Closing in on stick-slip

THE old chestnut of stick-slipping drillstrings is back again, but this time around we may come closer to understanding how this problem arises. Sarah Belfield writes for Australian Mining

One new approach to modelling stick-slip behaviour may help explain why some rotary drilling set-ups experience stick-slipping while others manage to avoid the problem.

Stick-slipping is where a rotary drill bit, instead of steadily cutting rock, alternates between coming to a halt (sticking) and entering a sliding mode in which cutting takes place (slipping).

This behaviour is a headache for drillers. The catalogue of knock-on effects includes drill bit damage and increased bit wear, accelerated accumulation of fatigue in drilling equipment, unintended drilling direction changes, drillstring failure, and damaged electronics — not to mention the slabs of time lost in trying to work around the stick-slip problem itself.

It’s said the effects of stick-slipping, and of the twisting vibrations that give rise to stick-slipping, can sometimes be seen at the surface, even to the point where drill rigs shake and rotary-table motors stall.

Back with a vengeance

After a period of relative obscurity during the 1990s, the issue of stick-slipping is gaining attention again, according to CSIRO drilling mechanics research scientist Thomas Richard.

He says in the early days of polycrystalline diamond drill bits, stick-slipping was a severe problem. Later, in the 90s, the advent of mud motors (which sit near the drill bit and use the hydraulic force of drilling fluids to help drive the bit forward) meant higher angular velocities could be imposed on the bit. It helped keep stick-slipping at bay.

Now, with the rise of directional drilling technology known as rotary steerable systems — which usually involve no additional motors — stick-slip behaviour is creeping back onto the agenda.

But despite having been around for decades in drilling contexts, stick-slipping remains something of a mystery.

However, in a bid to rectify this, Richard and his research colleagues Christophe Germay, based at the University of Liège in Belgium, and Emmanuel Detournay, at the University of Minnesota in the USA, have taken a novel approach to try to explain how and why the phenomenon arises.

What matters most

Unlike previous analyses of stick-slipping in rotary drilling, Richard and his colleagues have taken great pains to closely model the processes taking place at the rock-bit interface. They use results originally obtained at the University of Minnesota in the late 1990s.

People previously looking at the stick-slip phenomenon tended to trivialise the rock-bit interface, Richard says. They focused instead on the drillstring itself — overly complicating it, he suggests.

He says that when trying to find the root cause of stick-slipping, drill bit modelling must be detailed because the complexity seen in stick-slipping originates at the bit. Richard’s model involves drag bits, which feature cutter blades that shear rock in a scraping action.

Modelling of other bit types, such as tri-cone bits, would be very complex because of the moving parts involved, he says. Even though drag bits are relatively simple in their design, the processes occurring between a drag bit and the rock it’s cutting are not straightforward. For instance, friction involves non-linear relationships and rock-cutting with multiple blades is what Richard calls “history dependent”.

It seems Richard’s approach may have paid off. What he and his colleagues found in their modelled drilling world is that two distinct scenarios are possible. The first is that along-string vibrations lead to twisting vibrations, which in turn morph into permanent stick-slipping. The second scenario is that the vibrations gradually settle down over time.

One particular parameter contained in their model seems to be the only thing controlling which scenario arises. They call the parameter the “rock-bit interaction number” and assign it the Greek symbol t.

Richard says the significance of t is that if you change its value, you change the evolution of the drill bit’s response to the rock. In turn, changing this evolution changes how easily vibrational energy is transferred from an along-the-rods direction to an around-the-rods direction.

A t value of 1 seems to be a watershed value. If t is greater than or equal to 1, around-the-rods vibrations are damped, and stick-slipping has no chance of arising. But if t is less than one, these vibrations are amplified and stick-slipping can set in.

“This is purely a model,” Richard cautions.

“We are currently designing an experiment in the lab to try to validate that model.”

He anticipates finishing in about six months from October.

Avoiding stick-slipping

While the results of the laboratory tests are awaited, the million dollar question is what can drillers do now to prevent stick-slip behaviour from arising.

According to the model devised by Richard and his colleagues, stick-slipping is not necessarily inevitable if your drilling set-up possesses a t value of less than one.

Along-the-rods vibrations feed into around-the-rods vibrations, so anything you can do to limit the former should go some way to limiting the latter.

“If somehow you’re able to limit the axial vibration of the bit, so for example by adding friction to the side of the bit — what people call an active gauge — it will maybe limit the vibration,” Richard says.

“It would certainly help, according to the model.”

Another approach is to put a drill bit’s cutting blades closer together. Blades that are relatively far apart have more of an opportunity to create an undulated surface in the rock. The undulations become more pronounced over time in a manner similar to how dirt roads develop corrugations. And undulations perpetuate along-rod vibrations, something to be avoided.

A similar measure (from the rock’s point of view) is to increase the angular velocity imposed on the drill bit. This stick-slip prevention method is often seen to work in the field, Richard says.

Stiffening the drilling string is also a possibility he says, but this can be difficult to achieve in practice.

Thomas Richard

CSIRO

thomas.richard@csiro.au

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