Mechanical sticking occurs as a result of one or more of the following 8 conditions:

1. inadequate hole cleaning

2. formation instability

3. key seating (not often a problem but has happened occasionally)

4. running into underagaugehole

5. accumulation of cuttings

6. drilling plastic formations

7. large boulders falling into the hole

8. running tools with too large a gauge for the hole diameter.

1. Inadequate Hole Cleaning

Cuttings which are not adequately removed will settle on the lower side of the hole and may cause the hole to pack off, resulting in stuck pipe.

Due to the tendency of the cuttings to fall to the lower side of the hole, high angle holes are more difficult to clean than vertical wells. In addition, for effective hole cleaning, the circulation rate must be higher than the slip velocity of the cuttings. This is more difficult to achieve in deviated hole than vertical hole.

The major indications of inadequate hole cleaning are:

• mass balance incorrect while drilling
• increase in pump pressure and connection drag
• sudden smooth increase in drag whilst POOH.

2. Formation Instability

Weak and Unconsolidated Formations

Some formations are particularly weak and unconsolidated and are prone to swelling (eg., shale) and/or sloughing (eg., coal or shale). During drilling operations, swelling and sloughing can grip the pipe and cause sticking.

In general, shale is the formation that causes the most problems regarding stuck pipe.

Shales - Variable Hole Stability

Shales are weaker along the formation bedding plane than across it. For this reason, hole stability may vary greatly in holes through the same formation but at different inclinations and directions.

Brittle/Sloughing Shale

These shales slough into the hole after breaking into pieces. Sloughing shale indications are:

• large amounts of shale at the shaker screens after circulating bottoms up
• excess drag during trips
• high levels of fill.

Swelling Shales

Some shales chemically react with water; a process called hydration which results in formation swelling. All swelling shales are potential causes of stuck pipe whether they are highly reactive gumbos, or shales which hydrate very quickly.

Even though the hole may be cleaned by sufficient flow rates during drilling, it may need to be re-drilled after each stand as the swelling continues.

The indications of swelling are:

• torque increases during drilling
• difficulty in sliding the BHA when orienting
• drag in connections
• excessive drag when POOH
• requirement for repeated reaming of sections
• increase in MBT of water based muds.

3. Key Seating

Key seating is most likely to occur when changes in hole direction are too severe. It is caused when the drillpipe, which is of smaller diameter than the drill collars, rubs against the side of the hole and wears a slot, (see Figure 13.2).

It is still possible to drill ahead when key seating has occurred. However, when pulling out of hole, the larger diameter drill collars will hang up on the ‘key seat’ and the pipe will become stuck..

The greater the dogleg severity, the greater the side load and the faster the key seat can be developed. Key seats can also be developed at ledges and casing shoes where the groove is worn into the metal instead of the formation. The development of key seats is directly affected by the number of rotating hours.

4. Undergauge Hole and Assembly Changes

The gauge of bits and stabilisers will be reduced by abrasive hole sections. Therefore, maximising a bit run in abrasive formations could prove to be a false economy due to the inevitable result of increased reaming operations which may cause the bit to wear out very quickly and create an undergauge hole.

If a full gauge BHA is tripped into an undergauge hole, a stuck pipe situation can occur. Greater care in keeping the hole full gauge will prevent this problem.

Undergauge hole is indicated when the bit becomes stuck but when full circulation is still possible. Preventative measures taken to keep the hole full gauge will reduce this problem.

A dog leg which presents an obstruction to a stiff BHA may present no difficulty to a flexible assembly which can “snake” around doglegs. Formations which have been drilled with a flexible BHA may present no problem when pulling out of the hole, but when a new stiffer BHA or string of casing is introduced, the newly drilled hole will act as if it is undergauge. This will result in the risk of stuck pipe. Check trip assemblies prior to running casing should be selected carefully with this point in mind.

Undergauge hole is indicated by:

• an increase in down drag on RIH
• increasing or erratic torque
• bit stalling
• a reduction in ROP.

5. Accumulation of Cuttings

Cuttings usually accumulate during conditions of low annular velocity.

Cuttings accumulation is indicated by:

• reduced cuttings on the shale shaker
• increased overpull
• loss of circulation

6. Drilling Plastic Formations

Stuck pipe can result from the plastic nature of some formations, including, but not limited to salts. Stresses can be relieved in the formation allowing the formation to extrude into the borehole. Although this extrusion may be measured in mere fractions of an inch, it can be sufficient to cause a bit or stabiliser to become stuck in the hole.

Flowing formations and abnormal pressures may be encountered almost anywhere where there are unequally relieved stresses, but will be experienced most often at the top of a formation or on the flanks of a salt dome or clay diapir. Increasing the mud weight may reduce flowing of plastic formations.

Plastic formations are indicated by:

• increasing and erratic torque.
• a higher overpull with increased hours between trips.

7. Large Boulders Falling into the Hole

This condition usually only occurs while drilling top hole in unconsolidated formations. Boulders can be dislodged and fall into the hole or against the drill string.

Indications of boulders are:

• erratic torque
• erratic drag on connections
• tendency to stick when pulling out of hole.

8. Running Large Gauge Tools

This situation is similar to the condition Undergauge Hole and Assembly Changes. Running large gauge tools will most likely result in stuck pipe and is normally associated with a mistake in the selection of a downhole component. The Drilling Supervisor is responsible for all downhole equipment run into the hole and as such all downhole equipment must be callipered prior to running in the hole.

Indications of large gauge tools are:

• string standing up when running into open hole
• string stuck immediately after running into open hole.