Drilling & Extraction Activities
A common drilling rig
After the discovery of an adequate petroleum reserve, companies must sign agreements and leases, if applicable, to begin their drilling process. Scientists may be called upon to do environmental impact surveys as necessary depending on where drilling is taking place. If the drilling site is offshore, companies must determine legal jurisdiction (i.e. if the site is within US territorial waters, or another country depending on where the company is located). Once these things are sorted out, the drilling and extraction process begins.
The crew will first have to set up the land. Land must be level for extraction to start, so the ground may have to be leveled out first. Because water is a critical part of extraction, if there is no natural source close to the site, crew members may have to drill a water well. Then a pit is created to dump any mud or rocks into as the drilling takes place. This pit is lined with plastic to minimize environmental impact.
A drill bit attached to a drill string is driven by a motor that creates the borehole, which is a very deep and narrow hole into the ground. The borehole depth varies, but on average are about 6,000 feet deep for an oil reserve–more than one mile deep. Casing is set around the internal circumference of the borehole to provide structural integrity. The borehole works as a well with a diameter that can range from 12 centimeters to an entire meter. It is often made of successively smaller holes, with the initial diameter at ground level being the largest.
After the borehole is drilled and coated, the next stage is to complete the well so that it can be properly used. Mud is circulated throughout the drilling process, pushing crushed rock to the top to be removed from the hole. Eventually, the crew will hit the bottom of the well, which they can usually realize when they hit sand from the reservoir rock. Then they remove the drill from the ground and run tests to confirm if this has happened. The inside of the well is then coated in cement. Tests to determine if they have actually hit the bottom include:
- Well logging: electrical and gas sensors are used to measure the rock formations to see if the bottom has been hit.
- Drill system testing: using a sensor to measure the pressure at the bottom of the hole.
- Core samples: removing samples of the rock below to determine whether or not it is the reservoir rock.
Perforations are then made in the casing around the inside of the hole so that the oil can flow through freely to the top. A tube is placed down the hole to push the oil to the top, and a casing is placed around the tube by use of a device called a packer. This is to completely encase the newly perforated hole. The tubing and cement is then topped with something called a Christmas Tree, which is a device that allows the crew to control the oil flow. The oil is then extracted in three ways:
- Primary Recovery: underground well pressure is sufficient to push some crude oil to the surface. This is only part of the recovery and does not get the majority of the oil moving.
- Secondary Recovery: external energy, such as water injection methods, are used to increase the well pressure. This pushes more of the crude oil to the surface. This is the point that gets most of the oil moving. Usually acid is used to activate the pressure that pushes the oil up, as it dissolves the rock keeping the oil down.
- Enhanced Recovery: steam injection methods are used to heat the crude oil in the well. This decreases the viscosity of the crude oil, allowing it to flow freely to the surface. This is often down after secondary recovery.
Once the oil is flowing, the rig is removed from the site so that they can pump. The pump is attached to the opening of the well and is run using an electric motor that controls a lever. The lever moves up and down, creating air movement that propels the oil upward.
These methods are used on both land surfaces and offshore with some variations, and offshore drilling usually requires more equipment. Offshore drilling also uses an oil rig, but it must be carried out to sea on a barge. There are additional issues that may arise, such as blowouts. Extra precautions are taken for offshore drilling. Onshore drilling is still far more common.
Critical Investigation: How and Why
Drilling for oil is a huge environmental concern. Although steps are taken to prevent damage to the environment, such as scientists running environmental impact surveys and plastic lining being used to contain some of the trash, it does not change the fact that cement and acid are being put into the ground. The 2010 BP oil spill brought attention to offshore drilling when a rig exploded off the coast of the Gulf of Mexico. 170 million gallons of oil flowed into the gulf, leaving over 8,000 birds, sea turtles, and marine mammals dead or injured. Nearly 2 million gallons of chemical dispersants were put into the ocean to break the oil into smaller droplets, but they are highly toxic to humans and animals alike. If not properly controlled, oil drilling can be catastrophic both onshore and offshore; however, environments and ecosystems are still disturbed from drilling, even without spills taking place.
Otter covered in crude oil after BP spill