Trans.,95, Series B, No. 4, 1093.Bradley, W.B. 1975. Factors Affecting the Control of Borehole Angle In Straight and Directional Wells. SPEJournal of Petroleum Technology 27 (6): 679-688. SPE-5070-PA. http://dx.doi.org/10.2118/5070-PA(http://dx.doi.org/10.2118/5070-PA).Bradley, W.B. 1979. Failures of Inclined Boreholes. J. of Energy Resources Tech. 101 (4): 232-239.Bradley, W.B., Jarman, D., Plott, R.S. et al. 1991. A Task Force Approach to Reducing Stuck Pipe Costs.Presented at the SPE/IADC Drilling Conference, Amsterdam, Netherlands, 11-14 March. SPE-21999-MS.http://dx.doi.org/10.2118/21999-MS (http://dx.doi.org/10.2118/21999-MS).Brakel, J.D. and Azar, J.J. 1989. Prediction of Wellbore Trajectory Considering Bottomhole Assembly andDrill-Bit Dynamics. SPE Drill Eng 4 (2): 109-118. SPE-16172-PA. http://dx.doi.org/10.2118/16172-PA(http://dx.doi.org/10.2118/16172-PA).Cagle W.S. and Mathews, H.D. 1977. An Improved Lost Circulation Slurry Squeeze. Petroleum Engineer(July): 26.Chenevert, M.E. 1970. Shale Alteration by Water Adsorption. J Pet Technol 22 (9): 1141-1148. SPE-2401-PA.http://dx.doi.org/10.2118/2401-PA (http://dx.doi.org/10.2118/2401-PA).Civan, F., Knapp, R.M., and Ohen, H.A. 1989. Alteration of permeability by fine particle processes. J. Pet. Sci.Eng. 3 (12): 65-79. http://dx.doi.org/10.1016/0920-4105(89)90033-8 (http://dx.doi.org/10.1016/0920-4105(89)90033-8).Clancy, L.W. and Boudreau, M. Jr. 1981. High-Water Loss High-Solids Slurry Stops Lost Circulation with OilMud. Oil & Gas J. (January): 99.Clark, D.A. 1982. An experimental investigation of the mechanics of bit teeth-rock interaction. MS thesis,University of Tulsa, Tulsa, Oklahoma.

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da Fontoura, S.A.B. and dos Santos, H.M.R. 1989. In-Situ Stresses, Mud Weight and Modes of Failure AroundOil Wells. Proc., 15th Canadian Rock Mechanics Symposium, University of Toronto, 235.Detournay, E. and Cheng, A.H.-D. 1988. Poroelastic response of a borehole in a non-hydrostatic stress field.International Journal of Rock Mechanics and Mining Science & Geomechanics Abstracts 25 (3): 171182.http://dx.doi.org/10.1016/0148-9062(88)92299-1 (http://dx.doi.org/10.1016/0148-9062(88)92299-1).Dunbar, M.E., Warren, T.M., and Kadaster, A.G. 1986. Bit Sticking Caused by Borehole Deformation. SPEDrill Eng 1 (6): 417-425. SPE-14179-PA. http://dx.doi.org/10.2118/14179-PA (http://dx.doi.org/10.2118/14179-PA).Dykstra, M.W., Chen, D.C.-K., Warren, T.M. et al. 1996. Drillstring Component Mass Imbalance: A MajorSource of Downhole Vibrations. SPE Drill & Compl 11 (4): 234241. SPE-29350-PA. http://dx.doi.org/10.2118/29350-PA (http://dx.doi.org/10.2118/29350-PA).Gill, J.A. 1989. How borehole ballooning alters drilling responses. Oil Gas J. 87 (11): 43-50.Gnirk, P.F. 1972. The Mechanical Behavior of Uncased Wellbores Situated in Elastic/Plastic Media UnderHydrostatic Stress. SPE J. 12 (1): 4959. SPE-3224-PA. http://dx.doi.org/10.2118/3224-PA (http://dx.doi.org/10.2118/3224-PA).Goins, W.C. Jr. 1952. How to Combat Circulation Loss. Oil & Gas J (June): 71.Hale, A.H., Mody, F.K., and Salisbury, D.P. 1993. The Influence of Chemical Potential on Wellbore Stability.SPE Drill & Compl 8 (3): 207-216. SPE-23885-PA. http://dx.doi.org/10.2118/23885-PA (http://dx.doi.org/10.2118/23885-PA).Hansford, J.E. and Lubinski, A. 1966. Cumulative Fatigue Damage of Drill Pipe in Dog-Legs. J Pet Technol 18(3): 359-363. http://dx.doi.org/10.2118/1258-PA (http://dx.doi.org/10.2118/1258-PA).Helmick, W.E. and Longley, A.J. 1957. Pressure-Differential Sticking of Drillpipe and How It Can Be Avoidedor Relieved. Oil Gas J. 55 (17 June): 132.Hempkins, W.B., Kingsborough, R.H., Lohec, W.E. et al. 1987. Multivariate Statistical Analysis of StuckDrillpipe Situations. SPE Drill Eng 2 (3): 237244; Trans., AIME, 283. SPE-14181-PA. http://dx.doi.org/10.2118/14181-PA (http://dx.doi.org/10.2118/14181-PA).Howard, G.C. and Jr., P.P.S. 1951. An Analysis and the Control of Lost Circulation. J Pet Technol 3 (6):171-182. http://dx.doi.org/10.2118/951171-G (http://dx.doi.org/10.2118/951171-G).Hsiao, C. 1988. A Study of Horizontal-Wellbore Failure. SPE Prod Eng 3 (4): 489494. SPE-16927-PA.http://dx.doi.org/10.2118/16927-PA (http://dx.doi.org/10.2118/16927-PA).Hussaini, S.M. and Azar, J.J. 1983. Experimental Study of Drilled Cuttings Transport Using Common DrillingMuds. Society of Petroleum Engineers Journal 23 (1): 11-20. SPE-10674-PA. http://dx.doi.org/10.2118/10674-PA (http://dx.doi.org/10.2118/10674-PA).Krueger, R.F. 1986. An Overview of Formation Damage and Well Productivity in Oilfield Operations. SPEJournal of Petroleum Technology 38 (2): 131-152. SPE-10029-PA. http://dx.doi.org/10.2118/10029-PA(http://dx.doi.org/10.2118/10029-PA).Larsen, T.I., Pilehvari, A.A., and Azar, J.J. 1997. Development of a New Cuttings-Transport Model forHigh-Angle Wellbores Including Horizontal Wells. SPE Drill & Compl 12 (2): 129-136. SPE-25872-PA.http://dx.doi.org/10.2118/25872-PA (http://dx.doi.org/10.2118/25872-PA).Lubinski, A. 1961. Maximum Permissible Dog-Legs in Rotary Boreholes. J Pet Technol 13 (2): 175194.

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SPE-1543-G. http://dx.doi.org/10.2118/1543-G (http://dx.doi.org/10.2118/1543-G).Lubinski, A. and Woods, H.B. 1953. Factors Affecting the Angle of Inclination and Doglegging in RotaryBoreholes. API Drilling and Production Practice (1953): 222-242.Lummus, J.L. and Azar, J.J. 1986. Drilling Fluids Optimization: A Practical Field Approach. Tulsa, Oklahoma:PennWell Publishing Company.Ma, D. and Azar, J.J. 1986. A study of rock-bit interaction and wellbore deviation. Journal of Energy ResourceTechnology 108 (3): 228-233.Ma, D. and Azar, J.J. 1985. Dynamics of Roller Cone Bits. ASME Journal of Energy Resources Technology 107(4): 543-548.McKinney, L.K. 1986. Formation Damage Due to Oil-Base Drilling Fluids at Elevated Temperature andPressure. MS thesis, University of Tulsa, Tulsa, Oklahoma.McLamore, R.T. 1971. The Role of Rock Strength Anisotropy in Natural Hole Deviation. SPE Journal ofPetroleum Technology 23 (11): 1313-1321. SPE-3229-PA. http://dx.doi.org/10.2118/3229-PA (http://dx.doi.org/10.2118/3229-PA).Miller, T.W. and Cheatham JR., J.B. 1971. Rock/Bit-Tooth Interaction for Conical Bit Teeth. Society ofPetroleum Engineers Journal 11 (2): 162-170. SPE-3031-PA. http://dx.doi.org/10.2118/3031-PA(http://dx.doi.org/10.2118/3031-PA).Moore, P.L. 1986. Drilling Practices Manual, second edition. Tulsa, Oklahoma: PennWell PublishingCompany.Morita, N. and Gray, K.E. 1980. A Constitutive Equation for Nonlinear Stress-Strain Curves in Rocks and ItsApplication to Stress Analysis Around a Borehole During Drilling. Presented at the SPE Annual TechnicalConference and Exhibition, Dallas, Texas, 21-24 September. SPE-9328-MS. http://dx.doi.org/10.2118/9328-MS(http://dx.doi.org/10.2118/9328-MS).Muecke, T.W. 1979. Formation Fines and Factors Controlling Their Movement in Porous Media. J Pet Technol31 (2): 144150. SPE-7007-PA. http://dx.doi.org/10.2118/7007-PA (http://dx.doi.org/10.2118/7007-PA).Nicholson, R.W. 1974. Acceptable Dogleg Severity Limits. Oil & Gas J. (April): 73.Nyland, T., Azar, J.J., Becker, T.E. et al. 1988. Additive Effectiveness and Contaminant Influence on Fluid-LossControl in Water-Based Muds. SPE Drill Eng 3 (2): 195-203. SPE-14703-PA. http://dx.doi.org/10.2118/14703-PA (http://dx.doi.org/10.2118/14703-PA).O'Brien, D.E. and Chenevert, M.E. 1973. Stabilizing Sensitive Shales With Inhibited, Potassium-Based DrillingFluids. SPE Journal of Petroleum Technology 25 (9): 1089-1100. SPE-4232-PA. http://dx.doi.org/10.2118/4232-PA (http://dx.doi.org/10.2118/4232-PA).Okrajni, S. and Azar, J.J. 1986. The Effects of Mud Rheology on Annular Hole Cleaning in Directional Wells.SPE Drill Eng 1 (4): 297-308. SPE-14178-PA. http://dx.doi.org/10.2118/14178-PA (http://dx.doi.org/10.2118/14178-PA).Outmans, H.D. 1958. Mechanics of Differential-Pressure Sticking of Drill Collars. SPE-963-G. Trans., AIME,213: 265274.PETERSON, C.R. 1970. Roller Cutter Forces. Society of Petroleum Engineers Journal 10 (1): 57-65.SPE-2393-PA. http://dx.doi.org/10.2118/2393-PA (http://dx.doi.org/10.2118/2393-PA).Plcido, J.C.R., Azar, J.J., Jr., J.R.S. et al. 1994. Drillpipe Fatigue Life Prediction Model Based On Critical

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Plane Approaches. Presented at the Offshore Technology Conference, Houston, Texas, 2 May-5 May.OTC-7569-MS. http://dx.doi.org/10.4043/7569-MS (http://dx.doi.org/10.4043/7569-MS).Rabia, H. 1985. Oil Well Drilling Engineering. London: Graham and Trotman Limited.Roegiers, J.C. and Detournay, E. 1988. Consideration on Failure Initiation in Inclined Boreholes. Proc., 29th USSymposium on Rock Mechanics, University of Minnesota, 461-469.Rollins, H.M. 1966. Drill Stem Failures Due to H 2 S. Oil & Gas J. (January).Sanchez, R.A., Azar, J.J., Bassal, A.A. et al. 1999. Effect of Drillpipe Rotation on Hole Cleaning DuringDirectional-Well Drilling. SPE J. 4 (2): 101108. SPE-56406-PA. http://dx.doi.org/10.2118/56406-PA(http://dx.doi.org/10.2118/56406-PA).Sanner, D.. 1989. Effect of Drilling Fluid Filtrates on Flow Properties of Various Rocks. Tulsa, Oklahoma:University of Tulsa.Sharma, M.M. and Wunderlich, R.W. 1985. The Alteration of Rock Properties Due to Interactions With DrillingFluid Components. Presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada,22-26 September. SPE-14302-MS. http://dx.doi.org/10.2118/14302-MS (http://dx.doi.org/10.2118/14302-MS).Storli, C. 1987. Formation Damage of Primary and Secondary Emulsifiers and Amine Compounds at ElevatedTemperatures Using Various Concentrations. Tulsa, Oklahoma: University of Tulsa.Thomas, R.P., Azar, J.J., and Becker, T.E. 1982. Drillpipe Eccentricity Effect on Drilled Cuttings Behavior inVertical Wellbores. SPE Journal of Petroleum Technology 34 (9): 1929-1937. SPE-9701-PA. http://dx.doi.org/10.2118/9701-PA (http://dx.doi.org/10.2118/9701-PA).Tomren, P.H., Iyoho, A.W., and Azar, J.J. 1986. Experimental Study of Cuttings Transport in Directional Wells.SPE Drill Eng 1 (1): 4356. SPE-12123-PA. http://dx.doi.org/10.2118/12123-PA (http://dx.doi.org/10.2118/12123-PA).Tovar, J. 1990. Formation Damage Studies Using Whole Drilling Muds in Simulated Boreholes. MS thesis,Tulsa, Oklahoma: University of Tulsa.Veeken, C.A.M., J.V.Walters, C.J.Kenter et al. 1989. Use of Plasticity Models For Predicting Borehole Stability.Presented at the 30 August-2 September 1989. ISRM-IS-1989-106.Wolfson, L. 1974. Three-dimensional Analysis of Constrained Directional Drilling Assemblies in a CurvedHole. MS thesis, Tulsa, Oklahoma: University of Tulsa.Lubinski, A. and Woods, H.B. 1955. Use of Stabilizers in Controlling Hole Deviation. Drilling and ProductionPractice. API-55-165.Zoback, M.D., Moos, D., Mastin, L. et al. 1985. Well bore breakouts and in-situ stress. J. Geophys. Res. 90(B7): 55235530. http://dx.doi.org/10.1029/JB090iB07p05523 (http://dx.doi.org/10.1029/JB090iB07p05523).

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)

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