Junk Retrieve Operations

 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 work￾over operations (Figure 2.14). These non-drillable items must be retrieved 

before operations can be continued. It is noted that junk retrieving opera￾tions 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 equip￾ment, 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 


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 elimi￾nate 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 


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 drill￾string, 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 pres￾sures 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 drill￾ing 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 eval￾uation, 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 displace￾ment, 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 sig￾nificant challenge for oil and gas operating companies worldwide is to max￾imize 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 clean￾ing 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 pen￾etration 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