Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Prevention of Lost Circulation
Remedial Measures

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moderate lost-circulation zones. In the case of severe lost circulations, the use of various plugs to
seal the zone becomes mandatory. It is important, however, to know the location of the lost-
circulation zone before setting a plug. Various types of plugs used throughout the industry
include bentonite/diesel-oil squeeze, cement/bentonite/diesel-oil squeeze, cement, and barite.
Squeeze refers to forcing fluid into the lost-circulation zone.
Hole deviation is the unintentional departure of the drill bit from a preselected borehole
trajectory. Whether drilling a straight or curved-hole section, the tendency of the bit to walk
away from the desired path can lead to higher drilling costs and lease-boundary legal problems.
Fig. 10.6 provides examples of hole deviations. 
Fig. 10.6—Example of hole deviations.
It is not exactly known what causes a drill bit to deviate from its intended path. It is, however,
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.
Drillstring characteristics, specifically the 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.
Improper hole cleaning.
Hole Deviation
Definition
Causes

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It is known that some resultant force acting on a drill bit causes hole deviation to occur. The
mechanics of this resultant force is complex and is governed mainly by the mechanics of the
BHA, rock/bit interaction, bit operating conditions, and, to some lesser extent, by the drilling-
fluid hydraulics. The forces imparted to the drill bit because of the BHA are directly related to
the makeup of the BHA (i.e., stiffness, stabilizers, and reamers). The BHA is a flexible, elastic
structural member that can buckle under compressive loads. The buckled shape of a given
designed BHA depends on the amount of applied WOB. 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, thus
causing the deviation. 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. 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. 
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); and 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.
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, respectively; or fatigue as a result of mechanical cyclic loads with
or without corrosion.
Pipe failure as a result of twistoff occurs when 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.
Pipe-parting failure occurs when the induced tensile stress exceeds the pipe-material ultimate
tensile stress. This condition may arise when pipe sticking occurs, and an overpull is applied in
addition to the effective weight of suspended pipe in the hole above the stuck point.