Technology Update: A Holistic Approach To Controlling Torsional Dynamics in the Drillstring
Authors
Liam Lines (Weatherford)
Publisher
SPE - Society of Petroleum Engineers
Publication Date
October 1, 2016
Source
Journal of Petroleum Technology
Paper ID
SPE-1016-0020-JPT
Abstract
Technology Update
Understanding drilling dynamics, the downhole shock and vibration that occur while drilling, is a crucial step to improving drilling efficiency and reducing nonproductive time (NPT).
Industry statistics suggest that 20% to 30% of all downhole failures are related to adverse drilling dynamics (SPE 127413 and SPE 98150), and that drilling tool providers spend roughly USD 750 million per year on repair and maintenance. This amount pales compared with the untold costs borne by exploration and production companies as the result of drilling and completion inefficiencies, NPT, and tools lost in the hole.
Preventing adverse dynamics requires a strong understanding of the fundamentals, particularly in addressing torsional dynamics: time-based variations in downhole rotational speed and torque.
A common misconception is that torsional dynamics is limited to stick/slip. However, stick/slip is only one of three distinct modes of torsional dynamics, each with its own unique indicators, potential for damage, effect on drilling efficiency and—importantly—most effective means of mitigation.
Differing Dynamics
In addition to full stick/slip (FSS), the modes include low-frequency torsional oscillation (LFTO) and high-frequency torsional oscillation (HFTO) (Fig. 1). LFTO and FSS both typically occur at less than 2 Hz; HFTO occurs at between 50 and 250 Hz.
LFTO, probably the most common of all three modes, is the excitation of a fundamental torsional natural frequency of the drillstring. The excitation causes the drillstring to act as a long torsional spring, which results in sinusoidal oscillations in bit speed and torque. The oscillation frequency is a function of the drillstring length and stiffness, and is typically less than 0.5 Hz for all drillstrings (SPE 21945). LFTO is characterized by a maximum revolutions per minute (RPM) of up to twice the average drillstring RPM and a bit that may stop, but only instantaneously.