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66 Chapter 6 Drilling Technology and Costs about the formation, even before field testing. Logs that identify key formation characteristics other than temperature, flow, and fractures are not widely used for geothermal resources. Logging trucks equipped with hightemperature cables are now more common, but not without additional costs. Geothermal logging units require wirelines that can withstand much higher temperatures than those encountered in everyday oil and gas applications. This has encouraged the growth of smaller logging companies that are dedicated to geothermal applications in California and Nevada. Thermal expansion of casing. Thermal expansion can cause buckling of the casing and casing collapse, which can be costly. Also, thermal contraction due to cooling in injection wells, or thermal cycling in general, can also lead to damage and eventual tensile failure of casing. It is customary in U.S. geothermal drilling to provide a complete cement sheath from the shoe to surface on all casing strings. This provides support and stability to the casing during thermal expansion as the well heats up during production – and shields against corrosion on the outside of the casing. In contrast, thermal expansion is much less of an issue in oil and gas completions. Oil and gas casings and liners are often only tagged at the bottom with 150 to 300 m (500 to 1,000 ft) of cement to “isolate” zones, and do not require a complete sheath from shoe to the surface. The oil and gas liner laps are also squeezecemented for isolation purposes. Thermal expansion and contraction of casing and liners is an issue that has been adequately addressed for wells with production temperatures below 260°C (500°F). Fullsheath cementing and surfaceexpansion spools can be employed in this temperature range with confidence. Above operating temperatures of 260°C (500°F), greater care must be taken to accommodate thermal expansion or contraction effects. Drilling fluids/“mud” coolers. Surface “mud coolers” are commonly used to reduce the temperature of the drilling fluid before it is pumped back down the hole. Regulations usually require that mud coolers be used whenever the return temperature exceeds 75°C (170°F), because the high temperature of the mud is a burn hazard to rig personnel. The drilling fluid temperature at the bottom of the well will always be higher than the temperature of the fluid returning to the surface through the annulus, because it is partly cooled on its way upward by the fluid in the drill pipe. High drilling fluid temperatures in the well can cause drilling delays after a bit change. “Staging” back into the well may be required to prevent bringing to the surface fluid that may be above its boiling temperature under atmospheric conditions. Drill bits and increased rate of penetration. While many oil and gas wells are in sedimentary column formations, geothermal operations tend to be in harder, more fractured crystalline or granitic formations, thus rendering drilling more difficult. In addition to being harder, geothermal formations are prone to being more fractured and abrasive due to the presence of fractured quartz crystals. Many EGS resources are in formations that are igneous, influenced by volcanic activity, or that have been altered by high temperatures and/or hot fluids. Drilling in these formations is generally more difficult. However, not all geothermal formations are slow to drill. Many are drilled relatively easily overall, with isolated pockets of hard, crystalline rock. In these conditions, drill bit selection is critical. Bits used in geothermal environments are often identical to those used in oil and gas environments, except that they are more likely to come from the harder end of the specification class range. The oil and gas industry tends to set the market price of drill bits. Hard tungsten carbidebased roller cone bits, the most commonly used type for geothermal applications, comprise less than 10% of this market. Hard formation bits from the oil and gas industry generally do not provide sufficient cuttingPDF Image | Drilling Technology and Costs
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