AST
about Stick-slip
The stick-slip vibration phenomenon has become an important risk element to evaluate in the planning of oil and gas well drilling. The reason for this is the widespread use of new, highly efficient drill bits using Polycrystaline Diamond Cutters (PDC) that cut the rock by sheer rotary force compared to the previous roller-cone bits that crushed the formations and required only a limited amount of energy to turn.
The stick-slip action is characterized by the absorption and release of energy as a function of the difference between static and dynamic friction. When stick-slip takes place at the end of a long drill-string, the phenomenon will produce accumulation and release of energy stored as several turns of twist in the string.
In the slip or release phase, the string spins out of control and this is what creates stick-slip-associated destructive vibrations. Stick-slip occurring where the PDC cutters meet the rock has the potential to create the longest stick and most violent slip periods. Consequently, stick-slip initiated at the rock-cutting interface is highly feared, and is responsible for most down-hole tool and tool joint overload failures in the industry. The patented Anti Stick-slip Technology from Tomax targets this form of stick-slip.
Stick-slip can also be produced by the friction between the hole wall and the drill-string itself. In this interface, there is no potential for holding up the rotation for a long stick period and the stick-slip from friction is typically less threatening.
Stick-slip can easily induce harmonic oscillations, which will appear as such on down-hole recordings. These are self-excited oscillations in string rotation, but with stable torsion at the bit. Harmonic oscillations of this kind can be observed for example in a clockwork spring. For minor variations of stick-slip and self-excited harmonics, the destructive capacity is minimal. But the variations in string rotation can pose challenges to the directional steering systems that rely on a relatively steady rotation for their guidance systems to work.
Cutter-induced stick-slip can occur inbetween other, less harmful, stick-slip effects and abrupt loads can trigger heavy stick-slip out of nowhere. Such abrupt loads may derive from sea swells causing heave on a drillship or from friction causing the bit to advance in leaps in deviated wellbores.