DRILL STRI NG-

Operating principles: Cable Tool drilling is usually

recognised as a process of advancing the hole by:

I. breaking or stirring the formation,2. mixing the resulting cuttings into a slurry with

water in the bottom of the hole,

3. removing the cuttings by bailing.

Other cable tool methods include the tube sampling or"shelJ" method, which advances the hole bv driving a tube

into the formation and removing the tube w;th the retainedsolid cylinder cut from the forq1ation. This method may:

• drive the sample tube by repeated blows using areciprocating or "spudding" action,

• work the sample tube down using a repeated

raising and dropping action,• impact the sample tube into the bottom using one

long drop.

The last two methods are frequently used on light cable

machines which are not equipped with a powered reciprocat­ing action.

Another similar system is the Banka drill, developed inIndonesia in the 1880s to drill and sample alluvial deposits.

In this system, the casing is driven or rotated down and thenthe contents are bailed out, leaving a5-10 cm (2-4 inch) plugin' the bottom.

The semi mechanical Banka is basically a cable tool rig

with a single winch supplying the percussive, chopping, and

bailing actions. The Banka driller manualJY performs thepercussive snapping provided by the spudding arm on a full­mechanical cable tool rig. The casing is hammered down byfree-falJ action with the bailer attached as a guide. It is thenbailed with the drive hammer unclamped. A sinker bar

provides chopping and hammering weight and keeps thehole aligned. The swivel allows the tool string to spin andturn.

Cable tool percussion drilling: The early cable toolreciprocating action was achieved using a "walking beam".

Today the required reciprocating motion is imparted bya "spudder". Usually a spudding arm working on the bight

of the drilling cable (the bend around the spudding armpulleys), lifts the tool string quickly and allows it to dropfreely. The spudding arm must tighten the cable to "catch"the drill string just before it reaches the bottom, so that

impact with the bottom is achieved with the cable at "fullstretch ".

III Suitability of cable tool drilling

This type of drilling has been in use throughout the

modern drilling era. In 1859 an early form of this type ofdrilling was used to drill the first oil well (in the state ofPennsylvania, USA).

Despite many new developments bringing different types

of drilling, cable tool spudding remains a useful techniquein many drilJing situations.

Advantages of the cable tool: Among its advantagesare:

• low capital investment and cheap maintenance.Capital cost is less than a third of rotarymachines of similar"capacity. Maintenance does

not require so much precision fitting andexpensive parts as more complex machines. Manycomponents can be fabricated in a machine shopand are all available "recycled" worldwide.

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EASY ACCESS TO MACHINE PARTS

• it produces high quality samples in unconsolidated

formations. Well suited for geological samplingtasks. Generally regarded as the most reliable

sampling technique for alluvial (placer) mineralinvestigations.

• it is particularly suited to water well work as itpermits collection of detailed information on eachwater horizon as it is penetrated.

• it is suited to remote area operation as only smallsupplies of water, fuel or other materials are necessary.

Fuel consumption is on the order of 15-20 litres/day(4-5 U.S gallons/day) vs. 500 litres/day (130 U.S.

gallons/day) for air rotary water well rigs.• it can drill economically and obtain samples in

cavernous formations.

• in general, it can drill in a greater variety oflithologies with a single tool string than other drilltypes. Bits can be renewed on-site. Highlysuitable for remote settings.

JOBS SUITED TO CABLE TOOL DRILLING

e it suit-SleTS

CLA~S• DRILLING INI- UNCONSOLID"'TE.D

FORIN.TIOt{S~~51\NDS- SOFT 5E.DIMENTI\R'I OR 1O.T.R.WEATHERED

ROCKs- "lOST CIRCUL"'TlON" FOI<.MJ\TIONS

I-JL~-~.r

• COU:E.CTING GOOD SI'\MPLES OFUNCONSOLID"'TED FORM~TIONS

• IDENT\F)'1NG- WI'\TER SHOWS

• INS-rl'\LLING EQUIPMENT IN HOloES

• CONDUC.TING TE~TS AND T ••••I:.INGMe.I'\SUR!i:MENTS

PLUMBL1N':

SURVEY

Disadvantages of the cable tool: The cable tool has

some disadvantages (which accounts forthe variety of otherdrilling types).

if• productive output measured in hole produced per

day is relatively low in most cases.• ard rock penetration rates may be very low.• when casing is required. deep drilling presents

problems in keeping the casing free.• the heavy hammering action causes disturbance

and damage in some formations, resulting inmixed samples.

• geophysical logging of a cased hole is restricted

to neutron and gamma logs.

Shell (drive tube drilling): The normal drill stem assem­bly is fitted with a driving tube tool rather than a chopping/mixing bit.

Mud drilling: Usually the casing is driven to prevent thehole caving. Sometimes, particularly when drilling large

diameter holes, it is preferable to stabilise the formation byfilling the hole with mud. The drilling proceeds using churn

drilling techniques .

lChop Pump, OR Drive Tube, OR Earth Socket

/4.

SHEll (DRIVE TUBE DRilLING)

Drill string is assembled with:

1. Rope Socket

2. Drill Stem

3. Jars

Normal cable tool operations (consolidated forma­

tions): The spudding action is operated to:

• allow the tools to drop freely,• catch the tools before the bottom so that when the

bit strikes the bottom the cable is stretched,

• quickly accelerate the tools upwards,• allow the weight to come off the cable so that the

swivel turns to rotate the bit.

The cable tool spudding machine is capable of handling

many drilling applications. The technique used must beadjusted to suit the purpose of drilling. Many techniqueshave been developed. Here we look at the more commontechniques.

Drilling unconsolidated formations (churn drilling):The mast must be fitted with an effective recoil system. The

recoil provides extra strength for the rapid upward accelera­tion of the bit after it drops into the material at the bottomof the hole.

II Cable tool techniques

• Other cable types

Tube or shell sampling, commonly used for site investi­

gation, employs a very portable lightweight mast or tripodand a light winch unit.

Casing plug drilling: The technique requires the driv­

ing of heavy casing to cut the hole and retain the "core" inthe casing. The rig must have a good casing-pulling capac­ity.