【正文】 hoisting loads of 1,500,000 lb (680,440 kg). The number of land rigs operating in the United States and Canada is currently about 2000 to 2500. This total has fluctuated between 2000 to almost 5000 over the last 30 years. The ponent parts of the onshore drilling rig will be discussed in more detail later.Offshore drilling rigs fall into one of several categories, each designed to suit a certain type of offshore environment: we need to be able to circulate a drilling fluid down thedrillstem, through the bit, and back up the annulus between pipe and hole。 drilling fluid properties and cost.In addition to increasing temperature and pressure, other factors associated with the types of formations encountered and the types of fluids they contain can increase the difficulty of dealing with subsurface environments. Salt and anhydrite formations can adversely affect mud systems and, in some cases, cause difficulties in cementing casing. Massive salt formations may even shift laterally, causing casing collapse. Highpermeability gravels and vugular limestones can lead to lost circulation problems. In the Gulf Coast, “gumbo” shales can cause sticking problems and plugged bits. Carbon dioxide and hydrogen sulfide contaminants in formation fluids can reduce the endurance of drillpipe and bits through corrosion and sulfide embrittlement. In almost all cases, these problems are aggravated by increasing temperature and pressure. This chapter will return to the important topic of controlling subsurface pressures later. Drilling EquipmentThe basic drilling equipment of the early rotary rig has not changed much is far as function is concerned, although its capabilities have been largely expanded to meet modern drilling needs. Rather simply stated, we need to be able to transmit torque from a prime power source to a bit via a drillstem。 formation evaluation equipment capabilities and the operation of certain downhole equipment such as logging tools and special drilling tools. cement setting time, when cement is used to set casing inside the borehole.F, equals 360 176。C), assuming an average geothermal gradient. That is, 15,000 ft times 2 176。C) would have a bottomhole temperature of 360 176。This is because the reservoir fluids transmit the greatest pressure applied to them, much like hydraulic fluid in an automobile brake line. Figure shows how the normal pore pressure in the watersaturated portion of the reservoir is transmitted to the shallow end of the formation. The pressure at the top of the reservoir equals the pore pressure at the deep end, minus the fluid head exerted by the lighter (less dense) hydrocarbons.Structural Movement: If a normally pressured sand is lifted rather rapidly relative to geologic time, the overburden pressure may decrease more rapidly than the pore pressure can dissipate. This can occur as a result of piercement salt domes, plumes of lowdensity salt deposited by ambient seas, which force their way upward through the sediment, displacing the deposited layers.Charging: Abnormal pressures can be encountered in sands which, although originally normally pressured, have been placed in fluid contact with an abnormally pressured zone via a conduit (leaking fault, fracture, aquifer, borehole, or binations of these). Such a sand is said to be “charged” with a pressure greater than normal for its depth. This type of overpressure can be generated relatively quickly in the case of an underground blowout, as described in section , Well Control.There are several other mechanisms that contribute to the overpressuring of reservoirs. Clay can act as a semipermeable membrane, allowing osmosis to inhibit the flow of water from pacting shales as a result of an increase in ion concentration. The thermal expansion of water and thermal cracking of hydrocarbons both act to increase the volume of saturating fluids, and can help cause abnormal pressures in a confined reservoir rock. Figure Abnormal pressure as a result of fluid density contrastsThese contributions are thought to be minor, however.Another variable parameter that contributes to the hostility of subsurface environments is temperature. The temperature gradient, or geothermal gradient, is generally constant for a given borehole, although it may vary from area to area. Typically, the geothermal gradient is expressed in degrees per unit depth (℉/100 ft or ℃/100 m). The average is about 2 ℉/100 ft, or ℃/100 m, although ℉/I00ft and less than ℉/100 ft gradients are found (Levorsen 1967). A change in geothermal gradient can sometimes be an indicator pressured shales, foreshadowing the potential for an overpressured reservoir. In determining a bottomhole temperature from a known gradient, or vice versa, an adjustment must be made for the mean annual surface temperature. For example, a 15,000 ft (4572 m) well in an area with a mean annual surface temperature of 60 176。 structural movement chemical diagenesis rapid sedimentations Prudhoe Bay and Beaufort Sea, the annual cycles of ice formation and breakup can plicate drilling activities. Beaufort Sea drilling has been carried out from natural barrier islands and artificial gravel islands that are serviced by a variety of vehicles, depending on ice conditions. Rigs and equipment can be moved by barge during the icefree period of late autumn. After the sea ice has formed between the islands and mainland, thick ice roads can be constructed on top of the sea ice. Prior to road construction, helicopter and STOL (shorttakeoffandlanding) aircraft are utilized. Transportation during the time period in which the ice is breaking up requires the use of a “hoverbarge,” an aircushioned vehicle with a flat barge deck, or an AST (Archimedian Screw Tractor), an amphibious vehicle capable of towing cargo through water and broken ice on two screwshaped pontoons.Offshore drilling in deeper waters must be conducted from icedefended drillships or from stationary platforms built