Fig.
10.11—Cuttings-bed
buildup
in
directional wells.
Inadequate 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 drillstring,
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.
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.
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.
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