Though the processes are similar, each explanation which follows speaks to the process of drilling oil wells rather than natural gas unless otherwise noted.
The process begins with the assembly of a drilling rig. In the old days of drilling for oil, drilling rigs, sometimes called derricks, were somewhat permanent in nature as they were often left in place after the completion of the well. Many of these can still be seen in older oil development areas in Texas and California. As the industry grew and became more reliant on technology, expensive custom-built machines that could be moved from well to well were created. Today, the mobile drilling rig is the standard. But the term “mobile” is a bit misleading as these complex machines are made of dozens of component parts.
Without laboring too long here, the basics of site preparation include clearing and levelling the site. Typically, a drill site is roughly the size of two football fields set side by side including the end-zone and out of bounds areas (about 3 acres in size, though some are a little larger). The area size accommodates the drilling rig, equipment, holding pits for drilling mud, room enough for incoming and outgoing trucks, out buildings, and space for needed materials.
Once the site is prepped, a rectangular pit called the “cellar” is dug around the location of where the well will be drilled. This provides work space around the hole, under the rig. Once the site is prepared, the rig is hoisted into place. The rig is comprised of several moving parts but we’ll only detail a few (see diagram on page 27) to keep our explanation moving.
The Vertical Hole
A starter hole is drilled to provide an opening for the main drilling process. Usually, a tricone tipped drill bit is used to drill the main hole. This heavy bit has three studded “rollers” attached to it which roll together to dig through rock as it is spun. The bit is attached to a drill “pipe” which also has other installed components to provide added weight to the bit as well as stabilizers which keep the hole centered as it is drilled. Drilling “mud” is circulated through the center to act as both a lubricant and a flushing system for drilled rock.
The pipe is attached to the drill rig at the Kelly Drive – aka, the “Kelly”. The Kelly is a polygonal tubing which turns when rotated by the rotary table which is the mechanical portion of the rig which spins the pipe. The pipe, which comes in lengths of 30-33 feet each is hollow and varies in outside diameter but is usually 5 ½ inches. Each pipe has threaded ends for connecting to one another. Up to three pipes are joined together to create a “stand” to make the process quicker than joining each pipe together during drilling. Each 99 foot stand is then hoisted into place to be connected to the pipe which has already been sunken into the earth behind the drill bit. In this manner, as each section of pipe is connected, a “string” of pipes is created. As each “stand” of pipe sinks into the surface, the process stops to disconnect the line from the rotary in order to hoist a new section of pipe into place. The next section of pipe is hoisted into place from the ground up a ramp through an opening in the rig floor called the “mouse hole.” The crew then swings the Kelly over the new pipe in the mouse hole to connect it. Pipe is connected and disconnected using an “Iron Roughneck.” Once connected, the Kelly and attached pipe are then hoisted back to the top of the drilling rig so it can be connected to the pipe already in the ground. This new pipe is then spun to continue the drilling process. Occasionally, the entire string has to be completely pulled out of the hole (called tripping out) to change the drill bit as it wears down. The entire process of reconnecting the string is then repeated to continue the drilling process. The vertical drilling process, which reaches depths of 10,000 feet in the Bakken, is thus made up of a string that can be comprised of over 300 pipes.
The pipes are hollow to facilitate the use of drilling mud which is pumped down the center of the string as the drill spins. Mud serves several purposes. It provides cooling lubricant for the drill bit; it “flushes” out the rock material being drilled out of the hole (called “cuttings”); it provides “downhole” pressure to keep any other fluids or gasses trapped in various rock layers from being rapidly released; and it keeps the hole from caving in. As the mud flows down the middle of the pipe, it is pushed through the center of the head of the spinning tricone drill bit, flushing the drilled cuttings away and to the sides of the pipe. Since the bit is larger in diameter than the pipe, the mud and cuttings are allowed to flow upwards and past the outside of the string (the pipes) to the surface. A “Shale Shaker” separates rock from the mud. Rock refuse is directed to a pit while the mud is re-circulated into the pump for reuse.
“Casing” is hollow tubing placed in the hole in order to protect and support the hole and serves as permanent containment for the flow of oil during pumping once the well is completed. The casing is smaller than the hole drilled so that cement can be placed around it to both hold it into place and provide an extra barrier between the pumped oil and the earth. Casing looks similar to drilling pipe but is wider and comes in 40 foot sections. The casing is held center in the hole by spacers, or springs, located on the outside of each pipe. After casing is placed in the hole, cement is pumped in and through it and down to the last point of drilling where it is forced outside and around the casing back toward the surface. Once reaching the surface, the casing becomes fully encased in cement - similar to how drilling mud works. The cement that remains inside the casing is pressed out using a “wiper plug” that stops at the bottom of the casing. Casing strings are formed in a fashion similar to how pipe strings are formed, with the exception that “casing seals” are added with a collar for permanence.
The casing process actually contains several sections. Larger casings are placed in the upper part of the hole and can be as wide as 16 inches in diameter. This is known as the conductor casing, or surface casing. This casing drops 30 to 60 feet below the surface. Its main purpose is to provide stability to the well during the pumping process and also serves as the foundation of the “blowout preventer.” A smaller casing, simply called “intermediate casing”, is placed inside the conductor casing. This first intermediate casing drops anywhere from 200 – 500 feet below the surface. Other intermediate casings can also be placed in the event there is a need to seal off shallow gas bearing zones. A “last” intermediate casing is usually placed to run the depth of the well past the water aquifer. The final vertical casing is called the “production casing” and runs the depth of the well down the final point of vertical drilling. Each casing is cemented into place. In the Bakken, the vertical can reach depths of 10,000 feet.
The Blowout Preventer
A blowout preventer, which sits below the rig in the cellar, is placed at the beginning of the drilling operation. Its purpose is simple: to prevent extreme pressure from gas zones from surging back through the hole. Such surges are referred to as “kicks”. If the downhole pressure created by heavy drilling mud becomes less than the upward pressure of gasses or oil, a kick occurs. When this happens, the blowout preventer, (or BOP – pronounced B-O-P… not “Bop”) which is a system of control valves, will automatically close off the well by various processes. These can include sheering through the casing with overlapping steel shears that both cut through the casing and cap it off (known as a Ram BOP), or through a pinching process where hydraulic pistons force a flexible rubber seal wedge into the casing (known as an Annular BOP). Most BOPs actually contain redundant systems which utilize both Ram and Annular mechanisms. If the BOP fails, a blowout occurs. This catastrophic event can launch piping in the hole out and through the floor of the rig and can result in an explosion and fire.
Once vertical drilling is complete, the drilling rig is removed and the horizontal drilling process begins. Horizontal drilling is used in the same sentence as Hydraulic Fracturing but they are not one in the same. Horizontal drilling has actually been in practice for nearly 100 years. It is used in several “non-fracturing” applications such as drilling relief wells or for placing utility lines. The horizontal drilling process begins by lowering the directional drill into the vertical well hole using giant reels of line, or tubing – not piping. Instead of rotating the line, the hydraulic motorized drilling machine (Mud Motor) rotates in the hole. The machine contains several components for controlling the drilling angle, monitoring, and measuring during the drilling process, also known as “Logging While Drilling” (LWD) and Measurement While Drilling” (MWD). These sensors allow for accurate control. This machine, like in vertical drilling, also allows for drilling mud to be pumped through via the tubing.
The “Kick-Off” point is where the horizontal drilling process begins (see the diagram on pages 32-33.) In the Williston Basin, the kick-off point begins about 500 feet above the Bakken Shale layer (which is also where the vertical drilling process stops). The drilling machine travels at a downward angle to reach vertical. This curved diagonal path, known as the “lateral”, travels vertical to horizontal along the lateral about ¼ of a mile. Bends in the drilling machine allow it to angle along the lateral. By pumping mud through the mud motor, the bit turns while the drillstring wire does not rotate, allowing the bit to drill in the direction it points. Steering is accomplished through controls at the surface. Several methods to determine that the source rock has been reached are also used. The testing of rock samples that flow to the surface, measurement of pressures in the hole, and monitoring of sensors all fill the bill. Once it reaches the target Bakken layer, it travels horizontally, drilling for up to 10,000 feet.
The entire drilling process can last up to three weeks. Once complete, the hydraulic fracturing process can begin.