Junk is usually described as small items of non-drillable metals that fall or
are left behind in the borehole during the drilling, completion, or workover operations (Figure 2.14). These non-drillable items must be retrieved
before operations can be continued. It is noted that junk retrieving operations may be recognized as part of a fishing operation too. It is important
to remove the fish or junk from the well as quickly as possible. The longer
these items remain in a borehole, the more difficult these parts will be to
retrieve. Further, if the fish or junk is in an open hole section of a well the
more problems there will be with borehole stability.
Junk in the hole such as metal fragments or broken-off or dropped equipment, may lodge between the hole wall and drillpipe, tool joints, or drill
collars (Figure 2.14). Except when the drillstring pulls around the object or
the object can be pushed into the hole wall,
serious fishing problems can
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develop. This is especially true if the drillpipe gets jammed to one side in a
cased hole. To avoid junk, never leave the hole unprotected or leave loose
objects lying around the rotary area. Junk in the hole, smaller fish, lost in
the hole may include i) bit cones, bearings, or other parts lost when a bit
breaks, ii) broken reamer or stabilizer parts, iii) metal fragments lost in a
twist-off, iv) metal fragments produced by milling the top of a fish to aid in
its retrieval, v) naturally occurring pieces of hard, crystalline, or abrasive
minerals such as iron pyrite, vi) tong pins, wrenches, or other items that fall
into the hole because of rig equipment failure or by accident, vii) equipment
such as packer, core barrels, and drill stem test (DST) tools that become
lodged downhole, and viii) wire line tools and parted wire line.
Twist-off
Twist-off is a parting of the drillstring caused by metal fatigue or washout
(Figure 2.15). If the drillpipe twists off, this means that the pipe was twisted
along its vertical axis. As a result, the fluid cycle will stop leading to expose
the bit into heat (i.e., no cooling + no lubrication). Twist-off will also eliminate the nozzles fluid pressure which supports the drilling operation. It can
lead to a drillpipe fatigue failure (Figure 2.16). This typically happens when
lower sections of the pipe get stuck. There are early symptoms of twist-off
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such as the torque indication. The higher the torque deflection during
drilling operation, the more it is likely to get twist-off. Therefore, the driller
should be aware of the situation.
Twist-off is usually the result of not moving the whole pipe in the same
rotation speed. It is also the result of: i) rough pipe handling, ii) faulty drillstring, iii) stress reversals in a sharply deviated hole drilling with drillpipe
in compression, iv) poorly stabilized drill collars scarring by tong dies,
v) improper makeup torque, vi) erosion caused by washout, vii) other
damage that creates weak spots where cracks can form and enlarge under
the constant bending and torque stresses of routine drilling, and viii) other
damage that creates weak spots where cracks can form and enlarge under
the constant. The pipe often separates in a
helical break or in a long tear
or split. The surface signs of a twist-off include i) loss of drillstring weight,
ii) lack of penetration, iii) reduced pump pressure, iv) increased pump
speed, v) reduced drilling torque, and vi) increased rotary speed.
2.1.5 Difficult-to-drill Rocks
In general, it is difficult to drill rocks especially if it is hard rock. Problems
related to drilling hard rock are very frustrating. Interpretation of pore
pressure for hard rock is mysterious. However, it is assumed that pore pressures are close to “normal” over long depth intervals because drilling in
hard rock is slow and there is no kick (i.e., in overpressured shale sections).
As a result, many hard rock drilling problems cannot be logically explained.
For overbalance drilling, hard rock drilling problems are: (i) slow drilling rate; (ii) lost circulation; (iii) differential sticking; (iv) stress-corrosion
fatigue – twisted-off drillpipe, lost bit cone, drillstring wash-outs; (v) poor
directional control; (vi) severe dog legs; (vii) deep invasion – poor log evaluation, irreparable formation damage. For underbalance drilling, hard rock
drilling problems are: (i) sloughing shale – bridges and fill (i.e., lost time
kicks); (ii) corrosive gas entrainment – drillpipe and bit embrittlement;
(iii) borehole enlargement – difficult fishing jobs, poor cement displacement, and casing collapse (no cement sheath); (iv) plastic flow (i.e., shale or
salts) – excessive torque, lost circulation beneath pack-off, and stuck pipe.
Hard rocks are difficult to drill because of the extreme zig-zags from over
pressured shales to sub-normally pressured sands and carbonates.
A better understanding of the presence and magnitude of the pressure
shifts may help us minimize the worst extremes of imbalance and more
intelligently strike an optimum compromise, realizing that mud density
and, especially, mud chemistry can never completely solve these hard rock
drilling problems. Well log pressure plots in these erratic stratigraphies are
so difficult to interpret that they often have been considered useless. A significant challenge for oil and gas operating companies worldwide is to maximize drill bit run intervals within interbedded, hard-to-drill rock sections.
In these more challenging applications, polycrystalline diamond compact
(PDC) and roller cone insert designs are pushed to their limits and often
fail due to PDC thermal fatigue, severe abrasion, bearing failures, or impact
damage. This translates into additional trips for replacing bit types or cleaning the hole from junk left in the hole, representing significant added costs.
2.1.6 Resistant Beds Encountered
Once a resistant bed is encountered resulting in dramatic drop of penetration rate, a decision needs to be made whether to stop drilling or to
continue. If the resistant bed is comprised of gravels, the drilling fluid may
need to be thickened to lift-out the cuttings. If the resistant bed is hard
granite, drilling with the LS-100 should cease. Other drilling methods
should be found or drilling should be attempted at another location