Onshore wells
Onshore wells are boreholes drilled into the earth primarily for the exploration and extraction of oil and gas. The drilling process has evolved over centuries, with two main techniques: cable-tool drilling and rotary drilling. Cable-tool drilling, an older method, involves a heavy metal bit that pounds through rock, making it effective for shallow wells but slower than modern techniques. In contrast, rotary drilling, introduced in the 1890s, uses a rotating drill bit attached to a string of pipe, allowing for faster penetration and better management of borehole stability through the use of drilling fluids.
Once a borehole is drilled, various tests and analyses are conducted to assess the presence and quantity of hydrocarbons. This includes well logging, where instruments are lowered into the borehole to gather data on the rock and fluid characteristics. If economically viable deposits are found, the well undergoes completion procedures, allowing hydrocarbons to be extracted and transported. Onshore drilling is subject to environmental considerations, regulatory requirements, and economic factors, especially as exploration on land becomes increasingly limited and attention shifts toward offshore and environmentally sensitive areas. The success of onshore wells significantly impacts the energy economy and is influenced by global market conditions and political factors.
Onshore wells
Several million wells have been drilled in the exploration for oil and gas within every environment of the earth. Testing, completion, and evaluation technologies of a completed well are similar whether the borehole is dug by the cable-tool or by the rotary method. Downhole, wire-line analyses have evolved from the simple electric log of the 1920s to a modern array of evaluative services.
Cable Drilling
The location of the first drilling operation is lost to history, although it is known that the Chinese drilled for brine and water two thousand years ago using crude cable-tool methods. Similar methods of drilling were still being employed in the 1850s. By this process, a well is created by raising and lowering into the borehole a heavy metal bit suspended from a cable or rope. Gradually, the bit will pound its way through the rocks. With the addition of a jar, a mechanical device that imparts a sharp vertical stress to the bit, the process is greatly improved. Surface equipment, contained within a wooden derrick, or rig, was commonly steam driven and repeatedly withdrew the bit from the hole, allowing it to be again dropped to the bottom of the well. As the bottom of the hole fills with rock chips, a bailer is periodically used to remove this debris.
Cable drilling is a slow process. Its greatest advantage is easy identification of oil- and gas-producing rock units. Because minimal drilling fluids are used, the uncontrolled surface flows of encountered hydrocarbons (occurrences known as blowouts) are frequent. For this reason, cable drilling is most applicable within depths of 1,000 meters. As late as 1920, cable-tool rigs drilled as many as 85 percent of all wells completed in the United States.
Rotary Drilling
Introduced to the industry at Corsicana, Texas, in 1895, the rotary method was used to drill 90 percent of American wells in the 1950s. In rotary drilling, the drill bit is attached to connected sections of steel pipe, or drill string, and lowered into the borehole. Pressure is placed on the bit and the drill pipe is rotated, causing the bit to grind against the bottom of the borehole. In contrast to the cable method, new borehole depths are created by the rock being torn rather than pounded. When the drill bit becomes dull, the drill string is removed from the borehole, disassembled, and stacked within the tall mast, or derrick. A new bit is attached, and the drill string is reassembled.
The application of a drilling fluid system is a key element of the rotary method. Originally ordinary mud, drilling fluids have become a carefully formulated solution of water, clays, barite, and chemical additives. These fluids are circulated under pressure down the center of the drill string, extruded through the drill bit, and pumped back to the surface through the space between the drill string and the borehole. These fluids serve several important functions: to lubricate and cool the drill bit, remove rock chips from the borehole, and protect the borehole from dangerous blowouts. Because of its mechanical advantages, the rotary method is approximately ten times faster than the cable method in drilling a borehole.
After the borehole is completed, and assuming commercial deposits of oil or gas are discovered, completion procedures are initiated. Because surface instruments cannot detect the presence of subsurface hydrocarbons, the rock units exposed in the wall of a borehole must be evaluated for the presence and quality of contained oil and gas. A preliminary analysis is conducted on the rock chips continuously brought to the surface by the drilling fluid system. A key component of this analysis is the identification of microscopic fossils, nicknamed “bugs,” which indicate the age of the rock layers being drilled. The rock chips are far too small to preserve most fossils, but microscopic fossils survive the drilling process. Later, instruments lowered into the borehole determine the physical and chemical characteristics of the penetrated rocks and their contained fluids and gases. Should the presence of hydrocarbons be indicated, further testing is conducted to determine the economic value of the discovery. Finally, if economic payout is indicated, the borehole undergoes final completion procedures. Special production tubing systems are installed, and the oil or gas is pumped, or flows under its own pressure, from the rocks up the borehole and into a pipeline or surface storage system.
Rotary drilling procedures vary little with geographic location or climate. In urban areas, the derrick is covered with soundproof material and sometimes even disguised for aesthetic purposes. In sensitive areas, such as arctic regions and offshore operations, safety and environmental preservation precautions are mandated by state and federal law.
Other Drilling Methods
With the turbodrill method, or the downhole tool method, the drill string remains stationary while the drill bit rotates under the influence of circulating drilling fluid. Drilling very straight boreholes with minimum mechanical problems, this process excels at directional drilling, especially horizontal drilling. It also greatly reduces wear on the drill stem pipe. The use of a downhole tool is replacing the rotary drilling method. Although a rotary drill rig is still installed, its rotary capability is not used. The hammer drill, a combination of slow rotary motion coupled with percussion impact, produces a faster rate of rock penetration, but this method has not been widely accepted into practice. Experiments with vibration and sonic drills have proved unsuccessful or uneconomic thus far.
History of a Borehole
After the location for a borehole is determined, rotary drilling equipment is taken to the chosen site, an area of about 0.5 to 1 hectare. When the drill rig is assembled, sections of drill pipe, or drill string, are connected within the derrick. The drilling fluid hose is connected to the upper end of the drill string, while a drill bit is attached to the bottom. The rig is now ready to “make hole.” The history of a borehole begins with its “spud-in” time, that moment when the ground is broken by the rotating bit.
The rotary table, located in the center of the rig floor and connected to powerful engines, rotates the drill string and attached bit. As the bit rotates, drilling fluid (termed mud) is pumped down the inside of the drill string and through openings in the bit. The density of this mud is carefully controlled so that as it exits the bit, it is capable of lifting rock fragments, or cuttings, from the bottom of the borehole, allowing the bit to rotate against a fresh rock surface. The drilling mud, with its contained cuttings, is circulated up the annulus (passage) between the wall of the borehole and the outside of the drill string. At the surface, cuttings are separated by flowing the drilling mud through a vibrating sieve. Periodically, a sample of cuttings is collected for geologic analysis. Finally, the cleansed mud circulates through the “mud pit,” where, after cooling to surface temperature, it is pumped through the drilling fluid hose back into the drill string. While the borehole is being drilled, this mud system is in continuous circulation. Every 9 meters approximately, as the borehole becomes deeper, a new section of drill pipe is added to the drill string, increasing the depth capability of the rig.
At shallow depths, where the bit is penetrating loose soils and poorly consolidated rock formation, the drilling speed is measured in tens to hundreds of meters per day. With increased depths, penetration rates will diminish to as little as a meter per day, depending on rotation pressure and velocity and rock characteristics. At the surface, “conductor pipe” is driven 6 to 10 meters into the ground to protect the borehole against collapse. At depths below the conductor pipe, rock units containing fresh water are protected from drilling fluid contamination by lowering “surface casing” through the conductor pipe and into the borehole and injecting cement to hold it in place. At greater depths, progressively smaller radius “intermediate casing” may be cemented into the borehole, keeping the newly drilled hole open while sealing off unusually high-pressure or unusually low-pressure rock strata. Because each new series of casing must fit into the prior cemented casing, the borehole diameter becomes smaller with increased depth. When the borehole reaches programmed total depth (TD), the drilling process is complete. The next phase of activity involves testing for the presence and quantity of oil and gas.
The borehole is protected by cementing “production casing” through the depth of the production zone. Perforating guns, multibarrel firearms designed to fit into the borehole, are lowered to the target production depth and fired electrically. High-velocity bullets penetrate the casing cement and become embedded in the rock strata, creating pathways through the strata to the wall of the borehole. In some cases the rocks are fractured by explosives or high-pressure fluids. The holes are prevented from collapsing under the weight of the overlying rocks by injecting coarse sand into the holes. Oil or gas emitting from the rock through these pathways flows into installed production tubing and to the surface, where the hydrocarbons are either temporarily stored or directed to a nearby pipeline. At this point, the well is completed and “on line.”
Testing and Analyses
Drill cuttings are periodically collected from the drilling fluid and analyzed in the field in converted mobile-home vehicles. These field tests determine rock type, contained minerals, density, pore space percentage, and association with either natural gas or crude oil (petroleum). Since drilling is an expensive operation, commonly costing millions of dollars, the majority of boreholes are subjected to additional analyses, termed well logging. Conducted by contracted specialists, well-logging operations involve lowering an elongated instrument called a sonde to the total depth of the borehole. As the sonde is slowly pulled up the hole, it records various characteristics of the rocks exposed within the wall of the borehole and their contained fluids and gases. These characteristics, which include electrical resistivity, conductivity, radioactivity, acoustic properties, and temperature, are transmitted to the surface, where they are recorded and filed for future use. A basic property is the diameter of the borehole, which indicates the hardness or softness of the layers. Logging the rate of drilling also indicates the hardness or softness of the layers. It is common for four or five different logs to be recorded, while on a very important borehole, more than twice this number may be taken.
In the office, individuals trained in geology and engineering study the cuttings analyses and logging data and determine the presence and economic extent of oil or gas by calculating rock porosity, permeability, density, thickness, lateral extent, inclination, and pressure at various depths in the borehole. Should these analyses be pessimistic, the borehole is declared “dry and abandoned” and permanently sealed at several depths by cement plugs. Such is the fate of approximately six out of seven boreholes drilled in frontier (new) geographic regions or to unproven depths; such boreholes are termed wildcat wells. For the one in seven wildcat wells in which logging analyses indicate a chance of success, verification analyses in the form of drill-stem testing (DST) will be conducted.
Drill-stem testing equipment is attached to the base of the drill string and lowered to the rock depth to be tested. After this depth is physically isolated from the rest of the borehole, assuring a valid test, the DST tool is activated, allowing fluids or gases contained within the isolated rocks to flow into the drill string and to the surface. From DST, rock pressures and flow capacities are calculated. When DST verifies positive economic results determined by logging analyses, the commercial quantities of either oil or gas, or both, are declared, and the well is prepared for its final completion phase.
Economic and Political Considerations
The fortunes of the American oil and gas drilling industry are closely tied to the market value of a barrel (42 US gallons, or about 159 liters) of crude oil. As that value increases, so generally does the number of drilling rigs under contract. Adding confusion to this economy-to-rig-use relationship are such considerations as international politics, governmental policies, environmental concern, and marketplace competition for high-risk investment dollars. After a century and a half of drilling wells in the search for new reserves of oil and gas, terrestrial portions of the United States are considered a mature exploration province. The chances of discovering large new reserves of hydrocarbons on land are very small. The future lies in drilling within the offshore provinces (deep water of the Gulf of Mexico and the Atlantic eastern seaboard) and environmentally protected regions (northern Alaska and national parks and forestlands). In order to maintain a hydrocarbon-based energy economy while reducing dependence upon foreign hydrocarbons, oil and gas well-drilling and production programs may have to take place in these frontier exploration regions. Such programs must be governed by consensus regulatory, environmental, and economic policies until solar, nuclear, or some unforeseen resource assumes the dominant energy position and oil and gas wells no longer need be drilled into the earth.
Principal Terms
cable-tool drilling: a repetitive, percussion process of secondary use in the boring of relatively shallow oil and gas wells
completion procedures: all methods and activities necessary in the preparation of a well for oil and gas production
downhole tool: a drill bit and motor mounted on the end of the drill string; fluid pumped into the drill string drives the downhole tool, and the drill string is not rotated
drilling fluids: a carefully formulated system of fluids used to lubricate, clean, and protect the borehole during the rotary drilling process
drilling rig: the collective assembly of equipment, including a derrick, power supply, and draw-works, necessary in cable-tool and rotary drilling
drill string: the length of steel drill pipe and accessory equipment connecting the drill rig with the bottom of the borehole
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: historically, the principal method of boring a well into the earth using a fluid-circulating, generally diesel-electric generated, rotating process
well log: a graphic record of the physical and chemical characteristics of the rock units encountered in a drilled borehole
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