The Salton Sea geothermal system is overlain by a thermal cap of low permeability rocks that restricts the upward movement of the high-temperature reservoir brines. Petrographic and fluid inclusion data from two wells show that the thermal cap in the southern part of the field consists of an upper layer of lacustrine and evaporite deposits with low initial permeabilities and a lower layer of deltaic sandstones. The sandstones were incorporated into the thermal cap as downward percolating fluids deposited anhydrite and calcite in the pore space of the rocks, reducing their permeabilities. During development of the thermal cap, base-metal sulfides, potassium feldspar and quartz veins were deposited by brines from higher temperature portions of the system.

The maximum wellhead pressure at which hot water wells discharge is an important parameter for geothermal power and as it slowly declines with years of exploitation presents a moving target for project designers. It can also decrease rapidly for newly closed-in wells (within days or even hours) to a point at which auto-discharge is impossible and tedious techniques have to be employed to restart flow. The common cause of this phenomenon is reduction in the temperature of the hot water feeding the well; in the former case is the result of a general decline in the reservoir water enthalpy, and in the latter is due to cooler denser water from higher in the uncased part of the well percolating down and flooding the lower more permeable levels from which a discharging well mainly draws its fluids. The inter-relationship of feed water temperature, depth and maximum discharging-pressure is determined in this study with illustrated examples demonstrating application.

This section covers analyses performed on gas. Chemical analyses can only be related to well performance if the quantity of the various fluids are known. The IGT on-line data computer system measures the flowrate, the pressures, and the temperatures every 10 seconds. These values are automatically recorded over operator selected intervals both on magnetic media and on paper. This allows review of samples versus operating conditions. This paper covers analyses performed on gas, including: An approximate sampling schedule during flow tests; On-site sample handling and storage of gas samples; Addresses of laboratories that perform off site analyses; Sample shipping instructions; Data archiving; and Quality Control/Quality Assurance. It is expected that the above procedures will change as the flow test progresses, but deviations from the written procedures should be approved by C. Hayden of IGT and noted on the results of the analysis.

An independent analysis of lost circulation materials for geothermal applications has been completed using unique laboratory tools developed for the purpose. Test results of commercial materials as well as mathematical models for evaluating their performance are presented. Physical attributes that govern the performance of lost circulation materials are identified and correlated with test results. 9 refs., 27 figs., 4 tabs.

Scientific core hole VC-2A was drilled into the western ring-fracture zone at Sulphur Springs in the Valles caldera, New Mexico. VC-2A, the second scientific core hole in the caldera, was cored through a faulted and brecciated sequence of intracauldron tuffs and volcaniclastic rocks to a depth of 528 m. As of November 1, 1986, the unequilibrated bottom-hole temperature was 212/degree/C. The rocks penetrated are intensely altered and host sub-ore grade stockwork molybdenite mineralization between 25 and 125 m. This report contains a detailed core log to aid researchers in their studies of the Valles caldera magma hydrothermal system. 3 refs., 2 figs.

Chloride (Cl) concentrations of 10-120 ppm{sub w} have been measured in superheated steam produced by wells at The Geysers, a vapor-dominated geothermal field in northern California. Corrosion of the well casing and steam-gathering system has been recognized in some parts of The Geysers, and is apparently related to the presence of Cl. Cl in the steam is in a volatile form, generated with the steam at reservoir temperatures, and probably travels to the wellhead as HCl gas. Published experimental data for partial pressures of HCl in steam over aqueous HCl solutions and for dissociation constants of HCl were used to calculate distribution coefficients for HCl. Reservoir liquid Cl concentrations capable of generating steam with the observed Cl concentrations were then calculated as a function of pH and temperatures from 250 to 350º C. Equilibrium mineral/liquid reactions with the K-mica and K-feldspar assemblage found in the wells limit the reservoir liquid pH values at various Cl concentrations to about 5 to 6 (near neutral at 250 to 350º C). Within this pH range, liquid at 250º C could not produce steam containing the high Cl concentrations observed. However, liquid at higher temperatures (300 to 350º C) with chloride concentrations greater than …

If analysis of the microseismicity accompanying fluid injections is to be of maximum use in predicting hot dry rock (HDR) reservoir performance, it should lead to the determination of both the rock volume and active flowing surface area of the reservoir. In the granitic rock at the HDR geothermal site at Fenton Hill, New Mexico, the micro-earthquakes located during hydraulic fracturing occur in large three-dimensional volumes called seismic clouds. Cores cut from the region prior to fracturing show numerous planar fractures, some mineral-filled, at virtually random orientations. Evidence supports the hypothesis that only a few of these planes make up the flow path between wells for most of the injected fluid. If this is indeed the case, then it is necessary to be able to distinguish between fractures that accept flow from those which do not. We accomplish this by defining “flow-probable” planes to be those which have seismicity located relatively farther away from lines where other planes intersect. We show that these flow probable planes intercept wellbores at locations where other data confirm the presence of hydraulically active fractures.

Virtually all wells drilled into bedrock intercept a water-bearing fracture, but cavities occur only in areas underlaid by limy rocks. Multiple cavities are common in wells in the Conasauga and Knox Groups but are rare in the Rome Formation and the Chickamauga Group. The geometric mean height (vertical dimension) of the cavities is 0.59 m, the geometric mean depth is 14 m, the average lateral spatial frequency is 0.16, and the average vertical spatial frequency is 0.019. Differences in cavity parameter values are caused partly by geologic factors such as lithology, bed thickness, and spatial fracture frequency. However, hydrologic factors such as percolation rate, recharge amount, aquifer storage capacity, and differences between lateral and vertical permeability may also be important. Tracer tests show that groundwater velocity in some cavities is in the range 20-300 m/d, and relatively rapid flow rates occur near springs. In contrast, wells that intercept cavities have about the same range in hydraulic conductivity as wells in regolith and fractured rock. The hydraulic conductivity data indicate a flow rate of less than 1.0 m/d. This difference cannot be adequately explained, but rapid groundwater movement may be much more common above the water table than below. Rapid groundwater …

A correlation is developed to predict thermal conductivity of drill cores from the Los Azufres geothermal field. Only andesites are included as they are predominant. Thermal conductivity of geothermal rocks is in general scarce and its determination is not simple. Almost all published correlations were developed for sedimentary rocks. Typically, for igneous rocks, chemical or mineral analyses are used for estimating conductivity by using some type of additive rule. This requires specialized analytical techniques and the procedure may not be sufficiently accurate if, for instance, a chemical analysis is to be changed into a mineral analysis. Thus a simple and accurate estimation method would be useful for engineering purposes. The present correlation predicts thermal conductivity from a knowledge of bulk density and total porosity, properties which provide basic rock characterization and are easy to measure. They may be determined from drill cores or cuttings, and the procedures represent a real advantage given the cost and low availability of cores. The multivariate correlation proposed is a quadratic polynomial and represents a useful tool to estimate thermal conductivity of igneous rocks since data on this property is very limited. For porosities between 0% and 25%, thermal conductivity is estimated with a maximum …

A high-temperature vapor-dominated reservoir underlies a portion of the Northwest Geysers area, Sonoma County, California. The high-temperature reservoir (HTR) is defined by flowing fluid temperatures exceeding 500º F, rock temperatures apparently exceeding 600º F and steam enthalpies of about 1320 BTU/lb. Steam from existing wells drilled in the Northwest Geysers is produced from both a “typical” Geysers reservoir and the HTR. In all cases, the HTR is in the lower portion of the wells and is overlain by a “typical” Geysers reservoir. Depth to the high-temperature reservoir is relatively uniform at about -5900 ft subsea. There are no identified lithologic or mineralogic conditions that separate the HTR from the “typical” reservoir, although the two reservoirs are vertically distinct and can be located in most wells to within about 200 ft by the use of downhole temperature-depth measurements. Gas concentrations in steam from the HTR are higher (6 to 9 wt %) than from the “typical” Geysers reservoir (0.85 to 2.6 wt %). Steam from the HTR is enriched in chloride and the heavy isotopes of water relative to the “typical” reservoir. Available static and dynamic measurements show pressures are subhydrostatic in both reservoirs with no anomalous differences between the two: …

Three geothermal wells tapping the Dogger aquifer were studied in detail for their variations in chemical composition with time or conditions of exploitation. Analytical improvements for the determination of Cl, SO{sub 4}, Ca, Mg, Na and K make it possible to detect variations respectively of 0.15, 0.8, 0.6, 1.8, 1.8 and 1.4 %. Despite the fact that the natural flow may be important in some parts of the basin aquifer, we conclude that this factor is not responsible for the small variations noticed in mineralization within the one year survey period. The results concerning reactive and nonreactive species are best explained if a vertical heterogeneity of the chemistry of the fluid is assumed. A number of calcareous sub-layers, already demonstrated by geological studies, contribute to varying degrees to the production of the hot water. The changes in pumping rates, which are fixed according to external requirements, play a major role in the hydrodynamic and chemical disequilibrium of the wells. The consequences for the geothermal exploitations are emphasized.

The present exploitation of Palinpinon reservoir has revealed valuable information on the subsurface characteristics of the reservoir under large scale production. The initial behavior of the field has given signals that there is a need to implement appropriate strategies to optimise its capacity without jeopardizing the supply of adequate steam to the power plant. Some of the problems encountered such as reinjection returns, mineral deposition, ingress of acid fluid and other phenomena indicated the need to pursue an aggressive monitoring capability and timely appraisal of the field response to design an approach which will best suit the optimum management of the reservoir. The results of reservoir monitoring are discussed as well as the policies applied in operating the field.

Computer modeling of the Beijing low temperature, basement aquifer system has shown that secular natural convection of meteoric waters down to depths greater than 5 km can produce a temperature field which is similar to that observed in deep wells. Secular convection occurs within a crustal block with the approximate dimensions of 45 km {times} 60 km {times} 10 km; the Beijing system is probably one of the largest secular convecting systems described so far. It is driven entirely by the crustal heatflow which appears to be slightly lower (i.e. 54 mW/m{sup 2}) than the average continental heat flux. Several geophysical implications are described.

The Nesjavellir geothermal field in Iceland is being developed to provide the capital city of Reykjavik and surrounding areas with hot water for space heating. In the last few years, many wells have been drilled at the site and various geothermal studies have been conducted. The main upflow to the system is underneath the nearby Hengill volcano, and the natural recharge rate and enthalpy are estimated to be 65 kg/s and 1850 kJ/kg, respectively. An extensive vapor zone is believed to be present in the upflow region. Permeabilities and porosities of the system range between 1 and 50 md and 1 and 10 percent, respectively. In this paper, the characteristics of the Nesjavellir field are described and a three-dimensional numerical model of the resource is discussed.

The Nesjavellir geothermal field in Iceland is being developed to provide the capital city of Reykjavik and surrounding areas with hot water for space heating. In the last few years, many wells have been drilled at the site and various geothermal studies have been conducted. The main upflow to the system is underneath the nearby Hengill volcano, and the natural recharge rate and enthalpy are estimated to be 65 kg/s and 1850 kJ/kg, respectively. An extensive vapor zone is believed to be present in the upflow region. Permeabilities and porosities of the system range between 1 and 50 md and 1 and 10 percent, respectively. In this paper, the characteristics of the Nesjavellir field are described and a three-dimensional numerical model of the resource in discussed. 15 refs., 11 figs., 1 tab.

H. C. Helgeson noted in 1968 that the salinity of the brine in the geothermal reservoir within the Salton Sea geothermal system generally increases from the top to the bottom and from the center to the sides. He also noted that pressure measurements at perforations in cased wells seemed to indicate that the formation fluids at the depths of production have a specific density about equal to 1, and that hot concentrated brines apparently exist in pressure equilibrium with comparatively cold dilute pore waters in the surrounding rocks. Since 1968 there have been no published reports that dispute these observations. However, a very high heat flux through the top of the system seems to require a substantial component of convective transfer of heat beneath an impermeable cap, whereas the apparent salinity gradient with depth seems to require little or no free convection of brine. This paradox may be resolved if double-diffusive convection is the main process that controls the depth-temperature-salinity relations. Such convection provides a mechanism for transferring heat from the bottom to the top of the hydrothermal system while maintaining vertical and horizontal salinity gradients—densities remaining close to unity. In 1981, Griffiths showed experimentally that layered double-diffusive convection cells …

The complete list of HDR objectives is provided in Reference 10, and is tabulated below in Tables 1 and 2 for the reader's convenience. The primary, level 1, objective for HDR is ''to improve the technology to the point where electricity could be produced commercially from a substantial number of known HDR resource sites in a cost range of 5 to 8 cents/kWh by 1997''. A critically important milestone in attaining this cost target is the level II objective: ''Evaluate the performance of the Fenton Hill Phase II reservoir''. To appreciate the significance of this objective, a brief background is helpful. During the past 14 years the US DOE has invested $123 million to develop the technology required to make Hot Dry Rock geothermal energy commercially useful. The Governments of Japan and the Federal Republic of Germany have contributed an additional $32 million to the US program. The initial objectives of the program were met by the successful development and long-term operation of a heat-extraction loop in hydraulically-fractured hot dry rock. This Phase I reservoir produced pressurized hot water at temperatures and flow rates suitable for many commercial uses such as space heating and food processing. It operated for more …

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