Page 17 - A Primer of Oilwell Drilling, 7th Edition

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here are several special operations used in oilwell drilling:
Tdirectional drilling, fishing, and well control.
Petroleum Extension-The University of Texas at Austin

dIRECTIONaL dRILLINg
No well is ever perfectly vertical. Even wells meant to be drilled
vertically will wander a few degrees from vertical and move in dif- Special
ferent directions. Routine measurements are taken during drilling
to determine if a well is deviating from vertical by more than the Operations
allowed amount (normally less than 5 degrees). if so, careful drilling
practices, such as changing the placement of stabilizers in the BHA
or adjusting the rotary speed or weight on bit, will bring the well
back within the tolerances normally allowed for vertical wells.
Directional drilling is used when a well is intentionally devi-
ated to reach a bottomhole location (BHL) that is different from the
surface location (SL). Directional drilling is done for many reasons.
The BHL might be under an obstruction such as a building or
lake where rigging up over the required BHL is not possible. it
might be necessary to drill several wells from a fixed place, such
as an offshore platform or an onshore drilling island (fig. 215), to
different bottomhole locations.
Part of an existing well might become blocked with lost drill-
ing tools that are unrecoverable, or a well might have been drilled
into an unproductive part of the reservoir. it is possible to set a
plug in the lower part of the well and deviate, or kick off, the well
to a new BHL. Some reservoirs are more efficiently produced by
wells drilled at a very high angle. These wells are known as hori-
zontal wells because the inclination angle from vertical reaches 90
degrees or more.
Older directional drilling methods placed inclined wedges,
called whipstocks, in the well to force the bit to move in the de-
sired direction. in soft sediments, it is possible to place a large bit
nozzle or jet in the desired direction and simply erode the well’s
starting path. Although time consuming, these methods are still
used at times.
The two faster and often more reliable methods of directional
drilling are:
• Slide drilling with a motor
• Drilling with a rotary steerable assembly

DIRECTIONAL
WELL
Figure 215. Several directional wells
tap an offshore reservoir.

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