The source of this article is here: http://www.energyindustryphotos.com/how_oil_and_gas_wells_are_drille.htm
Peter
How Oil And Gas Wells Are Drilled Horizontally
Today's oil companies now have the ability to drill wells sideways, to reach specific pockets of oil and gas or "targets" which may be beneath cities or environmentally sensitive area where a rig cannot be set up. Multiple wells can be drilled from one pad site on land or from an offshore platform, reducing costs and the environmental impact of oil and gas exploration.
Horizontal drilling techniques are also used to expose more surface area of oil bearing rock so that the overall production of the well is increased in either a new well or old well that has been producing for some time.
You may be asking yourself, how do you get heavy, rigid drill pipe to turn and go sideways? The truth is that when so much pipe is strung together it does
Mud Motor
Illustration of a Mud Motor as Used In Horizontal Oil and Gas Wells
A directional assembly (parts that make up the drill string that the rig lowers into the hole to drill the well) consists of a "mud motor" which is a machine that contains a rotor and stator inside it which are turned by the force of the drilling fluid or "mud" that is pumped down the drill pipe. This motor turns the drill bit so instead of turning the entire length of drill pipe, the motor, which is bent at a certain angle, can remain in a fixed position drilling the hole in whatever direction the bend of the motor is aimed toward.
MWD 
MWD tool on the surface. (Is lowered into a special section of drill pipe or " Monel collar" which is non magnetic.
A probe full of instruments is placed inside a nonmagnetic length of drill pipe just above the mud motor to show the driller on the surface which direction he is drilling in and at what angle or inclination he is heading. This instrument is an MWD or "Measure While Drilling" probe. It typically consists of instruments that measure inclination utilizing accelerometers, and instruments that measure azimuth or compass heading using magnetometers. This information is sent via a series of pressure pulses in the column of drilling fluid flowing down to the bit from pumps on the surface. At the surface a device called a transducer picks up these increases in pressure (similar to Morse code signals) and they are decoded by surface equipment in the MWD company's mobile lab that is placed on deck of the offshore rig, or near a the rig on land. This equipment is connected to a display on the rig floor where the driller can see which way the bent motor is sliding.
The directional driller can then turn the entire length of drill pipe ever so slightly in the direction he wants the well to go and the motor which is being turned by the flow of drilling mud coming down the drill pipe causes the bit to chew away at the rock and drill ahead. MWD equipment can also be combined with sensors that record gamma rays given off from the rock formations below, instruments that measure resistivity and conductivity, etc. This is known as "logging while drilling" or LWD. It enables the oil company geologists to see if they are staying in, or have reached, the oil or gas bearing zones in real time instead of having to pull all the drill pipe out of the hole and run a wireline based logging tool down the hole.
Enhanced Recovery
The ability of oil companies to drill horizontally is being used in fields that have been depleted to the point where very little oil or gas is flowing into the original vertical well and to drill multiple horizontal wells from the same offshore platform to a pocket of oil or gas as seen in the illustration below. See What is Enhanced Oil Recovery?
Illustration of an Offshore Horizontal Oil Well from a Drilling Platform
In a well that was originally drilled vertically in the past and is not producing as much as it once was, a "re-entry" will be done whereby a device called a whipstock is lowered and fixed at a certain depth.
A special bit or "mill" is used at the end of the regular drill pipe, deflected into the side of the casing by the whipstock to cut a hole in the heavy metal casing and out the side of the well. This is done somewhere just above the productive formation. Then, after this "window" has been cut the rig will pull the milling bit out of the hole and then lower the directional assembly consisting of the mud motor and MWD equipment down into the hole and begin to drill a horizontal hole. Horizontal drilling techniques are being used in a variety of rock formations including the Barnett Shale of Texas - Oklahoma, and the Bakken Formation in North Dakota and Canada. In the Barnett Shale and Marcellus Formation for example, horizontal wells can extract the very low volume of gas given off by the dense shale. In the past such formations of low porosity had been passed over and considered not economically viable with vertical wells. Now large areas of the United States are being opened up to natural gas exploration in shale and limestone formations of low porosity. These dense formations are drilled with horizontal drilling methods and then "fracked" with high pressure water to create more fissures that will allow gas to escape from the rock into the wellbore. Coal bed methane, or natural gas can be extracted from coal seams too deep to mine using horizontal drilling techniques.
Recent Advances In Technology
New techniques to drill wells horizontally are being developed all the time. One such technique is called "Rotary Steerable". With this system the entire drill string can be rotated, enabling faster penetration and quicker drilling of the well. It utilizes the same components, MWD and a mud motor, but it is a very specialized type of mud motor that has kick pads that orient the drill bit in the right angled direction instead of using a bent mud motor.
Leaders in the industry include companies such as Weatherford, Halliburton (Sperry Sun), Pathfinder Energy Services, Schlumberger and Baker Hughes.
The YouTube video below, while without sound, illustrates the drilling of a hypothetical horizontal well. It shows first the drilling of a horizontal hole, drilling the "curve" of the well with directional equipment, pulling out to cement in a "liner" or casing to protect the upper layers of the well bore, then drilling out of this casing again into rock horizontally and finally set the production casing. (click on the following link to see the video on youtube)
While these explanations are simplified I hope that they give the reader some idea of how this technology works. As the world requires more and more oil and gas new advances in drilling technology are being developed to work in deeper holes, hotter temperature zones and pressures.
For more information on how oil wells are drilled the two books that I recommend the most are The Nontechnical Guide To Petroleum Geology, Exploration, Drilling and Production and A Primer Of Oilwell Drilling by Ron Baker. Both used by many colleges and technical schools as well as Oilfield Service companies in their training programs. For production only try Oil and Gas Production In Nontechnical Language. Drilling Technology In Nontechnical Language
is a lower cost alternative to "A Primer Of Oilwell Drilling". All can be found in The Oilfield Bookstore,For More Information on any of these books and customer reviews click on the book covers below...
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