Exploratory drilling methods

Dr A K Mishra

Introduction

There are a variety of drilling techniques which can be used to sink a borehole into
the ground. Each has its advantages and disadvantages, in terms of the depth to
which it can drill, the type of sample returned, the costs involved and penetration
rates achieved. There are two basic types of drills: drills which produce rock chips,
and drills which produce core samples.

Figure 1: Simple diagram of a drilling rig and its basic operation

Drilling rig classification

There are many types and designs of drilling rigs, with many drilling rigs capable of switching
or combining different drilling technologies as needed. Drilling rigs can be described using
any of the following attributes:
By power used

 Mechanical — the rig uses torque converters, clutches, and transmissions powered by
its own engines, often diesel
 Electric — the major items of machinery are driven by electric motors, usually with power
generated on-site using internal combustion engines
 Hydraulic — the rig primarily uses hydraulic power
 Pneumatic — the rig is primarily powered by pressurized air
 Steam — the rig uses steam-powered engines and pumps (obsolete after middle of 20th
Century)

By pipe used

 Cable — a cable is used to raise and drop the drill bit

 Conventional — uses metal or plastic drill pipe of varying types
 Coil tubing — uses a giant coil of tube and a downhole drilling motor
By height
(All rigs drill with only a single pipe. Rigs are differentiated by how many connected pipe they
are able to "stand" in the derrick when needing to temporarily remove the drill pipe from the
hole. Typically this is done when changing a drill bit or when "logging" the well.)

 Single — can pull only single drill pipes. The presence or absence of vertical pipe
racking "fingers" varies from rig to rig.
 Double — can hold a stand of pipe in the derrick consisting of two connected drill pipes,
called a "double stand".
 Triple — can hold a stand of pipe in the derrick consisting of three connected drill pipes,
called a "triple stand".
By method of rotation or drilling method

 No-rotation includes direct push rigs and most service rigs

 Rotary table — rotation is achieved by turning a square or hexagonal pipe (the "Kelly") at
drill floor level.
 Top drive — rotation and circulation is done at the top of the drill string, on a motor that
moves in a track along the derrick.
 Sonic — uses primarily vibratory energy to advance the drill string
 Hammer — uses rotation and percussive force
By position of derrick

 Conventional — derrick is vertical

 Slant — derrick is slanted at a 45 degree angle to facilitate horizontal drilling

Auger drilling

Auger drilling is done with a helical screw which is driven into the ground with
rotation; the earth is lifted up the borehole by the blade of the screw. Hollow stem
Auger drilling is used for environmental drilling, geotechnical drilling, soil engineering
and geochemistry reconnaissance work in exploration for mineral deposits. Solid
flight augers/bucket augers are used in construction drilling. In some cases, mine
shafts are dug with auger drills. Small augers can be mounted on the back of a utility
truck, with large augers used for sinking piles for bridge foundations.

Auger drilling is restricted to generally soft unconsolidated material or weak

weathered rock. It is cheap and fast.

Figure 2 : Auger drilling

Rotary Air Percussion drilling (Air Percussion)

Air Percussion drilling is used most frequently in the mineral and water exploration
industry. The drill uses a pneumatic reciprocating piston-driven 'hammer' to
energetically drive a heavy drill bit into the rock. The drill bit is hollow, solid steel and
has ~20 mm thick tungsten rods protruding from the steel matrix as 'buttons'. The
tungsten buttons are the cutting face of the bit.

The cuttings or "chips" are blown up the outside of the rods and collected at surface.
Air or a combination of air and foam lift the cuttings.
Air Percussion drilling is used primarily for mineral exploration, water borehole
drilling and blast-hole drilling in mines, there are other applications such as
engineering solutions, etc. Air Percussion produces lower quality samples because
the cuttings are blown up the outside of the rods and can be contaminated from
contact with other rocks. Air Percussion drilling to depths of more than 200metres
and could present quite a challenge to the drilling team as encountering water could
rapidly clog the outside of the hole with debris, precluding removal of drill cuttings
from the hole. Experienced drillers will however understand these limitations and
through the use of drill enhancing fluids and proper drilling techniques, not only
ensure that the hole stays open until completion, but also ensure further drilling.
Depths in excess of 500m has been recorded in Southern Africa.

The use of multiple high-powered air compressors, which push 900-1150cfm of air at
300-350psi down the hole ensures drilling of a deeper holes up to ~1250m due to
higher air pressure which pushes all rock cuttings and any water to the surface. This,
of course, is all dependent on the density and weight of the rock being drilled, as well
as the state of equipment used.

Core drilling

Core drilling and related methods use hardened steel or tungsten blades to bore a
hole into unconsolidated ground. The drill bit has three blades arranged around the
bit head, which cut the unconsolidated ground. The rods are hollow and contain an
inner tube which sits inside the hollow outer rod barrel. The drill cuttings are removed
by injection of compressed air into the hole via the annular area between the
innertube and the drill rod. The cuttings are then blown back to surface up the inner
tube where they pass through the sample separating system and are collected if
needed. Drilling continues with the addition of rods to the top of the drill string. Air
core drilling can occasionally produce small chunks of cored rock.

This method of drilling is used to drill the weathered regolith, as the drill rig and steel
or tungsten blades cannot penetrate fresh rock. Where possible, air core drilling is
preferred over RAB drilling as it provides a more representative sample. Air core
drilling can achieve depths approaching 300 meters in good conditions. As the
cuttings are removed inside the rods and are less prone to contamination compared
to conventional drilling where the cuttings pass to the surface via outside return
between the outside of the drill rob and the walls of the hole. This method is more
costly and slower than RAB.
Figure 3 : Diamond core drill bits

Figure 4: Direct rotary circulatory system

Cable tool drilling
Cable tool rigs are a traditional way of drilling water wells internationally and and on
many farms in Southern Africa stories could be related on how the jumper rigs drilled
on various sites for months. The majority of large diameter water supply wells,
especially deep wells completed in bedrock aquifers, were completed using this
drilling method. Although this drilling method has largely been replaced in recent
years by other, faster drilling techniques. It is still in many cases- especially low
yielding aquifers - the most practicable drilling method for large diameter, deep
bedrock wells, and in widespread use for small rural water supply wells. The impact
of the drill bit fractures the rock and in many shale rock situations increases the
water flow into a well over rotary.

Figure 5: Cable tool drilling

Also known as ballistic well drilling and sometimes called "spudders", these rigs raise
and drop a drill string with a heavy carbide tipped drilling bit that chisels through the
rock by finely pulverizing the subsurface materials. The drill string is comprised of the
upper drill rods, a set of "jars" (inter-locking "sliders" that help transmit additional
energy to the drill bit and assist in removing the bit if it is stuck) and the drill bit.
During the drilling process, the drill string is periodically removed from the borehole
and a bailer is lowered to collect the drill cuttings (rock fragments, soil, etc.). The
bailer is a bucket-like tool with a trapdoor in the base. If the borehole is dry, water is
added so that the drill cuttings will flow into the bailer. When lifted, the bailer closes
and the cuttings are then raised and removed. Since the drill string must be raised
and lowered to advance the boring, casing (larger diameter outer piping) is typically
used to hold back upper soil materials and stabilize the borehole.

Figure : Two types of bailer (a) Dart Valve (b) Flat bottom

Cable tool rigs are simpler and cheaper than similarly sized rotary rigs, although loud
and very slow to operate. The world record cable tool well was drilled in New York to
a depth of almost 12,000 feet. The common Bucyrus Erie 22 can drill down to about
1,100 feet. Since cable tool drilling does not use air to eject the drilling chips like a
rotary, instead using a cable strung bailer, technically there is no limitation on depth.

Reverse circulation (RC) drilling

RC drilling is similar to air core drilling, in that the drill cuttings are returned to surface
inside the rods. The drilling mechanism is a pneumatic reciprocating piston known as
a hammer driving a tungsten-steel drill bit. RC drilling utilises much larger rigs and
machinery and depths of up to 500 metres are routinely achieved. RC drilling ideally
produces dry rock chips, as large air compressors dry the rock out ahead of the
advancing drill bit. RC drilling is slower and costlier but achieves better penetration
than RAB or air core drilling; it is cheaper than diamond coring and is thus preferred
for most mineral exploration work.

Reverse circulation is achieved by blowing air down the rods, the differential
pressure creating air lift of the water and cuttings up the inner tube which is inside
each rod. It reaches the bell at the top of the hole, then moves through a sample
hose which is attached to the top of the cyclone. The drill cuttings travel around the
inside of the cyclone until they fall through an opening at the bottom and are
collected in a sample bag.

The most commonly used RC drill bits are 5-8 inches (12.7–20.32 cm) in diameter
and have round metal 'buttons' that protrude from the bit, which are required to drill
through rock and shale. As the buttons wear down, drilling becomes slower and the
rod string can potentially become bogged in the hole. This is a problem as trying to
recover the rods may take hours and in some cases weeks. The rods and drill bits
themselves are very expensive, often resulting in great cost to drilling companies
when equipment is lost down the bore hole. Most companies will regularly 'sharpen'
the buttons on their drill bits in order to prevent this, and to speed up progress.
Usually, when something is lost (breaks off) in the hole, it is not the drill string, but
rather from the bit, hammer, or stabiliser to the bottom of the drill string (bit). This is
usually caused by a blunt bit getting stuck in fresh rock, over-stressed metal, or a
fresh drill bit getting stuck in a part of the hole that is too small, due to having used a
bit that has worn to smaller than the desired hole diameter.

Although RC drilling is air-powered, water is also used, to reduce dust, keep the drill
bit cool, and assist in pushing cutting back upwards, but also when collaring a new
hole. A mud called liqui-pol is mixed with water and pumped into the rod string, down
the hole. This helps to bring up the sample to the surface by making the sand stick
together. Occasionally, 'super-foam' (AKA 'quik-foam') is also used, to bring all the
very fine cuttings to the surface, and to clean the hole. When the drill reaches hard
rock, a collar is put down the hole around the rods which is normally PVC piping.
Occasionally the collar may be made from metal casing. Collaring a hole is needed
to stop the walls from caving in and bogging the rod string at the top of the hole.
Collars may be up to 60 metres deep, depending on the ground, although if drilling
through hard rock a collar may not be necessary.

Reverse circulation rig setups usually consist of a support vehicle, an auxiliary

vehicle, as well as the rig itself. The support vehicle, normally a truck, holds diesel
and water tanks for resupplying the rig. It also holds other supplies needed for
maintenance on the rig. The auxiliary is a vehicle, carrying an auxiliary engine and a
booster engine. These engines are connected to the rig by high pressure air hoses.
Although RC rigs have their own booster and compressor to generate air pressure,
extra power is needed which usually isn't supplied by the rig due to lack of space for
these large engines. Instead, the engines are mounted on the auxiliary vehicle.
Compressors on an RC rig have an output of around 1000 cfm at 500 psi (500 L·s-1
at 3.4 MPa). Alternatively, stand-alone air compressors which have an output of 900-
1150cfm at 300-350 psi each are used in sets of 2, 3, or 4, which are all routed to the
rig through a multi-valve manifold.
Diamond core drilling

Multi-combination drilling rig (capable of both diamond and reverse circulation

drilling). Rig is currently set up for diamond drilling.
Diamond core drilling (Exploration diamond drilling) utilises an annular diamond-
impregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of
solid rock. The diamonds used are fine to microfine industrial grade diamonds. They
are set within a matrix of varying hardness, from brass to high-grade steel. Matrix
hardness, diamond size and dosing can be varied according to the rock which must
be cut. Holes within the bit allow water to be delivered to the cutting face. This
provides three essential functions; lubrication, cooling, and removal of drill cuttings
from the hole.

Diamond drilling is much slower than reverse circulation (RC) drilling due to the
hardness of the ground being drilled. Drilling of 1200 to 1800 metres is common and
at these depths, ground is mainly hard rock. Diamond rigs need to drill slowly to
lengthen the life of drill bits and rods, which are very expensive.

Core samples are retrieved via the use of a lifter tube, a hollow tube lowered inside
the rod string by a winch cable until it stops inside the core barrel. As the core is
drilled, the core lifter slides over the core as it is cut. An overshot attached to the end
of the winch cable is lowered inside the rod string and locks on to the backend,
located on the top end of the lifter tube. The winch is retracted, pulling the lifter tube
to the surface. The core does not drop out the inside of the lifter tube when lifted
because a "core lifter spring," located at the bottom of the tube allows the core to
move inside the tube but not fall out.
Diamond core drill bits

Once a rod is removed from the hole, the core sample is then removed from the rod
and catalogued. The Driller's offsider screws the rod apart using tube clamps, then
each part of the rod is taken and the core is shaken out into core trays. The core is
washed, measured and broken into smaller pieces using a hammer to make it fit into
the sample trays. Once catalogued, the core trays are retrieved by geologists who
then analyse the core and determine if the drill site is a good location to expand
future mining operations.

Diamond rigs can also be part of a multi-combination rig. Multi-combination rigs are a
dual setup rig capable of operating in either a reverse circulation (RC) and diamond
drilling role (though not at the same time). This is a common scenario where
exploration drilling is being performed in a very isolated location. The rig is first set
up to drill as an RC rig and once the desired metres are drilled, the rig is set up for
diamond drilling. This way the deeper metres of the hole can be drilled without
moving the rig and waiting for a diamond rig to set up on the pad

Direct Push Drilling

Direct push technology includes several types of drilling rigs and drilling equipment
which advances a drill string by pushing or hammering without rotating the drill
string. This should perhaps not properly be called drilling, however the same basic
results (i.e. a borehole) are achieved. Direct push rigs include both cone penetration
testing (CPT) rigs and direct push sampling rigs such as a Geoprobe. Direct push
rigs typically are limited to drilling in unconsolidated soil materials and very soft rock.

CPT rigs advance specialized testing equipment (such as electronic cones), and soil
samplers using large hydraulic rams. Most CPT rigs are heavily ballasted (20 metric
tons is typical) as a counter force against the pushing force of the hydraulic rams
which are often rated up to 20kn. Alternatively, small, light CPT rigs and offshore
CPT rigs will use anchors such as screwed-in ground anchors to create the reactive
force. In ideal conditions, CPT rigs can achieve production rates of up to 250-300
meters per day.

Geoprobe rigs use hydraulic cylinders and a hydraulic hammer in advancing a hollow
core sampler to gather soil and groundwater samples. The speed and depth of
penetration is largely dependent on the soil type, the size of the sampler, and the
weight and power the rig. Direct push techniques are generally limited to shallow soil
sample recovery in unconsolidated soil materials. The advantage of direct push
technology is that in the right soil type it can produce a large number of high quality
samples quickly and cheaply, generally from 50 to 75 meters per day. Rather than
hammering, direct push can also be combined with sonic (vibratory) methods to
increase drill efficiency.

Hydraulic-rotary drilling

Oil well drilling utilises tri-cone roller, carbide embedded, fixed-cutter diamond, or
diamond-impregnated drill bits to wear away at the cutting face. This is preferred
because there is no need to return intact samples to surface for assay as the
objective is to reach a formation containing oil or natural gas. Sizable machinery is
used, enabling depths of several kilometres to be penetrated. Rotating hollow drill
pipes carry down bentonite and barite infused drilling muds to lubricate, cool, and
clean the drilling bit, control downhole pressures, stabilize the wall of the borehole
and remove drill cuttings. The mud travels back to the surface around the outside of
the drill pipe, called the annulus. Examining rock chips extracted from the mud is
known as mud logging. Another form of well logging is electronic and is frequently
employed to evaluate the existence of possible oil and gas deposits in the borehole.
This can take place while the well is being drilled, using Measurement While Drilling
tools, or after drilling, by lowering measurement tools into the newly-drilled hole.

The rotary system of drilling was in general use in Texas in the early 1900s. It is a
modification of one invented by Fauvelle in 1845, and used in the early years of the
oil industry in some of the oil-producing countries in Europe. Originally pressurized
water was used instead of mud, and was almost useless in hard rock before the
diamond cutting bit.[1]. The main breakthrough for rotary drilling came in 1901, when
Anthony Francis Lucas combined the use of a steam-driven rig and of mud instead
of water in the Spindletop discovery well.[2]

The drilling and production of oil and gas can pose a safety risk and a hazard to the
environment from the ignition of the entrained gas causing dangerous fires and also
from the risk of oil leakage polluting water, land and groundwater. For these reasons,
redundant safety systems and highly trained personnel are required by law in all
countries with significant production.

Sonic (Vibratory) Drilling

A sonic drill head works by sending high frequency resonant vibrations down the drill
string to the drill bit, while the operator controls these frequencies to suit the specific
conditions of the soil/rock geology.

Resonance magnifies the amplitude of the drill bit, which fluidizes the soil particles at
the bit face, allowing for fast and easy penetration through most geological
formations. An internal spring system isolates these vibrational forces from the rest
of the drill rig.