Oil Well Drilling
Oil Well Drilling
Oil and natural gas have been found from the surface to depths exceeding 30,000 ft (9,144 m) beneath Earth’s surface. 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 American 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 sometimes depicted in movies. Before the advent of well control technology, gushers were common and hazardous events.
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 studded with hard metal teeth rotates to pulverize the rock. Rotary drilling equipment is complicated, but the essential components of a rotary drilling rig include: a rotary table, a bit, the drill string, a derrick, draw works, mud handling system, prime movers, and drill line.
A rotary table is a platform through which the drill string is passed into the hole, and which can be mechanically engaged with the drill string to cause it to rotate. 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 or industrial grade diamonds. When forced against and rotated on a rock surface, the bit shatters the rock into small pieces known as cuttings.
The modern drill string is the pipe that is used to lower the drill bit into the borehole. It may contain thick steel drill collars for added weight, as well as sophisticated monitoring and surveillance equipment to provide real time information about drilling conditions.
The derrick is the frame structure from which the drill string in suspended using a system of pulleys known as the crown and traveling blocks. The draw works 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 that a heavier fluid be circulated into and out of the hole to cool and lubricate the bit, flush rock cuttings out of the hole, stabilize the walls of the borehole against collapse, control fluid loss into the penetrated rock, and to contain formation pressure. Drilling fluid is referred to as mud. Mixtures of water, high density minerals such as barite, and additives such as polymers are now use for the safe and efficient drilling of oil and gas wells. The mud handling system is a series of tanks, pumps, valves, pipes and hoses that enables the mud to be pumped down the inside of the drill string, out the drill bit, and circulated back to the surface in a controlled manner. The power source for the rig may consist of 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 conditions. 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 were drilled as relatively straight holes directly down below the surface location. Today, many wells are directionally drilled to subsurface locations from a single 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 and expensive. Directional drilling allows many wells to fan out from a single offshore drilling platform, or for wells to be drilled beneath buildings where it would be impossible to place a drilling rig.
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 a steel casing. The size and quality of the casing 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 borehole and pressure integrity throughout the system. The design of a casing program is very complicated. 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 oil and gas in a controlled manner. Production tubing is commonly run from the completed interval back to the surface and is secured in place by a tubing hanger 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 uppermost 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 because of its elaborate branching structure. Wellheads 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 process is similar. The differences in location conditions, design criteria, logistical considerations and related cost are enormous. In deepwater, the process can approach the most sophisticated technical operations known to man and an individual well can cost many tens of millions of dollars.
See also Hydrocarbon; Oil spills.
Resources
BOOKS
Gow, S. Roughnecks, Rock Bits and Rigs: The Evolution of Oil Well Drilling Technology in Alberta, 1883-1970. Calgary, Alberta: University of Calgary Press, 2006.
Lyons, W.C. and G.J. Plisga. Standard Handbook of Petroleum & Natural Gas Engineering. Woburn, MA: Butterworth-Heinemann, 2004.
William Engle
K. Lee Lerner
Leonard C. Bruno