Page 324 - Drilling Technology in Nontechnical Language
P. 324
Chapter 13 – DRILLING PROBLEMS AND SOLUTIONS 315
can occur if a more highly stabilized assembly is run straight into the
same section. It is a geometry-related problem. It is necessary to ream in
first with the new BHA. Reaming is like drilling but instead of rotating
on bottom, the bit drills on its gauge area only to smooth out the curved
section and enlarge it slightly. The work that the bit does is monitored by
watching—and limiting—the torque required to turn the drillstring.
Where hard and soft layers of formation alternate, the soft parts can
enlarge more than the harder formations. Ledges form. It is possible for the
pipe to get stuck at changes of diameter, such as on the bit, on stabilizers,
or at the top of the BHA.
Solids-related stuck pipe
Solids particles in the annulus can cause the pipe to get stuck (fig. 13–4).
Mostly, these solid particles will be drilled cuttings or wellbore cavings.
However, there are other possibilities. Solids-related problems normally
occur when pulling pipe out of the hole, or if the pipe is left in one place
without any circulation. In most cases, circulation will not be possible
because the solid particles will block off the annulus. If circulation is
impossible due to solids, the hole is said to be packed off. This stuck pipe
situation is the most difficult to cure, but most of the time, it is luckily not
difficult to avoid.
Before the drillstring is pulled out after drilling, circulation should
continue after drilling stops for long enough to lift all the cuttings to the
surface. This is not always as straightforward as it sounds. In inclined
wellbore sections, cuttings move upwards by a combination of lifting
(vertically upwards movement) and rolling along the low side of the hole.
As inclination increases, more rolling and less lifting takes place; in a
horizontal well, the vertical lift component is zero. Making solids roll
along the hole takes more energy (higher flow rates) than lifting them.
Also, as inclination increases, the drillpipe will tend to rest on the low side
of the hole. This reduces the flow rate along the bottom, as the flow will
preferentially go to the largest available area—above the drillpipe. Under
these conditions, cuttings beds form easily and are not easily moved if the
drillstring is not rotated (when drilling with a directional motor). Modern
rotary steerable tools avoid this.
_Devereux_Book.indb 315 1/16/12 2:13 PM