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Drilling Operations


Topics

  1. The Rotary Drilling Rig

  2. Drilling Geometries

  3. The Rig Crew

  4. Drilling Cost

  5. Drilling Activity

  6. Classes

  7. References


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

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.

Please see the Merriam-Webster Visual Dictionary for an excellent interactive diagram.


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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Drilling Geometries

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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The Rig Crew

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 monkey board) 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.

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Drilling Costs

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 2007 it cost $4 million to drill an oil well and $3.9 million to drill a natural gas well. The average cost to drill a well was $574/foot of depth.

Please see Costs of Crude Oil and Natural Gas Wells Drilled for a history of U.S. drilling costs.

Please see Oil and Gas Lease Equipment and Operating Costs 1988 through 2009 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 $17.01/BOE from 2005-2007.  The cost for onshore development was $13.38/BOE and for offshore development was $49.54/BOE. BOE is the barrels of oil equivalent. 

Please see Oil and Gas Production from the Energy Information Administration for more on these costs.

Drilling costs have increased as oil prices and drilling activity have risen.  In 2000, when oil prices were $26.72 /B, drilling costs were $593 thousand per oil well and $126/foot.  For 2006, when prices averaged $59.69/B, drilling costs were $2.238 million per oil well and $402/foot. 

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Drilling Activity

Drilling activity is reported in terms of the number of drilling rigs operating and the number of wells drilled. PetroStrategies reports these values in two graphs Rig Count and Wells Drilled. Also see Crude Oil and Natural Gas Drilling Activity.

Is there a difference between oil and gas rigs?   There is no physical difference in the rigs.  The designation "oil rig" or "gas rig" is used when the drilling rig is used to drill a well into an oil or gas reservoir.

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Classes  

To learn more about oil and gas exploration and production, please check out the following classes:

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A Layman's Guide to the Oil & Gas Industry

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The Oil & Gas Industry in Perspective

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Fundamentals of Upstream Operations

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References

Check out the following references to learn more about drilling operations:

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Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production, Norman J. Hyne, Pennwell Publishing.

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Other Books About Drilling Operations

Click on the following links to learn more about drilling:

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International Association of Drilling Contractors

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Baker Hughes Rig Count

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Baker Hughes Interactive Rig Counts

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International Petroleum Museum and Exposition

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The American Petroleum Institute

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Independent Petroleum Association of America

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Schlumberger Oilfield Glossary

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Learn About Energy - Society of Petroleum Engineers

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Drilling Rig Video from You Tube

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Offshore Energy Center

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