Page 324 - Drilling Technology in Nontechnical Language
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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
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