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 spreadsheet is a compilation of Active Fault features compiled by the California Geological Survey, published as a Web feature service for the National Geothermal Data System. The document contains 7 worksheets, including information about the template, instructions on using the template, notes related to revisions of the template, Resource provider information, the data, a field list (data mapping view) and a worksheet with vocabularies for use in populating the spreadsheet (data valid terms). Each feature in an active fault data set (row in this spreadsheet) should be characterized by a unique combination of name, URI, geologic history, slip, locatability and orientation properties, as well as being physically connected or inferred to be connected in the Earth. For mapped active faults ,which are the scope of this scheme, the deformation style is assumed to be brittle (as opposed to ductile). Fields in the data table include FeatureURI, Name, FullName, ParentFeatureURI, Label, Description, Symbol, OtherID, SpecificationURI, FeatureType, GeologicHistory, RepresentativeAgeURI, YoungerAgeURI, OlderAgeURI, IntervalSinceMovement, Shape, ObservationMethod, PositionAccuracyMeters, PositionAccuracy, Displacement, SlipRate, SlipAccumulationInterval, MovementType, MovementSense, DipDirection, DateMostRecentEvent, RecurrenceInterval, TotalSlip, Source and MetadataURI.

This spreadsheet is a compilation of Aqueous Well Chemistry observations compiled by the California Geological Survey, published as a Web feature service for the National Geothermal Data System. The document contains 12 worksheets, including information about the template, notes related to revisions of the template, Resource provider information, the data, a field list (data mapping view), An Analyte Dictionary and a worksheet with vocabularies for use in populating the spreadsheet (data valid terms). Data from 468 wells are included.

This spreadsheet is a compilation of Aqueous Spring Chemistry observations compiled by the Arizona Geological Survey, published as a Web feature service for the National Geothermal Data System. The document contains 12 worksheets, including information about the template, notes related to revisions of the template, Resource provider information, the data, a field list (data mapping view), An Analyte Dictionary and a worksheet with vocabularies for use in populating the spreadsheet (data valid terms). Data from 210 springs are included.

Franciscan rocks in The Geysers Geothermal Resources Area (GRA) constitute a greatly deformed and chaotic assemblage of rocks known as melange which, under natural conditions, poses problems to slope stability. Unstable conditions inherent in the melange are accentuated by the steepness of slopes, chemical alteration of the rocks, and numerous faults and shear zones. Adverse effects that may be .experienced- with development of melange terrain can, however, be mitigated if potential problem areas are recognized before development begins and if good engineering geology·and engineering practices are followed.