Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Fig. 10.12—Fluid velocity profile in eccentric
annulus (after Hzouz et al.
[3]
).
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. 
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. 

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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. 
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 nonvarying 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.
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.
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 overpull in case of a sticking problem, and well-control systems (ram
preventers, annular preventers, internal preventers) that 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 that will ensure that the
mud properties are designed for their intended functions are also necessary.
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