Oilwelldrillingproblemsandsolutions.pptx

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1. Oil well drilling problems & The Solutions (Work Shop) Dr.Abdulhussain Neamah Shnawa College of Petroleum Engineering Alayen University

2. Oil well drilling Problems Out line ------------------------------------ ● Introduction. ● What potential problems may occur during drilling? ● Why do these problems occur? ● What to do and how to act quickly to minimize their economic impact on the drilling budget. ● How recognizing signals before problems become eminent will help reduce rig downtime and cost.

3. Introduction 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. 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. \ .

4. Types of Drilling Problems Pipe sticking Loss of circulation Hole deviation Pipe failure Borehole stability Mud contamination Formation damage Hole cleaning H2S-bearing zones Shallow gas zones Equipment and personal- related prolems

5. (1):Stuck pipe ●Stuck Pipe: pipe is considered stuck if it cannot be freed from the hole without damaging the pipe, and without exceeding the drilling rig’s maximum allowed hook load Classified into two-categories 1-Deffrential pipe sticking. 2-Mechanical Pipe sticking

6. Deferential Sticking

7. If the pm (Mud pressure)> formation-fluid pressure, ( pff ), then the pipe is said to be differentially stuck

8. Pull Force required. The Def.Pressure acting on portion of(D.P) Fp (Pull force)required to free St. Pipe is a function of Δp ; the coefficient of friction, f; and the area of contact, Ac, between the pipe and mud cake surfaces( Ref (1).

9. Area of Contact

10. Indicators of differential-pressure-stuck pipe while drilling permeable zones or known depleted-pressure . An increase in torque and drag An inability to reciprocate the drill string and, in some cases, to rotate it Uninterrupted  drilling fluid  circulation

11. If sticking does occur, common field practices for freeing the stuck pipe include : Mud-hydrostatic-pressure reduction in the annulus. Oil spotting around the stuck portion of the drill string. Washing over the stuck pipe. Reducing mud weight by dilution Reducing mud weight by gasifying with nitrogen Placing a packer in the hole above the stuck point  

12. Prevention or mitigation of differential stuck pipe : 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 tripping operations. Select a mud system that will yield smooth mudcake (low coefficient of friction ). Maintain drillstring rotation at all times, if possible

13. (2)- Lost circulation Loss of circulation is the uncontrolled flow of whole mud into a formation, sometimes referred to as a “thief zone.” A-Partial losses( Contineous to flow with som losses) 1-( 2-4 m3/Job simple ). 2-medIum10-15 m3/Job 3-Safere10m3/ hr B. Total lost circulation, however, occurs when all the mud flows into a formation with no return to surface .(1_ with filling the hole2) without filling ). If drilling continues during total lost circulation, it is referred to as blind drilling.

14. Causes of lost-circulation zones Formations that are inherently fractured, cavernous, or have high permeability Improper drilling conditions Induced fractures caused by excessive downhole pressures and setting intermediate   Casing too high Induced fractures Induced or inherent fractures may be horizontal at shallow depth or vertical at depths greater than approximately 2,500 ft. Excessive wellbore pressures are caused by high flow rates (high annular-friction pressure loss) or tripping in too fast (high surge pressure), which can lead to mud equivalent circulating density (ECD). Induced fractures can also be caused by: Improper annular  hole cleaning Excessive mud weight Shutting in a well  in high-pressure shallow gas

15. Maintenances to avoid Fracturing Eqs . 1 and 2  show the conditions that must be maintained to avoid fracturing the formation during drilling and tripping in, respectively . ………….(1) and ………..(2) Where λ eq  = equivalent circulating density of mud and λ mh  = static mud weight, Δλ af  = additional mud weight caused by friction pressure loss in annulus, Δλ s  = additional mud caused by surge pressure, λ frac  = formation-pressure fracture gradient in equivalent mud weight ,.

16. Cavernous formations Cavernous formations are often limestones with large caverns. This type of lost circulation is quick, total, and the most difficult to seal. High-permeability formations that are potential lost-circulation zones are those of shallow sand with permeability in excess of 10 darcies . Generally, deep sand has low permeability and presents no loss-of-circulation problems. In noncavernous thief zones, mud level in mud tanks decreases gradually and, if drilling continues, total loss of circulation may occur.

17. Prevention of lost circulation complete prevention is impossible Maintaining proper mud weight Minimizing annular-friction pressure losses during drilling and tripping in Adequate hole cleaning Avoiding restrictions in the annular space Setting casing to protect upper weaker formations within a transition zone Updating formation pore pressure and fracture gradients for better accuracy with log and drilling data If lost-circulation zones are anticipated, preventive measures should be taken by treating the mud with loss of circulation materials (LCMs) and preventive tests such as the leakoff test and formation integrity test should be performed to limit the possibility of loss of circulation.

18. Preventive test …...........+. Remedial measures 1-leak of test(LOT) 2-Formation integrity test ( FIT). When an operator chooses to perform an LOT or an FIT, if the test fails, some remediation effort—typically a cement squeeze—should be carried out before drilling resumes to ensure that the wellbore is competent . Treat it by. Fibrous Flaked Granular A combination of fibrous, flaked, and granular materials Various types of plugs used throughout the industry include : Bentonite/diesel-oil squeeze Cement/bentonite/diesel-oil squeeze Cement Barite

19. LCM A variety of LCM is available, and combining several types and particle sizes for treatment purposes is common practice. Conventional—and relatively inexpensive—materials include: Sized calcium carbonate Paper Cottonseed hulls Nutshells Mica Cellophane

20. (3):H ole deviation Hole deviation is the unintentional departure of the drill bit from a preselected borehole trajectory. walk away from the desired path can lead to  drilling problems  such as higher drilling costs and also lease-boundary legal problems

21. Causes of hole deviation 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 bottom hole assembly (BHA) makeup Stabilizers (location, number, and clearances) Applied weight on bit (WOB) Hole-inclination angle from vertical Drill-bit type and its basic mechanical design Hydraulics at the bit. о Improper hole cleaning

22. The contribution of the rock/bit interaction to bit deviating forces is governed by: Rock properties Cohesive strength Bedding or dip angle Internal friction angle Drill-bit design features Tooth angle Bit size -Bit type -Bit offset in case of roller-cone bits -Teeth location and number -Bit profile -Bit hydraulic features

23. Drilling parameters Tooth penetration into the rock and its cutting mechanism The mechanics of rock/bit interaction is a very complex subject and is the least understood in regard to hole-deviation problems. Fortunately, the advent of  downhole measurement-while-drilling  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.

24. (4):D rill pipe failures Drillpipe failures is a prevalent  drilling problem . It 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 , respectively; or fatigue as a result of mechanical cyclic loads with or without corrosion.

25. Types of drill pip Failure Twist off: The induced shearing stress caused by high torque exceeds the pipe-material ultimate shear stress. In vertical-well drilling, excessive torques are not generally encountered under normal drilling practices. In directional and extended-reach drilling, however, torques in excess of 80,000 lbf-ft are common and easily can cause twistoff to improperly selected drillstring components . Parting: The induced tensile stress exceeds the pipe-material ultimate tensile stress . ( pipe sticking&Over pull ). Collapse and burst: Itis rare; however, under extreme conditions of high mud weight and complete  loss of circulation , pipe burst may occur . Fatigue: It is Adynamic phenomena beginning with microcracks developed to macrocraks caused also by string vibration,bending ( Dog-leg) then developed to corrosion specialy in presence of O2,Chloride, Co2,H2s.

26. Pipe-failure prevention Fatigue failures can be mitigated by minimizing induced cyclic stresses and insuring a noncorrosive environment during the drilling operations . Cyclic stresses can be minimized by controlling dogleg severity and drillstring vibration s. Corrosion can be mitigated by corrosive scavengers and controlling the mud pH in the presence of H 2 S . The proper handling and inspection of the drillstring on a routine basis are the best measures to prevent failures .

27. (5):Borehole instability Borehole instability  is the undesirable condition of an open hole interval that does not maintain its gauge size and shape and/or its structural integrity. This articles discusses the causes, types, effects, and possible prevention of borehole instability.

28. Causes , types and associated problems Causes: Mechanical failure caused by in-situ stresses Erosion caused by fluid circulation Chemical caused by interaction of borehole fluid with the formation Types: Hole closure or narrowing Hole enlargement or washouts Fracturing Collapse

29. Types and associated problems Hole closure, Problems associated(1-Increase in torque and drag, 2 - Increase in potential  pipe sticking , 3) Increase in the difficulty of casings landing. Hole enlargement(Washout).Caused by( Hydraulic erosion,Mechanical abrasion by string,Inherently Slouphing Shale). The associated problems are(Increase in cementing difficulty , Increase in potential hole deviation , Increase in hydraulic requirements for effective hole cleaning, Increase in potential problems during logging operations

30. Types and associated problems • Fracturing • Fracturing occurs when the wellbore drilling-fluid pressure exceeds the formation-fracture pressure. 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 integrity of the drilled hole. The associated problems are pipe sticking and possible loss of well.

31. Principles of borehole instability Before drilling, the rock strength at some depth is in equilibrium with the in-situ rock stresses (effective overburden stress, effective horizontal confining stresses). While a hole is being drilled, however, the balance between the rock strength and the in-situ stresses is disturbed. In addition, foreign fluids are introduced, and an interaction process begins between the formation and borehole fluids. The result is a potential hole-instability problem. Although a vast amount of research has resulted in many borehole-stability simulation models, all share the same shortcoming of uncertainty in the input data needed to run the analysis. Such data include :

32. Principles of borehole instability Data •In-situ stresses • Pore pressure • Rock mechanical properties • Formation and drilling-fluids chemistry Mechanical Rock-Failure Mechanisms Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive or the tensile strength of the rock. Compressive failure is caused by shear stresses as a result of low mud weight, while tensile failure is caused by normal stresses as a result of excessive mud weight.

33. Shale Instability * More than 75% of drilled formations worldwide are shale formations. * Reported cost attributed to problems of Shale instability >1.5billion$/year. * The cause of shale instability is two-fold: mechanical (stress change vs. shale strength environment) and chemical (shale/fluid interaction—capillary pressure, osmotic pressure, pressure diffusion, borehole-fluid invasion into shale).

34. Causes of Shale instability *Mechanical Instability.   As stated previously, mechanical rock instability can occur because the in-situ stress state of equilibrium has been disturbed after drilling. The mud in use with a certain density may not bring the altered stresses to the original state; therefore, shale may become mechanically unstable .

35. Chemical Instability: drilling-fluid/shale interaction which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the this problem include capillary pressure, osmotic pressure, pressure diffusion in the vicinity of 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 through the pore-throat interface. This results in the development of capillary pressure, p cap  , which is expressed as : …………..…………...........5-1(where σ is the interfacial tension, ϴ 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.

36. Chemical instability *Osmotic Pressure : When the energy level or activity in shale pore fluid, a s , is different from the activity in drilling mud, a m , water movement can occur in either direction across a semipermeable membrane as a result of the development of osmotic pressure , p os , or chemical potential, μ c . To prevent or reduce water movement across this semipermeable membrane that has certain efficiency, E m , the activities need to be equalized or, at least, their differentials minimized. If a m is lower than a s , it is suggested to increase E m and vice versa. The mud activity can be reduced by adding electrolytes that can be brought about through the use of mud systems such as seawater , saturated-salt / polymer , KCl / NaCl /polymer , and lime/gypsum .

37. Pressure Diffusion.   Pressure diffusion: is a phenomenon of pressure change near the borehole walls that occurs over time. This pressure change is caused by the compression of the native pore fluid by the borehole-fluid pressure,  p wfl , and the osmotic pressure,  p os . Borehole Fluid Invasion into Shale .  In conventional drilling, a positive differential pressure (the difference between the borehole-fluid pressure and the pore-fluid pressure) is always maintained. As a result, borehole fluid is forced to flow into the formation (fluid-loss phenomenon), which may cause chemical interaction that can lead to shale instabilities. To mitigate this problem, an increase of mud viscosity or, in extreme cases, gilsonite is used to seal off micro fractures.

38. Borehole-Instability Prevention Total prevention of borehole instability is unrealistic because restoring the physical and chemical in-situ conditions of the rock is impossible. However, the drilling engineer can mitigate the problems of borehole instabilities by adhering to good field practices. These practices include proper mud-weight selection and maintenance, the use of proper hydraulics to control the ECD, proper hole-trajectory selection, and the use of borehole fluid compatible with the formation being drilled. Additional field practices that should be followed are minimizing 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

39. Mud Contamination Definition A mud is said to be contaminated when a foreign material enters the mud system and causes undesirable changes in mud properties, such as density , viscosity , and filtration . Common Contaminants, Sources, and Treatments: The 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 and carbonates (HCO3−, CO3—); soluble sulfides (HS−, S—); and salt/salt water flow (Na+ , Cl − ).

40. Solids Contamination.   Material added to make up a mud system (bentonite, barite) and materials that are drilled (active and inert). Excess solids of any type are the most undesirable contaminant to drilling fluids. They affect all mud properties. mechanical separating equipment ( shakers( 140μ or larger ), desanders (down to 50 μ) , desilters ( down to 20 μ) , and centrifuges ). When solids become smaller than the cutoff point of desilters , centrifuges may have to be used. تُستخدم مواد الندف الكيميائية أحيانًا لتلبد المواد الصلبة الدقيقة إلى حجم أكبر بحيث يمكن إزالتها بواسطة معدات إزالة المواد الصلبة. لا تميز المواد الندفية الإجمالية بين أنواع مختلفة من المواد الصلبة ، في حين أن المواد الندفية الانتقائية سوف تعمل على تلبد المواد الصلبة المحفورة ولكن ليس المواد الصلبة المضافة من الباريت . كملاذ أخير ، يستخدم التخفيف أحيانًا لخفض تركيز المواد الصلبة.

41. Calcium-Ions Contamination.   *The source (gypsum, anhydrite, cement, lime, seawater, and hard/brackish makeup water ). The calcium ion is a major contaminant to freshwater-based sodium-clay treated mud 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 . 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 preferred treatment if the calcium ion is from lime or cement. If treatment becomes economically unacceptable, break over to a mud system, such as gypsum mud or lime mud, that can tolerate the contaminant.

42. Biocarbonate , Carbonate& Hydrogen Sulfide Contamination. Biocarbonate and Carbonate Contamination . The contaminant ions (CO 3 — , HCO 3 − ) are from drilling a CO 2 -bearing formation, thermal degradation of organics in mud, or over treatment with soda ash and bicarbonate. 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 H 2 S-bearing formation. Hydrogen sulfide is the most deadly ion to humans and is extremely corrosive to steel used during drilling operations . (It causes severe embrittlement to drillpipe .) Scavenging of H 2 S is done by use of zinc, copper, or iron.

43. Salt/Saltwater Flows.   The ions, Na+ Cl − , 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, high fluid loss, and pH decrease. Some actions for treatment are dilution with fresh water, the use of dispersants and fluid-loss chemicals, or conversion to a mud that tolerates the problem if the cost of treatment becomes excessive.

44. Producing Formation Damage Producing formation damage has been defined as the impairment of the unseen by the inevitable, causing an unknown reduction in the unquantifiable. In a different context, formation damage is defined as the impairment to reservoir (reduced production) caused by wellbore fluids used during drilling/completion and workover operations. It is a zone of reduced permeability within the vicinity of the wellbore (skin) as a result of foreign-fluid invasion into the reservoir rock. 

45. Fig. 5—Formation skin damage.

46. Damage Mechanisms Formation damage is a combination of several mechanisms including solids plugging , clay-particle swelling or dispersion , saturation changes , wettability reversal , emulsion blockage , aqueous-filtrate blockage, and mutual precipitation of soluble salts in wellbore-fluid filtrate and formation water . Solids Plugging.  Fig. 6.  shows that the plugging of the reservoir-rock pore spaces can be caused by the fine solids in the mud filtrate or solids dislodged by the filtrate within the rock matrix. To minimize this form of damage, minimize the amount of fine solids in the mud system and fluid loss .  reservoir-rock pore spaces can be caused by the fine solids in the mud filtrate or solids dislodged by the filtrate within the rock matrix. To minimize this form of damage, minimize the amount of fine solids in the mud system and fluid loss.

47. Solids Plugging .(Fig.6)  

48. Damage Mechanisms Clay-Particle Swelling.  This is an inherent problem in sandstone that contains water-sensitive clays. When a fresh-water filtrate invades the reservoir rock, it will cause the clay to swell and thus reduce or totally block the throat areas . Saturation Change.   Production is predicated on the amount of saturation within the reservoir rock. When a mud-system filtrate enters the reservoir, it will cause some change in water saturation and, therefore, potential reduction in production. Fig . 7.  shows that high fluid loss causes water saturation to increase, which results in a decrease of rock relative permeability .

49. Fig7.—Formation damage caused by saturation

50. Wettability Reversal& Emulsion Blockage . Wettability Reversal.  Reservoir rocks are water-wet in nature. It has been demonstrated that while drilling with oil-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% in production loss can be caused by this mechanism. Therefore, to guard against this problem, the amount of excess 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 enter the rock and can form an emulsion within the pore spaces, which hinders production through emulsion blockage

51. Aqueous-Filtrate Blockage& Precipitation of Soluble Salts .   Aqueous-Filtrate Blockage.  While drilling with water-based mud, the aqueous filtrate that enters the reservoir can 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 or from formation water or both, can cause solids blockage and hinder production.

52. Hole Cleaning 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 . Fig. 8  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. In adequate hole cleaning can lead to costly drilling problems such as mechanical pipe sticking, premature bit wear, slow drilling, formation .   fracturing, excessive torque and drag on drill string, difficulties in logging and cementing, and difficulties in casings landing. The most prevalent problem is excessive torque and drag, which often leads to the inability of reaching the target in high-angle/extended-reach drilling.   

53. Fig.8 —Cuttings-bed buildup in directional wells.

54. Factors in Hole Cleaning Annular-Fluid Velocity .  Flow rate is the dominant factor in cuttings removal while drilling directional wells. An increase in flow rate will result in more efficient cuttings removal under all conditions. However, how high a flow rate can be increased may be limited by the maximum allowed ECD, the susceptibility of the openhole section to hydraulic erosion, and the availability of rig hydraulic power.

55. Factors in Hole Cleaning Hole Inclination Angle.  Laboratory work has demonstrated that when hole angle increases from zero to approximately 67° from vertical, hole cleaning becomes more difficult, and therefore, flow-rate requirement increases. The flow-rate requirements reach a maximum at approximately 65 to 67° and then slightly decrease toward the horizontal. Also, it has been shown that at 25 to approximately 45°, a sudden pump shutdown can cause cuttings sloughing to bottom and may result in a mechanical pipe-sticking problem. Although, hole inclination can lead to cleaning problems, it is mandated by the needs of drilling inaccessible reservoir, offshore drilling, avoiding troublesome formations, and side tracking and to drill horizontally into the reservoir. Objectives in total field development (primary and secondary production), environmental concerns, and economics are some of the factors that intervene in hole angle selection

56. Factors in Hole Cleaning Drillstring Rotation.  Laboratory studies have shown ,The level of enhancement is a combined effect of pipe rotation, mud rheology, cuttings size, flow rate, and, very importantly, the string dynamic behavior. 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. Also, 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. However , pipe rotation can cause cyclic stresses that can accelerate pipe failures due to fatigue, casing wear, and, in some cases, mechanical destruction to openhole sections. In slimhole drilling, high pipe rotation can cause high ECDs due to the high annular-friction pressure losses .

57. Factors in Hole Cleaning Hole/Pipe Eccentricity .  In the inclined section of the hole, the pipe has the tendency to rest on the low side of the borehole because of gravity. This creates a very narrow gap in the annulus section below the pipe, which causes fluid velocity to be extremely low and, therefore, the inability to transport cuttings to surface. As  Fig. 9.  illustrates, when eccentricity increases, particle/fluid velocities decrease in the narrow gap, especially for high-viscosity fluid. However, because eccentricity is governed by the selected well trajectory, its adverse impact on hole cleaning may be unavoidable.

58. Fig. 9—Fluid velocity profile in eccentric annulus (after Hzouz et al. [3] ).

59. Factors in Hole Cleaning Rate of Penetration .  Under similar conditions, an increase in the drilling rate always results in an increase in the amount 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 only alternative 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.

60. Factors in Hole Cleaning Mud Properties.  The functions of drilling fluids are many and can have unique competing influences. The two mud properties that have direct impact on hole cleaning are viscosity and density. The main functions of density are mechanical borehole stabilization and the prevention of formation-fluid intrusion into the annulus. Any unnecessary increase in mud density beyond fulfilling these functions will have an adverse effect on the ROP and, under the given in-situ stresses, may cause fracturing of the formation. Mud density should not be used as a criterion to enhance hole cleaning. Viscosity, on the other hand, has the primary function of the suspension of added desired weighting materials such as barite. Only in vertical-well drilling and high-viscosity pill sweep is viscosity used as a remedy in hole cleaning.

61. Factors in Hole Cleaning Cuttings Characteristics .  The size, distribution, shape, and specific gravity of cuttings affect their dynamic behavior in a flowing media. The specific gravity of most rocks is approximately 2.6; therefore, specific gravity can be considered a no varying factor in cuttings transport. The cuttings size and shape are functions of the bit types (roller cone, polycrystalline-diamond compact, diamond matrix), the regrinding that takes place after they are generated, and the breakage by their own bombardment and with the rotating drillstring . It is impossible to control their size and shape even if a specific bit group has been selected to generate them. Smaller cuttings are more difficult to transport in directional-well drilling; however, with some viscosity increase and pipe rotation, fine particles seem to stay in suspension and, therefore, are easier to transport.

62. Hydrogen-Sulfide-Bearing Zones and Shallow Gas Drilling H 2 S-bearing formations poses one of the most difficult and dangerous problems to humans and equipment. If it is known or anticipated, there are very specific requirements to abide by in accordance with Intl. Assn. of Drilling Contractors rules and regulations. Shallow gas may be encountered at any time in any region of the world. The only way to combat this problem is to never shut in the well; divert the gas flow through a diverter system instead. High-pressure shallow gas can be encountered at depths as low as a few hundred 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, underground blow.

63. Equipment and Personnel-Related Problems Equipment: The integrity of drilling equipment and its maintenance are major factors in minimizing drilling problems. Proper rig hydraulics (pump power) for efficient bottom and annular hole cleaning, proper hoisting power for efficient tripping out, proper derrick design loads and drilling line tension load to allow safe over pull in case of a sticking problem, and well-control systems (ram preventers, annular preventers, internal preventers) t hat allow kick control under any kick situation are all necessary for reducing drilling problems. Proper monitoring and recording systems that monitor trend changes in all drilling parameters and can retrieve drilling data at a later date, proper tubular hardware specifically suited to accommodate all anticipated drilling conditions, and effective mud-handling and maintenance equipment t hat will ensure that the mud properties are designed for their intended functions are also necessary.

64. Equipment and Personnel-Related Problems Personnel Given equal conditions during drilling/completion operations, personnel are the key to the success or failure of those 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 to successful drilling/completion practices.