Oil drilling

Oil or petroleum (also known as crude oil) is a fossil fuel found largely in vast underground deposits. Oil and its byproducts (natural gas, gasoline, kerosene, asphalt, and fuel oil, among others) did not have any real economic value until the middle of the nineteenth century when drilling was first used as a method to obtain it. Today, oil is produced on every continent but Antarctica. Despite increasingly sophisticated methods of locating possible deposits and improved removal techniques, oil is still obtained by drilling.

History

Oil was known in the ancient world and had several uses. Usually found bubbling up to Earth's surface at what are called oil seeps, oil was used primarily for lighting, as a lubricant, for caulking ships (making them watertight), and for jointing masonry (for building). The Chinese knew and used oil as far back as the fourth century B.C.

By the 1850s, crude oil was still obtained by skimming it off the tops of ponds. Since oil from whales was becoming scarce as the giant mammals were hunted almost to extinction, oil producers began to look elsewhere to extract oil. In 1859, while working for the Seneca Oil Company in Titusville, Pennsylvania, Edwin L. Drake and his crew drilled the first modern oil well. They struck oil almost 70 feet (21 meters) down. America's oil boom, and the world's oil industry, was launched.

Oil would be a minor industry for some time, since the only product of crude oil that was thought to be useful was kerosene. The remainder was simply thrown away. Fifty years later, with the invention of the internal-combustion engine and with greater knowledge of the varied applications of petroleum, the oil industry was born in earnest. This market soon became international in scope, and drilling for oil became very serious and sometimes very financially rewarding.

Drilling for oil

The method Edwin Drake used to drill oil wells is called cable-tool or percussion drilling. A hole is punched into the ground by a heavy cutting tool called a bit that is attached to a cable and pulley system. The cable hangs from the top of a four-legged framework tower called a derrick.

The cable raises and drops the drill bit over and over again, shattering the rock into small pieces or cuttings. Periodically, those cuttings

Deep drilling oil rig at Naval Petroleum Reserve's Elk Hills site near Bakersfield, California. (Reproduced courtesy of the

U.S. Department of Energy

.)

have to be wetted down and bailed out of the hole. By the late 1800s, steam engines had become available for cranking the drill bit up and down and for lowering other tools into the hole.

Although this method is sometimes still used for drilling shallow wells through hard rock, almost all present-day wells are bored by rotary drilling equipment, which works like a corkscrew or carpenter's drill. Rotary drilling originated during the early 1900s in Europe.

Rotary drilling process. In rotary drilling, a large, heavy bit is attached to a length of hollow drill pipe. As the well gets deeper, additional sections of pipe are connected at the top of the hole. The taller the derrick, the longer the sections of drill pipe that can be strung together. Although early derricks were made of wood, modern derricks are constructed of high-strength steel.

The whole length of pipe, or drillstring, is twisted by a rotating turntable that sits on the floor of the derrick. When the drill bit becomes worn, or when a different type of drill bit is needed, the whole drillstring must be pulled out of the hole to change the bit. Each piece of pipe is unscrewed and stacked on the derrick. When the oil-bearing formation is reached, the hole is lined with pipe called casing, and finally the well is completed or made ready for production with cementing material, tubing, and control valves.

Throughout the rotary drilling process, a stream of fluid called drilling mud is continuously forced to the bottom of the hole, through the bit, and back up to the surface. This special mud, which contains clay and chemicals mixed with water, lubricates the bit and keeps it from getting too hot. The drilling mud also carries rock cuttings up out of the hole, clearing the way for the bit and allowing the drilling crew's geologists to study the rock to learn more about the formations underground. The mud also helps prevent cave-ins by shoring up the sides of the hole.

Offshore drilling. Offshore drilling processes and equipment are essentially the same as those on land, except that special types of rigs are used depending on water depth. Jackup rigs, with legs attached to the ocean floor, are used in shallow water with depths to 200 feet (61 meters). In depths up to 4,000 feet (1,220 meters), drilling takes place on semisubmersible rigs that float on air-filled legs and are anchored to the bottom. Drillships with very precise navigational instruments are used in deep water with depths to 8,000 feet (2,440 meters). Once a promising area has been identified, a huge fixed platform is constructed that can support as many as 42 offshore wells, along with living quarters for the drilling crew.

Many advancements have been made in oil-drilling technology. The most advanced rotary cone rock bits presently available can drill about 80 percent faster than bits from the 1920s. At that time, well depths reached about 8,200 feet (2,500 meters). Today's drills can reach down more than 30,000 feet (9,150 meters).

[ See also Natural gas ; Oil spills ; Petroleum ]