Excess vibration of the drillstring is a serious concern in drilling because it accelerates the fatigue process and reduces the life of the bottomhole assembly (BHA). In extreme cases, excess vibration can lead to premature failure of the equipment. Drillstring vibration can also produce borehole washout, increase the risk of sticking pipe, and reduce the rate of penetration (ROP). Although weight-on-bit (WOB) and rotary speed are the primary parameters used to control vibration, the study of other parameters, such as ballooning, contact and frictional forces between the drillstring and the wellbore, and the finite displacement of the drillstring may provide additional tools and methods for controlling and minimizing drillstring vibration during drilling. The dynamic behavior of an active drilling assembly is complex to simulate and analyze, and finite element numerical models are used to investigate the influence of various factors on drillstring vibration.
The objective of this study is to derive additional options that will improve ROP by providing more flexibility in BHA design than those provided by traditional WOB-rotary speed methods. We focus on the influence of mud density, borehole inclination angle, and the choice of drilling fluid rheology model on the drillstring vibration.
The current increase in the oil demands pushes the oil and gas industry to implement new technologies and increase the efficiency of the drilling operation. Vibration is considered to be one of the causes of decreased efficiency of the operation. Technically, however, resonance, rather than vibration, is the major problem. Vibration refers to the mechanical oscillations about an equilibrium point. This oscillation may be periodic, such as the motion of pendulum, or total random motion, such as what occurs during the drilling operation. Resonance, which was first observed in 1602 by Galileo Galilei, is the tendency of the system to oscillate at larger amplitudes at some frequencies than at others. In the vibrational control analysis, resonance is the main problem and the goal is to avoid operating in the resonance regions. Although vibration can waste energy, create washouts and hole integrity problems, and lead to tool failure, stuck pipe, and decreased ROP, resonance magnifies the damage and can lead to complete failure of the drilling operation.
This section defines several terms used in this paper to discuss the issues associated with vibration control.
Drilling system refers to the definitive lithology, drillstring and BHA components, and hole conditions at a certain depth, as well as various parameters that regulate the internal wall of the hole, such as friction factor and linear capacity. Minor changes within these parameters can lead to a change in their interaction and alter the results of the observation and the conclusion.
Resonance frequency of the drilling system refers to the frequency resonance that the system vibration does not change with the changing amplitude, but reaches a state in which even small periodic driving forces can produce large excessive amplitude of vibrations. Resonance occurs because, at this state, the system stores the vibration energy during the other vibration frequencies and then re-produces when the vibration reaches the resonance frequency. It creates a great deal of damage, or even a failure, of the drillstring and BHA, as well as hole integrity problems.
Free vibration occurs when a mechanical system is set off with an initial input and then allowed to vibrate freely; it is rarely exists in the drilling operation and has not shown any threat to the drillstring as a result of the use of a viscous drilling fluid medium (mud).
Forced vibration occurs when an alternating force is applied to a mechanical system, which results in a vibration.