How to produce?

Production consists of bringing the hydrocarbons contained in the substratum to the

surface. This requires the use of a large number of wells. A field spreads over a vast
area, at least several km² and sometimes more than 100 km². A traditional well (vertical
or slightly deviated) only draws oil or gas from a radius of a few tens of meters.
Moreover, such wells only cross the reservoir over the limited height of a vertical or
near vertical cross section. A large number of vertical wells would therefore be
necessary to completely extract the contents of a reservoir. The horizontal well
technique has totally revolutionized the industry, because such wells have a much
greater length of contact with the reservoir. Thus, the technique enables a significant
reduction in the number of wells necessary for a given development. Production drilling
presents a number of challenges.

What is the principle of extraction?

The basic principle is to generate pressure at the bottom of oil or gas wells, inferior to
the pressure in the reservoir. As a consequence of this pressure difference, the
hydrocarbons will move towards the well and thence to the surface. In practical terms,
the well is totally lined with tubing right down to the reservoir. This tubing, difficult to
move once it is fixed into position, guarantees the operational effectiveness of the well
throughout its working life. The oil and gas are brought to the surface via another tube,
in oil field jargon the (extraction) tubing, placed in the lining. This tubing is detachable
and can be changed whenever corrosion or deposition problems appear.

Sometimes, the oil field pressure is sufficient for the hydrocarbons to make their own
way to the surface; in this case the well is said to be “eruptive”. In other cases wells are
never eruptive. And in all cases, the oil field pressure diminishes gradually as production
continues. After a certain time, it is no longer sufficient for eruptive extraction and it
becomes necessary to stimulate production, what is called assisted recovery.

On arrival at the surface, the output from the extraction wells begins its journey through
the surface installations.

The creation and life of a well can be divided up into five segments:

 Planning
 Drilling
 Completion
 Production
 Abandonment

The well is created by drilling a hole 5 to 36 inches (127.0 mm to 914.4 mm) in diameter
into the earth with a drilling rig that rotates a drill string with a bit attached. After the
hole is drilled, sections of steel pipe (casing), slightly smaller in diameter than the
borehole, are placed in the hole. Cement may be placed between the outside of the casing
and the borehole. The casing provides structural integrity to the newly drilled wellbore, in
addition to isolating potentially dangerous high pressure zones from each other and from
the surface.

With these zones safely isolated and the formation protected by the casing, the well can
be drilled deeper (into potentially more-unstable and violent formations) with a smaller
bit, and also cased with a smaller size casing. Modern wells often have two to five sets of
subsequently smaller hole sizes drilled inside one another, each cemented with casing.
Mud log in process, a common way to study the lithology when drilling oil wells.

To drill the well

 The drill bit, aided by the weight of thick walled pipes called "drill collars" above
it, cuts into the rock. There are different types of drill bit; some cause the rock to
disintegrate by compressive failure, while others shear slices off the rock as the bit
turns.
 Drilling fluid, a.k.a. "mud", is pumped down the inside of the drill pipe and exits
at the drill bit. Drilling mud is a complex mixture of fluids, solids and chemicals
that must be carefully tailored to provide the correct physical and chemical
characteristics required to safely drill the well. Particular functions of the drilling
mud include cooling the bit, lifting rock cuttings to the surface, preventing
destabilization of the rock in the wellbore walls and overcoming the pressure of
fluids inside the rock so that these fluids don't enter the wellbore.
 The generated rock "cuttings" are swept up by the drilling fluid as it circulates
back to surface outside the drill pipe. The fluid then goes through "shakers" which
strain the cuttings from the good fluid which is returned to the pit. Watching for
abnormalities in the returning cuttings and monitoring pit volume or rate of
returning fluid are imperative to catch "kicks" early. A "kick" is when the
formation pressure at the depth of the bit is more than the hydrostatic head of the
mud above, which if not controlled temporarily by closing the blowout preventers
 and ultimately by increasing the density of the drilling fluid would allow
formation fluids and mud to come up through the drill pipe uncontrollably.
 The pipe or drill string to which the bit is attached is gradually lengthened as the
well gets deeper by screwing in additional 30-foot (9 m) sections or "joints" of
pipe under the kelly or top drive at the surface. This process is called making a
connection. Usually, joints are combined into three joints equaling one stand.
Some smaller rigs only use two joints and some rigs can handle stands of four
joints.

This process is all facilitated by a drilling rig which contains all necessary equipment to
circulate the drilling fluid, hoist and turn the pipe, control downhole, remove cuttings
from the drilling fluid, and generate on-site power for these operations.

Modern driller Argentina.

Completion (oil and gas wells)

After drilling and casing the well, it must be 'completed'. Completion is the process in
which the well is enabled to produce oil or gas.

In a cased-hole completion, small holes called perforations are made in the portion of the
casing which passed through the production zone, to provide a path for the oil to flow
from the surrounding rock into the production tubing. In open hole completion, often
'sand screens' or a 'gravel pack' is installed in the last drilled, uncased reservoir section.
These maintain structural integrity of the wellbore in the absence of casing, while still
allowing flow from the reservoir into the wellbore. Screens also control the migration of
formation sands into production tubulars and surface equipment, which can cause
washouts and other problems, particularly from unconsolidated sand formations in
offshore fields.

After a flow path is made, acids and fracturing fluids are pumped into the well to fracture,
clean, or otherwise prepare and stimulate the reservoir rock to optimally produce
hydrocarbons into the wellbore. Finally, the area above the reservoir section of the well is
packed off inside the casing, and connected to the surface via a smaller diameter pipe
called tubing. This arrangement provides a redundant barrier to leaks of hydrocarbons as
well as allowing damaged sections to be replaced. Also, the smaller cross-sectional area
of the tubing produces reservoir fluids at an increased velocity in order to minimize liquid
fallback that would create additional back pressure, and shields the casing from corrosive
well fluids.

In many wells, the natural pressure of the subsurface reservoir is high enough for the oil
or gas to flow to the surface. However, this is not always the case, especially in depleted
fields where the pressures have been lowered by other producing wells, or in low
permeability oil reservoirs. Installing smaller diameter tubing may be enough to help the
production, but artificial lift methods may also be needed. Common solutions include
downhole pumps, gas lift, or surface pump jacks. Many new systems in the last ten years
have been introduced for well completion. Multiple packer systems with frac ports or port
collars in an all in one system have cut completion costs and improved production,
especially in the case of horizontal wells. These new systems allow casings to run into the
lateral zone with proper packer/frac port placement for optimal hydrocarbon recovery.
 Types of oil wells

A natural gas well in the southeast Lost Hills Field, California, US.

Fossil-fuel wells come in many varieties. By produced fluid, there can be wells that
produce oil, wells that produce oil and natural gas, or wells that only produce natural gas.
Natural gas is almost always a byproduct of producing oil, since the small, light gas
carbon chains come out of solution as they undergo pressure reduction from the reservoir
to the surface, similar to uncapping a bottle of soda pop where the carbon dioxide
effervesces. Unwanted natural gas can be a disposal problem at the well site. If there is
not a market for natural gas near the wellhead it is virtually valueless since it must be
piped to the end user. Until recently, such unwanted gas was burned off at the wellsite,
but due to environmental concerns this practice is becoming less common. Often,
unwanted (or 'stranded' gas without a market) gas is pumped back into the reservoir with
an 'injection' well for disposal or repressurizing the producing formation. Another
solution is to export the natural gas as a liquid. Gas-to-liquid, (GTL) is a developing
technology that converts stranded natural gas into synthetic gasoline, diesel or jet fuel
through the Fischer-Tropsch process developed in World War II Germany. Such fuels
can be transported through conventional pipelines and tankers to users.

Another obvious way to classify oil wells is by land or offshore wells. There is very
little difference in the well itself. An offshore well targets a reservoir that happens to be
underneath an ocean. Due to logistics, drilling an offshore well is far more costly than an
onshore well. By far the most common type is the onshore well.

Another way to classify oil wells is by their purpose in contributing to the

development of a resource. They can be characterized as:

 production wells are drilled primarily for producing oil or gas, once the producing
structure and characteristics are determined
 appraisal wells are used to assess characteristics (such as flow rate) of a proven
hydrocarbon accumulation
 exploration wells are drilled purely for exploratory (information gathering)
purposes in a new area
 Wildcat wells are those drilled outside of and not in the vicinity of known oil or
gas fields.

At a producing well site, active wells may be further categorized as:

 Oil producers producing predominantly liquid hydrocarbons, but mostly with

some associated gas.
 Gas producers producing almost entirely gaseous hydrocarbons.
 water injectors injecting water into the formation to maintain reservoir pressure or
simply to dispose of water produced with the hydrocarbons because even after
treatment, it would be too oily and too saline to be considered clean for dumping
overboard, let alone into a fresh water source, in the case of onshore wells.
Frequently water injection has an element of reservoir management and produced
water disposal.
 Aquifer producers intentionally producing reservoir water for re-injection to
manage pressure. This is in effect moving reservoir water from where it is not as
useful to where it is more useful. These wells will generally only be used if
produced water from the oil or gas producers is insufficient for reservoir
 management purposes. Using aquifer produced water rather than water from other
sources is to preclude chemical incompatibility that might lead to reservoir-
plugging precipitates.
 Gas injectors injecting gas into the reservoir often as a means of disposal or
sequestering for later production, but also to maintain reservoir pressure.

Types of drilling fluids

Many types of drilling fluids are used on a day-to-day basis. Some wells require that
different types be used at different parts in the hole, or that some types be used in
combination with others. The various types of fluid generally fall into a few broad
categories:

 Air: Compressed air is pumped either down the bore hole's annular space or
down the drill string itself.
 Air/water: The same as above, with water added to increase viscosity, flush the
hole, provide more cooling, and/or to control dust.
 Air/polymer: A specially formulated chemical, most often referred to as a type of
polymer, is added to the water & air mixture to create specific conditions. A
foaming agent is a good example of a polymer.
 Water: Water by itself is sometimes used.
 Water-based mud (WBM): A most basic water-based mud system begins with
water, then clays and other chemicals are incorporated into the water to create a
homogenous blend resembling something between chocolate milk and a malt
(depending on viscosity). The clay (called "shale" in its rock form) is usually a
combination of native clays that are suspended in the fluid while drilling, or
specific types of clay that are processed and sold as additives for the WBM
system. The most common of these is bentonite, frequently referred to in the
oilfield as "gel". Gel likely makes reference to the fact that while the fluid is being
 pumped, it can be very thin and free-flowing (like chocolate milk), though when
pumping is stopped, the static fluid builds a "gel" structure that resists flow.
When an adequate pumping force is applied to "break the gel", flow resumes and
the fluid returns to its previously free-flowing state. Many other chemicals (e.g.
potassium formate) are added to a WBM system to achieve various effects,
including: viscosity control, shale stability, enhance drilling rate of penetration,
cooling and lubricating of equipment.
 Oil-based mud (OBM): Oil-based mud can be a mud where the base fluid is a
petroleum product such as diesel fuel. Oil-based muds are used for many
reasons, some being increased lubricity, enhanced shale inhibition, and greater
cleaning abilities with less viscosity. Oil-based muds also withstand greater heat
without breaking down. The use of oil-based muds has special considerations.
These include cost and environmental considerations.
 Synthetic-based fluid (SBM): Synthetic-based fluid is a mud where the base fluid
is synthetic oil. This is most often used on offshore rigs because it has the
properties of an oil-based mud, but the toxicity of the fluid fumes are much less
than an oil-based fluid. This is important when men work with the fluid in an
enclosed space such as an offshore drilling rig.

The main functions of a drilling mud can be summarized as follows:

Remove cuttings from well

Control formation pressures

Seal permeable formations

Maintain wellbore stability

Minimizing formation damage

Cool, lubricate, and support the bit and drilling assembly

Transmit hydraulic energy to tools and bit

Ensure adequate formation evaluation

Control corrosion (in acceptable level)

Facilitate cementing and completion

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 (see Down-The-Hole Drill)

By position of derrick

 Conventional — derrick is vertical

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

Drill types
There are a variety of drill mechanisms 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.

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.
Cable tool water well drilling rig in Kimball, West Virginia. These slow rigs have mostly been replaced by
rotary drilling rigs in the U.S.

Percussion rotary air blast drilling (RAB)

RAB drilling is used most frequently in the mineral exploration industry. (This tool is also
known as a Down-The-Hole Drill.) 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 are blown up the outside of the rods and collected at surface. Air or a combination
of air and foam lift the cuttings.

RAB drilling is used primarily for mineral exploration, water bore drilling and blast-hole drilling
in mines, as well as for other applications such as engineering, etc. RAB produces lower quality
samples because the cuttings are blown up the outside of the rods and can be contaminated from
contact with other rocks. RAB drilling at extreme depth, if it encounters water, may rapidly clog
the outside of the hole with debris, precluding removal of drill cuttings from the hole. This can
be counteracted, however, with the use of "stabilisers" also known as "reamers", which are large
cylindrical pieces of steel attached to the drill string, and made to perfectly fit the size of the hole
being drilled. These have sets of rollers on the side, usually with tungsten buttons, that constantly
break down cuttings being pushed upwards.

The use of high-powered air compressors, which push 900-1150 cfm of air at 300-350 psi down
the hole also ensures drilling of a deeper hole up to ~1250 m 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, and on how worn the drill bit is.
Air core drilling

Air 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.

Cable tool drilling

SpeedStar cable tool drilling rig, Ballston Spa, New York

Cable tool rigs are a traditional way of drilling water wells. 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 supplanted in recent years by
other, faster drilling techniques, it is still 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.

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 composed 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 trapdoor
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.

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.

Cable tool rigs now are nearly obsolete in the United States. They are mostly used in Africa or
Third-World countries. Being slow, cable tool rig drilling means increased wages for drillers. In
the United States drilling wages would average around US$200 per day per man, while in Africa
it is only US$6 per day per man, so a slow drilling machine can still be used in undeveloped
countries with depressed wages. A cable tool rig can drill 25 feet to 60 feet of hard rock a day. A
newer rotary top head rig equipped with down-the-hole (DTH) hammer can drill 500 feet or
more per day, depending on size and formation hardness.[2]

Reverse circulation (RC) drilling

Reverse Circulation (RC) rig, outside Newman, Western Australia

Track mounted Reverse Circulation rig (side view).

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 (13–20 cm) in diameter and have round
metal 'buttons' that protrude from the bit, which are required to drill through shale and abrasive
rock. 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 re-grind 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, owing 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" (a.k.a. "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) utilizes 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 or sawn through 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 rigs

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. While this does
not meet the proper definition of drilling, it does achieve the same result — a borehole. Direct
push rigs include both cone penetration testing (CPT) rigs and direct push sampling rigs such as
a PowerProbe or 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 20 kN.
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.

Direct push drilling 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.[3] 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.[4]

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. Vibrations may also be generated within the drill head. The frequency is
generally between 50 and 120 hertz (cycles per second) and can be varied by the operator.

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.

Limits of the technology

An oil rig

Drill technology has advanced steadily since the 19th century. However, there are several basic
limiting factors which will determine the depth to which a bore hole can be sunk.

All holes must maintain outer diameter; the diameter of the hole must remain wider than the
diameter of the rods or the rods cannot turn in the hole and progress cannot continue. Friction
caused by the drilling operation will tend to reduce the outside diameter of the drill bit. This
applies to all drilling methods, except that in diamond core drilling the use of thinner rods and
casing may permit the hole to continue. Casing is simply a hollow sheath which protects the hole
against collapse during drilling, and is made of metal or PVC. Often diamond holes will start off
at a large diameter and when outside diameter is lost, thinner rods put down inside casing to
continue, until finally the hole becomes too narrow. Alternatively, the hole can be reamed; this is
the usual practice in oil well drilling where the hole size is maintained down to the next casing
point.

For percussion techniques, the main limitation is air pressure. Air must be delivered to the piston
at sufficient pressure to activate the reciprocating action, and in turn drive the head into the rock
with sufficient strength to fracture and pulverise it. With depth, volume is added to the in-rod
string, requiring larger compressors to achieve operational pressures. Secondly, groundwater is
ubiquitous, and increases in pressure with depth in the ground. The air inside the rod string must
be pressurised enough to overcome this water pressure at the bit face. Then, the air must be able
to carry the rock fragments to surface. This is why depths in excess of 500 m for reverse
circulation drilling are rarely achieved, because the cost is prohibitive and approaches the
threshold at which diamond core drilling is more economic.

Diamond drilling can routinely achieve depths in excess of 1200 m. In cases where money is no
issue, extreme depths have been achieved, because there is no requirement to overcome water
pressure. However, circulation must be maintained to return the drill cuttings to surface, and
more importantly to maintain cooling and lubrication of the cutting surface.
Without sufficient lubrication and cooling, the matrix of the drill bit will soften. While diamond
is the hardest substance known, at 10 on the Mohs hardness scale, it must remain firmly in the
matrix to achieve cutting. Weight on bit, the force exerted on the cutting face of the bit by the
drill rods in the hole above the bit, must also be monitored.

A unique drilling operation in deep ocean water was named Project Mohole.

Causes of deviation
Most drill holes deviate from the vertical. This is because of the torque of the turning bit working
against the cutting face, because of the flexibility of the steel rods and especially the screw joints,
because of reaction to foliation and structure within the rock, and because of refraction as the bit
moves into different rock layers of varying resistance. Additionally, inclined holes will tend to
deviate upwards because the drill rods will lie against the bottom of the bore, causing the drill bit
to be slightly inclined from true. It is because of deviation that drill holes must be surveyed if
deviation will impact the usefulness of the information returned. Sometimes the surface location
can be offset laterally to take advantage of the expected deviation tendency, so the bottom of the
hole will end up near the desired location. Oil well drilling commonly uses a process of
controlled deviation called directional drilling (e.g., when several wells are drilled from one
surface location).

Rig equipment
Simple diagram of a drilling rig and its basic operation

Drilling rigs typically include at least some of the following items: See Drilling rig (petroleum)
for a more detailed description.

 Blowout preventers: (BOPs)

The equipment associated with a rig is to some extent dependent on the type of rig but (#23 &
#24) are devices installed at the wellhead to prevent fluids and gases from unintentionally
escaping from the borehole. #23 is the annular (often referred to as the "Hydril", which is one
manufacturer) and #24 is the pipe rams and blind rams.

 Centrifuge: an industrial version of the device that separates fine silt and sand from the drilling
fluid.
 Solids control: solids control equipments for preparing drilling mud for the drilling rig.
 Chain tongs: wrench with a section of chain, that wraps around whatever is being tightened or
loosened. Similar to a pipe wrench.
 Degasser: a device that separates air and/or gas from the drilling fluid.
 Desander / desilter: contains a set of hydrocyclones that separate sand and silt from the drilling
fluid.
 Drawworks: (#7) is the mechanical section that contains the spool, whose main function is to
reel in/out the drill line to raise/lower the traveling block (#11).
 Drill bit: (#26) device attached to the end of the drill string that breaks apart the rock being
drilled. It contains jets through which the drilling fluid exits.
 Drill pipe: (#16) joints of hollow tubing used to connect the surface equipment to the bottom
hole assembly (BHA) and acts as a conduit for the drilling fluid. In the diagram, these are
"stands" of drill pipe which are 2 or 3 joints of drill pipe connected together and "stood" in the
derrick vertically, usually to save time while Tripping pipe.
 Elevators: a gripping device that is used to latch to the drill pipe or casing to facilitate the
lowering or lifting (of pipe or casing) into or out of the borehole.
 Mud motor: a hydraulically powered device positioned just above the drill bit used to spin the
bit independently from the rest of the drill string.
 Mud pump: (#4) reciprocal type of pump used to circulate drilling fluid through the system.
 Mud tanks: (#1) often called mud pits, provides a reserve store of drilling fluid until it is required
down the wellbore.
 Rotary table: (#20) rotates the drill string along with the attached tools and bit.
 Shale shaker: (#2) separates drill cuttings from the drilling fluid before it is pumped back down
the borehole.