Drilling guides › Stuck pipe: causes, first actions and free point from stretch
Stuck pipe: causes, first actions and free point from stretch
Pipe is stuck when it can no longer be pulled out of the hole without exceeding the safe overpull, whether or not it can still be rotated or circulated. Stuck pipe is one of the largest sources of lost time in drilling, and the chance of freeing the string falls quickly in the first hours. The most useful skill is therefore recognising which kind of sticking you have, because the right first action for one kind can make another kind worse.
The causes fall into two families. Differential sticking is caused by pressure: the string is pushed against a permeable wall by the overbalance. Mechanical sticking is caused by something physical in the way: cuttings, cavings, junk, or the shape of the hole itself.
Differential sticking
When the mud pressure is higher than the pore pressure across a permeable zone, mud filtrate flows into the rock and leaves a filter cake on the wall. If part of the string (usually the drill collars) lies still against that cake, the cake under the pipe is sealed off from the mud pressure. The full overbalance then acts on the contact area and holds the pipe against the wall. The longer the pipe stays still, the thicker the cake and the larger the contact area become.
Warning signs: the pipe was stationary (a connection, a survey, a repair), it is opposite a known permeable sand, circulation is normal with no pressure increase, and the string cannot be moved up, down or rotated. Rising overpull at connections across the same sand can come first.
The contact area and the friction coefficient are hard to know in a real well, so this is a teaching estimate that shows the scale of the problem, not a design number.
Worked example 1: how large the sticking force can be
Mud 11.5 ppg, a sand at 9,000 ft TVD with a pore pressure equivalent to 9.5 ppg. Assume a contact band 2 in wide along 30 ft of collar, and a friction coefficient of 0.2.
One short contact length can hold more than many strings can safely overpull. This is why excess mud weight, long stationary periods and large slick collars in permeable sections are all avoided.
Mechanical sticking
Pack-off and bridging
Cuttings that are not carried out, cavings from unstable shale, or broken formation collapse around the BHA and pack the annulus. Warning signs are rising pump pressure, partial or total loss of returns, erratic torque, increasing drag on trips, and fill on bottom. Pack-offs are most common in high-angle sections where cuttings beds form, when pulling out with the pumps off, and in reactive or fractured shales.
Wellbore geometry
The hole is a shape the string cannot pass. Typical cases are a stiff BHA run into a section drilled with a more limber one, ledges between hard and soft layers, micro-doglegs, and key seats, where the drill pipe has worn a slot into the high side of a dogleg that is narrower than the collars. Geometry sticking usually happens while the pipe is moving, at a definite depth, with circulation unaffected.
Undergauge hole
An abrasive formation wears the bit and stabilizers so the hole is drilled smaller than gauge. A new full-gauge bit run in fast then jams in the undergauge section. Mobile salts and swelling clays can also close the hole over time. Measuring the gauge of the bit and stabilizers when they come out, and reaming the last stands to bottom, avoid most of these cases.
Telling them apart
| Question | Differential | Pack-off / bridge | Geometry |
|---|---|---|---|
| Pipe moving before sticking? | No, stationary | Moving up or static | Moving up or down |
| Circulation | Free | Restricted or lost | Free |
| Rotation | Not possible | Restricted | Often possible |
| Pump pressure | Normal | Higher | Normal |
First actions
Follow the rig's stuck pipe procedure and the company policy, and respect the ratings of the pipe, the jar and the rig. The general principles are:
- Pack-off: reduce the pump pressure straight away to avoid packing the annulus tighter and breaking down the formation, keep low pressure trapped, apply moderate torque and work the pipe in the direction opposite to the one it stuck in, usually down. Bring circulation back slowly once the string moves.
- Differential: circulate at the normal rate, apply torque and slack off the maximum allowable set-down weight, then jar. Act quickly, since the force grows with time. If this fails, spotting a release fluid or reducing the hydrostatic pressure may be planned, but lowering the mud weight is a well control decision and must be checked against the kick margin.
- Geometry: if stuck moving up, jar down; if stuck moving down, jar up, with no torque in the direction that could make it worse. Once free, ream or backream the section.
Jarring basics
A drilling jar stores energy as stretch in the string and releases it suddenly as an impact. To jar up, the driller pulls to a set overpull above the string weight at the jar, the jar trips after its delay, and the stretched pipe above accelerates and strikes. To jar down, weight is slacked off. The jar must be "cocked" (reset) between blows. Hydraulic jars fire after a timed delay, mechanical jars at a preset load. Common mistakes are using overpull figures that ignore drag, so the jar never fires or fires far harder than planned, firing beyond the pipe or connection limits, and placing the jar where it is itself in the stuck zone or in the neutral point.
Free point from stretch
If the pipe will not come free, it is useful to know how deep the free pipe goes, so that a back-off or cut can be planned just above the stuck point. The surface estimate uses Hooke's law: free pipe stretches in proportion to the extra pull and its own length. Pull the string to a reference load just above its free weight and mark the pipe at the rotary table. Pull a known extra load, mark again, and measure the distance between the marks.
Here A is the steel cross-section of the pipe body in in² and W is the pipe weight in lb/ft. Steel weighs about 3.4 lb/ft per in² of cross-section, which is where the 3.4 comes from. The body (plain end) weight is the right one; nominal weight includes an allowance for tool joints and gives a longer free length than the true one. In metric units, use E = 207 GPa and the body area in m², and keep the units consistent.
Worked example 2: free point from stretch
5 in, 19.5 lb/ft drill pipe, ID 4.276 in. After the reference mark, an extra 40,000 lb pull gives 24 in of stretch.
The nominal-weight answer is almost 700 ft too deep, which could put a back-off below the stuck point. Take two or three readings with different pull increments and average them.
This estimate assumes a vertical, uniform string with no drag. In deviated wells, friction resists the pull and makes the free length appear shorter; with mixed pipe sizes, each section stretches differently. A wireline free point tool gives the actual stuck depth and should be used before a back-off.
Good practice
- Keep the pipe moving across permeable zones and minimise the time it is still.
- Keep the mud weight no higher than the well needs, with a thin, slick filter cake.
- Circulate the hole clean before tripping, especially at high angle.
- Record free pick-up and slack-off weights so that overpull is measured against a real baseline.
- Note what the pipe was doing when it stuck. It is the key to choosing the first action.