With the threat of depletion of conventional sources of energy there has been increased interest in harnessing geothermal energy for mans use in many countries. However, there are comparatively few people with wide experience in the science and technology of developing this new energy resource. Geothermal training programs are therefore being organized by many governments, universities, and industries, and there is a need for literature that summarizes the current state of knowledge in various facets of the work. We have drawn upon our background as geochemists with experience in many New Zealand and overseas geothermal areas, to provide information on the theories and methods used in the geochemical aspects of geothermal work. We also describe and seek to place in context the contributions from other sciences in a geothermal development program and provide, as a background, descriptions of the nature and occurrence of geothermal areas. While aimed at geothermal specialist training, the book may also provide useful material in graduate earth science courses, as well as be of general interest to geologists, geophysicists, chemists, and chemical engineers. The first two chapters review the extent of geothermal development throughout the world and outline the geological and hydrological features of geothermal areas. With this introduction, the chemistry of geothermal fluids is then described in detail, and the reasons for the various fluid compositions are interpreted through the chemical and mineralogical reactions that occur in the geothermal systems. A brief description of the physical and chemical nature of high-temperature water solutions follows to aid this understanding. Procedures of geochemical field and laboratory work are considered in detail in Chapters 5 and 6 with particular reference to New Zealand experiences. The basic information, theory, and data required for the chemical work are provided. The approach is practical with many examples. Recomended sequences are given for the geological, geophysical, and geochemical work required in investigating a new geothermal area. Chapter 7 gives examples of the calculations necessary to interpret geochemical results, and methods of storing, retrieving, and interpreting data. The use of chemistry to monitor changes in a geothermal fields is described.
The key element in the characterization, assessment and development of geothermal energy systems is the resource type. Throughout the past 30 years many resource type schemes and definitions were published, based on temperature and thermodynamic properties. An alternative possibility to cataloging geothermal energy systems is by their geologic characteristics, referred to as geothermal plays. Applied to worldwide case studies, a new catalog is developed based on the effects of geological controls and structural plate tectonic positions on thermal regime and heat flow, hydrogeologic regime, fluid dynamics, fluid chemistry, faults and fractures, stress regime, and lithological sequence. Understanding geologic controls, especially of geothermal plays without surface expression, allows the comparison with hydrocarbon reservoirs through their ratio of porosity and permeability. This analog has implications on site-specific, first class exploration strategies and reservoir improvement through technologies specifically suitable for unconventional sustainable energy reservoirs. This article aims to introduce geothermal plays to a wide geoscientific community and to initiate a geologically based cataloging of geothermal resources. With this new catalog of geothermal plays, it will be ultimately possible to transfer lessons learned not only within one specific catalog type, but also technology from geothermal plays to unconventional hydrocarbon plays and vice versa.
The aim of this book is to demonstrate how volcanological concepts can be applied to the evaluation and exploration of geothermal energy resources. In regard to the geothermal content of the book, some of the information comes from the first-hand experience gained during the authors' exploration work in Middle America and with the Los Alamos Hot Dry Rock program. Other cases discussed come from classic geothermal systems in many regions and settings. The book begins with a summary of recent practical advances in volcanology, and then moves on to describe the considerable importance of pyroclastic rocks as a took to evaluate geothermal systems, including an in-depth treatment of hydrovolcanism. Following chapters deal with surface manifestations of geothermal systems, and systems associated with calderas, silicic lava domes, and basaltic volcanoes. The last chapter is on geothermal systems in maturing composite volcanoes. The Appendices include a broad overview of field methods in volcanic regions, volcanic rock classifications and properties, thermodynamic properties of water vapor (steam tables), and the use of cuttings in geothermal well logs. A two-dimensional heat flow code used for estimating geothermal resources is also given. The book makes two significant contributions: first, in its treatment of eruption dynamics, focusingmore » on quantitative and theoretical analysis of volcanic processes, and second, in its comprehensive treatment of the fundamentals of hydrovolcanism, including fuel-coolant interactions and hydrofracturing.« less
Abstract Laboratory measurements and field data indicate that self-potential anomalies comparable to those observed in many areas of geothermal activity may be generated by thermoelectric or electrokinetic coupling processes. A study using an analytical technique based on concepts of irreversible thermodynamics indicates that, for a simple spherical source model, potentials generated by electrokinetic coupling may be of greater amplitude than those developed by thermoelectric coupling. Before more quantitative interpretations of potentials generated by geothermal activity can be made, analytical solutions for more realistic geometries must be developed, and values of in-situ coupling coefficients must be obtained.If the measuring electrodes are not watered, and if telluric currents and changes in electrode polarization are monitored and corrections made for their effects, most self-potential measurements are reproducible within about + or -5 mV. Reproducible short-wavelength geologic noise of as much as + or -10 mV, primarily caused by variation in soil properties, is common in arid areas, with lower values in areas of uniform, moist soil. Because self-potential variations may be produced by conductive mineral deposits, stray currents from cultural activity, and changes in geologic or geochemical conditions, self-potential data must be analyzed carefully before a geothermal origin is assigned to observed anomalies.Self-potential surveys conducted in a variety of geothermal areas show anomalies ranging from about 50 mV to over 2 V in amplitude over distances of about 100 m to 10 km. The polarity and waveform of the observed anomalies vary, with positive, negative, bipolar, and multipolar anomalies having been reported from different areas. Steep potential gradients often are seen over faults which are thought to act as conduits for thermal fluids. In some areas, anomalies several kilometers wide correlate with regions of known elevated thermal gradient or heat flow.
The formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling, dilution, and condensation. In this paper we show how these processes control the occurrence, distribution, and stable isotope composition of calcite based on a study at Broadlands-Ohaaki. The two principal calcite occurrences in the Broadlands-Ohaaki geothermal system are: (1) as replacement of rock forming minerals and volcanic glass; and (2) as platy crystals infilling voids. Both are stable over a broad temperature range from 160 degrees C to 300 degrees C. Replacement calcite is widespread and forms through hydrolysis reactions involving calcium alumino-silicates and sub-boiling liquids that contain 0.3 to 0.75 m CO ~2~ . Platy calcite, in contrast, forms over a restricted vertical interval of a few hundred meters within the upflow zone. It precipitates from boiling fluids through exsolution of carbon dioxide as indicated by coeval liquid-rich and vapor-rich fluid inclusions and its formation in the two-phase zone. Fluid inclusion data help to define the boiling paths of fluids from which platy calcite formed. Homogenization temperatures range from 160 degrees C to 310 degrees C and are consistent within the present geothermal regime. Ice melting temperatures range from 0.0 to -1.0 degrees C and indicate the presence of up to 0.5 m dissolved carbon dioxide. Model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fluid dictates the depth of first boiling. Most fluid inclusion data lie along a model boiling path characteristic of the centre of the upflow zone, in which the rising fluid (initially containing 0.75 m CO ~2~ ) begins to boil at approximately 320 degrees C and approximately 2000 m depth; data from well Br-18 instead matches a curve in which the rising fluid (initially containing 0.53 m CO ~2~ ) begins boiling at approximately 245 degrees C and approximately 900 m depth. The shallowing of the depth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water, as it rises and mixes with marginal waters. Calcite precipitates from both shallow formed steam-headed groundwater and deeply derived chloride water, and these waters are isotopically distinct. At Broadlands-Ohaaki, the delta ^18^ 0 values of calcite at 200 degrees C range from 0.5 to 7.5 per mil, whereas delta ^18^ 0 values of calcite at 200 degrees C range from 4 to 10 per mil. Taking appropriate temperature dependent fractionation factors into account, these data indicate equilibration with chloride water (delta ^18^ 0 ~H2O~ = -4.5 per mil) and steam-heated groundwater (delta ^18^ O ~H2O~ = -7.0 per mil), respectively. Oxygen isotopes of hydrothermal calcites in the nearby Wairakei and Waiotapu geothermal systems show similar patterns, consistent with the occurrence of both chloride and steam-heated waters there. Calcite formation is explained by a model that describes the distribution of two-phase conditions and aqueous carbon dioxide concentrations in a column of hydrothermal fluid rising through a rock matrix of isotropic permeability. In this ideal situation, platy calcite forms along the inner margin of the two-phase zone, having the shape of an inverted cone, whereas replacement calcite mostly forms in the surrounding one-phase liquid-only zone. The sparse occurrence of calcite at less than or equal to 800 m depth in the central upflow of the Ohaaki sector at Broadlands-Ohaaki is compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling deeper in the system.
All cells contain much more potassium, phosphate, and transition metals than modern (or reconstructed primeval) oceans, lakes, or rivers. Cells maintain ion gradients by using sophisticated, energy-dependent membrane enzymes (membrane pumps) that are embedded in elaborate ion-tight membranes. The first cells could possess neither ion-tight membranes nor membrane pumps, so the concentrations of small inorganic molecules and ions within protocells and in their environment would equilibrate. Hence, the ion composition of modern cells might reflect the inorganic ion composition of the habitats of protocells. We attempted to reconstruct the "hatcheries" of the first cells by combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. These ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K(+), Zn(2+), Mn(2+), and phosphate. Thus, protocells must have evolved in habitats with a high K(+)/Na(+) ratio and relatively high concentrations of Zn, Mn, and phosphorous compounds. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in marine settings but is compatible with emissions of vapor-dominated zones of inland geothermal systems. Under the anoxic, CO(2)-dominated primordial atmosphere, the chemistry of basins at geothermal fields would resemble the internal milieu of modern cells. The precellular stages of evolution might have transpired in shallow ponds of condensed and cooled geothermal vapor that were lined with porous silicate minerals mixed with metal sulfides and enriched in K(+), Zn(2+), and phosphorous compounds.
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Research Article| September 01, 1986 Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas H. Martin H. Martin 1Centre Armoricain d'Etude Structurale des Socles, Institut de Géologie, Université de Rennes, 35042 Rennes Cédex, France Search for other works by this author on: GSW Google Scholar Geology (1986) 14 (9): 753–756. https://doi.org/10.1130/0091-7613(1986)14<753:EOSAGG>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. Martin; Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology 1986;; 14 (9): 753–756. doi: https://doi.org/10.1130/0091-7613(1986)14<753:EOSAGG>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The comparative study of Archean and post-Archean granitic rocks shows significant changes with time. The high rare-earth element fractionation and the low Yb content of the Archean granitoids indicate the major role of garnet and hornblende, whereas these two minerals do not play a prominent part in the genesis of modern granitic rocks. This difference is a direct consequence of the cooling of Earth.In Archean time the subducted oceanic crust was young and warm, so it reached the conditions of melting before dehydration had occurred, leaving a garnet- and hornblende-bearing residue. In contrast, the modern subducted oceanic slab is generally old and cold, so it is dehydrated before it reaches the melting conditions of hydrous tholeiite; therefore, in the absence of a hydrous phase, it cannot melt at shallow depth. The fluids produced by dehydration reactions of modern crust rehydrate the overlying mantle wedge, which, in consequence, can undergo partial melting and give rise to calc-alkaline magmas; in this case, olivine and pyroxene are the most important residual phases. The location of calc-alkaline magma genesis in subduction-zone environments has migrated over time from the subducted Archean oceanic crust to the mantle wedge, a migration attributed to the progressive cooling of Earth. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Responding to the need to reduce atmospheric emissions of carbon dioxide, Donald Brown (2000) proposed a novel enhanced geothermal systems (EGS) concept that would use CO{sub 2} instead of water as heat transmission fluid, and would achieve geologic sequestration of CO{sub 2} as an ancillary benefit. Following up on his suggestion, we have evaluated thermophysical properties and performed numerical simulations to explore the fluid dynamics and heat transfer issues in an engineered geothermal reservoir that would be operated with CO{sub 2}. We find that CO{sub 2} is superior to water in its ability to mine heat from hot fractured rock. CO{sub 2} also has certain advantages with respect to wellbore hydraulics, where larger compressibility and expansivity as compared to water would increase buoyancy forces and would reduce the parasitic power consumption of the fluid circulation system. While the thermal and hydraulic aspects of a CO{sub 2}-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. An EGS system running on CO{sub 2} has sufficiently attractive features to warrant further investigation.
Abstract An analytical solution of the transient temperature response in a semi‐infinite medium with a line source of finite length has been derived, which is a more appropriate model for boreholes in geothermal heat exchangers, especially for their long‐duration operation. The steady‐state temperature distribution has also been obtained as a limit of this solution. An erratic approach to this problem that appears in some handbooks and textbooks is indicated. Two representative steady‐state borehole wall temperatures, the middle point temperature and the integral mean temperature, are defined. Differences between them are compared, and concise expressions for both are presented for engineering applications. On this basis the influence of the annual imbalance between heating and cooling loads of the geothermal heat exchangers is discussed regarding their long‐term performance. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 558–567, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10057
Enhanced (or engineered) geothermal systems (EGS) have evolved from the hot dry rock concept, implemented for the first time at Fenton Hill in 1977. This paper systematically reviews all of the EGS projects worldwide, based on the information available in the public domain. The projects are classified by country, reservoir type, depth, reservoir temperature, stimulation methods, associated seismicity, plant capacity and current status. Thirty five years on from the first EGS implementation, the geothermal community can benefit from the lessons learnt and take a more objective approach to the pros and cons of ‘conventional’ EGS systems.
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We review ten historical Enhanced Geothermal Systems (EGS) projects and find that typically, during injection: (1) flow from the wellbore is from preexisting fractures, (2) bottomhole pressure exceeds the minimum principal stress, and (3) pressure-limiting behavior occurs. These observations are apparently contradictory because (1) is consistent with shear stimulation, but (2) and (3) suggest propagation of new fractures. To reconcile these observations, we propose that, in many cases, new fractures do not form at the wellbore, but away from the wellbore, and new fractures initiate from open and/or sliding natural fractures and propagate through the formation. Fracture initiation from natural fractures is aided by concentrations of stress caused by the fractures׳ opening and sliding. The propagating fractures may terminate against natural fractures, forming a complex network of both new and preexisting fractures. We perform computational modeling with a discrete fracture network simulator that couples fluid flow with the stresses induced by fracture deformation. The modeling results demonstrate that several geological conditions must be in place for stimulation to occur only through induced slip on preexisting fractures and to avoid significant opening of new or preexisting fractures. These conditions cannot be expected to be present at every EGS project, and our review of the literature shows that they typically are not. The simulation results indicate that pure shear stimulation is more likely to be possible in locations with thick faults present, and our review of the literature shows that EGS field experience is consistent with this hypothesis. We discuss field experiences from several EGS projects and describe how they are consistent with the idea that significant propagation of new fractures has occurred.
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The geochemical energy budgets for high-temperature microbial ecosystems such as occur at Yellowstone National Park have been unclear. To address the relative contributions of different geochemistries to the energy demands of these ecosystems, we draw together three lines of inference. We studied the phylogenetic compositions of high-temperature (>70 degrees C) communities in Yellowstone hot springs with distinct chemistries, conducted parallel chemical analyses, and carried out thermodynamic modeling. Results of extensive molecular analyses, taken with previous results, show that most microbial biomass in these systems, as reflected by rRNA gene abundance, is comprised of organisms of the kinds that derive energy for primary productivity from the oxidation of molecular hydrogen, H2. The apparent dominance by H2-metabolizing organisms indicates that H2 is the main source of energy for primary production in the Yellowstone high-temperature ecosystem. Hydrogen concentrations in the hot springs were measured and found to range up to >300 nM, consistent with this hypothesis. Thermodynamic modeling with environmental concentrations of potential energy sources also is consistent with the proposed microaerophilic, hydrogen-based energy economy for this geothermal ecosystem, even in the presence of high concentrations of sulfide.
Graph showing worldwide installed geothermal electrical capacity as a function of time ----------
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