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1993 | Buch

Geothermal Systems Kapitel lesen Erstes Kapitel lesen

Geothermal Fluids

Chemistry and Exploration Techniques

verfasst von: Dr. Keith Nicholson

Verlag: Springer Berlin Heidelberg

Enthalten in: Professional Book Archive

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SUCHEN

Über dieses Buch

This book introduces aqueous geochemistry applied to geothermal systems. It is specifically designed for readers first entering into the world of geothermal energy from a variety of scientific and engineering backgrounds, and consequently is not intended to be the last word on geothermal chemistry. Instead it is intended to provide readers with sufficient background knowledge to permit them to subsequently understand more complex texts and scientific papers on geothermal energy. The book is structured into two parts. The first explains how geothermal fluids and their associated chemistry evolve, and shows how the chemistry of these fluids can be used to, deduce information about the resource. The second part concentrates on survey techniques explaining how these should be performed and the procedures which need to be adopted to ensure reliable sampling and analytical data are obtained. A geothermal system requires a heat source and a fluid which transfers the heat towards the surface. The fluid could be molten rock (magma) or water. This book concentrates on the chemistry of the water, or hydrothermal, systems. Consequently, magma-energy systems are not considered. Hot-dry rock (HDR) systems are similarly outside the scope of this text, principally because they contain no indigenous fluid for study. Both magma-energy and HDR systems have potential as energy sources but await technological developments before they can be exploited commercially. Geothermal systems based on water, however, are proven energy resources which have been successfully developed throughout the world.

Inhaltsverzeichnis

Frontmatter

Geothermal Fluid Chemistry

Frontmatter
Chapter 1. Geothermal Systems
Abstract
Geothermal fields are found throughout the world in a range of geological settings, and are increasingly being developed as an energy source. Each of the different types of geothermal system has distinct characteristics which are reflected in the chemistry of the geothermal fluids and their potential applications. However, they all have in common a heat source at a few kilometres depth, and it is this which sets water, present in the upper sections of the Earth’s crust, into convection. Most geothermal resources can be used for space heating applications (eg. urban district heating schemes, fish farming, greenhouse heating), but it is only the hotter systems (>~180°C) which are used to generate electricity through the production of steam (see Rowley, 1982, for a review of systems worldwide). Since aqueous geochemistry is involved in all stages of the exploration, evaluation, and production of a geothermal field, an understanding of the chemistry of the fluids is essential for the development of a resource. The chemistry of the geothermal waters and gases contains important information about the hydrology of the field and conditions in the reservoir. These aspects of geothermal fluid chemistry are discussed in the following chapters. However, before looking at specific aspects of geothermal chemistry, it will be useful to first place the fluids in context by briefly describing the different types of geothermal system.
Keith Nicholson
Chapter 2. Water Chemistry
Abstract
The most common type of fluid found at depth in high-temperature geothermal systems is of near-neutral pH, with Chloride as the dominant anion. Other waters encountered within the profile of a geothermal field are commonly derived from this deep fluid as a consequence of chemical or physical processes. These waters, the characteristics of which are described below, are classified according to the dominant anions. Although not a formal genetic scheme, this descriptive Classification does permit some generalisations to be made on the likely origins of the waters. Examples of the composition of the different water types are given in Table 2.1.
Keith Nicholson
Chapter 3. Gas Chemistry
Abstract
The following gases, with steam, are invariably present in geothermal discharges from both natural features and wells: CO2, H2S, NH3, N2, H2, CH4. These gases are often collectively referred to as the “non-condensible gases”. Despite the near ubiquitous presence of these speeies, the exploration and evaluation of geothermal systems has traditionally placed greater emphasis on the sampling, analysis and interpretation of water chemistry, with gas chemistry rarely considered in the same detail. The greater care which gas sampling requires, coupled with the variability of gas compositions within a field and the sensitivity of gas equilibria to pressure, vapour-liquid Separation processes and temperature has led to a lack of confidence in the interpretation of gas chemistry relative to water compositions. However, improved analytical procedures and an increasing database of geothermal gas compositions, particularly from well discharges, mean that it is possible to give gas chemistry the detailed consideration it deserves in all phases of the geothermal resource development.
Keith Nicholson
Chapter 4. Isotope Chemistry
Abstract
Elements are defined by differences in the number of protons in the nucleus (atomic number, Z). The isotopes of an element therefore have the same atomic number, but are distinguished on the basis of different atomic masses (mass number, A = number of protons + number of neutrons). These differences in atomic mass are caused by the gain or loss of neutrons. Isotopes of an element display slight differences in their chemical and physical behaviour. However, these differences are so slight that for most elements chemical reactions do not lead to a measurable Separation of the isotopes. Such Separation is only significant in the lighter elements where a difference of one or two neutrons represents a significant fraction of the total atomic mass. It is this property of mass fractionation that is employed in the application of stable isotopes to hydrological studies.
Keith Nicholson

Exploration Techniques & Surveys

Frontmatter
Chapter 5. Exploration Techniques
Abstract
This chapter is an introduction to Part II, and provides an overview of exploration techniques for both active and extinct or fossil geothermal systems. The latter are of interest to geothermalists in charting the history of thermal activity in a region, and to economic geologists searching for epithermal gold deposits.
Keith Nicholson
Chapter 6. Water Surveys
Abstract
Too often in the preparation and Interpretation of geochemical surveys great emphasis is placed on the analytical methods used and recognition of any assodated sources of error. However, before a sample can be analysed it must be collected, and it is this aspect of the analytical programme than can be neglected; Bad sampling techniques can invalidate a survey and are far more costly to correct than analytical errors. Laboratory errors or mistakes can often be corrected by reanalysis of the sample, but if the sample is contaminated or unrepresentative, the analytical result will lack significance, regardless of the accuracy and precision of the analytical method used. This is not usually discovered until after the samples have been analysed and the data interpreted on a field-wide basis, possibly necessitating a re- sampling programme: bad sampling techniques are very costly. It is therefore worthwhile to adopt standardised methods of water sampling. This includes standardising the equipment to be used, and the correct preparation of that equipment in the laboratory several days before any field surveys are undertaken.
Keith Nicholson
Chapter 7. Gas Surveys
Abstract
An effective sealing valve is essential for any vessel used in gas sampling, as air contamination must be avoided. Hirabayashi (1986) describes a simple arrangement for gas sampling using a double-mouthed syringe. The simplicity of the equipment is attractive, but no details regarding the seals are provided and it appears that the mouth of the syringe is sealed by a screw clip; the preservation of sample integrity must be a concern with such apparatus. Giggenbach (1975) described a 300mL-volume cylindrical, round-bottom glass flask equipped with a Quickfit Rotaflo teflon stopcock. This valve is resistant to attack by acid gases, and preserves the seal despite large temperature variations during sample collection and storage.
Keith Nicholson
Chapter 8. Soil and Soil-Gas Surveys
Abstract
Water and gas sampling of natural discharges are the most common types of survey over known geothermal areas. However, where discharge features are few and where the extent of the field is not known, soil and soil-gas surveys can prove helpful. These surveys can identify permeable regions in a field and possible upflow or boiling zones. They can also delineate the margins of a geothermal system, and therefore often complement geophysical surveys particularly where interpretation of geophysical data is difficult, eg. due to topographic effects.
Keith Nicholson
Backmatter
Metadaten
Titel
Geothermal Fluids
verfasst von
Dr. Keith Nicholson
Copyright-Jahr
1993
Verlag
Springer Berlin Heidelberg
Electronic ISBN
978-3-642-77844-5
Print ISBN
978-3-642-77846-9
DOI
https://doi.org/10.1007/978-3-642-77844-5