Stuck pipe can be a costly problem in the oilfield. When the drill string gets stuck in the well, all drilling operations grind to a halt as workers attempt to free the pipe. Delays add significantly to drilling costs and is another example of NPT (non-productive time).
Clearly, stuck pipe is best when avoided. By studying the geologic formations and developing a well-thought-out drilling plan, the risk of stuck pipe can be reduced. However, in the real world it is impossible to prevent occasional mechanical breakdown and material failure. Because of this, stuck pipe will probably always be a part of drilling.
There are four common causes of stuck pipe: key seating, cuttings accumulation, formation instability, and differential pressure sticking.
Let’s take a look at the most common causes of stuck pipe and what drillers can do to troubleshoot the pipe in each case.
Key Seating
Key seating occurs when the drill string erodes a slot into the inside edge of a curving directional borehole. This slot is likely the diameter of the drill pipe. Since this diameter can be significantly smaller than the BHA, when tripping and the BHA reaches this slot, it gets wedged into the slot, hence it is stuck! Key seating is most likely to occur in soft formations located near points of high borehole curvature where the tension forces in the drillstring are high
The usual solution to key seating is to lower a tool into the borehole that can be used to ream out the notch created by the drill string, allowing the pipe to rotate freely. The goal is to allow the pipe to follow its natural curvature without digging into the side of the borehole. However, it is challenging to accomplish this so it is best to avoid the situation if at all possible.
Cuttings Accumulation
When cuttings are produced by the drill bit, the drilling mud is supposed to carry them to the surface for disposal. Cuttings accumulation occurs when cuttings begin to build up in the annulus. If the cuttings build up enough, they can put a lot of friction on the rotating drill string or even completely block the annulus preventing circulation.
Fortunately, pipe that is stuck due to cuttings accumulation can often be freed simply by increasing the mud pressure and torque on the rotation system while attempting to move the drill string up and down. Drill pipe stuck due to cuttings accumulation can usually be freed without exceeding the maximum safe rotation torque and mud pumping rate of the rig equipment.
Formation Instability
Some rock types, such as salt and shale, tend to expand into the hole over time. Sometimes, the formation expands enough to pinch the drill string, leading to stuck pipe.
The primary way to counteract the effects of formation instability is to ensure that the mud weight is high enough to counteract the forces acting to deform formation rocks. If the pipe is stuck in a salt formation, fresh water can sometimes be used to dissolve the salt and free the pipe although that can leave the borehole walls with significantly varying diameters. A borehole that has varying diameters is called “rugose”.
Differential Pressure Sticking
Key seating, cuttings accumulation, and formation instability are referred to as mechanical sticking mechanisms because they are related to changes in the shape of the borehole or increased friction. In contrast, differential pressure sticking is caused by a pressure effect, which causes the pipe to become embedded in the mud cake lining the borehole.
Differential pressure sticking typically occurs in formations with exceptionally high permeability and low pore pressure. If drilling fluid has high pressure relative to the formation pressure and the drill string embeds itself into the wall cake from the mud solids, a sealed large differential pressure forms. Recall that force is a pressure over an area. This differential pressure exerts a high force into the borehole wall such that the friction force generated is greater than the strength of the drill string. If this happens over a large enough segment of the pipe, it will eventually get stuck.
Differential pressure sticking can be resolved by decreasing the mud weight (with subsequent well control problems if one isn’t careful) or modifying the mud to reduce the friction between the pipe and the mud cake.
What if you can’t fix stuck pipe?
If, after trying the above methods, an operator is unable to resolve a stuck pipe issue, they may have no choice but to abandon that part of the drillstring in the hole and go around the problem area. To do this, they need to steer the drill out of the borehole and onto a new path that bypasses the stuck pipe.
Typically, this requires plugging the old hole with cement and using a whipstock to divert the bit from the hole. A whipstock forces the drill bit to cut into the side of the hole and start on a new path.
While the whipstock itself isn’t expensive, the stuck pipe and any tools or drill bits attached to it are usually lost. The loss of materials and equipment, combined with the rig time required to bypass the stuck pipe, can be quite expensive for an operator dealing with stuck pipe.
Images: “Jogger” by Michael Black