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Page 1




Larry W. Lake,

Volume II - Drilling

Robert F. Mitchell,

Copyright 2006,
Society of Petroleum

Chapter 10 - Drilling
Problems and


By J.J. Azar,
University of Tulsa

Pgs. 433-454

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PEH:Drilling Problems and Solutions


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.

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, H2S-bearing
formation and shallow gas, and equipment and personnel-related problems.

Understanding and anticipating drilling problems, understanding their causes, and
planning solutions are necessary for overall-well-cost control and for successfully
reaching the target zone. This chapter addresses these problems, possible solutions,
and, in some cases, preventive measures.

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1 Pipe Sticking
1.1 Differential-Pressure Pipe Sticking
1.2 Mechanical Pipe Sticking

2 Loss of Circulation
2.1 Definition
2.2 Lost-Circulation 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 Pipe-Failure Prevention

5 Borehole Instability
5.1 Definition and Causes
5.2 Types and Associated Problems
5.3 Principles of Borehole Instability
5.4 Mechanical Rock-Failure Mechanisms
5.5 Shale Instability
5.6 Wellbore-Stability Analysis
5.7 Borehole-Instability 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 Hydrogen-Sulfide-Bearing Zones and Shallow Gas
10 Equipment and Personnel-Related Problems

10.1 Equipment
10.2 Personnel

11 Nomenclature
12 References
13 General References
14 SI Metric Conversion Factors

Pipe Sticking

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

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maximum-normal-stress criterion, failure is said to occur when, under the action of combined stresses, one of
the acting principal stresses reaches the failure value of the rock tensile strength. In the maximum of energy of
distortion criterion, failure is said to occur when, under the action of combined stresses, the energy of distortion
reaches the same energy of failure of the rock under pure tension.

Shale Instability

More 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 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).

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.

Chemical Instability. Chemical-induced shale instability is caused by the drilling-fluid/shale interaction, which
alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. The
mechanisms that contribute to this problem include capillary pressure, osmotic pressure, pressure diffusion in
the 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 through
the pore-throat interface. This results in the development of capillary pressure, pcap , which is expressed as


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.

Osmotic Pressure. When the energy level or activity in shale pore fluid, as , is different from the activity in
drilling mud, am , water movement can occur in either direction across a semipermeable membrane as a result of
the development of osmotic pressure, pos , or chemical potential, μc . To prevent or reduce water movement
across this semipermeable membrane that has certain efficiency, Em, the activities need to be equalized or, at
least, their differentials minimized. If am is lower than as , it is suggested to increase Em 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.

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, pwfl, and the osmotic pressure, pos.

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 microfractures.

Wellbore-Stability Analysis

Several models in the literature address wellbore-stability analysis.[2] These include very-simple to
very-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 safety
windows for mud selection and design.

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.

Mud Contamination


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. Generally, water-based mud systems are the
most susceptible to contamination. Mud contamination can result from overtreatment of the mud system with
additives or from material entering the mud during drilling.

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−).

Solids Contamination. Solids are materials that are 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. 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 part
of the mud makeup. The removal of drilled solids is achieved through the use of mechanical separating
equipment (shakers, desanders, desilters, and centrifuges). Shakers remove solids in the size of cuttings
(approximately 140μ or larger). Desanders remove solids in the size of sand (down to 50μ). Desilters remove
solids in the size of silt (down to 20μ). 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 bigger
size so that they can be removed by solids-removal equipment. Total flocculants do not discriminate between
various 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, 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. It also causes added thinners
to 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 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.

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Plane Approaches. Presented at the Offshore Technology Conference, Houston, Texas, 2 May-5 May.
OTC-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 US
Symposium 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 During
Directional-Well Drilling. SPE J. 4 (2): 101–108. SPE-56406-PA.

Sanner, D.Ø. 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 Drilling
Fluid Components. Presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada,
22-26 September. SPE-14302-MS. (

Storli, C. 1987. Formation Damage of Primary and Secondary Emulsifiers and Amine Compounds at Elevated
Temperatures 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 in
Vertical Wellbores. SPE Journal of Petroleum Technology 34 (9): 1929-1937. SPE-9701-PA.
/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): 43–56. SPE-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 Curved
Hole. MS thesis, Tulsa, Oklahoma: University of Tulsa.

Lubinski, A. and Woods, H.B. 1955. Use of Stabilizers in Controlling Hole Deviation. Drilling and Production
Practice. 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): 5523–5530. (

SI Metric Conversion Factors

ft × 3.048* E − 01 = m

gal × 3.785 412 E − 03 = m3

in. × 2.54* E + 00 = cm

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in.2 × 6.451 6* E + 00 = cm2

lbf × 4.448 222 E + 00 = N
lbm × 4.535 924 E − 01 = kg
= kPa

*Conversion factor is exact.
Category (/Special%3ACategories): PEH (/Category%3APEH)


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