Offshore wells

Offshore wells are drilled in favorable oil and gas areas that are inundated with oceanic (salt) or inland lake (fresh) waters. Most of the world’s future hydrocarbon reserves may be discovered in offshore provinces.

Establishing Offshore Drilling Locations

Offshore drilling is the extension of on-land oil and gas drilling techniques in waters that either cover or lie adjacent to landmasses of the earth, such as the Great Lakes of North America and the continental margins that surround each continent. The continental margins are the principal arena in which offshore drilling is conducted; they constitute approximately 21 percent of the surface area of the oceans and may contain a majority of the world’s future reserves of oil and gas.

As of the last decade of the twentieth century, oil and gas production had been established off the coasts of more than forty countries in the continents of Africa, Australia, South America, North America, Asia, and Europe, and offshore drilling had been completed in the waters of more than half the nations on Earth. Drilling platforms capable of operating in water deeper than 1,800 meters probe the hydrocarbon potential of the outer continental margin. Even farther offshore, self-propelled drill ships capable of drilling more than 2,000 meters into the sea floor within waters as deep as 6,000 meters have been analyzing the deeper portions of the oceans since the early 1960s.

The specific offshore drilling location must be established at the outset. This is the responsibility of the geologist, who, by studying the various rock exposures and their structures, decides on the best surface site for the drilling equipment. Such exposures are submerged offshore, and geophysical methods of determining the geology of the sea floor are used. Offshore geophysics is an indirect technique of studying the rocks composing the sea floor by measuring their physical properties. A magnetometer towed behind an aircraft flying low over the water can be used to measure the magnetic properties of underlying rocks. With a gravimeter mounted in a slow-moving boat or aircraft, the gravity field associated with the sea floor can be analyzed. A combination of these methods will help the geologist locate a site underlain by rocks that may contain hydrocarbons. By far the most common offshore geophysical tool is the seismic reflection survey. Seismology depends on the artificial generation of an elastic sound wave and its transmission through the layered, sedimentary rocks underlying the sea floor within the continental margin zone. These waves reflect off sedimentary rocks and are transmitted back to the surface of the water, where a vessel will record the time difference between transmission and reflection. Millions of such combined reflection arrival times are interpreted as a cross-sectional view of the underlying sea floor. A series of intersecting seismic sectional views presents a simple three-dimensional portrayal of the best site for offshore drilling equipment.

Drilling Methodology

The sole method used for drilling offshore wells is the rotary method. Rotary motion is supplied by diesel engines to a length of interconnected drill pipe (the drill string), to the bottom of which is attached the drill bit. Drilling bits come in a variety of styles designed to drill through differing types of sedimentary rock. Each type of drill bit contains an arrangement of high-strength alloy teeth that tear through the rock when rotated under pressure. As the hole deepens, new sections of drill pipe are added. Periodically, the inside of the borehole is lined with cemented casing, which prevents the hole from caving in. During drilling, drilling fluid is circulated through the pipe and hole. This circulating fluid, a mixture of water, special clays, and other minerals and chemicals, is necessary to maintain a safe temperature and pressure in the borehole and to clean the hole of newly created rock chips. The drilling fluid can be formulated to stabilize chemically active rock layers. Since it is dense, the drilling fluid supports (partially floats) 10 to 20 percent of the weight of the drill pipe—a significant consideration when drilling a deep well. All drilling activities take place within the derrick, an open steel structure that often is 60 meters tall. The derrick holds the draw works, whereby the drill string can be drawn out of the borehole and disassembled, one length at a time.

Should a drilling operation discover new reserves of oil or gas, production platforms must be constructed on-site after the movable drilling platform is deployed elsewhere. From these fixed production platforms, as many as fifty or more additional wells are drilled to determine the new hydrocarbon field’s size and volume. Each of these boreholes is drilled from the same general location on the production platform; however, at a predetermined depth below the sea floor, individual boreholes veer away from one another, allowing different sectors of the field to be economically developed from one production platform. This process of deviation is termed directional drilling; it requires the assistance of a specialist, as the bottom positions of the boreholes must be very carefully controlled for effective hydrocarbon production. After the final directional hole has been drilled and the limits of the field fully defined, the production derrick is replaced with equipment used to gather the oil and gas from the many active wells flowing into the platform. An offshore pipeline is laid, connecting the producing platform to onshore pipeline and refining systems, and the new field begins production.

Drilling Platforms

The type of platform that will be used to contain the drilling equipment is critical, considering the inhospitable weather periodically encountered offshore. There are four types of platforms: the submersible, semisubmersible, and jack-up platforms, and the drill ship. A submersible platform is stabilized by flooding the hollow legs and pontoons of the platform and establishing seabed moorings. Since these platforms are in contact with the sea floor, they cannot be used in excessively deep waters. In such deep waters, a semisubmersible design is employed; the hollow pontoons are only partially flooded, permitting the platform to settle below the surface of the water but not to rest on the bottom. Inherent in this design is buoyancy that, along with anchor moorings, is sufficient to maintain the platform safely over the drilling site. The ratio of semisubmersible to submersible units in use is approximately 4:1.

By far, the most popular offshore unit is the jack-up platform. These movable structures are towed to the drill location and stationed by lowering massive steel legs to the seabed. In essence, the platform is “jacked up” on the legs to a level sufficient to protect the unit from storm waves. The fourth design is the drill ship, a free-floating, usually self-propelled vessel that contains an open-water drilling unit in the ship’s center. The drill ship is kept on location by computer-activated propellers that maintain the horizontal and vertical motion of the standing ship within safe limits.

Regardless of the basic design, all offshore platforms contain a drilling derrick, storage, and machinery housing, living and eating quarters, recreation and basic health facilities, and a helicopter landing pad. The entire structure must be of a size sufficient to house and maintain a normal working crew of forty to sixty individuals.

Multiple Borehole Platforms

Because of the expense associated with the siting of an offshore drilling platform, more than one borehole is commonly drilled from the same surface location. More than sixty-five boreholes have been drilled from a single platform. Most are drilled into the seabed at a predetermined angle, allowing many wells to “bottom” in an oil or gas reservoir rock over a distance as much as 3 kilometers laterally from the platform site. In this manner, the hydrocarbon reservoir can be exploited economically with a minimum risk to the aquatic environment.

Multiple borehole platforms are designed to extend a safe height above sea level. Platforms in the Gulf of Mexico, the North Sea, and the Santa Barbara Channel of California have been designed to endure the most severe storms likely to occur within a one-hundred-year cycle, including hurricane-force storms and earthquake tremors. Special submersible drilling platforms are designed for operation in polar waters, where ice-free waters exist only for a short time period during summer. These structures, which must be able to withstand strong currents and floating pack ice, are protected below the waterline by a steel or cement caisson. Often the drilling deck is circular, allowing easy passage of floating ice.

Floating platforms, semisubmersibles, and drill ships move with wind and water currents. Unless compensated for, these motions affect drilling efficiency. When boreholes are drilled in very deep waters, compliant platforms that yield to weather and water currents are employed.

An International Priority

Offshore drilling for oil and gas, even with the very high costs dictated by its architectural, meteorological, engineering, and safety requirements, is an international priority; it is driven by need, economic return, and national security. Except for difficult-to-explore and environmentally sensitive regions, such as the Arctic, the South American interior, and central Africa, many onshore regions of the world have entered a mature stage of hydrocarbon exploration: Most of the accessible large-volume oil and gas fields have already been discovered. The United States is an excellent example, for it imports almost 50 percent of its daily petroleum requirements, leaving it vulnerable to foreign military and political disturbances. Studies have consistently shown that the best opportunities for increasing the reserves of American oil and gas lie in the continued exploration of the continental margins. Although not all offshore areas contain oil and gas, analyses conducted by the U.S. Geological Survey indicate that within the continental margin of the United States, as much as 40 billion barrels (a barrel equals 42 U.S. gallons, or about 159 liters) of oil and more than 5.7 trillion cubic meters of natural gas are yet to be discovered. By 2022, the US imported 8.23 million barrels of petroleum each day and exported more than 9.5 million barrels.

The degree and specific locations of offshore exploration and drilling activities depend on political, environmental, and economic factors, all of which affect petroleum products’ availability and price. Certain offshore drilling regulations have already been established. In the United States, states’ rights generally prevail to approximately 5,500 meters, or 3 nautical miles, offshore. From there out to designated international waters, federal government policies must be followed. The outer limits of American waters and those of other signatory nations are controlled by the 1982 Third United Nations Convention on the Law of the Sea, which allows nations to define Exclusive Economic Zones extending 200 nautical miles (370 kilometers) offshore. Because of economic and engineering constraints, these outer limits have not yet been tested widely with actual drilling. A common cause for the delay in exploration drilling is public concern for the marine environment. An example of a highly publicized and heavily studied American offshore oil spill is one that occurred in the Santa Barbara Channel, off California, in 1969; another is the 1989 tanker spill in Prince William Sound, in the Gulf of Alaska. In 2010, the Deepwater Horizon drill rig, operated by BP, sank in the aftermath of an explosion. Attempts to activate shutoff devices on the sea floor failed, and it took around six months to fully seal the wells. About 4.9 million barrels (210,000,000 U.S. gallons; 780,000 cubic meters) of oil were spilled—about 20 times the amount of the 1982 Alaska oil spill. The long-term effects of these spills are unknown. One study, however, has shown that between 1950 and 1980, the commercial fish-catch weight in the Gulf of Mexico increased fourfold, suggesting that offshore compatibility between the fishing and hydrocarbon industries is possible. Yet, as environmentally fragile continental margin areas, such as the shores of Alaska and Canada, continue to be evaluated, intensified concerns for the environment may delay exploration agendas. Finally, the costs of finding and producing oil and gas offshore will continue to climb as deeper waters are probed.

Principal Terms

continental margin: the offshore area immediately adjacent to the continent, extending from the shoreline to depths of approximately 4,000 meters

directional drilling: the controlled drilling of a borehole at an angle to the vertical and at an established azimuth

drilling fluids: a carefully formulated system of fluids and solids that is used to lubricate, clean, and protect the borehole

geophysics: the quantitative evaluation of rocks by electrical, gravitational, magnetic, radioactive, seismological, and other techniques

hydrocarbons: naturally occurring organic compounds that in the gaseous state are termed natural gas and in the liquid state are termed crude oil or petroleum

rotary drilling: a fluid-circulating, rotating process that is the chief method of drilling oil and gas wells

sedimentary rock: rock formed by the deposition and compaction of loose sediment created by the erosion of preexisting rock

seismology: the application of the physics of elastic wave transmission and reflection to subsurface rock geometry

Bibliography

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