The three main causes of stuck pipe are cuttings and cavings, keyseats, and differential sticking. Cuttings and cavings build up in the annulus when mud and hydraulics fail to keep the hole clean. Poor design, deteriorating mud systems, pump failure, holes in the pipe, or many other conditions may give the same result. The drill string may not move up or down, and circulation may be restricted or absent.
Keyseat sticking generally occurs while the pipe is moving upward. The top of the drill collars, the uppermost stabilizer, and the bit are the most likely parts of the drill string to hang up in the keyseat, or slot, cut into the dogleg by the downhole assembly. Complete circulation is nearly always present during keyseat sticking, and the pipe is more likely to have freer movement downward than upward.
Wall stuck pipe generally displays full circulation, but both up and down pipe movement are restricted equally. This type of sticking nearly always occurs in the bottom hole assembly. It comes about when the drill string has been stationary for a time. It is common when drilling highly deviated holes and when keyseated. Differential pressure across the drill collars forces strong contact between the drill collars and the side of the borehole. The mud properties conducive to wall sticking are overbalance and high water loss.
When the drill string sticks, work it in the direction opposite to which it was moving when it became stuck. Work it for an extended time, jarring if drilling jars are in the bottom hole assembly and above the free point; do not immediately call for the fishing equipment. Decide what type of sticking is involved, and use stretch table to determine where the free point. If the drill pipe is worn to less than nominal weight, stretch tables may not be accurate. It should be possible to determine if the free point is moving up the hole. If this is the case, it is time to do something else. Continue working the pipe while decisions are being made.
If the drill string is stuck above a depth of 600 m, it may be possible to move it downward with surface jars, although the author has had little success with surface jars unless the pipe is stuck above 400 m. If it is known that differential sticking has occurred, spot a lighter fluid, like diesel to lower the pressure differential, always considering up hole well control requirements. Spotting nitrogen can be instantly effective, but surface pipe and surface equipment must be more than adequate to control any possible kick. Nitrogen is hard on mud, so sufficient volume of good mud to displace the volume ruined by the nitrogen should be available. If carbonate cuttings stick the drill string, spotting acid may help; remembering that a considerable amount of acid gas will be produced.
Either of two devices may be chosen to obtain information about the downhole situation. The freepoint device records applied torque and tension by mechanical-electrical means. It can only determine the uppermost depth at which the pipe is free to move. The stuck-pipe log is a type of sonic log. The information it obtains is similar to that of a simple cement bond log. It is able to survey the stuck pipe all the way to bottom. It is of course more expensive than the freepoint device. The author recommends that the stuck pipe log be run first. The fishing job can be planned most effectively with complete information. After backing off once wireline operations wireline operations may be prevented by millings or cuttings within the fish.
The back-off tool consists of a rope socket, sinker bar, collar locator, and pigtail or rod. Primer cord is looped on the pigtail or rod. This assembly is known as a string shot. Torque is worked down to the tool joint selected for separation. Holding tension and torque, position the string shot in this tool joint and ignite the charge. Withdraw the back-off tool and then trip out of the hole. Always back off at least one joint above the tool joint. Use area logs to avoid backing off in known washout zones. Also, never back off within two hundred feet of the last casing shoe; this interval is known to be prone to washout. Rather than back off in this section, back off inside the casing.
If the stuck-pipe log shows only a small section of the fish to be stuck, a simple fishing assembly may be run rather than a wash-over assembly. Below the drill pipe run four to six drill collars (one for each inch of jar diameter), fishing jars, bumper sub, one drill collar, back-off sub, and tool to catch the fish. This assembly can be used to wash over with washpipe and rotary shoe substituted for the catch tool. A spear may be used between the back-off sub and the washpipe if it is thought possible to wash over to the bit and remove the fish on the same pass. Do not run over 150 m of washpipe; washing over is a dangerous procedure, although less so in open than in cased hole. Washing over in cased hole is one of the most risky of fishing operations.
Fishing jobs, which require successive back off and catch operations may find the path of the wireline tool to be blocked. This is particularly common after milling. After an irregular fish top has been dressed with a flat-bottomed mill, it sometimes pays to redress it with a tapered or pilot mill before making a catch tool run. Sometimes small workover tubing is used to drill out the impediment. Coiled tubing units are effective this task, but they are expensive.