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.

This paper presents the initial results of a survey of the occurrence and characteristics of geopressured fluid resources in California using the publicly-available database involving more than 150,000 oil and gas wells drilled in the State. Of the 975 documented on-shore oil and gas pools studied, about 42 % were identified as potentially geopressured. Geothermal gradients in California oil and gas fields lie within the normal range of 1°F to 2 OF per 100 feet. Except for the Los Angeles Basin, there was no evidence of higher temperatures or temperature gradients in geopressured pools. The porosity of geopressured pools shows the same normal distribution as for normal pressured pools, with a mode in the range of 20 to 25 %. The salinity distribution of both the geopressured and normal pressured pools appear to be bimodal, each with two peak ranges of o to 10,000 and 25,000 to 30,000 ppm. Compared to the U. S. Gulf Coast region, geopressured pools in California display much lower water salinity, and therefore, should have a higher solubility for methane.

This paper presents the results of a numerical simulation study of the performance of Enhanced Geothermal Systems (EGS), specifically, reservoirs with subcommercial permeability enhanced by hydraulic stimulation. The performance under consideration here is the net electrical power delivered as a function of time and the parameters in this exercise reflect conditions encountered at the Desert Peak EGS project in Nevada.

Existing geologic models of geothermal systems work reasonably well for the exploration of systems clearly associated with active, or recently active, volcanic centers. The poor results of exploration drilling undertaken during the last few years in the non-volcanic systems of the western United States, however, indicate that the model being used for exploring these systems requires significant modification to be useful. Based on considerations of local and regional geology, as well as on drilling results, it is proposed that the presence of large tilted fault blocks, characteristic of extended terrains, are essential to the development of this type of non-volcanic geothermal system.

The potentially exploitable geothermal energy reserves associated with an active or dormant volcano can be estimated using a methodology that combines principles of conductive heat transfer and volcanology to calculate temperature distribution in time and space following magma emplacement, then calculates recoverable geothermal reserves using principles of thermodynamics combined with reasonable values for rock and fluid properties and "recovery" and "utilization" factors. Four principal magma characteristics must be estimated for the calculation: volume, depth, age and temperature of emplacement. Since these four parameters and the recovery factor are the most important uncertain parameters in the reserve calculation, a probabilistic simulation is done by assigning to each a reasonable maximum and a reasonable minimum value and a probability distribution - usually triangular (if a most-likely value can be defined) or uniform (all values between the minimum and maximum being equally likely). The mean, standard deviation and most-likely values of reserves are then calculated statistically through Monte Carlo sampling of the uncertain parameters. The bases for assigning these maximum and minimum values are discussed, and an example of application of this methodology to a volcano in Nicaragua is presented. In a nationwide assessment of geothermal prospects in Nicaragua, this methodology has been applied to 14 different volcanoes, ranging in volume from 4 to 220 cubic km, depth from 3 to 7 km, age from 5,000 to 500,000 years, and temperature from 900°C to 1,100°C. With a uniform distribution of 0.05 to 0.1 for the recovery factor, and a typical utilization factor of 45%, the mean calculated reserves per volcano ranged from 61 MW to 676 MW for 30 years. In the absence of detailed exploration and drilling, this methodology provides a perfectly general, rigorous and internally consistent method of geothermal reserves estimation in the volcanic environment.

The Geysers geothermal field is the largest developed geothermal system in the world. The total installed capacity is presently 2,056 MW although actual production in early 1994 was estimated to be only 1,200 MW(net). The inability to produce at higher generation levels is due to the significant pressure decline that has occurred within the reservoir, particularly since the mid- 1980's. In 1992, a comprehensive field-wide numerical simulation model of The Geysers field was completed, based on drilling, well-test and production data provided by four of the five major field operators. The numerical simulation model was originally developed by UNO CAL, based primarily on data from the UNOCAL-NEC-Thermal (U-N-T) lease areas, and was extended in this study to incorporate data from the other operators.

This paper investigates the technical feasibility of improving the productivity of superheated steam production wells by low-rate water injection downhole. A combined numerical simulation model of the reservoir and well bore was developed to analyze the physical and thermodynamic processes associated with such downhole injection. The model consisted of a geometrically- increasing radial grid with 12 horizontal layers, the vertical stack of the central grid blocks representing the well bore. The model was calibrated against the temperature and pressure profiles from two flowing wells at The Geysers steam field in California. The modeling of low-rate downhole injection (through a tubing) so far indicates that up to a third of a megawatt (MW) of additional power can be easily gained by injection at 7,500 feet (ft) in a superheated steam well. Of this additional steam, about half results from de-superheating of steam and the rest from extraction of heat from the formation surrounding the well bore. In addition to increasing the power capacity, downhole injection allows neutralization of acidic steam through the addition of caustic soda to the injection water, scrubbing of chloride, and dilution of the non-condensable gases in the produced steam.

This is a data interchange content model for an observation of the chemical composition of an aqueous fluid. A header content model for sample characterization, location, and analysis metadata is included to assist users finding analyses for specific samples, locations, time intervals, etc. Typically water temperature at the time of sampling are recorded along with sample data. Several 'suites' of analytes representing common analysis results are defined, with the intention that an implementation of the content model would offer several observation feature types, each of which consists of the header fields combined with the analytes for a suite. A content type for reporting results for a single analyte is also proposed, with an abbreviated header; this could be implemented for services similar to the EPA storet data services. In this approach a single analysis result is delivered as a collection of observation records, each with the same AnalysisURI and reporting single analyte. AqueousChemicalAnalysis-BasicData contains fields for information used to discover and retrieve chemical analyses meeting various criteria, and to evaulate the reported results. These fields would be reported along with a collection of analytes from the different suites proposed on the suites table.