At the request of the United States Department of Energy, the author was asked by the Geothermal Energy Association (Washington, D.C.) to prepare a white paper on the subject (in connection with a new national assessment of geothermal resources). This paper offers a possible scheme in which geothermal resources are classified into seven categories based on temperature: non-electrical grade (<100°C), verylow temperature (100°C to <150°C), low temperature (150°C to 190°C), moderate temperature (190°C to <230°C), high temperature (230°C to <300°C), ultrahigh temperature (>300°C), and steam fields (approximately 240°C with steam as the only mobile phase). In the first four classes, liquid water is the mobile phase in the reservoir; in the “high” and “ultra-high” temperature reservoirs, the mobile fluid phase is either liquid or a liquid-vapor mixture.

Non-condensible gas and stable isotope data from the extreme northwest end of The Geysers steam field indicate that this part of the reservoir consists of a deeper zone, below about -8,000 feet msl, where gas concentrations arc as high as 100,000 ppm-wt, and a shallower zone with steam containing 15,000 ppm-wt gases, generally conforming to stratification of the reservoir previously reported from a production area several miles to the cast. Production wells may tap the shallow zone, the deep zone, or both, depending upon the positions of their production zones with respect to the interface. In addition to high gas content, the deeper zone is characterized by anomalously high temperatures, heavy isotopes of oxygen (and pcrhaps hydrogen) in the steam, and high total carbon in the gases. Shifts of gas composition during production can be interpreted in terms of dilution of steam in the shallow zone by boiling of reservoir liquid, and mixing between deep and shallow steam. Reservoir steam saturation after 1-1/2 years of production is caleulated to be about 0.3 in the shallow zone, and about 0.9 in the deep zone. There is some problem with saturation caleulations which involve the concentration of hydrogen, which yields deep zone saturation values as high as 2.5, which is impossible. This is discussed in relation to the possible presence of "excess" hydrogen in the steam.

Formal booking of geothermal energy reserves, for accounting purposes or annual reporting to shareholders or portfolio management, is not yet a common practice among geothermal companies. In the petroleum industry booking of oil and gas reserves is a routine practice, and at least two geothermal operators that are subsidiaries of petroleum companies book geothermal reserves. As in the petroleum industry, the reserves should be booked in appropriate resource uncertainty categories. To this end we propose three reserve categories with reference to the cumulative probability of exceeding the estimated reserves level: "proved" (equivalent to the 90th percentile), "proved-plus-probable" (equivalent to the lesser of the median and most-likely values), and "proved plus probable-plus-possible" (equivalent to the 10th percentile). However, before any reserves are booked in the proved category, we believe prospects for commercial productivity from the reservoir should be demonstrated. For the purpose of booking, reserves can be expressed in kilowatt-hours and also in equivalent barrels of oil.

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.

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.

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.

This paper presents an analysis of power generation prospects from Enhanced Geothermal Systems (EGS), specifically, reservoirs with subcommercial permeability enhanced by hydraulic stimulation. EGS is also known as “hot dry rock” or “hot fractured rock” systems. The performance under consideration here is the net electrical power delivered as a function of time over the 20-to-30 year life of a power plant. Although the parameters in this exercise generally reflect conditions encountered at the Desert Peak EGS project in the State of Nevada, United States, the conclusions are applicable, at least qualitatively, to any EGS project.

Most theoretical fundamentals of geothermal geochemistry were established by the mid-1980s, as were numerous practical applications of these fundamentals to geothermal resource evaluation and management. Since that time, these geochemical tools have been refined to various degrees. Advances are discussed in the categories of sampling and analysis, exploration and resource evaluation, and resource management. Noted developments include: widespread use of high-performance liquid chromatography (HPLC); advances in spectral analysis; new and refined chemical geothermometers (especially using non-condensible gas species); analysis tools that enable fluid inclusion stratigraphy; ground surface CO2 flux measurement; integration of geochemical reaction models into numerical reservoir simulation; scaling and wellflow chemistry modification; new reservoir tracers and flow-line tracer enthalpy technology. Emphasis is placed on commercially applied technology, but academic developments are included.

Drilling or recompletion of production wens with two or more producing wen bores is a technique that has been used successfully by several operators in the southwestern part of the Geysers field to increase well productivity and decrease the unit cost of steam production. 111e northwestern part of the field poses special difficulties for drilIing multiple-legged or "forked" wens, because of greater reservoir depth, higher reservoir temperatures, the need for narrower well completions, and less stable rock conditions. Three wells within the Adjacent project area of the northwest Geysers were recompleted as 2- and 3-legged producers during 1992-1993, using both existing and new methods for coping with the conditions in this part of the field. Total drilled depths of the producing legs were as much as 11,345 feet (3,458 m), and kickoff points were as deep as 7,895 feet (2,405 m) from 9-5/8-inch production casings. Reservoir temperatures in directionally drilled intervals reached in excess of 600°F (315°C).

A methodology is derived for calculating the static pressure normalized flow rate histories of wells from the usual production records kept by operators, namely, flow rate and flowing wellhead pressure as functions of time. Then, a generalized approach is developed for analyzing the flow rate decline trend to estimate the future decline in well productivity, make-up well requirement, remaining reserves, and well life. The authors have observed, based on data from several hundred wells, that the usual decline trend at The Geysers is "harmonic" with occasional episodes of "exponential" decline in response to new power plants coming on line. In the approach presented here, two decline curves are prepared for each well: flow rate versus cumulative production and the logarithm of flow rate versus cumulative production; the former plot shows a linear data trend if the decline trend is exponential and the latter if the decline trend is harmonic. The authors have observed from well histories as well as numerical simulation that forecasting based on either a linear p/z trend with cumulative production or an assumed exponential decline is conservative, while forecasting based on a harmonic decline trend is optimistic. Because of data scatter or too short a history, in many cases the flow rate decline trend of a well may be fitted to either exponential or harmonic equation; in such cases the lower and upper limits of the decline trend can be established.