8 minute read

Oil Well Drilling

Science EncyclopediaScience & Philosophy: Octadecanoate to Ovenbirds

Oil and natural gas has been found in geologic strata of Earth from the surface to depths exceeding 30,000 ft (9,144 m). Bogs and seeps in the ancient world were the initial source of oil and gas. As advancing economic systems and industries emerged with the development of nations and expanding populations, the need for plentiful and more efficient sources of energy were required. Hydrocarbon based fuels emerged as that more efficient energy source replacing wood, peat, and whale oil as primary sources of energy. As demand for energy increased it became necessary to gain access to deposits of oil and gas below those readily accessible on Earth's surface.

In the United States, one of the best-known oil seeps was at Titusville, Pennsylvania, and it was there that, in the eighteenth century, American entrepreneur, George H. Bissell, directed his attention. He hired a former railway conductor, Edwin L. Drake (1819–1880), to drill for oil, and on August 27, 1859, the oil well struck oil ("pay dirt" in the language of the drillers) at a depth of only 70 ft (21 m). The oil industry had begun. This market would prove to be international in scope and have seemingly limitless potential, thus drilling for oil became a very serious and sometimes very rewarding business.

Geologists and engineers had been drilling for water for quite some time and initially applied and adapted that technology in the early search for oil. The ancient Chinese practiced the simplest form of penetrating Earth by employing the "percussion method." The percussion or impact method penetrates the overlying earth by raising and dropping a heavy tool repeatedly in the same spot to break the dirt and rock enabling it to be removed or bailed out of the hole. The impact drilling technique was used to drill the first oil well in Pennsylvania, and it employed a chisel-like bit suspended from a cable to a lever on the surface. The up-and-down motion of the lever pounded the bit into the bottom of the hole and slowly chipped away pieces of rock. This was a slow process that had to be stopped periodically to remove the rock chips from the hole. For this method to work, the hole also had to be free of liquids, and it was this "dry" drilling that usually resulted in the "gushers" as often depicted in movies as a successful oil well strike. Before the advent of well control technology, gushers were a common and hazardous event.

Today, almost all oil wells are drilled by the rotary method. The rotary drilling method was first developed in Europe in the 1930s and soon replaced the percussion or cable-tool system. The method takes its name from the fact that a bit is rotated into the earth. The bit cuts or drills its way down. Rotary drilling equipment is complex, and a full course of study in its own right, but the essential components of a rotary drilling rig include: a rotary table, a bit, the drill string, a derrick, drawworks, mud handling system, prime movers and drill line.

A rotary table is a platform with a rotating device through which the drill string is passed into the hole that can be mechanically engaged with the drill string to cause it to rotate on bottom. Today, rotation is often provided by a "top drive" or "power swivel" mechanism fixed to the upper end of the drill string instead. A rotary bit may come in many configurations designed specifically for the type of rock it is to drill through but will typically have several roller cones with teeth of hardened metal and or industrial grade diamonds. When forced against and rotated on a rock surface, the bit shatters the rock into "cuttings."

The modern drill string is the pipe that is used to extend the drill bit into the borehole and may contain drill collars for added weight, sophisticated monitoring and surveillance equipment to provide real time information back to the surface and guidance systems.

The derrick is the frame structure from which the drill string in suspended on the drill line using a system of pulleys known as the "crown" and "traveling" blocks when working in the borehole. The drawworks or hoist is the key piece of equipment on the rig and is used to raise and lower the drill string and control the weight being applied by the drill bit on the rock face on bottom.

Rotary drilling may be performed under certain conditions with compressed air being circulated down the drill string and back to the surface through the annular space between the pipe and the rock wall. However, the more typical well requires a heavier fluid system to be circulated into and out of the hole to cool and lubricate the bit, flush rock cuttings out of the hole, to stabilize the borehole, control fluid loss into the penetrated rock and to contain formation pressure. This fluid is referred to as "mud." Originally that is exactly what it was in relatively simple shallow drilling operations—but very complex fluid systems are now required for the safe and efficient construction of most wells. The mud handling system is a series of tanks, pumps, valves, pipes and hoses that enables the mud to be pumped into the drill string, out the drill bit and circulated back to the surface in a controlled manner. The prime movers are the power source of the rig and may be diesel or natural gas fired engines, electric motors or a combination of engines and generators.

"Turbo-drilling" has proven to be an effective approach under special requirements governed by well depths and configurations. A turbo-drill is a mud driven turbine that is placed just above the drill bit in the drill string. The mud flowing through the turbine causes the bit to rotate without rotating the entire drill string back to the surface thus saving wear and tear on the borehole and the drill string itself. Most early wells drilled as relatively straight holes directly down below the surface location. Today a fair percentage of wells are directionally drilled to subsurface locations from a remote or common surface location for a variety of environmental and economic reasons. This is especially the case in offshore and marine environments where surface facilities are quite limited, restricted and extremely costly.

As a well is advanced toward its objective it becomes necessary to hold the hole open and to ensure the isolation of various substrata from one another. This is accomplished by lining the borehole with "casing" or pipe. The size and quality of the pipe and the number of strings to be run is determined by the target depth, anticipated producing characteristics of the well and geologic environment to be penetrated by the well. At shallow depths, surface pipe may be driven into the ground and cemented in place. One or more intermediate casing strings may be run to various depths as required by conditions within the borehole. Sometimes a liner or short casing string will be run to extend the hole and not run back to the surface. A producing string is often run into the target formation. The annular space between all casing strings is usually left fluid filled and many are cemented back to the surface or back to a sufficient level to ensure a fluid seal between the casing and the bore-hole and pressure integrity throughout the system. Obviously the design of a casing program is very complex. It must be made with the end state in mind as each string is run within the previous string; thus each string must be sized to accommodate anything that must be passed through it. Once in place, the string becomes a limiting factor as to what can be run into the hole should conditions change the objective or requirements of the well.

As casing strings are installed in the well, casing spools are installed on them at the surface to provide structural integrity and controlled access to the annular space between that string and the next smaller one. Each spool is bolted on top of the previous one. Upon completion of a successful well other equipment is run in to enable the well to produce well fluids in a controlled manner. This is yet another very complex system whose design will be tailored to the unique conditions of the individual well. Production tubing is commonly run from the completed interval back to the surface and is secured in place by a tubing hangar placed on top of the upper most casing spool. A series of valves are placed on top of the tubing hanger to control access to the interior of the production tubing. A block is then placed on top of the upper most valve to direct fluid flow either into a loop or at a desired angle depending on the requirements of the gathering system leading to the producing system. Additional valves are usually placed "downstream" of the block and a flow control device or choke system is installed to regulate fluid flow out of the well. This configuration of valves, blocks and control systems make up what is called the wellhead or "Christmas tree." Well-heads come in many configurations depending upon the nature of the well, its location and operational and maintenance needs. Once the wellhead is installed, the drilling and completion operation is complete.

While there are significant differences between what is required to drill a well on land and in a marine or offshore environment, the basic mission is very similar. The differences in location conditions, design criteria, logistical considerations and related cost are enormous. In deep-water, the process can approach the most sophisticated technical operations known to man and an individual well can cost in excess of sixty million dollars, often to be paid early in the project life well before surety of the results of the effort is known.

See also Hydrocarbon; Oil spills.



Howarth, S. A Century in Oil London: Wiedenfeld & Nicholson, 1997.

Jahn, F., M. Cook, and M. Graham. "Hydrocarbon Exploration and Production." Developments in Petroleum Science 46, The Netherlands: Elsevier Science, 2000.

Selley, R.C. Elements of Petroleum Geology. San Diego: Academic Press, 1998.


Hubbert, M. King. "Darcy's Law: its Physical Theory and Application to the Entrapment of Oil and Gas." History of Geophysics 3 (1987): 1 –27.

William Engle
K. Lee Lerner
Leonard C. Bruno

Additional topics