For the purposes of this paper, sustainability is defined as the ability to economically maintain the installed capacity, over the amortized life of a power plant, by taking practical steps (such as, make-up well drilling) to compensate for resource degradation (pressure drawdown and/or cooling). Renewability is defined here as the ability to maintain the installed power capacity indefinitely without encountering any resource degradation; renewable capacity is, however, often too small for commercial development. This paper also considers an additional level of commercial capacity (above the sustainable level) that is not planned to be maintained fully over the entire plant life as mitigation of resource degradation would become uneconomic or otherwise impractical at some point. This declining capacity above the sustainable level is considered commercial only if the levelized power cost is lower than that from alternative renewable, or environmentally benign, energy sources. Even if power cost at this unsustained commercial generation level proves higher than that from fossil fuels, this additional capacity can reduce fossil fuel usage if power from renewable or environmentally benign energy resources is given adequate tax breaks or price support. Displacement of fossil fuel usage is a social imperative that would reduce environmental pollution today and preserve these fuels as raw material for organic chemicals, and for potentially cleaner power generation in the future.

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.

This paper presents the case history of the first significant expansion of the installed power capacity in two decades at The Geysers steam field (California), which has been producing commercial electric power for nearly 50 years. Western GeoPower Corporation is constructing a 35 MW (net) power plant at The Geysers, where the generation capacity today has declined to about 900 MW from its peak of 1,800 MW in 1987. A 62 MW (gross) plant was operated at the Western GeoPower site from 1979 to 1989 but was shut down because of a rapid decline in well productivity. The development of a new 35 MW plant at this site has become possible today because: (a) a long production history and a large amount of resource data are available; (b) a substantial infrastructure exists at the site; and (c) the augmented injection in The Geysers field with treated municipal effluent over the last decade has sharply reduced well productivity decline. All four production wells drilled to date for this expansion have proven commercial; three of the four wells have shown much higher productivity than is typical for The Geysers field today, the fourth one being about average.

The Rye Patch geothermal field is a classic Basin-and-Range resource with a "blind" anomaly, moderate temperature and "deep circulation" as the source of heat. This paper presents a conceptual model of the field based on lithologic, geophysical and temperature logs as well as production, injection and pressure interference data from 8 deep wells drilled to date. For the hottest well in the field (405°F), we have conducted well bore heat transfer modeling to resolve a major observed discrepancy between the flowing and static temperature profiles, the latter being shown to be affected by downflow of cooler water. The conceptual model has been developed to fit all available resource information, and had been used to estimate reserves: a minimum of 36 M Wand a most-likely level of 64 MW for a 20-year project life. Based on well test data, the maximum gross power production capacity from the existing wells is estimated at 21 MW. The existing wells are capable of supplying the 12.5 MW plant installed at the site if one or two additional wells are drilled for injection.