Some proponents of geothermal energy have described this energy source as renewable, but many geothermal fields show declines in output as exploitation proceeds. On the other hand, those who would call geothermal a depletable energy source have to explain how some mature fields are able to produce with negligible declines and no apparent limit to the amount of recoverable energy. The confusion arises out of the attempt to describe geothermal resources using inappropriate conceptual models of how resources behave. This paper presents a conceptual model that is better suited to describe the expected performance of a geothermal field over its entire life cycle. The paper also describes a set of terms that can be used to quantify geothermal resources and to guide plans for development.

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.

The purposes of this paper are to review the stratigraphic and structural models which have been developed for The Geysers from surface mapping, propose a model which is the most compatible with drilling results and, based on the preferred model, show how stratigraphy and structure control the occurrence of steam.

This paper presents the results of a continuing study of geopressured resources in California. Three aspects of geopressured resources were examined in this study: the identification of geopressured zones in wells; quantification of the excess pressure from well logs; and the quantity of dissolved methane in California geopressured fluids.

Under the premise that the behavior of enhanced geothermal systems (EGS) will be dominated by fracture flow, this paper reviews the special features that would be required of any practical numerical simulator for EGS. These features, required in addition to the basic features of conventional geothermal simulators, namely, the ability to handle two-phase fluid flow, heat transfer and tracer transp0l1 in porous and fractured media, are: explicit representation of fractures, change in fracture aperture due to effective stress and shear, thermo-elastic effects, relation between fracture aperture and conductivity, and channeling of fluid flow within fractures. Chemical reaction between water and rock and coupling of the reservoir model with a well bore model would also be desirable features.

A numerical simulation model of The Geysers geothermal field, California, has been developed using reservoir and production data provided by UNOCAL, Calpine, NCPA and CCPA. The model was originally developed by UNOCAL and was based primarily on data from the Unocal-NEC-Thermal (U-N-T) lease areas; the purpose of this study was to extend the model by incorporating data from other field operators and to use the re-calibrated model to forecast future production trends.

The resource risk in connection with the financing of geothermal projects can be subdivided into questions of: resource existence, resource size, deliverability, cost of development and operation, environmental constraints, management and operational problems, and resource degradation. Except for the question of resource existence, these risks can change in perception, or in reality, over time. If resource risk cannot be managed or mitigated, it will result in increased cost, loss of revenue, or both, at times leading to economic failure and shutdown of the project. Costs accrue to the developer and the equity investors. In certain cases they can be passed on to lenders (default and rescheduling of payments), or to the public (increased power cost or increased taxes). This in turn may cause investors or lenders to shun future geothermal projects, or to increase the requirement for protective guarantees, with the result that future projects become more costly to finance, and thus less likely to succeed economically.

Geothermal power generation and mineral extraction from geothermal brines are affected by chemical processes within the reservoir. Until recently, numerical simulation technology for geothermal systems could not handle most chemical processes, except for tracking total salinity, one or two non-condensable gases, and non-reactive tracers. The Lawrence Berkeley National Laboratory (LBNL) has developed an enhanced version of their geothermal reservoir simulation software TOUGH2, developed with funding from the U.S. Department of Energy. This highly innovative software (TOUGHREACT) includes comprehensive chemical interactions between liquid, gaseous and solid phases that are coupled to the modeling of solute transport and subsurface multiphase fluid and heat flow.

An industry-DOE cost-shared project is underway to evaluate the technical feasibility of developing an EGS power generation project on the eastern side of the Desert Peak geothermal field. An existing well (DP 23-1) is the focus of much of the Phase I investigation, including re-interpretation of lithology, acquisition and analysis of a well bore imaging log, conducting and analyzing a step-rate injection test, performing a "mini-frac" to determine the magnitude of the least principal stress, and re-completing the well in preparation for hydraulic stimulation in Phase II. In addition, numerical modeling has been undertaken to estimate heat recovery and make generation forecasts for various stimulated volumes and well configurations.

The Department of Energy's EGS Strategic Plan anticipates that EGS experimentation in the United States will be underway by 2004 in areas within or adjaeent to commerciallydeveloped hydrothermal fields, and that an EGS demonstration plant will begin operating in 2008. Criteria for selecting sites for EGS experimentation focus on enhancing energy recovery at producing fields while advancing the EGS knowledge base incrementally towards a demonstration project. With input from field operators, basic characteristics and information on the type of EGS work that may be undertaken are presented for 15 producing fields and 2 unexploited fields. Six producing fields meet 90 - 100% of the site selection criteria and 9 meet 60 - 70%. The use of EGS techniques to supply an existing facility has the advantages of low cost, support from the geothermal industry and demonstration of applicability to a variety of conversion technologies. This approach seeks to reduce EGS risk and uncertainty, promoting the transition from research to commercial evelopment.