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 article attempts to critically evaluate the present state of the art of geothermal reservoir simulation. Methodological aspects of geothermal reservoir modeling are briefly reviewed, with special emphasis on flow in fractured media. Then we examine applications of numerical simulation to studies of reservoir dynamics, well test design and analysis, and modeling of specific fields. Tangible impacts of reservoir simulation technology on geothermal energy development are pointed out. We conclude with considerations on possible future developments in the mathematical modeling of geothermal fields. 45 refs., 4 figs., 2 tabs.

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.

This proceedings contains 20 technical papers and abstracts describing most of the research activities funded by the Department of Energy (DOE's) Geothermal Reservoir Technology Program, which is under the management of Marshall Reed. The meeting was organized in response to several requests made by geothermal industry representatives who wanted to learn more about technical details of the projects supported by the DOE program. Also, this gives them an opportunity to personally discuss research topics with colleagues in the national laboratories and universities.

This document discusses the development of quantitative simulation models for the investigation of geologic systems. The selection of variables, model verification, evaluation, and future directions in quantitative dynamic stratigraphy (QDS) models are detailed. Interdisciplinary applications, integration, implementation, and transfer of QDS are also discussed. (FI)

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.

Power plants which use geothermal heat to generate electric power produce a residual sludge in large quantities. This material precipitates from supersaturated brines and contains toxic metals, some of which are present in concentrations exceeding the non hazardous waste disposal regulations. Disposal of this waste as hazardous waste is costly. Work in this laboratory has shown that a biotreatment of the geothermal waste in which toxic metal resistant acidophilic organisms are used can serve as a basis for a new biotechnology for detoxification of geothermal residual brine sludges. Phase one studies have shown that an economically and technically feasible biotechnology can be developed. The efficiency of this technology depends on a number of parameters such as the bioreactor design, residence time, and the number and concentration of toxic metals to be removed. Further, the process, while rendering a detoxified material, produces a liquid phase which is enriched in toxic metals. This aqueous phase can be reinjected into the wells, or processed for the recovery of toxic metals, some of which are commercially valuable, for example, chromium. A parallel study in this laboratory has shown that a combined chemical and biochemical process for the recovery of these metals may also be …

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 …

PREFACE The Thirteenth Workshop on Geothermal Reservoir Engineering was held at Stanford University on January 19-21, 1988. Although 1987 continued to be difficult for the domestic geothermal industry, world-wide activities continued to expand. Two invited presentations on mature geothermal systems were a keynote of the meeting. Malcolm Grant presented a detailed review of Wairakei, New Zealand and highlighted plans for new development. G. Neri summarized experience on flow rate decline and well test analysis in Larderello, Italy. Attendance continued to be high with 128 registered participants. Eight foreign countries were represented: England, France, Iceland, Italy, New Zealand, Japan, Mexico and The Philippines. A discussion of future workshops produced a strong recommendation that the Stanford Workshop program continue for the future. There were forty-one technical presentations at the Workshop. All of these are published as papers in this Proceedings volume. Four technical papers not presented at the Workshop are also published. In addition to these forty five technical presentations or papers, the introductory address was given by Henry J. Ramey, Jr. from the Stanford Geothermal Program. The Workshop Banquet speaker was Gustavo Calderon from the Inter-American Development Bank. We thank him for sharing with the Workshop participants a description of the …

Program Review VI was comprised of six sessions, including an opening session, four technical sessions that addressed each of the major DOE research areas, and a session on special issues. The technical sessions were on Hydrothermal, Hot Dry Rock, Geopressured and Magma resources. Presenters in the technical sessions discussed their R and D activities within the context of specific GTD Programmatic Objectives for that technology, their progress toward achieving those objectives, and the value of those achievements to industry. The ''Special Issues'' presentations addressed several topics such as the interactions between government and industry on geothermal energy R and D; the origin and basis for the programmatic objectives analytical computer model; and international marketing opportunities for US geothermal equipment and services. The unique aspect of Program Review VI was that it was held in conjunction with the National Geothermal Association's Industry Round Table on Federal R and D. The Round Table provided a forum for open and lively discussions between industry and government researchers and gave industry an opportunity to convey their needs and perspectives on DOE's research programs. These discussions also provided valuable information to DOE regarding industry's priorities and directions.

The components of geothermal brines that cause corrosion and scaling problems are reviewed, especially brine pH, CO/sub 2/, H/sub 2/S, oxygen (from air), silicia, calcium, sulfides, and suspended particulates. Instrumental methods for on-line measurement are discussed to show how to keep costs low by operating a geothermal plant from a position of knowledge of what is occurring to the plant materials. The US Department of Energy research and development program in brine chemistry and on-line instrument development at Pacific Northwest Laboratory is discussed along with the strategy for commercial availability of new instruments to the geothermal industry.

The Triassic sandstone aquifer offers on a regional scale, a large potential for low-temperature geothermal exploitation in the Paris Basin. The Na-Cl water n the aquifer has highly variable mineralization (TDS = 4 to 110 g/l) and a wide range of temperature (50º to >100ºC). Chemical studies have been carried out on the Melleray site near Orléans, where a single wel was producing a Na-Cl geothermal water (TDS = 35 g/l) at a wellhead temperature of 72ºC to provide heat for greenhouses. The purpose of these studies is to understand the chemical phenomena occurring in the geothermal loop and to determine the treatment of the fluid and the exploitation procedures necessary for proper reinjection conditions to be achieved. During the tests performed after the drilling operations, chemical variations in the fluid were noticed between several producing zones in the aquifer. Daily geochemical monitoring of the fluid was carried out during two periods of differing exploitation conditions, respectively pumping at 148 m{sup 3}/h and artesian flow at 36 m{sup 3}/h. Vertical heterogeneities of the aquifer can explain the variations observed for the high flowrate. Filtration experiments revealed that the particle load varies with the discharge rate and that over 95 weight …

Sericite, either as illite or illite/smectite, is ubiquitous in geothermal systems. Theoretical Ca- and Na-smectite contents of non-expanding geothermal sericites have been calculated from published electron microprobe analyses. Geothermal sericites can be modeled as solid solutions of muscovite and smectite. For those sericites that fit the model, the amount of smectite in solid solution is related to temperature by the expression TºC = 1000/(0.45LogX{sub smectite} + 2.38) – 273. The temperature dependence of illite interlayer chemistry suggests a related temperature dependence of the K, Na and Ca content of geothermal fluids. The original data used by Fournier and Truesdell (1973) to derive the empirical Na-K-Ca geothermometer for geothermal fluids can be modeled equally well by an equation incorporating the equilibrium constant for the reaction of smectite to illite: T ºC = 1.145*10{sup 3}/([0.35LogNa + 0.175LogCa – 0.75LogK] + 1.51) – 273, where the concentration units are molalities. This supports the hypothesis that illite and illite/smectite are important controls on the concentrations of Na, K and Ca in geothermal fluids.

The Hot Dry Rock (HDR) geothermal energy program is a renewable energy program that can contribute significantly to the nation's balanced and diversified energy mix. The program was reviewed five times in the past three years. Three of these reviews were done by the US Department of Energy (DOE) and a fourth was conducted by the National Research Council at the request of DOE. In addition, HDR was evaluated in the Energy Research Advisory Board's Solid Earth Sciences Report. Recent economic studies for HDR have been performed by Bechtel National, Inc., the Electric Power Research Institute, and the United Kingdom. These studies are reviewed in light of recent progress at Fenton Hill in reducing drilling costs, and mapping and in identifying drilling targets. All of the attention focused on HDR has resulted in evaluating the way in which HDR fits within the nation's energy mix and in estimating when HDR will contribute to energy security. To establish a framework for evaluating the future of HDR, the status and progress of HDR are reviewed and the remaining Fenton Hill program is outlined. Recommendations are also made for follow-on activities that will lead to achieving full development of HDR technologies in the …