Directional drilling has pretty much become standard practice for many operators due to the increased amount of benefits that it adds. We'll go over these benefits in detail latter (Part 2). First we'll talk about how it's done...
So how do they do it? This was one of the first questions that baffled me whenever I entered the drilling industry. I knew that pipe could bend, but enough to create a 90 degree angle? That blew my mind. First off, you have to realize the magnitude of some these wells. These wells can reach lengths up to 25,000 feet. Over these great lengths, the pipe almost becomes like string. You can bend it any which way you want! Corkscrew, fishhook, horizontal; you name it they've done it.
As I just mentioned, directional wells can be drilled in several different configurations. It all depends on where the oil or gas is your trying to target. Below I've put together some images of a couple different types of classifications that are given to wells based off of their shape. These are just generalizations. Keep in my mind that these wells are drilled in a 3 dimensional plane, and the images below represent a 2 dimensional snapshot.
Last image you see there is a horizontal well. The horizontal well is probably the most well known due to the hype from media concerning the fracking process (these other wells can be fracked, not just horizontal wells). Currently, the longest horizontal well to date is a total depth of 40,320 feet, with a horizontal section (horizontal displacement from the vertical section) of 35,770 feet. This well was drilled by MOQ (Maersk Oil Qatar AS) in the Al Shaheen field offshore of Qatar in 2008. That's 6.77 miles. For all you Houstonians, just to give you an idea how far that is, that would be like placing a drilling rig inside the Astrodome and drilling a straight line all the way to the Club House at Memorial Park Golf Course! Amazing right?! (This is the point where some of you start thinking of me as nerdy)
To accomplish such an amazing feat, it has taken drilling companies years of innovating and honing of techniques. There are essentially two main tools used to drill a directional well. Both involve placing a mechanism behind the drill bit that allows the driller to create deflection (i.e. angling of the pipe). The two ways are either with a motor (Positive Displacement/Turbine) or with a Rotary Steerable System. I bet right you're wondering exactly how do these things work? Well don't worry, I'm going to tell you...
We'll first start off with the most primitive of the two, the positive displacement (PDM) motor (otherwise known as the mud motor). A PDM mud motor works essentially how it's name implies. It is a motor that is run off of mud. Basically, as the mud is positively displaced through the motor it turns a shaft within the motor, in return rotating the bit. Because of this, there is no need to rotate the pipe. Simply turning on the pumps to pump mud down the drill pipe will turn the bit. This is beneficial for a multiple reasons. One main reason being that it allows the oil company to not rotate inside of their casing (click for explanation of casing). Excessive rotation inside of the casing can lead to casing wear. Casing wear reduces the life of that casing, meaning the oil company will have less years to produce oil/gas out of the casing before having to repair it. Also, casing wear can lead to reduced strength of the casing. Optimal casing strength is needed for many wells that require fracking as the wells are put under extreme amounts of pressure. Casing is rated to a certain "burst pressure," or pressure needed to blow out the casing.
Sorry got on a tangent back to our topic, how do they build angle with the motor then? Well they place what is known as a bent sub above the motor. Please see the image below. Essentially it just orients the bit at an angle.
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| PDM on the rig floor Photo courtesy of Leistritz Corp. |
After the rig has drilled the straight hole, they will re-enter the well with a bent-sub motor. As sub refers an section of pipe that serves some purpose. Those purposes are basically infinite. They manufacture an array of different types of subs. Some subs will help to circulate the well, as other contain electronic components to monitor the drilling progress. The sub we are referring to simply provides angle to the bit. There are different subs depending on what type of angle you are wanting to drill.
The driller will orient this sub to whatever direction they are wishing to turn. He does this by rotating the pipe, which in turn rotates the orientation of the bent-sub. Once that direction is established, they turn on the pumps and begin to "slide drill." "Slide drilling" is whenever you are drilling with a mud motor and not rotating the pipe. It is known as "Slide Drilling" because the pipe is essentially sliding down the hole all while the bit is rotating. After the rig has built the curve, they will enter with a normal drilling assembly and drill the lateral. The lateral section refers to the base of the "L" in a horizontal well.
That covers it for the PDM! The second type of motor mentioned early is a turbine driven motor. This motor contains several stator rotor assemblies. Basically looks like several fans within the motor assembly. These motors can be built with several different angled orientations allowing for the drilling of directional wells. They are operated in a similar fashion as the PDMs .
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| Just an example of a turbine, not from a turbine drilling motor |
That concludes Part 1 of a 2 Part Series! I hope this was entertaining/educating. Please feel free to shoot me a comment if I used any lingo that the meaning couldn't be deducted from the post. Stop back by for Part 2 in the next week or so. In Part 2, I will cover the Rotary Steerable System along with the benefits of directional drilling.
"The Lord will fight for you, and you have only to be silent."
-Exodus 14:14
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