This paper investigates the practical range of net power capacity available from conventional and Enhanced Geothermal System (“EGS”) wells as a function of temperature. For a geothermal resource temperature up to about 190°C, which is the operating temperature limit of presently available downhole pumps, wells are typically pumped and power is usually generated in a binary-cycle plant, and in rare cases in a flash-cycle or hybrid-cycle plant. In this temperature range under the current state of downhole pump technology, the net MW capacity of a well has a practical upper limit of about 7.3 MW, irrespective of how high the well’s productivity index is. This capacity limit cannot be improved unless technology can be improved to allow pumping at a higher rate than the present practical limit of about 160 l/s (2,500 gallons per minute); improving the temperature tolerance of pumps, by itself, will not increase this capacity limit. For resource temperatures greater than 190°C, wells must be self-flowed, and power is generated from such wells in a flash-cycle or hybrid-cycle plant.

The technique of completing or recompleting production wells as multiple-legged or "forked" wells has been successfully applied in the Aidlin area of the northwest Geysers field. Wells in the Aidlin area are deeper, hotter and have smaller diameter casings than wells in the southeastern part of the field, where forked wells have been drilled previously. These factors, plus frequently unstable formation conditions at the production casing shoe, add further complications to the forking technique. During 1992-1993, two producing wells at Aidlin were worked over and recompleted as two-legged producers, and one new well was completed with three producing legs. Despite a variety of drilling problems, each of the recompletions resulted in a significant increase in well productivity, with an average increase per well of 58 %. In addition to yielding steam production at a cost that is relatively low compared with drilling new wells, multiple-legged recompletions allow for more flexible scheduling, budgeting and permitting of drilling programs.

This paper presents an analysis of the sensitivity of the cost of geothermal power to: (a) capital cost; (b) operations-and-maintenance (O&M) cost; (c) make-up well drilling cost; (d) resource characteristics (well productivity and its rate of decline); (e) development and operational options (installed plant capacity, number of years of make-up well drilling, and project life); and (f) macro-economic climate (interest and inflation rates). The power cost here represents levelized cost (in cents per kilowatt-hour) over the project life, the capital cost being amortized over 30 years; any royalties, tax burden, or tax credit are ignored. A range of development sizes, from 5 to 150 MW, is considered. The economy of scale in both capital cost and O&M cost, as well as the higher productivity decline rate due to increased installed capacity, are taken into account.