Oil and natural gas wells
Oil and natural gas wells are drilled structures used to extract hydrocarbons from beneath the Earth’s surface. The depth of these wells can range from a few hundred meters to over 6,000 meters, depending on the geological formations being targeted. The drilling process begins with an exploration team identifying potential sites through subsurface mapping and securing drilling rights from landowners, which could include individuals, governments, or foreign entities.
Once drilling commences, a rig is positioned, and a drill bit is used to penetrate the rock formations, with a specialized drilling fluid known as "mud" circulating to maintain pressure and remove debris. As the well is drilled deeper, steel casing is installed to stabilize the borehole and protect against fluid migration. The completion phase involves additional steps, such as installing production equipment and perforating casing to access the oil and gas.
After successful completion, the well produces hydrocarbons, which may initially flow using natural pressure but often require pumps as production declines over time. Eventually, wells reach an economic limit where production is no longer viable, leading to abandonment or conversion for other uses. The entire process emphasizes safety and environmental considerations, particularly in managing potential hazards like blowouts and maintaining site integrity post-abandonment.
Oil and natural gas wells
Wells drilled to produce oil and natural gas are designed to pump oil as long as the source is economically viable; they often provide many years of service. Drilling procedures are rigorous and exacting and are intended to avoid hazards such as blowouts.
Background
Oil and are recovered through drilled wells that are designed and constructed to ensure many years of service. These wells may vary from a few hundred meters to more than 6,000 meters in depth. They must recover oil and gas from their reservoirs in the subsurface. The location of the well is determined by an exploration team, which produces maps of the subsurface showing possible accumulation of oil and gas. A team of land agents investigates the ownership of the drill location and provides information so that the right to produce the oil and gas can be secured from the landowner—be it an individual, a state, the federal government, or a foreign nation. After the right to drill is secured, the drilling plan is converted into action.
Drilling Procedures
A suitable drilling rig is selected through the solicitation of information from drilling companies. After selection and transportation to the drilling site, the rig is positioned over the marked location, which has been accurately determined by surveying instruments. A drill bit is connected to drill pipe and drill collars. Drill collars are thick-walled cylinders about 9 meters in length used immediately above the drill bit to prevent the bit from wandering as it cuts through formations of varying strength and inclination. The drill pipe and collars are rotated by a rotary table at the surface, causing the drill bit to rotate. The weight of the drill string, as the downhole assembly is called, along with its rotation, causes the rock underneath the bit to be crushed. This crushed rock is circulated to the surface by drilling fluid. This fluid, called “mud,” is a mix of chemicals suited to the downhole environment. It is pumped down the well through the drill string, through the bit nozzles, and then back up to the surface in the annular space between the drill string and the wall of the drilled hole.
The drill bit eventually becomes dull and must be replaced. When this happens, the drill string must be unscrewed so that the bit can be brought to the surface. This process is called “tripping the bit.” The drilled hole must be lined with steel casing to prevent slumping of the borehole wall and unwanted migration and mixing of subsurface fluids. Casing is similar to the steel pipe seen in pipelines on the surface, but it is designed for the pressures and temperatures encountered in the subsurface. Casing setting depths are either predetermined or selected while drilling to control a hazardous condition such as a blowout or lost circulation.
After the first casing “string” is run, it is secured in the borehole by circulating a thin cement downward through the casing, then up the annular space between the casing wall and the borehole wall. Two casing strings are necessary in the simplest well, while several strings may be necessary for deep wells. As additional casing strings are run, each succeeding string must be smaller in diameter than its predecessor. In this way, an oil and gas well becomes smaller in diameter as its depth increases. For example, it is common to begin at the surface with a drill bit one-third meter in diameter, while the final well diameter at total depth may be as small as one-tenth meter. This concept is simple to understand by noting that as each casing string is secured in the wellbore, the succeeding bit size must be reduced in order to enter the newly secured casing.
When the borehole penetrates the rock formation containing oil and gas, the depths of interest are evaluated using electrical, acoustic, and radioactive techniques to determine the presence of oil and gas. If the evaluation indicates that oil and gas are there in commercial quantities, “completion” of the well is begun. Completion involves installing the final casing string, perforating the casing wall adjacent to the rock formation containing oil and gas by using gas jets or mechanical cutters, and installing production equipment. Depending on the initial success of the completion procedure, additional measures may be necessary to increase the rate of oil and gas produced. These measures include using reactive chemicals to dissolve the rock formation near the wellbore or using hydraulic pressure to fracture the rock formation. After the completion procedure, the well is tested to determine the rate of oil and gas being produced. Depending on the rate of production, a small string of pipe called tubing is placed inside the casing to provide a flow conduit for the produced fluids.
Most oil and gas wells are drilled as near to vertical as possible. The reason is that most regulatory agencies closely monitor the surface and bottomhole locations of wells in order to protect property rights. In many situations, however, it is impossible to locate the drilling rig over the desired bottomhole location. A river, lake, or building, for example, on the surface may necessitate the drilling of a well directionally to the desired bottomhole location. The progress of drilling is monitored by noting the azimuth (deviation from true north) and dip (deviation from vertical) of the well on a continual basis. Specialized directional drilling consultants oversee this complicated task. Sometimes wells are started vertically at the surface, then forced to dip all the way to the horizontal, then kept horizontal in the subsurface. Horizontal wells are much more expensive to drill than vertical wells, but good ones yield production rates far in excess of vertical wells.
A unique situation for well deviation exists offshore. A series of wells is drilled from an offshore location. The pattern of these wells, called a template, includes the deviation of all but the well immediately underlying the platform or floating rig. The proper locating of many wells from the same surface location ensures the broadest distribution of bottomhole locations and involves highly specialized technical knowledge.
Hazards
During the drilling of a well, potential hazards must be recognized by the drilling personnel. These include blowouts and lost circulation. A blowout is the uncontrolled escape of subsurface fluids to the surface. These spectacular events have been identified with the oil and gas industry since its beginnings, and they remain as one of its most newsworthy subjects. A properly drilled well should not encounter a blowout if adequate diagnosis and detection are made. The weight of the drilling fluid may be increased to control abnormal pressures in the subsurface. Blowout preventers, a type of valving used with the drilling rig, are designed to protect against blowouts until the well can be controlled and drilling resumed without spoiling the surface area adjacent to the well.
Lost circulation involves drilling fluid that is lost because it seeps into the pore space or fractures in the subsurface rock formations. If enough drilling fluid escapes downhole, well control can be lost and a blowout can occur. Lost circulation is controlled by decreasing the weight of the drilling fluid or using plugging agents circulated into the subsurface leak zones.
Oil and Gas Pumping and Production
When the well has been completed and production is assured, a wellhead is installed to replace the blowout preventers. The wellhead, nicknamed the “Christmas tree,” is a series of valves designed to seal the casing, its annular space, and the tubing to prevent leaks.
Crude oil is processed at the site only to remove unwanted foreign matter. Surface equipment used to process oil and gas includes dehydrators to remove water and water vapor, and separators to remove foreign matter, including rock particles, paraffin, and other debris prohibited by the buyer of the oil and gas. Large tanks are used to store oil prior to delivery.
Oil is transported by pipeline, truck, and train to the refinery for further breakdown into gasolines, motor oils, and other products and chemicals. Natural gas is odorized by placing a distinctive odorant in it, and its pressure is elevated by compression for delivery to the customer through a series of pipelines.
An ideal oil and gas well will flow to the surface using its internal energy. Oil wells eventually reach the point where their flowing energy is depleted and they must be pumped in order to continue producing. A variety of pumps have been used in the oil and gas industry; among them is the familiar beam pump, sometimes called a “horse’s head” or “nodding donkey,” that is seen in oil-producing areas around the world.
Depletion and Economic Limit
When a well’s energy is depleted, an enhanced oil recovery (EOR) project may be started. EOR techniques are used to produce additional oil from an oil and gas formation that has depleted its primary energy source. Various fluids ranging from to exotic liquids, gases, and even steam are injected into oil-producing rocks to force more oil from them. Often EOR projects can produce an amount of oil equivalent to that recovered during the well’s primary operating life.
All oil and gas wells eventually reach their “economic limit,” at which point economic production ceases. The economic limit is an arbitrary production rate that depends on the expenses associated with producing the well, the percentage of ownership of the well’s operator, and the price of the oil and gas. This limit may be reached in a short period after production begins for poorly performing wells, or it may exceed fifty years. Once the production rate falls below the economic limit, the well is either plugged and abandoned (cement plugs are used to seal the wellbore) or converted into a liquid disposal or EOR injection well. After the well’s operators have plugged the wellbore to the satisfaction of regulatory authorities, the surface location in the vicinity of the well is restored in an environmentally acceptable manner. Little or no trace of the well itself should be left.
Bibliography
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How Stuff Works
How Oil Drilling Works.