Horizontal Drilling Techniques, Directional Drilling Methods
Mud Motor Used In Directional Drilling. (note the slight bend to facilitate turning the bit slightly in a particular direction)
Horizontal drilling, often called directional drilling has revolutionized the way that oil and gas wells are drilled. The reason that horizontal drilling in changing the oil and gas business is that a well drilled sideways through a zone of oil and gas bearing rock, will produce many more times what a vertical well would. This is because a vertical oil or gas well only penetrates a few feet of the oil or gas zone whereas a well drilled horizontally may penetrate several thousand. This also means that previously unproductive rock formations such as the Barnett shale and Bakken oil shale can be productive.
A Continental Resources Rig Drilling a Horizontal Well In The Bakken Formation
Because these formations have poor porosity not much natural gas or oil could flow out, but with horizontal drilling a lateral, or directional hole can be drilled at around ninety degrees for a few thousand feet and hydraulic fracturing or "fracking" could be done. A frac job involves the pumping of high pressure fluid and a material such as rounded beads to act as a proppant and keep the fissures that are broken by the high pressure open and allow gas or oil to flow out.
Glass Beads Used As Proppant To Hold Fractured Fissures Open
The two technologies, horizontal drilling and hydraulic fracturing were made for each other. In addition to new wells such as in the Barnett shale, older wells can be drilled horizontally and stimulated with hydraulic fracturing. Once drilling has been finished and casing has been set in the well bore a service company such as Halliburton sets up dozens of large frac tanks and trucks with high pressure pumps pump thousands of gallons of fluid and proppant, under extreme pressure, into the oil or gas well. This is called a frac job, since the pressure of the fluid fractures the underground rock formation, and fissures and channels for oil and natural gas to seep out are increased.
Here is a photo of Halliburton equipment set up for a hydraulic fracturing or frac job.
Multiple pumping units are often used in stages to pump thousands of gallons of fluid held in frac tanks down the well bore after casing has been set, to fracture the formation.
For more on how hydraulic fracturing is done see: What Is A Frac Job
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Other Applications Of Horizontal Drilling
In addition to conventional oil and gas, directional drilling is being used to recover coal bed methane. Coal bed methane, which is essentially a more methane rich form of natural gas, is found throughout much of the country. This low volume, low pressure gas has been known about for years and farmers in some parts of the country as it bubbled from water wells but until horizontal drilling came along there was no way to get enough gas from a single well to make drilling for it profitable. Coal bed methane is now a growing source of natural gas thanks to horizontal drilling. Recent estimates are that coal bed methane accounts for over nine percent of total natural gas production in the U.S. Horizontal drilling is the primary method being used for shale gas deposits, coal bed methane and shale oil such as contained in the Bakken formation of North Dakota.
Illustration Of Coal Bed Methane Horizontal Drilling.
Horizontal Drilling For Coal Bed Methane Illustration Of Directional Drilling
The methods shown above are similar to that which is used in horizontal drilling for oil and gas. The illustration on the left incorporates Drill String Radar along with the LWD - MWD to detect the boundaries of the coal seam. The MWD or LWD equipment ( LWD if logging sensors are used) typically consists of sophisticated electronics including accelerometers to measure gravity and determine inclination, and magnetometers which detect the earth's magnetic field and provide compass direction of azimuth of the drilling assembly. In order for these instruments to read the earth's magnetic field, they are placed in nonmagnetic drill collars or "monel" collars behind the mud motor. The directional drilling assembly consists of the bit, mud motor, MWD -LWD sensors and nonmagnetic drill collars. LWD or Logging While Drilling instruments often read gamma radiation given off by the formation. This information is interpreted at the surface to determine if the drilling assembly is still within the shale or coal bed since each type of rock has a different signature based on it's emitted gamma radiation.
Having a reliable signal from these sensors is vital to horizontal drilling since they tell the driller at the surface which direction and at what angle he is drilling the well. Radio telemetry, which uses ultra low frequency radio waves that are picked up from an antenna on the surface, provide very fast updates of where the tool face, or bend in the mud motor is and compass direction or azimuth. This type of system is often used in shallow coal bed methane drilling but is often impractical at extreme depths that are often encountered in horizontal drilling for oil and gas.
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When depth prevents using radio telemetry a mud pulse system is used. Data from the MWD equipment is sent up hole in the form of pressure pulses inside the drill pipe. A transducer at the surface decodes these pressure spikes and the information from the sensors is displayed on a rig floor display that shows inclination and azimuth.
How A Mud Motor Works In Horizontal Drilling
The key component of horizontal drilling is the mud motor. This device is powered by the force of drilling fluid or "mud" that is pumped down the drill pipe. A mud motor contains a rotor, which is a spiraled shaft that turns the bit. It is surrounded by a fixed chamber called a stator. See the illustration below.
Illustration Of Mud Motor Used In Horizontal Drilling
The mud motor above appears straight but actually has a slight bend of two degrees to facilitate directional drilling. Often mud motors are adjustable to that directional drillers can make adjustments in te field according to how much angle they need to build. Often a rock bit will be used while building angle and then a PDC or polycrystalline diamond compact will be used for the lateral section.
Photo of a PDC or Polycrystalline Diamond Compact bit. Photo of a rock bit.
PDC bits are often used in shale and sometimes limestone. Rock bits are very effective for drilling in sandstone.
Since the mud motor can turn the bit without the drill pipe moving, and because there is a slight bend in the mud motor (often adjustable) the mud motor can be turned in a particular direction so the bit can chew away at the rock and slide forward. By knowing what direction the bent motor is sliding in, thanks to MWD or measure while drilling technology, horizontal drilling is possible.
This explanation of horizontal drilling as used in the oilfield was intended to describe the basics of how horizontal wells are drilled.. There are other new technologies in directional drilling such as rotary steerable that are revolutionizing the industry. Utilizing a system of sensors and kick pads, the drill string can be rotated from the surface and the kick pads bump the drilling assembly and bit in the desired direction. This allows a well to be drilled horizontally much faster since more weight and torque can be applied to the drill bit.
Other advances in horizontal drilling and MWD include hot hole tools. Hot hole tools are specialized MWD equipment and mud motors that can withstand temperatures of over 300 degrees Fahrenheit as is found in many locations.
For more on horizontal drilling techniques see the next page. How Wells Are Drilled Horizontally