Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Drillstring Rotation. Laboratory studies have shown and field cases have reported that
drillstring rotation has moderate to significant effects in enhancing hole cleaning. 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
Factors in Hole Cleaning

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hole are results of pipe whirling action. 
Although there is a definite gain in hole cleaning caused by pipe rotation, there are certain
limitations to its implementation. For example, during angle building with a downhole motor
(sliding mode), rotation cannot be induced. With the new steering rotary systems, this is no
longer a problem. 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. 
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. 10.12 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.