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HOW OIL DRILLING WORKS
In 2008 alone, the United States produced an estimated 4.9 million barrels of crude oil per day and imported 9.8 million barrels per day from other countries [source: U.S. Energy Information Administration]. This oil gets refined into gasoline,
Oil comes from the remains of tiny plants and animals (plankton) that died in ancient seas between 10 million and 600 million years ago. After the organisms died, they sank into the sand and mud at the bottom of the sea.
Over the years, the organisms decayed in the sedimentary layers. In these layers, there was little or no oxygen present. So microorganisms broke the remains into carbon-rich compounds that formed organic layers. The organic material mixed with the sediments, forming fine-grained shale, or source rock. As new sedimentary layers were deposited, they exerted intense pressure and heat on the source rock. The heat and pressure distilled the organic material into crude oil and natural gas. The oil flowed from the source rock and accumulated in thicker, more porous limestone or sandstone, called reservoir rock. Movements in the Earth trapped the oil and natural gas in the reservoir rocks between layers of impermeable rock, or cap rock, such as granite or marble.
These movements of the Earth include:
Folding - Horizontal movements press inward and move the rock layers upward into a fold or anticline.
Faulting - The layers of rock crack, and one side shifts upward or downward.
Pinching out - A layer of impermeable rock is squeezed upward into the reservoir rock.
kerosene, heating oil and other products. To keep up with our consumption, oil companies must constantly look for new sources of petroleum, as well as improve the production of existing wells.
How does a company go about finding oil and pumping it from the ground? You may have seen images of black crude oil gushing out of the ground, or seen an oil well in movies and television shows like "Giant," "Oklahoma Crude," "Armageddon" and "Beverly Hillbillies." But modern oil production is quite different from the way it's portrayed in the movies.
In this article, we'll examine how modern oil exploration and drilling works. We'll discuss how oil is formed, found and extracted from the ground.
Oil is a fossil fuel found in many countries around the world. On the next page, we'll discuss how oil is formed and how geologists find it.
Whether employed directly by an oil company or under contract from a private firm, geologists are the ones responsible for finding oil. Their task is to find the right conditions for an oil trap -- the right source rock, reservoir rock and entrapment. Many years ago, geologists interpreted surface features, surface rock and soil types, and perhaps some small core samples obtained by shallow drilling. Modern oil geologists also examine surface rocks and terrain, with the additional help of satellite images. However, they also use a variety of other methods to find oil. They can use sensitive gravity meters to measure tiny changes in the Earth's gravitational field that could indicate flowing oil, as well as sensitive magnetometers to measure tiny changes in the Earth's magnetic field caused by flowing oil. They can detect the smell of hydrocarbons using sensitive electronic noses called sniffers. Finally, and most commonly, they use seismology, creating shock waves that pass through hidden rock layers and interpreting the waves that are reflected back to the surface.
In seismic surveys, a shock wave is created by the following:
- Compressed-air gun - shoots pulses of air into the water (for exploration over water)
- Thumper truck - slams heavy plates into the ground (for exploration over land)
- Explosives - detonated after being drilled into the ground (for exploration over land) or thrown overboard (for exploration over water)
The shock waves travel beneath the surface of the Earth and are reflected back by the various rock layers. The reflections travel at different speeds depending upon the type or density of rock layers through which they must pass. Sensitive microphones or vibration detectors detect the reflections of the shock waves -- hydrophones over water, seismometers over land. Seismologists interpret the readings for signs of oil and gas traps.
Once geologists find a prospective oil strike, they mark the location using GPS coordinates on land or by marker buoys on water.
Oil Drilling Preparation
Once the site has been selected, scientists survey the area to determine its boundaries, and conduct environmental impact studies if necessary. The oil company may need lease agreements, titles and right-of way accesses before drilling the land. For off-shore sites, legal jurisdiction must be determined.
After the legal issues are settled, the crew goes about preparing the land:
- The land must be cleared and leveled, and access roads may be built.
- Because water is used in drilling, there must be a source of water nearby. If there is no natural source, the crew drills a water well.
- The crew digs a reserve pit, which is used to dispose of rock cuttings and drilling mud during the drilling process, and lines it with plastic to protect the environment. If the site is an ecologically sensitive area, such as a marsh or wilderness, then the cuttings and mud must be disposed of offsite -- trucked away instead of placed in a pit.
Once the land has been prepared, the crew digs several holes to make way for the rig and the main hole. A rectangular pit called a cellar is dug around the location of the actual drilling hole. The cellar provides a work space around the hole for the workers and drilling accessories. The crew then begins drilling the main hole, often with a small drill truck rather than the main rig. The first part of the hole is larger and shallower than the main portion, and is lined with a large-diameter conductor pipe. The crew digs additional holes off to the side to temporarily store equipment -- when these holes are finished, the rig equipment can be brought in and set up.
Depending upon the remoteness of the drill site and its access, it may be necessary to bring in equipment by truck, helicopter or barge. Some rigs are built on ships or barges for work on inland water where there is no foundation to support a rig (as in marshes or lakes).
In the next section, we'll look at the major systems of an oil rig.
Oil Rig Systems
Once the equipment is at the site, the crew sets the rig up. Here are the major systems of a land oil rig:
- large diesel engines - burn diesel-fuel oil to provide the main source of power
- electrical generators - powered by the diesel engines to provide electrical power
Mechanical system - driven by electric motors
- hoisting system - used for lifting heavy loads; consists of a mechanical winch (drawworks) with a large steel cable spool, a block-and-tackle pulley and a receiving storage reel for the cable
- turntable - part of the drilling apparatus
Rotating equipment - used for rotary drilling
- swivel - large handle that holds the weight of the drill string; allows the string to rotate and makes a pressure-tight seal on the hole
- kelly - four- or six-sided pipe that transfers rotary motion to the turntable and drill string
- turntable or rotary table - drives the rotating motion using power from electric motors
- drill string - consists of drill pipe (connected sections of about 30 feet (10 meters) and drill collars (larger diameter, heavier pipe that fits around the drill pipe and places weight on the drill bit)
- drill bit(s) - end of the drill that actually cuts up the rock; comes in many shapes and materials (tungsten carbide steel, diamond) that are specialized for various drilling tasks and rock formations
Casing - large-diameter concrete pipe that lines the drill hole, prevents the hole from collapsing, and allows drilling mud to circulate
Circulation system - pumps drilling mud (mixture of water, clay, weighting material and chemicals, used to lift rock cuttings from the drill bit to the surface) under pressure through the kelly, rotary table, drill pipes and drill collars
- pump - sucks mud from the mud pits and pumps it to the drilling apparatus
- pipes and hoses - connects pump to drilling apparatus
- mud-return line - returns mud from the hole
- shale shaker - shaker/sieve that separates rock cuttings from the mud
- shale slide - conveys cuttings to the reserve pit
- reserve pit - collects rock cuttings separated from the mud
- mud pits - where drilling mud is mixed and recycled
- mud-mixing hopper - where new mud is mixed and then sent to the mud pits
Derrick - support structure that holds the drilling apparatus; tall enough to allow new sections of drill pipe to be added to the drilling apparatus as drilling progresses
Blowout preventer - high-pressure valves (located under the land rig or on the sea floor) that seal the high-pressure drill lines and relieve pressure when necessary to prevent a blowout (uncontrolled gush of gas or oil to the surface, often associated with fire)
The Oil Drilling Process
The crew sets up the rig and starts the drilling operations. First, from the starter hole, the team drills a surface hole down to a pre-set depth, which is somewhere above where they think the oil trap is located. There are five basic steps to drilling the surface hole:
- Place the drill bit, collar and drill pipe in the hole.
- Attach the kelly and turntable, and begin drilling.
- As drilling progresses, circulate mud through the pipe and out of the bit to float the rock cuttings out of the hole.
- Add new sections (joints) of drill pipes as the hole gets deeper.
- Remove (trip out) the drill pipe, collar and bit when the pre-set depth (anywhere from a few hundred to a couple-thousand feet) is reached.
Once they reach the pre-set depth, they must run and cement the casing -- place casing-pipe sections into the hole to prevent it from collapsing in on itself. The casing pipe has spacers around the outside to keep it centered in the hole.
The casing crew puts the casing pipe in the hole. The cement crew pumps cement down the casing pipe using a bottom plug, a cement slurry, a top plug and drill mud. The pressure from the drill mud causes the cement slurry to move through the casing and fill the space between the outside of the casing and the hole. Finally, the cement is allowed to harden and then tested for such properties as hardness, alignment and a proper seal.