�Mystery� plague. It starts unexpectedly and robs the drilling operation of efficiency. Penetration rates drop, new bits are seriously worn in mere hours, drillstring components like MWD/LWD tools and pulsers cease to function and it all occurs during �normal� operations. Often, it can't be identified on the surface easily, or its effects are masked by mechanical systems. No, it's not a computer virus. These symptoms are caused by a more insidious thief—stick-slip.
Stick-slip is defined as energy storage in a moving system that, when released, results in some system component lunging forward at high velocity only to stop again shortly thereafter. Energy is stored again until it is released in another forward lurch. The process repeats itself in a cyclic pattern.
In stick-slip drilling, the bit stops rotating in the hole, usually because of excessive friction at the rock face or on some BHA component. As the rotating equipment on the surface continues to turn, the drillstring �twists up� storing energy until the friction on the bottom is overcome. Then, the bit and BHA spin wildly as this rotational energy is spent, after which the rotation on bottom stalls. The process repeats every few seconds for hours on end, depending on the system's characteristics.
At first, this doesn't seem too bad. One might guess that the high rotary speed of the bit could result in some very fast drilling for a few seconds followed by a period with no penetration. The average rate of penetration might just match or exceed that of a constantly turning bit.
Stick-slip drilling is highly inefficient. Often, when the bit is turning, it is doing so at multiples of normal rotary speed. There just isn't enough time for the cutting structure to dig into the rock face to remove rock. As the drill string �twists up� it often becomes slightly �shorter.� Weight on bit (or rather a constant reading on a surface weight indicator) does not remain constant at the bottom of the hole. Spinning drill collars and tools on the bottom of this �spring� create massive rotary inertia that can unscrew the BHA at some connection and leave a fish in the hole. Hopefully, that fish is not just the bit.
What's the answer? There are several ways to identify stick-slip during drilling. One is through the use of monitoring devices on the surface. Top-drive systems generally operate at a given rotary speed, but cyclic increases and decreases in torque can be detected. On electric motor-driven rotaries, where constant voltage is maintained, a cyclic amperage profile is usually generated. If the monitoring device is taking readings frequently enough, the sine wave of amperage versus time is a dead giveaway of stick-slip.
Rotary tables can sometimes speed up and slow down in response to stick-slip with a growling sound that indicates increased rotary torque, followed by a slight whistling sound as the torque is released and the table spins easily in response. An RPM counter will show a cyclic decrease/increase in rotary speed, another dead giveaway of stick-slip.
Another key indicator is a precipitous drop in penetration rate versus offset analog wells that did not experience stick-slip, while drilling in the same hole section. Once again, stick-slip is a highly inefficient drilling method.
The bit, the bit manufacturer and the poor guy that sold the bit are usually blamed for this poor performance. When the bit is pulled, sure enough, the outside ring of cutters or teeth is usually worn badly. This occurs when the bit spins at high rotary speed literally tearing the cutting structure away. These bits look like someone has ground the outside of the bit away with a bench grinder. That, in fact, is precisely what has happened, but it wasn't a bench grinder—it was the bottom of the hole!
Often, these symptoms are masked by variations in mechanical systems. Drillers and company men frequently misdiagnose stick-slip. As drilling goes into hotter, deeper, more abrasive rocks, using aggressive cutting structures like PDC bits, stick-slip is likely to become an increasingly serious problem.
Fortunately, most MWD tools have sensors that identify �shocks� or blows to the tool. Cyclic shocks experienced by the MWD tool can indicate stick-slip. Several MWD/LWD tool companies now supply a special sub to identify stick-slip. It contains multiple accelerometers that indicate decreasing downhole rotational speed followed by rapidly increasing rotational speed. Thus, stick-slip can be measured at the rig. These short, robust subs provide data to the surface through the mud pulser or electromagnetic telemetry, so folks on the rig can see stick-slip and take steps to halt the process. My, my, haven't we come a long way with MWD tools?
So, what are the cures for stick-slip? There are two major ones, which may, at first, seem counter intuitive.
The first involves two actions. One is to reduce the weight on the bit. This reduces friction, which stalls rotation, at the bottom of the hole. Thus, the bit rotates continuously and the cutting structure stays in contact with the formation face. When this is suggested, most drillers and company men scoff. How can one possibly increase penetration rate by decreasing weight on bit? Are you guys crazy? Well, no—not exactly.
The complement seems equally as silly—increase rotary speed. Here, the string is twisting up and releasing every 10 to 20 seconds, and you want me to increase rotary speed? Yep, that too will keep the bit turning continuously, if the WOB is low enough. It also helps to reduce downhole shocks and first-order sinusoidal drillstring vibration in many cases.
The second cure is to use less aggressive bits. Oh, now this is really getting silly from a driller's perspective. You want me to run a less aggressive bit to increase penetration rate. Yep, I sure do!
Speeding up the rotary table or top drive and decreasing WOB, or running less aggressive bits, just doesn't make a lot of sense to most rig personnel. In extreme cases, when the drillstring and rig seem to be on the verge of self-destruction, asking drillers to do something that would seem to aggravate the situation often results in ridicule or outright hostility.
The proof is in the results, however. Watch the dynamic MWD tool and see if it doesn't show reduced rotary speed and torque variations. Then, see how much longer the bit lasts. Finally, look at the penetration rate. When it goes up, you'll know that you've foiled the robbery and put this thief at bay.