PEH:Drilling Problems and SolutionsIntroduction It is almost certain that problems will occur while drilling a well, even in very carefully planned wells. For example, in areas in which similar drilling practices are used, hole problems may have been reported where no such problems existed previously because formations are nonhomogeneous. Therefore, two wells near each other may have totally different geological conditions. Publication Information In well planning, the key to achieving objectives successfully is to design drilling programs on the basis of anticipation of potential hole problems rather than on caution and containment. Drilling problems can be very costly. The most prevalent drilling problems include pipe sticking, lost circulation, hole deviation, pipe failures, borehole instability, mud contamination, formation damage, hole cleaning, H2Sbearing formation and shallow gas, and equipment and personnelrelated problems. Understanding and anticipating drilling problems, understanding their causes, and planning solutions are necessary for overallwellcost control and for successfully reaching the target zone. This chapter addresses these problems, possible solutions, and, in some cases, preventive measures. Contents 1 Pipe Sticking 1.1 DifferentialPressure Pipe Sticking 1.2 Mechanical Pipe Sticking 2 Loss of Circulation 2.1 Definition 2.2 LostCirculation Zones and Causes 2.3 Prevention of Lost Circulation 2.4 Remedial Measures 3 Hole Deviation 3.1 Definition 3.2 Causes 4 Drillpipe Failures 4.1 Twistoff 4.2 Parting 4.3 Collapse and Burst 4.4 Fatigue 4.5 PipeFailure Prevention 5 Borehole Instability 5.1 Definition and Causes 5.2 Types and Associated Problems 5.3 Principles of Borehole Instability 5.4 Mechanical RockFailure Mechanisms 5.5 Shale Instability 5.6 WellboreStability Analysis 5.7 BoreholeInstability Prevention 6 Mud Contamination 6.1 Definition 6.2 Common Contaminants, Sources, and Treatments 7 Producing Formation Damage 7.1 Introduction 7.2 Borehole Fluids 7.3 Damage Mechanisms 8 Hole Cleaning 8.1 Introduction 8.2 Factors in Hole Cleaning 9 HydrogenSulfideBearing Zones and Shallow Gas 10 Equipment and PersonnelRelated Problems 10.1 Equipment 10.2 Personnel 11 Nomenclature 12 References 13 General References 14 SI Metric Conversion Factors Pipe Sticking (/File%3AVol2DECover.png) Petroleum Engineering Handbook Larry W. Lake, EditorinChief Volume II Drilling Engineering Robert F. Mitchell, Editor Copyright 2006, Society of Petroleum Engineers Chapter 10 Drilling Problems and Solutions By J.J. Azar, University of Tulsa Pgs. 433454 ISBN 9781555631147 Get permission for reuse (http://www.copyright.com/content/cc3/e [Collapse] During drilling operations, a pipe is considered stuck if it cannot be freed and pulled out of the hole without damaging the pipe and without exceeding the drilling rig ’ s maximum allowed hook load. Differential pressure pipe sticking and mechanical pipe sticking are addressed in this section. DifferentialPressure Pipe Sticking Differentialpressure pipe sticking occurs when a portion of the drillstring becomes embedded in a mudcake (an impermeable film of fine solids) that forms on the wall of a permeable formation during drilling. If the mud pressure, pm , which acts on the outside wall of the pipe, is greater than the formationfluid pressure, pff , which generally is the case (with the exception of underbalanced drilling), then the pipe is said to be differentially stuck (see Fig. 10.1). The differential pressure acting on the portion of the drillpipe that is embedded in the mudcake can be expressed as sloughing.35 for weighted waterbased mud with no added lubricants.. Ac. the shape of drill collars. Mechanical Pipe Sticking The causes of mechanical pipe sticking are inadequate removal of drilled cuttings from the annulus; borehole instabilities. Differentialpressure pipe sticking can be prevented or its occurrence mitigated if some or all of the following precautions are taken: Maintain the lowest continuous fluid loss adhering to the project economic objectives. The dimensionless coefficient of friction.png)... Some of the methods used to reduce the hydrostatic pressure in the annulus include reducing mud weight by dilution.(10....2 and 10. if economical..(/File%3ADevol2_1102final_Page_434_Image_0001.. (/File%3AVol2_page_0434_eq_002.4) In this formula... Differentialpressurepipesticking problems may not be totally prevented. required to free the stuck pipe is a function of the differential pressure... 10.. or.. In directionalwell drilling... Some of the indicators of differentialpressurestuck pipe while drilling permeable zones or known depletedpressure zones are an increase in torque and drag; an inability to reciprocate the drillstring and.png). Maintain the lowest level of drilled solids in the mud system.png).... (/File%3AVol2_page_0434_eq_001.(10. If this condition exists while tripping out.. reducing mud weight by gasifying with nitrogen.. Select a mud system that will yield smooth mudcake (low coefficient of friction).. Excessive drilledcuttings accumulation in the annular space caused by improper cleaning of the hole can cause mechanical pipe sticking... or the use of drill collars with spiral grooves and externalupset tool joints can minimize the sticking force..04 for oilbased mud to as much as 0. Fp.png) Fig... (/File%3AVol2_page_0434_eq_003. f. remove all drilled solids.. .. such as hole caving..3) where (/File%3AVol2_page_0434_eq_004. and washing over the stuck pipe... common field practices for freeing the stuck pipe include mudhydrostaticpressure reduction in the annulus.. Eqs. and placing a packer in the hole above the stuck point..1—Differentialpressure sticking. they are uncontrollable variables once they are selected to meet well design objectives.. lowlubricity mudcake (high coefficient of friction). can vary from less than 0. Increases in torque/drag and sometimes in circulating drillpipe pressure are indications of large accumulations of cuttings in the annulus and of potential pipesticking problems. to rotate it; and uninterrupted drillingfluid circulation. Maintain drillstring rotation at all times. These parameters are unnecessarily high differential pressure... If sticking does occur... particularly in directional well drilling.. such as square...(10. thick mudcake (high continuous fluid loss to formation). Although hole and pipe diameters and hole angle play a role in the pipesticking force. The settling of a large amount of suspended cuttings to the bottom when the pump is shut down or the downward sliding of a stationaryformed cuttings bed on the low side of a directional well can pack a bottomhole assembly (BHA).... This is why it is a common field practice to circulate bottom up several times with the drill bit off bottom to flush out any cuttings bed that may be present before making a trip... Lep is the length of the permeable zone.....png). and excessive embedded pipe length in mudcake (delay of time in freeing operations)....(10. Dop is the outside diameter of the pipe.. Use the lowest differential pressure with allowance for swab and surge pressures during tripping operations.3 show controllable parameters that will cause higher pipesticking force and the potential inability of freeing the stuck pipe. and hmc is the mudcake thickness. it is very likely that pipe sticking will occur. between the pipe and mudcake surfaces. oil spotting around the stuck portion of the drillstring..1) The pull force. Δp; the coefficient of friction.. 10...2).. 10. Dh is the diameter of the hole.. However. which causes pipe sticking.. or collapse; plastic shale or salt sections squeezing (creeping); and key seating... a stationary cuttings bed may form on the low side of the borehole (see Fig. if possible. f; and the area of contact........2) From Bourgoyne[1]. Drilled Cuttings. in some cases...... which causes mechanical pipe sticking. if cuttings accumulation or hole sloughing is the suspected cause. no fluid return to surface. when drilling through salt that exhibits plastic behavior under overburden pressure.. Fig. 10. an increase in torque... The mechanics of key seating involve wearing a small hole (groove) into the side of a fullgauge hole. This condition is created either in doglegs or in undetected ledges near washouts.. it is important to mention briefly the pipesticking issues associated with the boreholeinstability problems.4 illustrates pipe sticking caused by key seating.4—Pipe sticking caused by key seat. shale can slough in or plastically flow inward. and. is given by (/File%3ADevol2_1102final_Page_438_Image_0001. (/File%3ADevol2_1102final_Page_437_Image_0001. 10. and ϴdl is the abrupt change in hole angle (commonly referred to as dogleg angle). Depending on mud composition and mud weight. (/File%3AVol2_page_0436_eq_001. Key seating is a major cause of mechanical pipe sticking. a keyseat pipesticking problem is indicated when several stands of pipe have been pulled out. If the pipe is stuck in a keyseat area. Also. the use of a mud that is too low in weight can lead to the collapse of the hole. the use of stiffer BHAs tends to minimize severe dogleg occurrences. depending on what caused the sticking..png) Fig.. 10.. plug back..6; however. During tripping out of hole. and (b) cuttings jamming the drill bit during tripping out... long bit runs can cause key seats; therefore. Also. the most likely successful solution is backing off below the key seat and going back into the hole with an opener to drill out the key section. If hole narrowing as a result of salt is the cause. 10. then circulating fresh water can free the pipe.5) where Fl is the lateral force.. which can cause mechanical pipe sticking.3 illustrates pipe sticking caused by wellbore instability. the salt has the tendency of flowing inward. Generally. 10. the pipe is stuck. . if mud weight is not high enough.. This groove is caused by the drillstring rotation with side force acting on it. then an increase in mud weight may free the pipe.. in some cases.png) Fig. Fig. back off.png) Fig.(/File%3ADevol2_1102final_Page_436_Image_0001..(10.. Key Seating. and then sidetrack is an economic issue that generally is addressed by the operating company. which causes mechanical pipe sticking. T is the tension in the drillstring just above the keyseat area..3—Pipe sticking caused by wellbore instability. The lateral force that tends to push the pipe against the wall. The most troublesome issue is that of drilling shale. it is common practice to make wiper trips. Indications of a potential pipesticking problem caused by borehole instability are a rise in circulating drillpipe pressure. Freeing mechanically stuck pipe can be undertaken in a number of ways. then rotating and reciprocating the drillstring and increasing flow rate without exceeding the maximum allowed equivalent circulating density (ECD) is a possible remedy for freeing the pipe.. which causes mechanical erosion and thus creates a key seat. This topic is addressed in Sec.png). In all formation types..2—Mechanical pipe sticking caused by drilled cuttings: (a) cuttings bed during drilling. 10.. all of a sudden. For example. If hole narrowing as a result of plastic shale is the cause. This will lead to a fishing operation to retrieve the fish. and then. The decision on how long to continue attempting to free stuck pipe vs. Borehole Instability. and barite. cement/bentonite/dieseloil squeeze. which can lead to mud ECD.6 and 10... to know the location of the lostcirculation zone before setting a plug.. 10. 10. or highpermeability zones. . especially those related to induced fractures. flaked. This type of lost circulation is quick.. However.. which can cause lost circulation. Hole Deviation [Collapse] Definition Hole deviation is the unintentional departure of the drill bit from a preselected borehole trajectory. or have high permeability are potential zones of lost circulation.. induced fractures can become potential zones of lost circulation. there is no production. and granular materials. Whether drilling a straight or curvedhole section. excessive mud weight. In noncavernous thief zones. mud level in mud tanks decreases gradually and..Loss of Circulation [Collapse] Definition Lost circulation is defined as the uncontrolled flow of whole mud into a formation. flaked.5—Lostcirculation zones.... and updating formation pore pressure and fracture gradients for better accuracy with log and drilling data. LostCirculation Zones and Causes Formations that are inherently fractured. These precautions include maintaining proper mud weight...(10.. (/File%3ADevol2_1102final_Page_439_Image_0001... minimizing annularfriction pressure losses during drilling and tripping in. deep sand has low permeability and presents no lossofcirculation problems. Total lost circulation.. In addition..7) where λmh = static mud weight... medium. Blind drilling also may continue if it is economically feasible and safe. Remedial Measures When lost circulation occurs. Generally. if drilling continues.5 shows partial and total lostcirculation zones... however. This is not a common practice in the field unless the formation above the thief zone is mechanically stable.png). In addition.. occurs when all the mud flows into a formation with no return to surface. sometimes referred to as thief zone. or shutting in a well in highpressure shallow gas can induce fractures. Fig.png). respectively.7 show the conditions that must be maintained to avoid fracturing the formation during drilling and tripping in. and a combination of fibrous.6 provides examples of hole deviations. Eqs.6) (/File%3AVol2_page_0438_eq_002. Fig.. limiting circulation loss is possible if certain precautions are taken. total. Induced or inherent fractures may be horizontal at shallow depth or vertical at depths greater than approximately 2.. especially in the transition zone. The common LCMs that generally are mixed with the mud to seal loss zones may be grouped as fibrous.. Squeeze refers to forcing fluid into the lostcirculation zone. and the most difficult to seal. 10..(10. It is important. If lost circulation zones are anticipated.500 ft. Excessive wellbore pressures are caused by high flow rates (high annularfriction pressure loss) or tripping in too fast (high surge pressure). mud continues to flow to surface with some loss to the formation. If drilling continues during total lost circulation. These materials are available in course.. The major causes of induced fractures are excessive downhole pressures and setting intermediate casing. too high.png) Fig. cavernous.. 10. avoiding restrictions in the annular space... are not avoidable if the target zone is to be reached. setting casing to protect upper weaker formations within a transition zone. total loss of circulation may occur. improper annular hole cleaning. Δλs = additional mud caused by surge pressure.. it is referred to as blind drilling. Δλaf = additional mud weight caused by friction pressure loss in annulus. cavernous.. however. and fine grades for an attempt to seal lowtomoderate lostcirculation zones. Highpermeability formations that are potential lostcirculation zones are those of shallow sand with permeability in excess of 10 darcies. cement... sealing the zone is necessary unless the geological conditions allow blind drilling. granular. and the fluid is clear water. (/File%3AVol2_page_0483_eq_001... which is unlikely in most cases. Cavernous formations are often limestones with large caverns. λfrac = formationpressure fracture gradient in equivalent mud weight. and λeq = equivalent circulating density of mud. adequate hole cleaning.. preventive measures should be taken by treating the mud with lostcirculation materials (LCMs).. Prevention of Lost Circulation The complete prevention of lost circulation is impossible because some formations. under certain improper drilling conditions. In partial lost circulation. In the case of severe lost circulations. the tendency of the bit to walk away from the desired path can lead to higher drilling costs and leaseboundary legal problems. the use of various plugs to seal the zone becomes mandatory. Various types of plugs used throughout the industry include bentonite/dieseloil squeeze. such as inherently fractured. and clearances). stiffness. however. Regardless of what may have caused pipe failure. Failure under cyclic (repeated) loads is called fatigue failure. The mechanics of this resultant force is complex and is governed mainly by the mechanics of the BHA. Twistoff Pipe failure as a result of twistoff occurs when the induced shearing stress caused by high torque exceeds the pipematerial ultimate shear stress. The combined action of cyclic stresses and corrosion can shorten the life expectancy of a drillpipe by thousand folds. excessive torques are not generally encountered under normal drilling practices. and. 10. teeth location and number. Actions that can minimize the buckling tendency of the BHA include reducing WOB and using stabilizers with outside diameters that are almost in gauge with the wall of the borehole. Collapse and Burst Pipe failure as a result of collapse or burst is rare; however. The mechanics of rock/bit interaction is a very complex subject and is the least understood in regard to holedeviation problems. bit hydraulic features); and drilling parameters (tooth penetration into the rock and its cutting mechanism). Pipe stiffness and length and the number of stabilizers (their location and clearances from the wall of the wellbore) are two major parameters that govern BHA buckling behavior. The BHA is a flexible. specifically the BHA makeup. The significance of the BHA buckling is that it causes the axis of the drill bit to misalign with the axis of the intended hole path. The forces imparted to the drill bit because of the BHA are directly related to the makeup of the BHA (i. or abandonment of the alreadydrilled section below the fish. generally agreed that one or a combination of several of the following factors may be responsible for the deviation: Heterogeneous nature of formation and dip angle. Fortunately.6—Example of hole deviations. It is a process of localized progressive structural fractures in material under the action of dynamic stresses. Causes It is not exactly known what causes a drill bit to deviate from its intended path. stabilizers. Pipe corrosion occurs during the presence of O2. and reamers). Drillbit type and its basic mechanical design. The contribution of the rock/bit interaction to bit deviating forces is governed by rock properties (cohesive strength. by the drillingfluid hydraulics. It is well established that a structural member that may not fail under a single application of static load may very easily fail under the same load if it is applied repeatedly. under extreme conditions of high mud weight and complete loss of circulation. bit operating conditions. Drillstring fatigue failure is the most common and costly type of failure in oil/gas and geothermal drilling operations. bit profile. the cost of fishing operations and the sometimes unsuccessful attempts to retrieve the fish out of the hole can lead to the loss of millions of dollars in rig downtime. internal friction angle); drillbit design features (tooth angle. pipe burst may occur. Holeinclination angle from vertical. and an overpull is applied in addition to the effective weight of suspended pipe in the hole above the stuck point. bedding or dip angle. This condition may arise when pipe sticking occurs.e. loss of expensive tools downhole. Improper hole cleaning. Cyclic stresses are induced by dynamic loads caused by drillstring vibrations and bendingload reversals in curved sections of hole and doglegs caused by rotation. elastic structural member that can buckle under compressive loads. The buckled shape of a given designed BHA depends on the amount of applied WOB. and/or H2S. In directional and extendedreach drilling. rock/bit interaction. to some lesser extent. It is. however. H2S is the most severely corrosive element to steel pipe.png) Fig. bit size. thus causing the deviation. Fatigue Fatigue is a dynamic phenomenon that may be defined as the initiation of microcracks and their propagation into macrocracks as a result of repeated applications of stresses. CO2. Drillstring characteristics. and it is deadly to humans. In verticalwell drilling.000 lbfft are common and easily can cause twistoff to improperly selected drillstring components. the advent of downhole measurementwhiledrilling tools that allow monitoring the advance of the drill bit along the desired path makes our lack of understanding of the mechanics of hole deviation more acceptable. Applied weight on bit (WOB). chlorides. number. bit offset in case of rollercone bits. bit type. Hydraulics at the bit. . It is known that some resultant force acting on a drill bit causes hole deviation to occur.(/File%3ADevol2_1102final_Page_440_Image_0001.. Drillpipe Failures [Collapse] Drillpipe failures can be put into one of the following categories: twistoff caused by excessive torque; parting because of excessive tension; burst or collapse because of excessive internal pressure or external pressure. Stabilizers (location. Parting Pipeparting failure occurs when the induced tensile stress exceeds the pipematerial ultimate tensile stress. torques in excess of 80. respectively; or fatigue as a result of mechanical cyclic loads with or without corrosion. fracturing. In the maximum of energy of distortion criterion. The drilling cost attributed to shaleinstability problems is reported to be in excess of onehalf billion U. and an interaction process begins between the formation and borehole fluids. The associated problems are pipe sticking and possible loss of well. all share the same shortcoming of uncertainty in the input data needed to run the analysis. formation and drillingfluids chemistry. while tensile failure is caused by normal stresses as a result of excessive mud weight. shale strength environment) and chemical (shale/fluid interaction—capillary pressure. Hole Enlargement. Mechanical RockFailure Mechanisms Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive or the tensile strength of the rock.7—Types of hole instability problems. boreholefluid invasion into shale). osmotic pressure. The problems associated with hole enlargement are an increase in cementing difficulty. failure is said to occur when. Hole closure is a narrowing timedependent process of borehole instability. and an increase in the difficulty of casings landing. Principles of Borehole Instability Before drilling. Fatigue failures can be mitigated by minimizing induced cyclic stresses and insuring a noncorrosive environment during the drilling operations. 10. an increase in potential pipe sticking. Borehole collapse occurs when the drillingfluid pressure is too low to maintain the structural integrity of the drilled hole. foreign fluids are introduced. Problems associated with hole closure are an increase in torque and drag. and. PipeFailure Prevention Although pipe failure cannot be eliminated totally. mechanical abrasion caused by drillstring. In addition. however. one of the acting principal stresses reaches the failure value of the rock tensile strength. the energy of distortion reaches the same energy of failure of the rock under pure tension. under the action of combined stresses. hole enlargement or washouts. there are certain measures that can be taken to minimize it.S dollars per year. quality control during manufacturing. The cause of shale instability is twofold: mechanical (stress change vs. The result is a potential holeinstability problem. Fig. erosion caused by fluid circulation. In the maximumnormalstress criterion. rock mechanical properties. in the case of shale. the rock strength at some depth is in equilibrium with the insitu rock stresses (effective overburden stress. Hole enlargements are generally caused by hydraulic erosion. The proper handling and inspection of the drillstring on a routine basis are the best measures to prevent failures. This lack of understanding is attributed to the wide variations of statistical data in determining type of service and environment of the drillstring. Shale Instability More than 75% of drilled formations worldwide are shale formations.7 illustrates holeinstability problems. Corrosion can be mitigated by corrosive scavengers and controlling the mud pH in the presence of H2S.png) Fig. 10. an increase in hydraulic requirements for effective hole cleaning. the balance between the rock strength and the insitu stresses is disturbed.In spite of the vast amount of work that has been dedicated to pipe fatigue failure. and chemical caused by interaction of borehole fluid with the formation. Borehole Instability [Collapse] Definition and Causes Borehole instability is the undesirable condition of an openhole interval that does not maintain its gauge size and shape and/or its structural integrity. an increase in potential hole deviation. Cyclic stresses can be minimized by controlling dogleg severity and drillstring vibrations. accuracy of methods in determining the stresses. . Although a vast amount of research has resulted in many boreholestability simulation models. Hole Closure. pore pressure. Fracturing. pressure diffusion. (/File%3ADevol2_1102final_Page_442_Image_0001. and inherently sloughing shale. failure is said to occur when. Compressive failure is caused by shear stresses as a result of low mud weight. Types and Associated Problems There are four different types of borehole instabilities: hole closure or narrowing. under the action of combined stresses. While a hole is being drilled. Hole enlargements are commonly called washouts because the hole becomes undesirably larger than intended. and collapse. and it generally occurs in plasticflowing shale and salt sections. Collapse. Such data include insitu stresses. magnitude of operating loads and frequency of occurrence (load history). Fracturing occurs when the wellbore drillingfluid pressure exceeds the formationfracture pressure. effective horizontal confining stresses). it is still the least understood. The causes can be grouped into the following categories: mechanical failure caused by insitu stresses. The associated problems are lost circulation and possible kick occurrence. It sometimes is referred to as creep under the overburden pressure. The failure criteria that are used to predict holeinstability problems are the maximumnormalstress criterion for tensile failure and the maximum strain energy of distortion criterion for compressive failure. and the applicability of material fatigue data. and an increase in potential problems during logging operations. break over to a mud system. anhydrite. nonlinear. The calcium ion tends to replace the sodium ions on the clay surface through a base exchange. waterbased mud systems are the most susceptible to contamination.. strength. It also causes added thinners to the mud system to become ineffective. Em. Mud Contamination [Collapse] Definition A mud is said to be contaminated when a foreign material enters the mud system and causes undesirable changes in mud properties. pcap .. and boreholefluid invasion into the shale when drilling overbalanced. which may cause chemical interaction that can lead to shale instabilities. and filtration. Mg++ ); soluble bicarbonates and carbonates (HCO3− . HCO3− ) are from drilling a CO2bearing formation. Pressure Diffusion. As a result. inert); gypsum/anhydrite (Ca ++ ); cement/lime (Ca++ ); makeup water (Ca++ . Solids Contamination. pwfl. and hard/brackish makeup water. or chemical potential. an increase of mud viscosity or. cement. As stated previously. This results in the development of capillary pressure. It has been shown that fine solids. barite) and materials that are drilled (active and inert). the activities need to be equalized or. Additional field practices that should be followed are minimizing time spent in open hole; using offsetwell data (use of the learning curve); monitoring trend changes (torque. Shakers remove solids in the size of cuttings (approximately 140μ or larger). CalciumIons Contamination. CO3—); soluble sulfides (HS− . and the use of borehole fluid compatible with the formation being drilled. When the energy level or activity in shale pore fluid. water movement can occur in either direction across a semipermeable membrane as a result of the development of osmotic pressure. are the most detrimental to the overall drilling efficiency and must be removed if they are not a necessary part of the mud makeup. an increase in capillary pressure is required. The mud in use with a certain density may not bring the altered stresses to the original state; therefore. such as gypsum mud or lime mud. horizontal); porefluid pressure and chemistry; and mud properties and chemistry. desanders. lime. Osmotic Pressure.. the data needed include rock properties (Poisson ratio. and Treatments The most common contaminants to waterbased mud systems are solids (added. The calcium ion is a major contaminant to freshwaterbased sodiumclay treated mud systems. Generally. thus causing undesirable changes in mud properties such as rheology and filtration. μc . gilsonite is used to seal off microfractures. drag. that can tolerate the contaminant. or over treatment with soda ash and bicarbonate. their differentials minimized. Desilters remove solids in the size of silt (down to 20μ). borehole fluid is forced to flow into the formation (fluidloss phenomenon). Chemical Instability. the use of proper hydraulics to control the ECD. However. However. and lime/gypsum. saturatedsalt/polymer. Pressure diffusion is a phenomenon of pressure change near the borehole walls that occurs over time. dilution is sometimes used to lower solids concentration. In conventional drilling. in extreme cases. shale may become mechanically unstable.. a positive differential pressure (the difference between the boreholefluid pressure and the porefluid pressure) is always maintained. viscosity.. as . Sources. S—); and salt/salt water flow (Na+ .png). while selective flocculants will flocculate drilled solids but not the added barite solids.Mechanical Instability. WellboreStability Analysis Several models in the literature address wellborestability analysis. . Borehole Fluid Invasion into Shale. elastoplastic. To mitigate this problem. Other than the mud data. These practices include proper mudweight selection and maintenance. the drilling engineer can mitigate the problems of borehole instabilities by adhering to good field practices. Capillary Pressure. and the osmotic pressure. The mechanisms that contribute to this problem include capillary pressure.. Common Contaminants.. thermal degradation of organics in mud.. sensitivity analysis can be conducted by assuming data for the many variables to establish safety windows for mud selection and design. During drilling. The removal of drilled solids is achieved through the use of mechanical separating equipment (shakers. Biocarbonate and Carbonate Contamination. To prevent or reduce water movement across this semipermeable membrane that has certain efficiency.. micron and submicron sized. Regardless of the model.. seawater. ϴ is the contact angle between the two fluids. BoreholeInstability Prevention Total prevention of borehole instability is unrealistic because restoring the physical and chemical insitu conditions of the rock is impossible. pos . proper holetrajectory selection.8) where σ is the interfacial tension. The sources of calcium ions are gypsum. centrifuges may have to be used.. The contaminant ions (CO3—. Solids are materials that are added to make up a mud system (bentonite. which can be achieved with oilbased or other organic lowpolar mud systems. circulating pressure. which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. at least. Chemicalinduced shale instability is caused by the drillingfluid/shale interaction. Chemical flocculants are sometimes used to flocculate fine solids into a bigger size so that they can be removed by solidsremoval equipment.. the data are often compounded with problems of availability and/or uncertainties. sodium carbonate (soda ash) is used if the source is gypsum or anhydrite. and physicochemical. desilters.. and centrifuges). fillin during tripping); and collaborating and sharing information. which is expressed as (/File%3AVol2_page_0443_eq_001. pressure diffusion in the vicinity of the borehole walls. Excess solids of any type are the most undesirable contaminant to drilling fluids. As a last resort. These contaminants cause the mud to have high yield and gel strength and a decrease in pH. am . osmotic pressure. Cl− ). If treatment becomes economically unacceptable. This pressure change is caused by the compression of the native pore fluid by the boreholefluid pressure. drilled. and r is the porethroat radius.. Sodium bicarbonate is the preferred treatment if the calcium ion is from lime or cement. The mud activity can be reduced by adding electrolytes that can be brought about through the use of mud systems such as seawater.. Total flocculants do not discriminate between various types of solids. such as density.. They affect all mud properties. modulus of elasticity); insitu stresses (overburden. is different from the activity in drilling mud. Desanders remove solids in the size of sand (down to 50μ). Mud contamination can result from overtreatment of the mud system with additives or from material entering the mud during drilling. purely mechanical. it is suggested to increase Em and vice versa. Treating the mud system with gypsum or lime is recommended.. If am is lower than as . The treatment depends on the source of the calcium ion.(10. To prevent borehole fluids from entering the shale and stabilizing it. KCl/NaCl/polymer. For example. the mud in the borehole contacts the native pore fluid in the shale through the porethroat interface... When solids become smaller than the cutoff point of desilters. active. mechanical rock instability can occur because the insitu stress state of equilibrium has been disturbed after drilling.[2] These include verysimple to verycomplex models such as linear elastic. pos. bentonite/polymer) and oilbased (invert emulsion. Salt/Saltwater Flows. Solids Plugging.9—Formation damage caused by solids plugging. It is a zone of reduced permeability within the vicinity of the wellbore (skin) as a result of foreignfluid invasion into the reservoir rock. 10. or gasified mud. (/File%3ADevol2_1102final_Page_446_Image_0002. emulsion blockage. therefore. potential reduction in production. .8 illustrates formation skin damage. completion fluids.8—Formation skin damage. Fig. that enter the mud system as a result of drilling salt sections or from formation saltwater flow cause a mud to have high yield strength. which results in a decrease of rock relative permeability. When a freshwater filtrate invades the reservoir rock. When a mudsystem filtrate enters the reservoir. (It causes severe embrittlement to drillpipe.png) Fig. gas. The contaminant ions (HS− . oil). Completion and workover fluids are mostly brines and are solids free.Hydrogen Sulfide Contamination. Production is predicated on the amount of saturation within the reservoir rock. S—) generally are from drilling an H2Sbearing formation. In a different context. Saturation Change. and mutual precipitation of soluble salts in wellborefluid filtrate and formation water. See the chapter on transport properties in the General Engineering volume of this Handbook for additional information. or iron.) Scavenging of H2S is done by use of zinc. clayparticle swelling or dispersion. This is an inherent problem in sandstone that contains watersensitive clays. Fig. Borehole Fluids Borehole fluids are classified as drilling fluids.10 shows that high fluid loss causes water saturation to increase. formation damage is defined as the impairment to reservoir (reduced production) caused by wellbore fluids used during drilling/completion and workover operations. wettability reversal.png) Fig. (/File%3ADevol2_1102final_Page_446_Image_0001. The ions. the use of dispersants and fluidloss chemicals. 10. Fig. saturation changes. There are two types of mud: water based (pure polymer. To minimize this form of damage. causing an unknown reduction in the unquantifiable. Hydrogen sulfide is the most deadly ion to humans and is extremely corrosive to steel used during drilling operations. 10. Damage Mechanisms Formation damage is a combination of several mechanisms including solids plugging. minimize the amount of fine solids in the mud system and fluid loss. Producing Formation Damage [Collapse] Introduction Producing formation damage has been defined as the impairment of the unseen by the inevitable. or conversion to a mud that tolerates the problem if the cost of treatment becomes excessive. Drilling fluids are categorized as mud. and pH decrease.9 shows that the plugging of the reservoirrock pore spaces can be caused by the fine solids in the mud filtrate or solids dislodged by the filtrate within the rock matrix. pure bentonite. ClayParticle Swelling. aqueousfiltrate blockage. copper. it will cause some change in water saturation and. or workover fluids. high fluid loss. 10. it will cause the clay to swell and thus reduce or totally block the throat areas. Na+ Cl − . 10. Some actions for treatment are dilution with fresh water. Any precipitation of soluble salts. 10. However. can cause solids blockage and hinder production. The flowrate requirements reach a maximum at approximately 65 to 67° and then slightly decrease toward the horizontal. Also. the susceptibility of the openhole section to hydraulic erosion. Laboratory work has demonstrated that drilling at an inclination angle greater than approximately 30° from vertical poses problems in cuttings removal that are not encountered in vertical wells. Flow rate is the dominant factor in cuttings removal while drilling directional wells. It has been proved that the whirling motion of the string around the wall of the borehole when it rotates is the major contributor to hole cleaning enhancement. whether from the use of salt mud systems or from formation water or both. see the Formation Damage chapter in the Production Operations Engineering volume of this Handbook.10—Formation damage caused by saturation. cuttings size. many studies have been conducted to gain understanding on hole cleaning in directionalwell drilling. It has been demonstrated that while drilling with oilbased mud systems. For more information. 10. and side tracking and to drill horizontally into the reservoir. mechanical agitation of the cuttings bed on the low side of the hole and exposing the cuttings to higher fluid velocities when the pipe moves to the high side of the hole are results of pipe whirling action. Hole Cleaning [Collapse] Introduction Throughout the last decade.11—Cuttingsbed buildup in directional wells. 10. Objectives in total field development (primary and secondary production). which often leads to the inability of reaching the target in highangle/extendedreach drilling. how high a flow rate can be increased may be limited by the maximum allowed ECD. AqueousFiltrate Blockage.11 illustrates that the formation of a moving or stationary cuttings bed becomes an apparent problem if the flow rate for a given mud rheology is below a certain critical value. during angle building with a downhole . Wettability Reversal. Laboratory studies have shown and field cases have reported that drillstring rotation has moderate to significant effects in enhancing hole cleaning. flowrate requirement increases. Although. Fig. Emulsion Blockage. excessive torque and drag on drillstring.(/File%3ADevol2_1102final_Page_447_Image_0001. flow rate. a sudden pump shutdown can cause cuttings sloughing to bottom and may result in a mechanical pipesticking problem. difficulties in logging and cementing. and. it is mandated by the needs of drilling inaccessible reservoir. hole inclination can lead to cleaning problems. Hole Inclination Angle. formation fracturing. (/File%3ADevol2_1102final_Page_448_Image_0001. Laboratory work has demonstrated that when hole angle increases from zero to approximately 67° from vertical. An increase in flow rate will result in more efficient cuttings removal under all conditions. Inherent in oilbased mud systems is the use of excess surfactants. Drillstring Rotation. Precipitation of Soluble Salts. and therefore. very importantly. excess surfactants in the mud filtrate that enter the rock can cause wettability reversal. which hinders production through emulsion blockage. the amount of excess surfactants used in oilbased mud systems should be kept at a minimum. and economics are some of the factors that intervene in hole angle selection. the string dynamic behavior. and the availability of rig hydraulic power. hole cleaning becomes more difficult. mud rheology. It has been reported from field experience and demonstrated in laboratory tests that as much as 90% in production loss can be caused by this mechanism. For example. it has been shown that at 25 to approximately 45°. environmental concerns. The most prevalent problem is excessive torque and drag.png) Fig. premature bit wear. While drilling with waterbased mud. The level of enhancement is a combined effect of pipe rotation. and difficulties in casings landing. Factors in Hole Cleaning AnnularFluid Velocity. Also. Inadequate hole cleaning can lead to costly drilling problems such as mechanical pipe sticking. avoiding troublesome formations. to guard against this problem. slow drilling. offshore drilling. Therefore. These surfactants enter the rock and can form an emulsion within the pore spaces. there are certain limitations to its implementation. the aqueous filtrate that enters the reservoir can cause some blockage that will reduce the production potential of the reservoir. Although there is a definite gain in hole cleaning caused by pipe rotation.png) Fig. Reservoir rocks are waterwet in nature. and well control systems (ram preventers. rotation cannot be induced. continuing education and training for personnel directly or indirectly involved is essential to successful drilling/completion practices. the benefit of avoiding other drilling problems. this is no longer a problem. Assn. The functions of drilling fluids are many and can have unique competing influences. there are very specific requirements to abide by in accordance with Intl. its adverse impact on hole cleaning may be unavoidable.6; therefore. Personnel Given equal conditions during drilling/completion operations. are easier to transport. If the limits of these two variables are exceeded.12—Fluid velocity profile in eccentric annulus (after Hzouz et al. underground blow. the flow rate and/or pipe rotation have to be adjusted. It is impossible to control their size and shape even if a specific bit group has been selected to generate them. Hole/Pipe Eccentricity. diamond matrix). In the inclined section of the hole. the regrinding that takes place after they are generated. casing wear. Mud density should not be used as a criterion to enhance hole cleaning. mechanical destruction to openhole sections. proper hoisting power for efficient tripping out. Proper monitoring and recording systems that monitor trend changes in all drilling parameters and can retrieve drilling data at a later date. under the given insitu stresses. 10. Mud Properties. when eccentricity increases. The size. an increase in the drilling rate always results in an increase in the amount of cuttings in the annulus. 10. of Drilling Contractors rules and regulations. To ensure good hole cleaning during highrateofpenetration (ROP) drilling. therefore. and the breakage by their own bombardment and with the rotating drillstring. Cuttings Characteristics. especially for highviscosity fluid. polycrystallinediamond compact. may cause fracturing of the formation. such as mechanical pipe sticking or excessive torque and drag. annular preventers. high pipe rotation can cause high ECDs due to the high annularfriction pressure losses. However. proper tubular hardware specifically suited to accommodate all anticipated drilling conditions. Smaller cuttings are more difficult to transport in directional well drilling; however. which causes fluid velocity to be extremely low and. (/File%3ADevol2_1102final_Page_450_Image_0001. This creates a very narrow gap in the annulus section below the pipe. If it is known or anticipated. Only in verticalwell drilling and highviscosity pill sweep is viscosity used as a remedy in hole cleaning. The two mud properties that have direct impact on hole cleaning are viscosity and density. the pipe has the tendency to rest on the low side of the borehole because of gravity. personnel are the key to the success or failure of those operations. The danger is that if the well is shut in. Nomenclature [Expand] References [Expand] General References [Expand] .motor (sliding mode). However.png) Fig. Equipment and PersonnelRelated Problems [Collapse] Equipment The integrity of drilling equipment and its maintenance are major factors in minimizing drilling problems.[3]). HydrogenSulfideBearing Zones and Shallow Gas [Collapse] Drilling H2Sbearing formations poses one of the most difficult and dangerous problems to humans and equipment. Any unnecessary increase in mud density beyond fulfilling these functions will have an adverse effect on the ROP and. shape. has the primary function of the suspension of added desired weighting materials such as barite. which will result in the most severe blowout problem. formation fracturing is more likely to occur. and. on the other hand. In slimhole drilling. internal preventers) that allow kick control under any kick situation are all necessary for reducing drilling problems. Proper rig hydraulics (pump power) for efficient bottom and annular hole cleaning. therefore. the inability to transport cuttings to surface. distribution. The only way to combat this problem is to never shut in the well; divert the gas flow through a diverter system instead. can outweigh the loss in ROP. because eccentricity is governed by the selected well trajectory. Overall well costs as a result of any drilling/completion problem can be extremely high; therefore. proper derrick design loads and drilling line tension load to allow safe overpull in case of a sticking problem. Rate of Penetration. Shallow gas may be encountered at any time in any region of the world. As Fig. in some cases. pipe rotation can cause cyclic stresses that can accelerate pipe failures due to fatigue. and specific gravity of cuttings affect their dynamic behavior in a flowing media. The specific gravity of most rocks is approximately 2. Viscosity. With the new steering rotary systems. the only alternative is to reduce the ROP. Under similar conditions. Highpressure shallow gas can be encountered at depths as low as a few hundred feet where the formationfracture gradient is very low. fine particles seem to stay in suspension and. Although a decrease in ROP may have a detrimental impact on drilling costs.12 illustrates. and effective mudhandling and maintenance equipment that will ensure that the mud properties are designed for their intended functions are also necessary. particle/fluid velocities decrease in the narrow gap. The cuttings size and shape are functions of the bit types (roller cone. with some viscosity increase and pipe rotation. specific gravity can be considered a nonvarying factor in cuttings transport. The main functions of density are mechanical borehole stabilization and the prevention of formationfluid intrusion into the annulus. org/search?q=Drilling Problems and Solutions) (http://scholar.SI Metric Conversion Factors [Expand] Categories (/Special%3ACategories): PEH (/Category%3APEH) 1.google.6 Drilling operations (/Category%3A1.php? title=Special%3ASearch&redirs=1&fulltext=Search&ns0=1&ns4=1&ns500=1&redirs=1&title=Special%3ASearch&advanced=1&fulltext=Advanced+search&search=Drilling Problems and Solutions) (http://wiki.onepetro.ca/scholar?q=Drilling Problems and Solutions) (http://www.php? title=Special%3ASearch&profile=advanced&fulltext=Search&ns0=1&ns4=1&ns102=1&ns104=1&ns106=1&ns108=1&ns420=1&ns828=1&redirs=1&profile=advanced&search=Drillin Problems and Solutions) .org/search?q=Drilling Problems and Solutions) (http://wiki.org/index.6_Drilling_operations) (https://www.seg.aapg.worldcat.org/index.