MECHANICAL STICKING
CAUSES OF MECHANICAL STICKINGIn mechanical sticking the pipe is usually completely stuck with little or no circulation. In
differential sticking, the pipe is completely stuck but there is full circulation.Mechanical
sticking can occur as result of the hole packing off (or bridging) or due to formation & BHA
(wellbore geometry).
Hole pack off (bridging) can be caused by any one or a combination of the following
processes:
1. Settled cuttings due to inadequate hole cleaning
2. Shale instability
3. Unconsolidated formations
4. Fractured and faulted formations
5. Cement blocks
6. Junk falling in the well
The formation & BHA (wellbore geometry) can also cause mechanical sticking as follows:
1. Key seating
2. Mobile formations
3. Undergauge hole
4. Ledges and micro doglegs
Understanding the cause of the mechanical sticking problem is key to solving the problem.
This is because the cause determines the action required to free the pipe. For example, if the
pipe becomes stuck while running in an open hole, it is likely that the BHA has hit a ledge or
gone into an undergauge hole. In other words, the sticking problem is due to the geometry of
the wellbore. As will be seen later, the freeing action depends largely on identifying and
curing the problem that caused mechanical sticking.
A discussion of each of the above processes will now follow.
HOLE PACK OFF CAUSES
1- SETTLED CUTTINGS
Settled cuttings due to inadequate hole cleaning (Figure 12.4) is one of the major causes of
stuck pipe. Best hole cleaning occurs around large OD pipe such as drillcollars, while
cuttings beds can form higher up the hole where the pipe OD is smaller. The problem of
settled cuttings is particularly severe in horizontal and high directional wells. In these wells,
when the pipe is moved upwards, the cuttings may be compacted around the BHA. This can
result in complete packing off of the drillstring and eventual pipe sticking.
With increasing deviation of the wellbore, drilling fluid parameters, drilling practices and
hydraulics should be optimised in order to effectively clean the hole.
In vertical wells, good hole cleaning is
achieved by the selection and
maintenance of suitable mud parameters
and ensuring that the circulation rate
selected results in an annular velocity
(around 100-120 ft/min) which is greater
than the slip velocity of the cuttings.
Highly inclined wells are particularly
difficult to clean due to the tendency of
drilled cuttings to fall to the low side of
the hole.In a highly deviated well, the
cuttings have only a small distance to fall
before they settle on the low side of the
hole and form a cuttings bed. Cuttings
beds develop in boreholes with
inclinations of 30 degrees or greater, depending on the flow rates and suspension properties
of the drilling fluid.Complete removal of cuttings beds by circulation may be impossible.
Once cuttings beds have formed, there is always a risk that on pulling the pipe up the hole,
the cuttings are dragged from the low side of the hole forming a cuttings pile (Figure 12.4).
If this pile accumulates around the BHA, it may plug the hole and cause the pipe to
mechanically stuck.
Besides causing stuck pipe, settled cuttings can result in:
• formation break down due to increased ECD
• slow ROP
• excessive overpull on trips
• increased torque
Hole cleaning is controlled by a number of parameters which were discussed in Chapter 8.
These include:
1. mud rheology, in particular the YP and gel strength
2. flow rate
3. hole angle
4. mud weight
5. ROP
6. hole diameter
7. drillpipe rotation
8. presence of wash outs
2 - SHALE INSTABILITYShale represents 70% of the rocks encountered whilst drilling oil and gas wells. Also shale
instability is by far the most common type of wellbore instability. Shales are classified as
being either brittle or swelling.
Brittle Shales
Instability in brittle shales is caused
mainly by tangential stresses around the
wellbore which are induced as a result of
the well being drilled. The induced
stresses depend on the magnitude of the
in-situ stresses, wellbore pressure, rock
strength and hole angle and direction.
Formation dip may also be a
contributory factor to brittle shale
failure. A safe mud envelope may be
established which can be used to
determine the safe mud weights to
prevent either tensile failure or collpase (compressive) failure.
Brittle shales tend to fail by breaking into pieces and sloughing into the hole. Rig indications
of brittle shale failure include:
• large amounts of angular, splintery cavings when circulating the well
• drag on trips
• large amounts of hole fill.
3 - Swelling Shales
Shales swelling (Figure 12.6) can be caused by hydrational processes or by the osmotic
potential which develops between the pore fluid of the shale and drilling fluid salinity.
The swelling of shales (Figure 12.6) is controlled by several complex factors including:
• Clay content
• Type of clay minerals (ie hydratable or inert)
• Pore water content and composition
• Porosity
• In-situ stresses
• Temperature
The degree of clay hydration depends on
the clay type and the cation exchange
capacity (CEC) of the clay content. The
greater the CEC, the more hydratable is the
clay. In drilling operations the following
clay types are encountered:
• Smectite with CEC of 80-150
meq/100g. Most of the
hydratable shales (termed
gumbos) belong to this group.
Bentonite clays belong to the
smectite group.
• Illite with CEC of 10-40 meq/100g.
• Chlorite with CEC of 10-40 meq/100g.
• Kaolinite with CEC of 3-10 meq/100g.
To aid the understanding of shale swelling, the following points must be considered:
1. The permeability of shales is very low, typically in the range of 10 -9 to 10 -6 Darcy.
(1 md = 10 –13 m2)
2. Thus, filter cakes do not form on shale surfaces.
3. However, water can still migrate into the shale (helped by the mud overbalance).
4. Water infusion into the shale will allow chemical effects to start working inside the
shale and at the exposed surfaces of the wellbore.
5. The pore pressure inside the shale section will also increase, contributing to destabilisation.
6. The low permeability of shale means that swelling effects can take considerable
time and shale instability can be a delayed effect.
Water can flow into or out of the shale through several processes; the most important ones
are hydrational and osmotic forces:
1. Hydration: This is by far the most common cause of shale hydration where water
flows into the shale and hydrate the clay plates. Highly hydratable shales are
composed of predominantly smectite- based clays. These clays (e.g.
montmorillonite) absorb water into the inner-layer space due to the high negative
charge on the surface of the clay platelet. This process results in the expansion of
the clay to several times its original volume.
Hydratable shales are usually found near the surface ±7000
At grater depths, the process of diagenesis converts the clay minerals into more stable forms, However, hydratable shales have been found in some wells at depths
greater than 7000 ft due to the inhibition of the diagenetic processes.
2. Chemical osmosis: This type of flow occurs at semi-permeable membranes which
are permeable to water and impermeable to solute ions or molecules. Shale surface
acts as a semi-permeable membrane allowing water to flow into or out of the shale
depending on the solute concentration of the mud and pore water of the shale. Water
flows through the semi-impermeable membrane from the low concentration to high
concentration solution. In terms of chemical jargon, water flows from solutions of
high water activity to solutions of low water activity until the concentrations of the
two solutions are equalised. (Water activity (aw): ratio of vapour pressure of water
in a solution, drilling mud or shale pore water to the vapour pressure of pure water
at the same temperature.)
3. Chemical diffusion: This is caused by the flow of solutes (soluble solids) from
areas of high concentration to low concentration. Hence if the concentration of
certain ions or molecules in the drilling mud is greater than those in the formation
water of the shale then the solute will flow into the formation provided there are no
barriers to flow. Solutes can also flow out of the shale if their concentration is
greater than that in the drilling mud. No flow will occur if solute concentration is the
same in mud and shale.
4. Hydraulic diffusion; water flows in the direction of decreasing hydraulic pressure
gradient (Darcy’s Law). This flow can only occur if the rock has permeability.
Shale hydration – Rig Site Indications
• Soft, hydrated or mushy cuttings
• Clay balls in the flowline
• torque and drag fluctuations
• Shale shaker screens blind off
• Increase in LGS, filter cake thickness, PV, YP and MBT (Methlyene blue test)
• Increase or fluctuations in pump pressure
• Circulation is restricted or sometimes impossible
• Bit and stabiliser balling when POH
• Generally occurs while POH (Tight hole) and problems while logging
• Problems increase with time.
Shale hydration – Prevention and Cure
• Use Inhibited mud system or displace to OBM system if possible
• Maintain mud properties as planned
• Addition of various salts (potassium, sodium, calcium) will reduce chemical
attraction between shale & water
• Addition of encapsulating polymers to WBM
• Reduce exposure time and case off the hydrated shale as soon as possible
• Regular wiper trips
• Good hole cleaning (especially in extended reach wells, ERW)
3 - UNCONSOLIDATED FORMATIONS
Unconsolidated formations are usually encountered
near the surface and include: loose sands, gravel and
silts. Unconsolidated formations have low cohesive
strengths and will therefore collapse easily (Figure
12.7) and flow into the wellbore in lumps and pack off
the drillstring.
Surface rig indications of an impending stuck pipe
situation near top hole are: increasing torque, drag and
pump pressure while drilling. Other signs include
increased ROP and large fill on bottom.
A common remedial action is to use a mud system with
an impermeable filter cake to reduce fluid invasion into
the rock. Reduction of flow rate, and in turn annular velocity, will reduce erosion of the hole
and removal of the filter cake.
4 - FRACTURED AND FAULTED FORMATIONS
This is a common problem in limestone and chalk formations. Several symptoms can be
observed on surface including:
• large and irregular rock fragments on shakers
• increased torque, drag and ROP
• small lost circulation
These fractured and faulted formations may fall into the Wellbore as soon as they are drilled
as the stresses which originally held them together are relieved by the drilling of the hole. In
addition, excessive drillstring vibrations cause the pipe to whip downhole and break and
dislodge the exposed fractured/faulted rocks. Therefore it is important to reduce drillstring
whipping to prevent dislodging of rock fragments when drilling fractured and faulted rocks.
In all cases, it is imperative to keep the hole clean in order to reduce the chances of hole
packing off.
If the drillstring is stuck in limestone or chalk formations and cannot be freed by jarring, an
inhibited hydrochloric acid pill may be spotted around the stuck zone.The acid will react
with the chalk/limestones, dissolving the rock around the pipe.If the pill is successful the
pipe will be freed quickly.
5 - CEMENT BLOCKS
Stuck pipe can be caused by cement blocks falling from the rat hole beneath the casing shoe
or from cement plugs.
This problem may be prevented by minimising the rat hole to a maximum of 5 ft and also by
ensuring a good tail cement is placed around the shoe .
The drillstring can also be stuck in green cement which has not set properly. This usually
occurs after setting a cement plug inside the casing or open hole. If the drillstring is run too
fast into the top of the cement and if the cement is still green then the cement can flash set
around the pipe and cause the pipe to be permanently stuck.
The author has come across several situations where the top of the cement is soft when
tagged, but literally within seconds of tagging the cement, the cement flash sets around the
BHA causing mechanical sticking. One possible explanation for this sudden flash setting is
that the energy release while circulating and rotating is enough to cause flash setting. It is
recommended that circulation is started two to three stands above the expected top of cement
and that WOB should be kept to absolute minimum.
6 - JUNK
Several recorded incidents of pipe sticking occurred as a result of junk falling into the hole.
This include junk falling into the wellbore from the surface or from upper parts of the hole.
Typical junks dropped from surface include pipe wrenches, spanners, broken metal, hard
hats etc. This problem can be minimised by keeping the hole covered when no tools are run
in the hole.
Junks can also fall from within the well including broken packer elements, liner hanger slips
and metal swarf from milling operation.
