A rule of thumb that is often mentioned is that drilling expense accounts for 40%–60% of the costs of a geothermal project. Although the actual costs depend on many variables, there is a general consistency between drilling expenses and depth of the hole drilled. As shown in Figure 9.1, the drilling costs for wells that will be used for power generation run into millions of dollars. For comparison, the costs of completing oil and gas holes by the petroleum industry are also shown. Also shown is the approximate cost curve for completing boreholes for geothermal heat pumps (GHPs), which are discussed in Chapter 11. Although GHP boreholes are generally drilled to depths of only a few hundred feet, an extrapolated curve is shown in the gure in order to facilitate comparison.