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6.3.3 Updated geothermal well costs Drilling Technology and Costs The MITDD drilling cost index was used to update completed well costs to year 2004 U.S. $ for a number of actual and predicted EGS/HDR and hydrothermal wells. 617 Table A.6.3 (see appendix) lists and updates the costs of geothermal wells originally listed in Tester and Herzog (1990), as well as geothermal wells completed more recently. Actual and predicted costs for completed EGS and hydrothermal wells were plotted and compared to completed JAS oil and gas wells for the year 2004 in Figure 6.1. Actual and predicted geothermal well costs vs. depth are clearly nonlinear. No attempt has been made to add a trend line to this data, due to the inadequate number of data points. Similar to oil and gas wells, geothermal well costs appear to increase nonlinearly with depth (Figure 6.1). However, EGS and hydrothermal well costs are considerably higher than oil and gas well costs – often two to five times greater than oil and gas wells of comparable depth. It should be noted that several of the deeper geothermal wells approach the JAS Oil and Gas Average. The geothermal well costs show a lot of scatter in the data, much like the individual ultradeep JAS wells, but appear to be generally in good agreement, despite being drilled at various times during the past 30 years. This indicates that the MITDD index properly normalized the well costs. Typically, oil and gas wells are completed using a 6 3/4” or 6 1/4” bit, lined or cased with 4 1/2” or 5” casing that is almost always cemented in place, then shot perforated. Geothermal wells are usually completed with 10 3/4” or 8 1/2” bits and 9 5/8” or 7” casing or liner, which is generally slotted or perforated, not cemented. The upper casing strings in geothermal wells are usually cemented all the way to the surface to prevent undue casing growth during heat up of the well, or shrinkage during cooling from injection. Oil wells, on the other hand, only have the casing cemented at the bottom and are allowed to move freely at the surface through slips. The higher costs for larger completion diameters and cement volumes may explain why, in Figure 6.1, well costs for many of the geothermal wells considered – especially at depths below 5,000 m – are 25 times higher than typical oil and gas well costs. Largediameter production casings are needed to accommodate the greater production fluid flow rates that characterize geothermal systems. These larger casings lead to larger rig sizes, bits, wellhead, and bottomhole assembly equipment, and greater volumes of cement, muds, etc. This results in a well cost that is higher than a similardepth oil or gas well where the completed hole diameter will be much smaller. For example, the final casing in a 4,000 m oil and gas well might be drilled with a 6 3/4” bit and fitted with 5” casing; while, in a geothermal well, a 10 5/8” bit run might be used into the bottomhole production region, passing through a 11 3/4” production casing diameter in a drilled 14 3/4” wellbore. This trend of higher costs for geothermal wells vs. oil and gas wells at comparable depths may not hold for wells beyond 5,000 m in depth. In oil and gas drilling, one of the largest variables related to cost is well control. Pressures in oil and gas drilling situations are controlled by three methods: drilling fluid density, wellhead pressure control equipment, and well design. The well design change that is most significant when comparing geothermal costs to oil and gas costs is that extra casing strings are added to shut off highpressure zones in oil and gas wells. While overpressure is common in oil and gas drilling, geothermal wells are most commonly hydrostatic or underpressured. The Chapter 6PDF Image | Drilling Technology and Costs
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