The Circum-Pacific region is the focus for much of the current geothermal energy activity. Geothermal resources are typically accessed using conventional petroleum or water well drilling techniques. However, the uniqueness of the geothermal resource often causes problems and the impact such problems have on the costs of accessing geothermal reservoirs can be substantial.

Historical data demonstrate the significance of unexpected problems. In extreme cases, trouble costs are the largest component of well costs, or severe troubles lead to abandonment of a hole. Drilling experiences from several U.S. geothermal areas are used to analyze the frequency and severity of various problems. In addition, expected trouble costs are calculated, based on estimates of probabilities of occurrence as a function of depth for different wells.

The most frequent drilling and completion problem in geothermal wells is lost circulation. This is especially true for resources in underpressured, fractured formations. Serious loss of circulation can occur during drilling--because of this, the producing portions of many wells are drilled with air as a drilling fluid and the inherent corrosion/erosion problems are tolerated--but it can also affect the cementing of well casing. Problems in bonding casing to the formation result from many other causes as well, and are common in geothermal wells. Good bonds are essential because of the possibility of casing collapse due to thermal cycling during the life of the well. Several other problems are identified and their impacts are quantified and discussed.

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