Petroleum is refined to produce gasoline and other petroleum products from crude oil. The refining process begins with the fractional distillation of heated crude oil. The crude-oil components (gas, gasoline, naphtha, kerosene, light and heavy gas oils, and residuum) are separated into lighter and heavier hydrocarbons. Light hydrocarbons are drawn off the distilling column at lower temperatures than are heavy hydrocarbons. The components are then treated in many different ways, depending on the desired final products (shown at the bottom). The conversion processes (shown as blue boxes) are discussed in the article. For simplification, not all of the products of the conversion processes are shown in the diagram.

----------------------------

The inside of a fractional-distillation column consists of a set of perforated trays. Each perforation is fitted with a device called a bubble cap, which forces the oil vapor coming up through the tray to bubble through the liquid sitting on the top of the tray. As heat is transferred from the vapor to the liquid during bubbling, some of the heavier hydrocarbons in the vapor condense (liquefy). Meanwhile, vapor with lighter hydrocarbons moves up to the next tray, where the same process takes place. The amount of liquid on each tray increases as some of the hydrocarbons are removed from the rising vapor. Excess liquid overflows to the next lower tray through a downcomer. At several levels in the column, liquid is drawn off--lighter products from the top of the column and heavier products from the bottom.

----------------------------

The main object of catalytic, or cat, cracking is to extract gasoline from heavy cuts of hydrocarbons. The feed for cat cracking is usually straight-run heavy gas oil and flasher tops. The feed and fresh catalyst are pumped into a reaction chamber, where the cracking takes place. During the cracking process, coke (carbon) ends up as a deposit on the catalyst and the catalyst becomes spent, or inactive. To remove the coke, the spent catalyst is pumped to a regenerator, where it is regenerated and sent back to the reaction chamber. Meanwhile the cracked hydrocarbon is sent to a fractionator, where the cracked products are separated. The fractionator bottoms, called cycle oil, are usually mixed with fresh feed and run through the reaction process again.

-------------------

PETROLEUM

Petroleum is crude oil, a naturally occurring liquid that can be distilled or refined to make fuels, lubricating oils, asphalt, and other valuable products. It is composed of many molecules of different sizes. The word petroleum comes from the Latin petra, meaning "rock," and oleum, meaning "oil." Used in a broad sense, petroleum also refers to natural gas and solid asphalt, or tar.

Crude oil is a valuable raw material that is used in making a great variety of products. About 70 percent of the energy consumed in the United States comes from crude oil and natural gas. Crude oil is refined into fuels, including gasoline, kerosene, jet fuel, diesel fuel, furnace oil, and fuel oil. It is also the source of greases, waxes, and coke. Crude oil and natural gas are used to make feedstocks—chemicals that are the basis of hundreds of petrochemical products, including paints, plastics, synthetic rubbers and fibers, fertilizers, drugs, and explosives. (See also Explosive; Fertilizer; Paint and Varnish; Petrochemicals; Plastics; Rubber, Natural and Synthetic.)

Petroleum is considered a nonrenewable resource. Unlike trees, it cannot be replaced once it is removed from the ground. Crude oil forms very slowly by natural processes deep in subsurface rocks and has never been formed in commercial laboratories. It exists only in natural underground reservoirs. There is a limited amount of crude oil in existence.

Characteristics
There are many different varieties of crude oil, ranging from very fluid, volatile liquids to viscous, semisolid materials. Crude oil is usually either black or green, but it can also be light yellow or transparent. Crude oils vary considerably in density and are described as heavy, average, or light. The densities of different crude oils are usually compared using the degrees-API-gravity scale, devised by the American Petroleum Institute (API). Oils with 10° API gravity or less are heavier than water. Heavy oils have 5° to 20° API gravities. Average crude oils have 20° to 25° API gravities. Light oils have 25° to 55° API gravities; they are very fluid and can be produced from subsurface reservoirs faster and in greater quantities than can the heavy oils. Light oils are more valuable because they contain more gasoline—the most valuable product refined from petroleum.

Crude oil and natural gas are called hydrocarbons because they are composed of compounds made up almost entirely of carbon and hydrogen, along with some minor impurities—sulfur, nitrogen, and oxygen. The main difference between crude oil and natural gas is the size of the hydrocarbon molecules. Those in natural gas have one to four atoms of carbon each and, at the Earth's surface, they exist as a gas. Crude oil is composed of many different hydrocarbon molecules, each with from five to more than 60 carbon atoms. The molecules can be in the form of straight chains, circles, or chains with side chains. These molecules are liquid under surface conditions.

Liquid hydrocarbons exist in deep reservoirs, where it is very hot. At these high temperatures, some hydrocarbons that are normally liquid occur as gas and natural-gas mixtures. When the natural gas is brought to the surface, the gas cools and the liquid hydrocarbon molecules condense, forming a liquid called condensate. It is very fluid, lightweight, and transparent with a bluish or yellowish tint. Condensate is often called natural gasoline.

Sulfur exists to varying extents as an impurity in some crude oils. When a crude oil contains less than 1 percent sulfur, it is called a sweet crude. When it contains more than 1 percent sulfur, it is a sour crude. Sulfur is removed from sour crudes before refining. Because it requires this extra processing, sour crude is worth less than sweet crude.

Crude oils are often classified according to their content. There are three types: asphalt-base crudes, paraffin-base crudes, and mixed-base crudes. Asphalt-base crude oils are usually black in color. Once refined, they produce more high-quality gasoline and asphalt than do the other crudes. Paraffin-base crude oils are usually greenish in color. Once refined, they produce more paraffin wax and high-quality motor-lubricating oils. Mixed-base crude oils are a combination of the other two types of crude oil.

Crude-oil volume is usually measured in barrels. One barrel holds 42 gallons (159 liters). Volume is also measured in tons. A barrel of average crude oil weighs 0.150 ton.

PETROLEUM RESERVES, PRODUCTION, AND CONSUMPTION
The term oil reserves refers to the estimated amount of crude oil that is expected to be produced in the future from wells in known oil fields. The total amount of recoverable oil already found in the world's conventional oil fields amounts to about 1 trillion barrels. By 1990 about 43.4 percent of this oil had been produced and consumed. The United States produces about 13 percent of the world's oil; the Persian Gulf region, 27 percent; Western Europe, 6 percent; Venezuela, 4 percent; and Canada, Mexico, Libya, Nigeria, and Indonesia together produce about 15 percent. In the early 1990s more than 24 billion barrels of oil were produced annually. The annual world consumption of refined petroleum was more than 23 billion barrels. (For a comparison of primary energy consumption in various countries, see Fuel.)

Because the United States consumes more crude oil than it produces, it must import crude oil. In the early 1990s the United States imported about 50 percent of its petroleum. The world's oil reserves consisted of about 137 billion tons. The largest reserves were in Saudi Arabia, Iraq, Kuwait, Iran, Abu Dhabi, Venezuela, Russia, Azerbaijan, Africa, Mexico, the United States, China, Libya, Nigeria, Indonesia, Algeria, Norway, Canada, India, Oman, Qatar, and Yemen. About 67 percent of the world's reserves was in the Middle Eastern countries of Saudi Arabia, Kuwait, Iran, Iraq, and United Arab Emirates.

Petroleum Formation in Sedimentary Rock
In order for a substantial natural-gas or oil deposit to form, three geologic conditions must be met. First, somewhere in the subsurface there must be a source rock that will generate the gas and oil. Second, in that same general area, there must be a reservoir rock to hold the natural gas and oil. Finally, there must be a trap in the underground reservoir rock to concentrate the gas and oil into commercially useful quantities.

Most petroleum is found in sedimentary-rock basins. In these basins the sedimentary rocks are 10,000 to 50,000 feet (3,000 to 15,000 meters) thick. There are about 700 sedimentary-rock basins worldwide. About half of these have been at least partially explored and drilled. (See also Earth; Geology; Rock.)

Source rocks. Both crude oil and natural gas are formed from ancient dead plant and animal material that lies buried in layers of sedimentary rock. Black shales—the most common source rocks—were formed from very fine-grained muds. A minimum temperature of 120° F (49° C) is necessary to start the process of natural generation of crude oil. Temperatures increase with depth. Oil is generated at temperatures between 120° F (49° C) and 350° F (177° C), which occur at depths between about 5,000 feet (1,500 meters) and 21,000 feet (6,400 meters). The area in the crust of the Earth where oil is generated from the source rock is called the oil window. Heavy oils are generated at the lower temperatures found in shallower parts of the oil window, whereas lighter oils are generated at the higher temperatures found in the deeper levels.

The generation of crude oil from organic matter in a source rock requires at least several million years. When the temperatures at depths below the oil window exceed 350° F (177° C), natural gas is formed. Any crude oil buried deeper is almost instantaneously changed into natural gas and graphite.

Reservoir rocks. Large amounts of water exist in subsurface rocks. Once crude oil and natural gas have formed, they rise to the surface because they weigh less than the water. Oil and gas flow slowly through the natural fractures in the subsurface rocks. As they rise, they sometimes hit a layer of reservoir rock—a sedimentary rock that contains tiny spaces called pore spaces. Sandstone and limestone are common reservoir rocks. The percent volume of pore spaces in a rock is called porosity. Oil and gas flow from pore space to pore space up the angle of the reservoir rock layer toward the surface. Permeability is a term used to describe how easily a fluid flows through a rock.

Traps. A trap is a high area in the reservoir rock. It is as far toward the surface as the gas and oil can flow. Traps can be either structural or stratigraphic. A structural trap is formed by deformation of the reservoir rock. A stratigraphic trap is formed during the deposition of the reservoir rock. Common types of structural traps are anticlines (long arches) and domes (circular uplifts). A structural trap can also be formed by a fault, which is a break in rocks where the rocks on opposite sides of the break have moved relative to each other. Stratigraphic traps can be formed by limestone reefs, river-channel sandstones, or reservoir rock that was deposited so that it is pinched or wedged in an upward direction.

Once in the trap, the gas, oil, and water separate according to density. The gas rises to the top of the trap and forms the so-called free-gas cap, where the pores of the reservoir rock are filled with natural gas. The oil settles below the gas, and the water, which is heaviest, settles at the bottom. The oil layer contains large amounts of dissolved natural gas. The boundary between the free-gas cap and the oil is called the gas-oil contact. The boundary between the oil and the water is called the oil-water contact.

A sedimentary rock layer, called cap rock, sits above the reservoir rock in the trap. The cap rock acts as a seal and does not allow oil or gas to escape upward from the trap. Salt and shale are common cap rocks.

Other Sources of Petroleum
Crude oil can also be obtained from oil shales and tar sands. Shale is a common rock in the Earth's crust. It originates from fine-grained mud deposits on the bottom of ancient oceans and lakes. Oil shales are rich in ancient organic matter. Subsurface temperatures have changed the organic matter in oil shale into kerogen, a material intermediate between organic matter and oil. When oil shale is heated to 662° F (350° C), the kerogen forms oil. Heated oil shales commonly yield 25 to 30 gallons (95 to 114 liters) of oil per ton of shale. Oil-shale deposits are very large. They are thought to be capable of producing more crude oil than can conventional oil reservoirs. They are very expensive to mine and heat, however, and disposing of the shale after the oil has been removed poses environmental problems.

Tar sands are a mixture of sand and thick, viscous heavy oils. In some tar-sand deposits, the heavy oil is removed by steam injection. Steam is pumped down wells and into the tar sands to make the heavy oil less viscous so that it will flow. Steam is also used to push the heated heavy oil through the sands toward producing wells.

PETROLEUM EXPLORATION
Petroleum became a major commercial source of fuel in the mid-1850s. The first oil wells were drilled on natural oil seeps—areas where oil leaks up to the ground surface. Oil and gas seeps are common in all petroleum-bearing areas. They occur when small fractures form in the cap rock above the subsurface trap, which is filled with gas and oil. Small amounts of gas and oil escape to the surface to form natural gas and oil seeps directly above the leaky trap.

Development of Petroleum Exploration
Until the early 1900s, petroleum was thought to flow in subsurface rivers. Drilling was concentrated on the surface on oil and gas seeps. Some drillers thought that petroleum occurred under hills; others thought it occurred under creeks. In the early 1900s drillers finally realized that petroleum occurred in the tiny pore spaces in subsurface reservoir rocks and was concentrated in traps within the reservoir rocks. According to the so-called anticlinal theory, oil accumulates in large, upward folds formed in reservoir rocks. Anticlines are often identified by the pattern of rock layers cropping out onto the surface of the ground. In the early 1900s oil and gas were found by mapping and drilling anticlines and domes identified at the ground's surface.

Types of Exploration
Geologists are employed to explore for crude oil and natural gas and to help develop reservoirs. They map rock layers cropping out on the surface of the ground in order to locate anticlines and domes. They use mapping tools similar to those used by surveyors. A special hand-held compass is often used to determine the orientation of the rock layers. For more detailed work, a plane table with a telescopic alidade and a stadia rod are used. The plane table is a drawing board that is kept level. The telescopic alidade, positioned on the plane table, is used to sight the stadia rod, a vertical pole that is placed on rock outcrops. With these instruments geologists can record on a map the position and size of the rock outcrops.

Aerial photographs and satellite pictures of the ground's surface are also used for mapmaking. Geologists also use data from drilled oil wells to make subsurface maps of the reservoir rocks. Matching up rock layers between wells allows geologists to draw cross sections in order to find petroleum traps.

Geophysicists use three methods of oil exploration: magnetic, gravity, and seismic exploration. In magnetic exploration a magnetometer is used to determine the strength of the Earth's magnetic field at a specific point on the Earth's surface. In gravity exploration a gravity meter, or gravimeter, is used to determine the strength of the Earth's gravity at a location. The magnetometer and gravity meter are used to locate hidden, subsurface petroleum traps.

In seismic exploration sound is transmitted into the ground by an explosive, such as dynamite, or by a thumper truck. As the sound passes into the subsurface, it is reflected off subsurface rock layers and returns to the surface as echoes. The echoes are detected and recorded at the surface with microphones called geophones, or jugs. The recordings are processed to form a picture of subsurface rock layers. Seismic exploration also works well in the ocean. Computers are used to enhance the subsurface picture formed from sound waves.

DRILLING FOR OIL
Before a well may be drilled on private land in the United States or Canada, the land must be leased from the landowner who owns the subsurface oil and gas. In the United States, coastal states own the ocean bottom from the beach out to a limit of 3 nautical miles (5.6 kilometers). The states of Florida and Texas own the ocean bottom out to a limit of 9 nautical miles (16.7 kilometers). Beyond that the federal government owns the ocean bottom out to a water depth of 8,000 feet (2,438 meters). This area is called the outer continental shelf, and parts of it are often leased by the government. Permits must be obtained from various government agencies before a well can be drilled. One of the purposes of the permits is to ensure that the drilling company restores the land after the well is drilled and that it properly plugs and abandons nonproductive wells.

In countries other than the United States and Canada, oil and gas belong to the national government. In these areas oil companies have to negotiate a concession—an area of land that can be explored and drilled during a certain period of time.

The Drilling Rig
Before the well is drilled, the site must be accurately surveyed or staked. A bulldozer then builds an access road to the drilling site and levels off the area for the drilling rig. Pits are dug to hold the drilling mud.

Modern oil wells are drilled with rotary rigs. As the steel drill pipe on the rig is rotated, the bit at the end of the pipe rotates and drills the well. A rotary rig can drill with great precision, either straight down or at a predetermined angle. Deviation drilling is used to create a so-called crooked, or slant, hole. It is often used on wells drilled from offshore platforms. There are four major systems on a rotary rig: the engines, the hoisting system, the rotary system, and the circulating, or mud, system.

Engines. The engines on a modern rotary rig are either diesel or electric. They supply the power for both the hoisting and rotary systems.

Hoisting system. The hoisting system includes a steel tower called the derrick, or mast. On the drilling floor of the rig is a large reel of braided steel cable. This cable is let out or taken up by a device called the draw works, which is powered by the engines. The cable passes around the crown block—a series of pulleys at the top of the derrick. It then goes around the traveling block (another pulley system that hangs in the derrick) and back up to the crown block. When the drilling cable is wound onto the draw works, the traveling block rises in the center of the derrick, raising any equipment attached to it. When the drilling cable is let off the draw works, the traveling block is lowered in the derrick. A hook on the bottom of the traveling block is attached to the rotary system.

Rotary system. The well is drilled with the rotary system. The drill string is made by screwing sections of steel drill pipe together. It is connected to the hoisting system by the swivel and hook above it. At the top of the drill string is the kelly, which fits into the bushing on the rotary table. The engines turn the rotary table, which turns the bushing and the kelly. The kelly turns the drill string in the well. On the bottom of the drill string is a drill bit. The most common drill bit used is the tri-cone drill bit, made of three steel cones, each with hard teeth. The rotating drill string turns the tri-cone drill bit at the bottom of the well. As the cones rotate, the teeth chip away at the rocks, producing flakes called well cuttings. As the well is drilled deeper, 30-foot (9-meter) sections of steel drill pipe, each called a joint, are added to the top of the drill string below the kelly.

Circulating system. The purpose of the circulating system is to pump drilling mud down the well as the well is being drilled. Drilling mud is a mixture of water or diesel oil, clay, and chemicals. It is stored in steel mud pits next to the drilling rig. Pumps force the drilling mud down the center of the hollow, rotating drill string. The drilling mud squirts out at the bottom of the well onto the drill bit. There it picks up the well cuttings from the bottom of the well and returns to the surface in the space between the rotating drill string and the rock walls of the well. At the surface, the well cuttings are removed from the drilling mud and the drilling mud is returned to the mud pits. The drilling mud lubricates and cools the drill bit. By keeping the well filled with circulating drilling mud, subsurface water, gas, or oil are prevented from flowing out of the subsurface rocks and into the well. Such an event could cause the sides of the well to cave in. If oil or gas were to flow onto the rig floor, it could catch fire and destroy the rig. Consequently, as long as the well is being drilled, the mud is circulated.

Other equipment. A series of rams, called blowout preventers, are located below the drilling floor and on top of the well. They are designed to close the well in case of a blowout, when subsurface gas or oil flows into and up the well. The oil gushers seen in old photographs and movies were usually on early cable- tool rigs that lacked an efficient blowout-preventer system and did not use circulating drilling mud.

The most serious problem encountered in drilling a well is when abnormally high pressures—usually caused by a pocket of high-pressure gas—are encountered in the subsurface. When this happens, the gas flows into and up the well. If the blowout preventers are activated in time, heavier drilling mud can be pumped down the well to force the gas back into the subsurface rocks. If the blowout preventers are not activated in time, a blowout occurs that can destroy the rig.

Turbine drills are currently being tested. In these drills, the bit is turned by drilling mud that jets out of the bit at the bottom of the well. These drills promise to be more efficient than the normal rotary drilling process. Researchers are also testing synthetic diamond bits, which drill faster and last longer than normal tri-cone drill bits.

Drilling the Well
A well that is drilled in a search for a new oil or gas deposit is called a wildcat, or exploratory, well. A well that is drilled in a known oil or gas field is called a developmental well. If a well does not lead to the discovery of oil or gas, it is called a duster, or a dry hole. Dusters are plugged and abandoned. About 30 percent of exploratory wells and 80 percent of developmental wells are successful.

Exploratory wells in the ocean are drilled from jack-up rigs, semisubmersibles, and drill ships. A jack-up rig is used in water about 250 feet (75 meters) deep or less. It has a platform with three tall legs. It is towed out to the drill site, and the legs are let down to the ocean floor to support the platform above the level of the ocean. Once the well is drilled, the platform is jacked down and moved to another site. Semisubmersibles and drill ships are used in deeper waters. A semisubmersible is a floating platform that is held above the drill site by anchors. A drill ship is a ship with a drilling rig mounted on it. It drills through a hole in the bottom of the ship. With the use of an onboard computer, the ship's propellers and thrusters keep the ship directly above the drill site.

After an offshore oil or gas field has been discovered, a production platform is erected for the producing wells. The production platform has legs that go down to the sea bottom. Numerous wells are located on a platform. They are drilled using the deviation-drilling process so that the wells on one platform can drain an entire offshore oil field. The legs of some producing platforms are more than 1,000 feet (300 meters) long. For future deep-water oil fields, floating production platforms that are anchored above the oil field are being constructed.

Completing the Well
After the well is drilled, it is tested for gas and oil by lowering a probe down the well on a wire line. As this probe is brought up the well, it remotely senses the electric, acoustic, and radioactive properties of the well's rock layers and their fluids and records these properties on a well log. A geologist examines the well cuttings and the well log to determine if promising oil- and gas-bearing zones exist in the well. These zones are sampled by using a drill-stem test to determine how much gas and oil they contain.

A well is completed when a length of pipe, called the casing, is cemented to the well's sides. This stabilizes the well and prevents water from flowing into it. The casing is then perforated at the level of the oil or gas zone by shooting explosives through the casing, cement, and rock. Only fluids from the producing zone can flow through the perforations and into the well. It is uncommon for the oil to flow to the surface spontaneously. If it does, a series of valves, gauges, and chokes, called a Christmas tree, is installed on the surface of the well to control the flow.

Pumping Oil to the Surface
In most oil wells, the oil has to be pumped from the bottom of the well to the surface. This is done by lowering a down-hole pump on the bottom of a long string of small pipe, called tubing, to the bottom of the well. The down-hole pump is activated by a pumper, or pump jack, on the surface. The pumper is powered by a diesel or an electric engine that is attached to one end of a walking beam. The walking beam is a long steel beam mounted on a center pivot. The engine causes the ends of the walking beam to rise and fall. At the end of the walking beam opposite the engine is a sucker-rod string of solid pipe that goes down the well through the tubing to the down-hole pump. The rising and falling of the sucker-rod string activates the pump. The oil is pumped up the tubing to the surface, where it is put into a vertical or horizontal metal tank that separates the oil from the natural gas and water that usually comes up with the oil. The oil is then stored in stock tanks. The deepest producing well—slightly more than 24,000 feet (6,400 meters) deep—is in Oklahoma. The deepest well is the dry #1 Bertha Rodgers Well, drilled in the Anadarko Basin of Oklahoma. It is 31,440 feet (9,583 meters) deep.

Primary Oil Recovery
Oil flows from the subsurface reservoir rocks and into wells because of the pressure on it, called reservoir drive. There are four different types of natural reservoir drives, and every oil field has at least one of them. In a dissolved-gas-drive oil field, the pressure is caused by gas bubbles that form within the oil in the pores of the reservoir rock. As a well drilled into the deep subsurface reservoir relieves the pressure on the oil, this gas bubbles out of the oil. The expanding gas bubbles force the oil into the well. A dissolved-gas-drive oil field is very inefficient and will produce an average of only 25 percent of the oil contained in the reservoir before the pressure is depleted.

In a free-gas-cap-drive oil field, the oil is pressurized because of the natural gas in the overlying free-gas cap. This type of reservoir drive is more efficient than a dissolved-gas-drive and will produce an average of 35 percent of the oil. The most efficient type of reservoir drive is a water-drive oil field, in which the oil is forced up by high-pressure water located below the oil. The water sweeps the oil through the pores of the reservoir rock and produces an average of 60 percent of the oil in the reservoir. The least efficient reservoir drive is a gravity drive, in which the oil is drained by gravity into the well. The oil produced by a natural reservoir drive is called primary-production oil.

At the surface of the oil well, natural gas, called solution gas, bubbles out of the oil, usually with some salt water, called oil-field brine. The gas and brine are separated from the oil in a separator. Since oil-field brine is a potential pollutant, it is usually pumped down an injection or disposal well into a subsurface reservoir rock. If possible, it is injected just below the oil-water contact into the same reservoir rock that produces the oil. This maintains pressure on the water below the oil in the reservoir and helps produce more oil from that reservoir.

The solution gas is either sold to a pipeline or used in a pressure-maintenance system. In a pressure-maintenance system, the solution gas is injected back into the free-gas cap of the oil-producing reservoir rock. This maintains the pressure on the gas in the free-gas cap above the oil and helps in the production of more oil.

Waterflooding and Enhanced Oil Recovery
During primary production the average oil field produces 30 percent of the oil in the reservoir by the natural reservoir drive. Other engineering techniques may be used to recover some of the remaining oil. A waterflood is often tried first. During waterflooding, new injection wells are drilled into the depleted oil reservoir or old wells are converted into injection wells. Water is pumped down the injection wells and pushes some of the remaining oil through the pores of the reservoir rock toward producing wells. A waterflood can produce up to 25 percent of the original oil in the reservoir but still leaves at least 50 percent of the oil in the subsurface reservoir.

Enhanced oil-recovery methods are often tried after the waterflood. These include gas injection, chemical flood, steam flood, or fire flood. During inert-gas injection, carbon dioxide or nitrogen gas is pumped down injection wells to move some of the remaining oil in the reservoir toward producing wells. In a chemical flood, chemicals similar to detergents are pumped down injection wells to wash some of the remaining oil from the pores of the reservoir rock. A steam flood is used when the oil in the subsurface reservoir is too viscous to flow though the pores of the reservoir rock. Steam is pumped down injection wells to heat the heavy oil in order to make it more fluid. The steam also pushes the oil through the subsurface reservoir toward producing wells. In a fire flood some of the subsurface oil remaining in the reservoir is set afire. Air must then be pumped down injection wells to keep the subsurface fire going. The fire heats the oil in the reservoir, making it more fluid. The gases generated by the fire push the heated, fluid oil toward producing wells.

Waterfloods and enhanced oil-recovery projects are very expensive and are not profitable when the price of oil is low. Wells that produce just enough oil to make a profit over operating costs are called stripper wells. When the well becomes unprofitable, it is abandoned. The well is plugged by pouring cement down it and putting a cap on the top.

TRANSPORTATION AND DISTRIBUTION OF OIL
The most efficient way to transport crude oil across land is through pipelines (see Pipeline). These range from 2 to 48 inches (5 to 122 centimeters) in diameter. A gathering system of small-diameter pipes collects oil from various wells in an oil field and leads into a large-diameter trunk pipeline. The trunk pipeline then leads to a refinery or to a transportation center such as a port. The oil is kept moving by pumping stations that are placed along the pipeline. To prevent corrosion, the pipeline is buried in the subsurface and is coated with paint, chemicals, tar, and glass fiber.

The trans-Alaskan pipeline was the most expensive pipeline in the world—costing 9 billion dollars to build. It is 48 inches (122 centimeters) in diameter and 800 miles (1,300 kilometers) long. Oil moves southward through it, at a rate of 1.5 million barrels each day, from the giant Prudhoe Bay oil field on the northern coast of Alaska to the ice-free port of Valdez, where the oil is shipped by tankers to refineries on the West coast of the United States. The pipeline traverses three mountain ranges and 250 rivers and streams. For some 400 miles (640 kilometers) it is suspended on pylons above permanently frozen ground called permafrost. If the pipeline had been buried in the ground, the heat from the oil in the pipeline would have melted the permafrost, causing considerable environmental damage.

The most efficient way to transport petroleum long distances across the ocean is by tanker. The current supertankers include Very Large Crude Carriers and Ultra Large Crude Carriers. Because, when full, some of the large supertankers can dock only in deepwater ports, they are often lightened by transferring the petroleum in small batches to smaller tankers, which then bring it into port. On rivers, barges are often used to transport petroleum. (See also Ship and Shipping.)

Railroad tank cars were once commonly used to move petroleum from wells to refineries. This system became increasingly costly, and after World War II, rail transport was gradually replaced by pipeline transport. Rail tank cars are still commonly used, along with pipelines, to transport refinery products.

Many small oil fields are not connected to a pipeline that leads to a refinery. Instead, tank trucks periodically pick up the petroleum stored in stock tanks near the well and transport it to a pipeline. Tank trucks are also commonly used to distribute refinery products.

REFINING PETROLEUM

Petroleum is refined to produce gasoline ...
In a refinery, crude oil is separated into useful products such as gasoline, kerosene, diesel fuel, home heating fuel, lubricating oils, and asphalt. Because refineries use water for cooling, they are built along water sources, such as rivers.

Separating Crude Oil into Its Parts

The inside of a fractional-distillation column ...
Oil refining begins with two basic processes: vaporization and condensation. Crude oil is composed of many different liquids, called fractions. Each fraction boils into a vapor and condenses back into a liquid at a different temperature.

At the refinery the crude oil is pumped into a pipe still or furnace, where it is heated to 750° F (399° C). The fuel used in the refinery are the fuel oil and gases generated by the refining processes. If the crude contains salt, it is desalted before entering the furnace. There much of the oil boils off as a vapor. The heated vapor and liquid are then put in a distilling column or bubble tower—a vertical metal cylinder. The vapor rises, and the liquid falls to the bottom of the distilling column. The distilling column contains trays arranged on top of each other. The rising vapors bubble up through holes in the trays. Bubble caps above the holes force the vapors to bubble through the liquid that has already condensed on the tray. As the rising vapors cool, they condense on the trays.

The farther the vapors rise up the distilling column, the cooler they become as they bubble through the liquids on each tray. The cooling vapors condense on the trays until the trays overflow. The liquid is drawn off at the sides of the distilling column. Each batch of liquid removed from the distilling column is called a cut, or fraction. The cuts that come off at high temperatures are called heavy cuts, or heavy fractions; those at lower temperatures are called light cuts, or light fractions. The straight-run cuts or fractions that come off the distillation column are, in order of decreasing cut points, or temperatures: heavy gas oil, light gas oil, kerosene, naphtha, and gasoline. The gas that comes off the very top of the distilling tower is natural gas. The liquid that drops to the bottom of the distilling column is called residuum.

Thermal Cracking and Catalytic Cracking

The main object of catalytic, or ...
After they are removed from the distillation column, the cuts are purified and separated. A major problem with the distillation process is that it produces too little gasoline. Because, of the various petroleum products, the greatest demand is for gasoline, other processes were developed to produce gasoline from the more abundant, less valuable cuts. For example, heavy fuel oil can be converted into high-octane gasoline by dividing, or "cracking," the fuel oil's large, heavy molecules into small, light ones. This can be done by either the thermal or the catalytic cracking method. Thermal cracking was developed first. It uses high temperatures and pressures. Catalytic cracking is more efficient and is now more widely used. Most of the towers seen in a refinery are cracking towers. Catalytic, or cat, cracking uses high temperatures, moderate pressures, and catalysts—chemicals that speed up the chemical reactions.

In catalytic cracking, the cracking stocks and fresh catalyst are pumped into a reaction chamber, where the cracking occurs. Coke accumulates on the catalyst, causing the catalyst to become spent, or inactive. To remove the coke, the spent catalyst is pumped to a regenerator, where it is mixed with heated air. The regenerated catalyst is then ready to be mixed with more cracking stock. Meanwhile the cracked products leave the reaction chamber and are separated in a fractionator. The fractionator bottoms, called cycle oil, can be run through the reaction process again.

Vacuum Flashing, Coking, and Hydrocracking
If the straight-run residuum is heated and vaporized, it decomposes into carbon and hydrogen. However, if it is put into a vacuum flasher with very low pressure, some of it boils off and vaporizes at a much lower temperature. This vapor is called flasher tops. Some of the flasher tops, along with the heavy gas oil cut, are used to make lubricating oils. The rest of the flasher tops is put into a cat cracker with heavy gas oil. The liquid remaining in the flasher is called flasher bottoms, or the bottom of the barrel. Some of the flasher bottoms is used as asphalt to be used for road paving and roofing, and some is burned as fuel. The rest goes through either a coker or a thermal cracker. In a coker the bottoms are heated and exposed to very high, and then very low, pressures. This process changes the flasher bottoms into natural gas, gasoline, naphtha, gas oil, residual fuel, and bottoms. Various waxes often form in the flasher bottoms and heavy gas oil. They are removed with solvents and sold as a by-product of the refinery.

Hydrocracking is cat cracking in the presence of hydrogen. It is often used to increase the yield of gasoline blending components. It is also used to produce light distillates (jet fuel and diesel fuel) from heavy gas oil. Hydrocracking light gas oil produces naphtha, light gas oil, heavy gas oil, and kerosene.

Other Conversion Processes
The naphtha produced from hydrocracking is drawn into a catalytic reformer, where it is cooled and separated into gases and reformate. The reformate is used to make aviation fuel and to blend with straight-run gasoline to make high-octane gasoline. The gases produced by many of the refinery processes are sent to a gas plant, where they are separated. Propane gas from the gas plant is used to make liquefied petroleum gas (LPG), which is sold as a fuel. Butane gas is used as an additive in gasoline. Ethane is used as a petrochemical feedstock. Most of the rest of the gas is sent to an alkylation plant. Here the gases are chilled and treated with acid in a reactor to separate them into butane, alkylate, and a fuel gas. The alkylate is then used to make high-octane gasoline.

The finished gasoline is made in a gasoline plant. The straight-run gasoline, cat-cracked gasoline, gasoline from thermal cracking and hydrocracking, reformate, butane, alkylate, and other chemicals are blended to make various types of gasoline.

Before many of the cuts or their products can go to market, they must be purified. For example, straight-run kerosene is usually hydrotreated to remove sulfur. Other purification processes include desalting— the removal of salt—and dehydration—the removal of water. During solvent extraction, solvents are used to remove impurities from the gasoline. Some of the refinery products are used as petrochemical feedstocks.

THE ROLE OF PETROLEUM IN SOCIETY
Oil companies are classified as either integrated or independent oil companies. Integrated oil companies are engaged in all aspects of petroleum, including exploration, production, transportation, refining, and marketing. These are the large oil companies whose names are on service stations. They are corporations with hundreds of thousands of shareholders, who are the owners. The integrated companies are divided into many departments and divisions. The independent oil companies are involved in only certain aspects of the petroleum industry, such as exploration and production, and tend to be smaller in size. They can be either corporations or privately owned.

Crude-Oil Prices
There are two types of crude-oil prices: the posted price and the spot price. The posted price is the amount that an oil company or refinery will pay for a barrel of oil from a certain field or area. In the early days the price offered by a company was often posted on signs in the oil fields. The posted price changes periodically and is rounded off to the nearest ten cents—for example, $18.50 per barrel. The cost per barrel specified by the Organization of Petroleum Exporting Countries (OPEC) is similar to a posted price. The spot price is continuously changing with market supply and demand and can rise and fall rapidly. It refers to a standard crude oil, called a benchmark crude, such as West Texas Intermediate and Saudi Arabian Light. The spot price is stated to the cent—for example, $18.17 per barrel.

Service Stations
There are three different types of operators of service stations: operators of company-owned stations, independent marketers, and operators of businesses in which gasoline is just a part of the business. An operator usually becomes the owner of a service station by buying the original inventory, paying a rent that is based on the amount of gasoline sold, and agreeing to maintain the station to the company's standards. Company-owned stations have a recognizable brand name of one of the major integrated oil companies.

Petroleum products are delivered from the company's refinery by tank trucks. Gas stations often offer services that include automotive repairs. Independent marketers own their own stations and buy their petroleum products from a jobber, or wholesaler.

Petroleum Conservation
In the United States, oil and gas production is regulated by state governments. Before 1930 there were few regulations in the United States concerning how fast oil and gas could be produced from a field. Oil and gas ownership was based on the rule of capture: any oil and gas pumped up a well belonged to the operator of the well, even if they had been drained from beneath another owner's land. Because of this rule, many oil wells were drilled close together—a wasteful practice because rapid oil production often resulted in damaged reservoirs. Furthermore, the natural gas produced with the oil was often flared off to get rid of it. The natural gas, however, was part of the reservoir drive that supplied the energy to produce the oil.

With the discovery of the giant East Texas and Oklahoma City oil fields in 1930, unrestricted production resulted in an oil glut in the United States, causing the price of oil to fall. The states then took control of oil and gas production. They voided the rule of capture. Today, all oil-producing states and countries have agencies that control the oil industry. These agencies decide the distance between oil wells and the rate at which the gas and oil should be produced. Flaring of natural gas is now prohibited almost everywhere. The gas must either be removed from the site through a pipeline or be reinjected into the oil reservoir as part of a pressure-maintenance program. Oil companies that operate wells on an oil field often pool their resources and install a pressure-maintenance, waterflood, or enhanced oil-recovery program in order to produce more oil from the field.

OPEC and the Oil Crisis
As countries without large oil reserves became more dependent on oil-producing countries, oil became a bargaining chip in the world of politics. In 1938 the Mexican government seized the holdings of foreign oil companies in that country. Many countries followed Mexico's example and began to nationalize and control their oil fields. This gave the governments of those countries control over much of the world's oil supply. In Mexico the government formed Petróleos Mexicanos (PEMEX), a national oil company. In 1960 Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela formed OPEC. They were joined later by Qatar, Indonesia, Libya, the United Arab Emirates, Nigeria, Algeria, Ecuador, and Gabon. In the first years of its existence, OPEC was a relatively weak organization. In the early 1970s, however, OPEC countries began renegotiating a greater share of oil revenue from the oil companies operating in their countries. During the Arab-Israeli War of 1973, the Arab members of OPEC cut off oil shipments to countries friendly to Israel and raised the price of Middle East crude from $3.01 to $11.28 per barrel. This created a gasoline shortage and greatly increased the cost of gasoline in many countries. (See also Organization of Petroleum Exporting Countries.)

In 1979 oil shipments from a major oil exporter—Iran—came to a halt. Oil prices rose from 17 to 34 dollars per barrel. Conservation measures in the United States and Europe decreased the demand for oil, and new oil from recently discovered oil fields in non-OPEC countries entered the market. In 1982 the price of oil started to fall. OPEC tried to stabilize the price by setting limits on the amount of oil produced by each member. Many OPEC members overlooked the quotas, however, and overproduced. An oil glut developed and in early 1986 oil prices fell rapidly from 29 to less than 10 dollars per barrel. In the United States the price collapse caused great financial hardship in states that depended on the taxes paid by oil companies. Drilling for oil became unprofitable. Oil companies drastically reduced the number of wells that were drilled and released many employees.

THE HISTORY OF PETROLEUM USE
Petroleum was an accessible resource for ancient people because natural oil seeps occurred on the ground's surface. Archaeologists have discovered 6,000-year-old mosaics set into asphalt that was obtained from ancient Sumer. Asphalt from natural oil seeps was used in Mesopotamia before 3000 BC for construction of roads and buildings and for waterproofing boats. Egyptian morticians used bitumen from oil to make headdresses for mummies, and they wrapped the mummies in asphalt-soaked linen.

Asphalt was also used to cement stones, to construct pyramids, and to waterproof cisterns and silos. Both the ancient Egyptians and the Chinese used petroleum as a medicine. Along the western coast of the Caspian Sea, in the Baku region of Azerbaijan, are found numerous natural gas and oil seeps, some of which have been burning for thousands of years. They are called the eternal fires of Persia.

Greek fire was developed by the Byzantines in the 7th century for the defense of Constantinople, now called Istanbul. It was a mixture of crude oil, pitch, charcoal, sulfur, phosphorus, and other chemicals. Used in naval battles, it was set afire and spread on the surface of the water.

Oil Use in the Americas
Spanish explorers discovered oil seeps in Latin America. The Indians of Central America and Mexico used asphalt to cement materials for building, and the Toltecs set mosaics in asphalt. The Seneca and Iroquois Indians of North America used petroleum for medicine and as paint for their bodies.

The Beginning of the Petroleum Industry
In the mid-1850s two things occurred to stimulate the petroleum industry: machines that required lubricating oils were developed, and oil lamps were used to light homes and offices. The whale oil used in lamps had become expensive. In 1849 the Scotsman James Young patented a process for converting coal into coal oil. A similar process was developed at the same time by the Canadian Abraham Gesner. He named his product kerosene, after the Greek words for "oil" and "wax." Coal oil and kerosene were less expensive than whale oil but smoked and had a disagreeable odor. In 1857 A.C. Ferris, a lamp maker, produced a clean-burning kerosene that did not have a bad smell.

The Pennsylvania Rock Oil Company obtained oil for making kerosene by skimming the oil off natural seeps. After the company went bankrupt, Edwin L. Drake leased its lands and formed the Seneca Oil Company. Drake drilled into an oil seep on Oil Creek, near Titusville, Pa., with a drilling rig used for brine wells. The well produced oil at the rate of 20 barrels per day. This marked the beginning of an oil boom. During the 1860s oil drilling expanded to West Virginia, New York, Ohio, Kansas, Kentucky, Tennessee, Colorado, and California. In 1870 John D. Rockefeller founded the Standard Oil Company, which soon gained a near monopoly on oil production (see Rockefeller Family). From 1859 to 1900 the main petroleum product was kerosene for lamps. Lubricants and some fuel oils also came into use.

Primitive rotary drilling rigs were introduced in the 1880s. In 1901, the first modern rotary rig was used at the Spindletop oil field, on a salt dome in Texas. The discovery well alone—a gusher—increased oil production in the United States by 50 percent and world production by 20 percent.

By the following year 400 wells had been drilled on Spindletop and more than 100 oil companies had been formed to drill, produce, refine, and market Spindletop oil. Petroleum had become a plentiful resource. During the next few years, new cars were fitted with gasoline-fueled, internal-combustion engines; locomotive and ship engines were converted from coal to oil; and the first airplanes were being flown. With the invention of the electric light bulb, kerosene lamps became obsolete. Gasoline became the most important product of crude oil, and kerosene became a minor product.

In 1860 a new refinery was built on Oil Creek. Barrels of oil were carried on horse-drawn carts or on railroad cars or were floated down the river to the refinery. The first oil pipeline was constructed in 1865 from the Pithole City oil field to the Oil Creek Railroad. The first long pipeline was built in Pennsylvania in 1879. It was one of the greatest engineering feats of its time. The first offshore wells were drilled from wooden piers at Sunnerland, near Santa Barbara, Calif., in 1896. In 1948 the first platform was used to drill an offshore well in Louisiana.

CAREER OPPORTUNITIES
The petroleum industry employs workers in more than 2,000 different occupations, from drilling the first wells to pumping gasoline at the corner service stations. Workers may be employed in a small company with a few employees or in an integrated oil company with tens of thousands of employees. People employed in the petroleum industry include highly trained research scientists in laboratories, pumping-well workers, and marketing directors.

Occupations in Scientific Fields
The people who begin the scientific search for oil are geologists and geophysicists (see Geology). Geologists search for oil by studying rocks. They may work in the field studying rock outcrops, or they may use microscopes and other instruments to examine well cuttings while wells are being drilled. Geotechnicians assist geologists by drafting geologic maps and cross sections and by running analytical instruments.

Geophysicists are usually trained in physics, geology, and mathematics. They are often employed to locate drill sites. Computer programmers and mathematicians help analyze the geophysical data collected by geophysicists. Jug hustlers are workers that help geophysicists adjust seismic equipment in the field. Surveyors, draftsmen, and laboratory assistants also help in exploration.

A petroleum engineer is trained in the technology of drilling and in well logging, well completion, and reservoir mechanics. Mechanical engineers are often used to design and operate the mechanical equipment on the drilling rig. Chemists develop new products and uses for crude oil. Chemical engineers then develop the equipment and processes for making the new products. Both chemical and mechanical engineers also help to keep the equipment working. Biologists often conduct environmental studies at proposed drilling and refinery sites.

Occupations in the Mechanical Trades
Nearly all the mechanical trades engaged in the handling of tools and materials are employed by the petroleum industry. Some of these jobs have become specialized. Pumpers are used to keep the wells and field equipment working. A tool pusher is in charge of the drilling rig. Drillers operate the drilling equipment and give orders to the crew, called roughnecks, who work on the rig floor.

Maintenance men such as electricians and machinists keep the machinery repaired and running. Pipeliners lay the pipelines that transport crude oil and refined-petroleum products.

Other Occupations
Service-station operators and owners are engaged in selling refinery products. Sales engineers are trained chemical or mechanical engineers that serve as consultants who are hired to solve customer problems. Landmen locate landowners and negotiate leases for drilling. Scouts are used in the field to gather information about wells that other oil companies are drilling. Roustabouts are the people who do the work on the leases. Jobbers and distributors distribute refinery products.

The petroleum industry is like any other large business. It uses accountants, managers, lawyers, computer programmers, secretaries, salespeople, and technicians. Specialists are also used from other fields. Divers install offshore platforms; pilots fly over pipelines in search of leaks; economists analyze the feasibility of projects; and public-relations personnel make announcements to the public.

-----------------------------------

Back to Top

-----------------------------------

 

CONVERSION FACTORS