Most presently active or contemplated hot dry rock (HDR) projects vary significantly from the original HDR concept of creating an engineered reservoir in an effectively impermeable medium. Instead, they form part of a spectrum of geothermal resources that exhibit less-than-commercial productivity, but are not completely impermeable. A new working definition, "Enhanced Geothermal Systems" (EGS), is now being used to describe all but the high-grade hydrothermal portion of the geothermal resource spectrum. To promote the commercial development of these lower permeability and/or fluid-deficient resources, EGS research and development in the U.S. during the next few years is likely to include field experiments that target hot, low permeability zones within or near the margins of commercially developed hydrothermal systems. These systems are located in areas of elevated regional heat flow (Cascade Range, Basin and Range Province, Salton Trough, California Coast Ranges and Rio Grande Rift Zone), where temperatures of 250°C can be reached at depths of 4 km or less. Potential sites for EGS experiments were identified with the objective of yielding immediate benefits to the geothermal industry while also contributing to the commercial development of an incrementally broader range of geothermal resources.

This paper investigates the economics of steam production at The Geysers from the point of view of a field developer. We present a cash-flow analysis and the calculation of several profitability criteria for steam supply to a hypothetical 55 MW (gross) power plant starting in 1989. This paper assesses in two parts the economics of developing the steam supply: (1) a deterministic economic analysis to establish the sensitivity of the profitability criteria to steam price where each parameter is given a unique value, and (2) a probabilistic analysis to estimate the profitability criteria, and their sensitivity to steam price, when uncertain parameters are allowed to vary. The results of the study indicate that no new commercial project is economically feasible at The Geysers unless the steam price exceeds 2 cents/kw'hour, because of long payout and extremely low profitability. Only a steam price exceeding 2.7 cents/kw'hour ensures a reasonably short payout time and the minimum profitability typically expected by field developers. Above a steam price of 3 cents/kw'hr, the economics of field development are attractive and risks are low. The accelerated decline in well productivity in recent years has increased risks and reduced profitability.

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.

This paper presents a technical feasibility analysis of the Western Geopower Corporation’s (“WGP”) 25.5 MW Unit 1 power project at The Geysers steam field, California; the WGP leasehold covers 567 acres. A commercial power plant (P.G.&E. Unit 15) operated at this leasehold during 1979 to 1989; the plant was shut down and dismantled, and the wells were plugged and abandoned, mainly due to the unduly high rate of decline in well productivity then experienced throughout The Geysers field. A new geothermal power development at this site has several attractive attributes: (a) a long production history and a large amount of resource data are available; (b) a substantial infrastructure still exists intact at the site; and (c) the decline rates in reservoir pressure and well productivity throughout The Geysers field are far lower today than when the original plant operated. An assessment of the geological characteristics of the field indicates that at least 423 acres of the leasehold can potentially supply steam to a power plant. Based on the potentially productive acreage and the 10-year production history of the original wells, steam reserves within supply a 25.5 MW plant for a project life of at least 20 years.

This paper derives the relation between productivity and pressure decline trends in a geothermal steam reservoir. At all stages of a geothermal project, the economics is affected by the rate of productivity decline. The rate of pressure decline becomes a critical issue when the reservoir pressure declines to the point that wells have to be flowed wide open to supply the turbine; at that stage, maintaining reservoir pressure and well productivity by redistribution and/or augmentation of injection becomes a stratcgic issue in field management. Therefore, the issues considered in this paper have significant practical implications.