Discretizaci´ on basada en la entrop´ıa (ED)

The following is a list of the different types of pipe sticking which can occur due to:

• Differential sticking.

• Hole restriction.

• Caved in hole.

• Hole irregularities and/or change in BHA.

It is impossible to lay down hard rules which will successfully cover all the case, however, for each situation, indications about the possible causes of the problem, preventive measures and remedial actions are listed in the following subsections.

Detailed procedures should be based on each particular case, evaluating every aspect of the problem and applying any past experience gained in the area concerned.

13.1.1. Differential Sticking Causes

This phenomenon can occur, where there is case of high differential pressure between the mud hydrostatic pressure and the formation pore pressure. Some indications of pipe becoming differentially stuck may be:

• The string becomes stuck in front of a porous formation.

• Pipe has not been moved for a period of time before getting stuck i.e. during a pipe connection.

• Circulation is free with no pressure variation.

• A normal amount of cuttings is observed at the shaker.

Preventive Measures

When conditions for a potential differential sticking are encountered, the risk can be minimised by applying the following procedure:

a) Reduce the mud weight as much as possible, maintaining the minimum differential pressure necessary for a safe trip margin.

b) Reduce the contact surface by using spiral type drill collars also called NWS( No Wall Stick) and using properly a stabilised bottom hole assembly. A shorter BHA with a greater number of HWDPs could be considered.

c) Use mud with minimum solids content and low filtrate in order to obtain a thinner wall cake.

d) Reduce the friction factor by adding lubricants to the mud.

e) Keep the pipe moving and in rotate as much as possible.

f) Consider the use of a drilling jar/bumper.

Methods of Freeing Pipe

1) Work the pipe applying cyclic slack-off and overpull combined with torque Always check the reduction in the pipe yield stress due to the application of the torque.

2) Spot oil-base mud or oil containing a surfactant around the drill collars.

3) Reduce the mud weight, if possible.

4) Use a drilling jar/bumper.

5) Conduct a DST procedure.

Note: Quick reactions are fundamental in freeing the wall of stuck drill pipe, since the problem becomes worse through time.

13.1.2. Sticking Due To Hole Restrictions Causes

The most common causes of hole restriction:

• Too thick a wall cake due to the use of high solids/high filtrate mud across porous and permeable formations.

• Swelling of formations containing clay.

• Extrusion of gumbo shale into the wellbore in underbalance situations.

Preventive Measures

Problems are usually suspected by incurring increase drag during connections. Once the cause is recognised to be any of the three causes previously listed above, the following actions should be undertaken:

a) Reduce mud filtrate, cake and solids content.

b) Use inhibited mud.

c) Increase mud weight.

d) Increase mud clearing capacity.

e) Increase flow rate.

In all situations, frequent wiper trips can reduce the problem and provide information on the severity.

Methods of Freeing Pipe

1) Work the pipe applying slack-off if the string has become stuck pulling out, and overpull if it stuck while running in.

2) Spot a cushion to break and remove the mud cake around the drill collars.

3) Increase the mud weight, if possible.

4) Use a drilling jar/bumper.

13.1.3. Sticking Due To Caving Hole Causes

This problem is mainly experienced in shale sections. The most common causes are:

• Hydration and swelling of clay minerals when in contact with fresh mud filtrate.

• Insufficient supporting action of the mud hydrostatic column.

• Mechanical action of the drill string.

Preventive Measures

Depending on the various causes, there are different prevention possibilities, to reduce the severity of the problem and to avoid the consequences of sticking the string.

Possible mud changes are:

a) Reduce water losses.

b) Lower pH value to 8.5 to 9 (if needed).

c) Use inhibited mud.

d) Add mud stabilising compounds (mainly sodium asphalt sulphonate).

e) Increase the mud weight.

f) Increase the YP/PV ratio to create laminar flow on the wall after pipe.

g) Increase the gel value to obtain a good cutting suspension when circulation is stopped.

Note: It is not always drilling with underbalance which results in a caving hole.

Possible BHA changes are:

a) Use bits without nozzles, particularly when reaming, to avoid scouring the well.

b) Use the minimum acceptable number of stabilisers.

Possible changes in parameters are:

a) Reduce rotary speed, if possible, to 80rpm or less.

b) Reduce the mud flow rate to obtain laminar flow in the annulus between hole and drill collars.

c) Avoid long circulation times across unstable sections.

d) Do not rotate pipe when tripping. Use a spinner or chain out.

e) Trip out with care to avoid swabbing. If any swabbing occurs, pull out with the kelly on.

Methods of Freeing Pipe

1) If circulation is possible, keep circulating trying to expel the caving.

2) If the string becomes stuck across a carbonate formation, spot an acid pill.

3) If circulation is blocked, try to regain it by applying pressure shocks and working the pipe at the same time. Special care is required to avoid breaking the formation i.e.

overcoming fracture gradient below the stuck point.

4) Use a drilling jar/bumper.

Note: The problem of pipe sticking due to cuttings dropping out is not necessarily related to a caving hole. The origin of such problems can also be an excessive rate of penetration in large holes and inadequate carrying capacity of the mud. In this case, change the mud properties and flow rate and, if necessary, limit the rate of penetration.

It is good practice to spot high viscosity pills from time to time to keep the hole clean.

The methods of getting pipe free in this situation are the same as listed above.

13.1.4. Sticking Due To Hole Irregularities And/Or Change In BHA Causes

The causes for sticking, related to, hole conditions and change in BHA, are:

• Dog legs.

• Key seats.

• New bit is run following a dulled bit which was undersize.

• New stabilisers run to replace previous worn stabilisers..

• String is stiffer than the previous one..

• Rock bit run after a diamond or a core bit.

Preventive Measures

a) The formation of dog legs can be prevented by the use of packed bottom hole assemblies.

b) Dog legs can be eliminated by using very stiff BHA's and reamers.

c) A key seat can be eliminated by reaming it with a key seat wiper or an undergauge stabiliser installed on the top of the drill collars.

d) Always ream a whole interval drilled with the previous bit.

e) Ream always the cored section, even if a full gauge core bit was used.

Methods of Freeing Pipe

1) Work the pipe applying slack-off if dog leg or key seat (the string becomes stuck pulling out) and overpull if running a new BHA (the string becomes stuck while running in the hole).

2) Spot on oil-based mud or oil containing a surfactant around the stuck point.

3) If the stuck point is in a calcareous section, spot an acid pill.