How Do Engineers Drill for Oil and Gas?
It is necessary to drill a hole to obtain
crude oil and natural gas from under the
earth's surface. Engineers make this hole using a rotary
drilling rig.
 The
rotary drilling rig uses a drill bit to cut through
the earth and create a hole. As the hole gets deeper, pipe is added to the drill bit to
allow it to dig further. These lengths of drill pipe form the drill
string. This pipe is connected to an engine that turns the drill bit to cut the
hole. The rotary rig operates the same as a hand-held electric drill. The electric drill
has a motor that turns the drill bit and sufficient weight must be applied to keep the drill
in contact with the bottom of the hole.
There are four main operations in a drilling rig:
hoisting, rotating, circulating, and
power. The hoisting system is
used to raise and lower pipe in and out of the hole and to support the
drill string to control the weight on the drill bit during drilling.
Hoisting
System
 The hoisting system consists of the
derrick,
traveling and crown blocks, the
drilling
line, and the
drawworks. The drilling rig
uses a derrick to support the drill bit and pipe (drill string). The derrick is a steel
tower that is used to support the traveling and crown blocks and the drill string. There
may be no more identifiable symbol of the oil and gas industry than the derrick on a
drilling rig.
The crown and traveling blocks are a
set of pulleys that raise and lower the drill string. The crown block is a stationary
pulley located at the top of the derrick. The traveling block moves up and down and is
used to raise and lower the drill string. These pulleys are connected to the drill string
with a large diameter steel cable.
The cable is connected to a winch or
drawworks. The
drawworks contains a large drum around which the drilling cable is wrapped. As the drum
rotates one way or the other, the drilling cable spools on or off the drum and raises or
lowers the drill string.
Rotating Equipment
 The rotating equipment turns the drilling bit. This equipment consists of
the swivel,
the kelly,
the rotary table, the drill
pipe, the drill collars,
and the
bit. The swivel is attached to the bottom of the traveling block and permits the
drill string to rotate. The kelly is a square or hexagonal shaped section of pipe that is
attached to the swivel. The kelly fits in a matching slot in the rotary table. As the
rotary table turns the kelly is also turned. The movement of the kelly rotates the drill
string and the drill bit.
Drilling pipe is round steel
tubes about 30 feet long with a diameter of from 4 to 5 inches. The drill
collars are used to add weight on the bit. Drill pipe has threaded
connections on each end that allow the pipe to be joined together to form longer sections
as the hole gets deeper.
The drilling
bit is used to create the hole. Drilling bit sizes range from six inches to
three feet in diameter. The most common drill bits are roller
cone bits and
diamond bits. Roller cone
bits have three cones containing rows of teeth. The cones rotate on bearings and turn as
the drilling bit rotates.
The teeth cut and crush the rock to create the hole. The bit
also contains small nozzles that spray
drilling fluids
to remove the rock fragments from the bottom of the hole.
Diamond bits have a single fixed
head that contains many small diamonds. As the bit turns the diamonds cut the rock.
Diamond bits also have nozzles to wash away the broken pieces of
rock. Different drilling bits are used depending on the type of rock that is encountered.
Circulating System
The drilling operation uses fluids to reduce friction and remove rock
fragments or cuttings. The circulating system
pumps these drilling fluids down
the hole, out of the nozzles in the drilling bit, and returns them to the surface where
the debris is separated from the fluid.
Drilling fluid is also knows as drilling mud
because of its characteristic brown color. The drilling mud is mixed in tanks. The mud is
pumped through a hose to the swivel, down the kelly, and into the drill pipe. The mud goes
down the drill string and out of the drilling bit nozzles. The mud carries the cuttings
from the bottom of the hole to the surface in the space between the outside of the drill
string and the inside of the hole.
The cuttings are separated from the mud in a vibrating screen called a
shale shaker. The cuttings are trapped on the screen and
the mud passes through the screen into the mud pits. The circulating pumps pick up this
clean mud and send it back down the hole. The cuttings are collected in a plastic-lined
pit for disposal.
Drilling mud
is a
mixture of water, clay, and special minerals and chemicals. Drilling mud
removes cuttings from the hole and cools and lubricates the drilling bit.
Mud also maintains pressure in the hole to keep fluids in the formation
from entering the hole and producing a gusher of oil on the surface.
Different muds are used during the drilling process to adjust to rock
formations, temperature, and pressure.
Power System
A drilling rig needs power to operate the circulating, rotating, and
hoisting systems. This power comes from two or more diesel engines. Power is transmitted
to the drilling rig from either generators that provide electricity or mechanical drivers
that use a series of pulleys and belts to transmit power from the engines to the
components that require the power.
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Wells can be drilled in a number of
different geometries from the simplest vertical well to complicated
multilateral completions.
Directional
drilling technology allows the industry
to access deposits that would otherwise be inaccessible.
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 Drilling is usually done by a service company
or drilling
contractor. The drilling crew
is composed
of a toolpusher,
a driller,
a derrickman,
a motorman and several roughnecks
and
roustabouts.
The toolpusher,
the location supervisor for the drilling contractor, is
usually a senior, experienced individual who has worked his way up
through the ranks of the drilling crew positions. His job is largely
administrative, including ensuring that the rig has sufficient
materials, spare parts and skilled personnel to continue efficient
operations.
The driller
is the supervisor of the rig crew. The driller is responsible for the
efficient operation of the rigsite as well as the safety of the crew.
He typically has many years of rigsite experience and has worked his
way up from other jobs. While the driller must know how to perform
each of the jobs on the rig, his or her role is to supervise the work
and control the major rig systems. The driller operates the pumps,
drawworks, and rotary table via the drillers console-a control room of
gauges, control levers, rheostats, and other pneumatic, hydraulic and
electronic instrumentation. The driller also operates the drawworks
brake using a long-handled lever. Hence, the driller is sometimes
referred to as the person who is "on the brake.“
The derrickman
is in charge of the
mud-processing area during periods of circulation. The derrickman also
measures mud density. The derrickman reports to the toolpusher, but is
instructed in detail by the mud engineer on what to add to the mud,
how fast and how much.
His other
job is to handle pipe in the derrick while pulling out or running into
the hole. One of the rig crew members who gets his name from the fact
that he works on a platform attached to the derrick or mast, typically
85 ft [26 m] above the rig floor, during trips. In a typical trip out
of the hole, the derrickman wears a special safety harness that
enables him to lean out from the work platform (called the
monkeyboard) to reach the drillpipe in the center of the derrick or
mast, throw a line around the pipe and pull it back into its storage
location (the fingerboards) until it is time to run the pipe back into
the well. In terms of skill, physical exertion and perceived danger, a
derrickman has one of the most demanding jobs on the rig crew. Some
modern drilling rigs have automated pipe-handling equipment such that
the derrickman controls the machinery rather than physically handling
the pipe. In an emergency, the derrickman can quickly reach the ground
by an escape line often called the Geronimo line.
The
motorman
is responsible for maintenance of the engines. While all members of
the rig crew help with major repairs, the motorman does routine
preventive maintenance and minor repairs.
A
roughneck
is a low-ranking member of the drilling crew. The roughneck usually
performs semiskilled and unskilled manual labor that requires
continual hard work in difficult conditions for many hours. After
roughnecks understand how the rig operates and demonstrates their work
ethic, they may be promoted to other positions in the crew.
A
roustabout
is any unskilled manual laborer on the rigsite. A roustabout
may be part of the drilling contractor's employee workforce, or may be
on location temporarily for special operations. Roustabouts are
commonly hired to do the peripheral tasks, ranging from cleaning up
location to cleaning threads to digging trenches to scraping and
painting rig components.
The
operator
is the company that serves as the overall manager and decision-maker of
a drilling project. Generally, but not always, the operator will have
the largest financial stake in the project. As far as the drilling
contractor and service companies are concerned, the designated
operator is paying for the entire operation, and the operator is
responsible for recouping some of that expense from the partners.
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The drilling company operates in accordance
with a contract which specifies that the well will be drilled to a specific depth. The
contractor is paid on a per day or per foot drilling rate.
In 2004
it cost $1.4 million to drill an oil well and $1.7 million to drill a
natural gas well. The average cost to drill a well was $297/foot of
depth. Please see
Costs of Crude Oil and Natural Gas Wells Drilled 1960-2004
for a history of U.S. drilling costs.
please
see
Oil and Gas Lease Equipment and Operating Costs 1987 through 2005
for a more information on oil and gas costs.
The
average cost to find and develop an oil and gas property in the United
States was $10.33/BOE from 2002-2004. The cost for onshore
development was $7.18/BOE and for offshore development was $27.66/BOE. BOE is the barrels of oil equivalent. Please see
Behind the
Bottom Line from the Energy
Information Administration for more on these costs.
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 To
learn more about drilling, please check out the following
PetroStrategies
classes:
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For
more information about drilling, please check out the following references:
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