The term borehole generally designates the result of a drilling operation in the earth, either vertically, horizontally and/or deviated using a drill string, comprising a drill bit at its lower end. At its upper end or top end, the drill string is driven by a drive system at the surface, called a top drive or rotary table. The top drive or rotary table is driven by an electric motor, or any other type of drive motor, providing a rotational movement to the drill bit in the borehole.
Typically, the drill string is a very slender structure of a plurality of tubulars or pipes, threadedly connected to each other, and may have a length of several hundreds or thousands of meters.
The lower part of the drill string is called the bottom hole assembly, BHA, and consists of heavier thick-walled pipes, called drill collars, at which the drill bit rests.
The drill string is hollow, such that drilling fluid can be pumped down towards the bottom hole assembly and through nozzles in the bit, for lubrication purposes. The drilling fluid is circulated back up the annulus, i.e. the space between the outer circumference of the drill string and the borehole wall, to transport cuttings from the bit to the surface.
A borehole may be drilled for many different purposes, including the extraction of water or other liquid (such as oil) or gases (such as natural gas), as part of a geotechnical investigation, environmental site assessment, mineral exploration, temperature measurement or as a pilot hole for installing piers or underground utilities, for example.
The bottom hole assembly is rigid in torsional direction as it is relatively short and thick-walled and in use experiences lateral deflections due to compressive force. The drill string is an extreme flexible structure due to its long length and relative small wall thickness, such that during drilling numerous vibrations are induced in the borehole equipment and, in particular, in the drill string. In the case of a rotary drill string and bottom hole assembly, torsional, axial and longitudinal or lateral vibrations may be induced.
Axial vibrations can cause bit bounce, which may damage bit cutters and bearings. Lateral vibrations are very destructive and can create large shocks as the bottom hole assembly impacts the wall of the borehole. Lateral vibrations may drive the system into backward whirl, creating high-frequency large-magnitude bending moment fluctuations, that result in high rates of component and connection fatigue. Imbalance in an assembly may cause centrifugally induced bowing of the drill string, which may produce forward whirl and result in one-sided wear of components. Torsional vibrations result, among others, in stick-slip motions or oscillations of the drill string alongside the borehole.
Stick-slip is a phenomenon caused by frictional forces between surfaces of the drill bit and/or the drill string contacting the earth formation or the inner wall of the borehole. The surfaces alternatingly may stick to each other or slide over each other, with a corresponding change in the force of friction. In extreme cases, the friction may become so large that the drill bit, i.e. the bottom hole assembly, temporarily comes to a complete standstill, called the stick mode. During the stick mode, the continuing rotational drive speed or motion of the drive system winds-up the drill string. If the torque build-up in the drill string is large enough to overcome the friction, the bottom hole assembly starts rotating again, called the slip mode. This, however, may cause a sudden jump or a stepwise increase in the angular acceleration of the movement of the drill bit and may result in excessive wear thereof. Stick and slip modes may follow each other rather quickly in an oscillating like manner.
Stick-slip is also a major source of problems causing equipment failures if the drill string, due to the rotary oscillations induced therein, starts to build-up a negative torque, i.e. a torque in the opposite direction compared to the direction of rotation of the drive system. When negative torque exceeds a friction threshold, pipe-connections will tend to unscrew.
When stick-slip occurs, the effectiveness of the drilling process is affected, such that a planned drilling operation may be delayed over as much as a few days, with the risk of penalty fees and the like.
Accordingly, in various situations it is required to control the effect of stick-slip oscillations in borehole equipment, thereby mitigating as much as possible the above outlined problems.
Mitigating the stick-slip phenomenon has been the subject of many studies and patent publications. International patent application WO 2010/063982, for example, suggests damping of stick-slip oscillations based on a frequency or wave propagation transmission line approach, by operating the speed controller having its frequency dependent reflection coefficient of torsional waves set to a minimum at or near the frequency of the stick-slip oscillations.
A problem with this known approach is that in stick mode, in which the bottom hole assembly comes to a complete standstill, the frequency approach fails to correctly describe the physical behaviour of the borehole equipment, as the speed of the bottom hole assembly obviously equals zero. Further, in practice, the bottom hole assembly rotates at relative low speeds, which makes a sufficient accurate sinusoidal waveform approach more difficult, and because a real drilling system shows a non-linear behaviour.