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Geothermal Energy GEOLOGICAL SURVEY CIRCULAR 519 Geothermal Energy Geothermal Energy By Donald E. White GEOLOGICAL SURVEY CIRCULAR 519 Washington 1965 United States Department of the Interior STEWART l. UDALL, Secretary Geological Survey William T. Pecora, Director First printing 1965 Second printing 1966 Free on application to the U.S. Geological Survey, Washington, D.C. 20242 CONTENTS Page Page Abstract--------------------------- 1 Hydrothermal systems of composite Introduction------------------------ 1 type---------------------------- 9 Acknowledgments------------------- 2 General problems of utilization ----- 10 Areas of "normal" geothermal Domestic and world resources of gradient ------------------------- 2 geothermal energy--------------- 12 Large areas of higher-than-"normal" Assumptions# statistics, and geothermal gradient--------------- 3 conversion factors--------------- 14 Hot spring areas-------------------- 4 References cited___________________ 14 TABLE Page Table 1. Natural heat flows of some hot spring areas of the world--------------------- 5 III Geothermal Energy By Donald E. White ABSTRACT commercially developed hot spring areas at rates of five to more than 10 times their rates of natural heat flow prior to The earth is a tremendous reservoir of heat, most of which development. Such overdrafts, in at least some systems, can is too deeply buried or too diffuse to consider as recoverable continue for many years, the excess heat being suppli-ed energy. Some large areas are higher-than-"normal" in heat from the heat reservoir. Eventually, depending on the char­ content, particularly in regions of volcanic and tectonic ac­ acteristics of each individual system, the effects of sus­ tivity. Recovery of stored heat from these large areas may tained overdraft must become evident. be economically feasible in the future but cannot compete in cost now with other forms of energy. Present world utilization of geothermal energy is about 1 million kw (kilowatts) or 7.5 x 1015 cal/yr (calories per Certain hot spring areas, commonly near active or recently year), and this amount can probably be increased 10-100 active volcanoes, are cischarging heat at rates per unit times for at least the next 50 years. Potential reserves to area of 10-1,000 times the "normal" (1.5 x l<.i6 calories depths of 3 km (kilometers) recoverable at or near present per square centimeter per second) heat flow of the earth; costs (l percent of total assumed recoverable) £.re estimated to be 2 x 1019 cal; resources to depths of 10 km recoverable some of the largest and hottest areas have been explored 22 for geothermal energy. These areas are characterized by at much more than present costs are estimated to be 1 x 10 high permeability, at least locally on faults, fractures, and cal. Of the total world resources of geothermal energy, prob­ sedimentary layers; this high permeability permits fluid ably 5-,..10 percent occurs in the United States; the areas of circulation, most of the total heat flow being transported highest potential are in the Western States. upward in water or steam. The circulation has produced reservoirs of stored heat closer to the earth's surface than INTRODUCTION is normally possible by rock conduction alone. Local near­ Geothermal heat supplies only a very mi­ surface thermal gradients are typically very high, but the nor fraction of present domestic and world gradient decreases greatly, and even reverses, at greater depths in any single drill hole. use of energy. In the United States. a geo­ thermal steam powerplant generating 28,000 Some other geothermal systems are here considered as a kw (kilowatts) of electricity (McNitt 1963 composite type. Near-surface caprocks are low in permea­ p. 14) is now operating at The Gey~ers i~ bility, and temperatures in these rocks are therefore con­ California, and many other areas are being trolled dominantly by conduction. Beneath the insulating actively explored; hot spring water is also caprocks, permeable reservoir rocks may permit convective used locally for space heating. Total utiliza­ transfer of fluids and heat. Natural hot springs are small and relatively unimpressive in a composite system because tion of geothermal energy in the world today little fluid escapes. Temperatures near the surface are low is roughly equivalent to a production capac­ but increase steadily with depth, in contrast to hot spring ity of 1 million kw, and approximately half of systems that have vigorous near-surface circulation and, this total has been developed in the past 15 therefore, high near-surface temperatures. years. In the future, geothermal energy is expected to be of considerable local signif­ Some undiscovered geothermal systems may be composite icance, and of a much greater total quantity in type. Because the near-surface permeabilities are so low that no fluid escapes, the principal evidence of such a sys­ than now. but it is not likely to rank as one tem will be abnormally high geothermal gradients and heat of the major sources of energy. flows distributed over a relatively large area. This brief survey of geothermal energy is Geothermal energy stored in the upper 10,000 feet of hampered by a serious lack of reliable data. many hot spring systems is 1,000-10,000 times the annual natural heat flow at the surface. Most of these hot spring Scattered bits of information, generally con­ systems, therefore, have appreciable age and stability rela­ sisting of a few temperature measurements, tive to human activity. E:aergy is being recovered from some are available for individual wells; heat flow 1 2 GEOTHERMAL ENERGY has also been estimated or measured semi­ AREAS OF "NORMAL" GEOTHERMAL GRADIENT quantitatively in a few thermal spring sys­ terns. The earth as a whole is a tremendous res­ ervoir of thermal energy but most of this· In this report, four types of thermal sys­ energy is too deeply buried or too diffuse tc terns are considered; each is gradational into justify consideration as an energy resource. at least one of the other types: Calculations of internal earth temperatureE' and of total quantity of heat are very differ­ 1. Areas of "normal" geothermal gradient ent, depending upon assumptions concernin~ and heat flow ... the origin of the earth, the total amount anc~ distribution of radioactivity, and other fac­ 2. Large areas of higher-than-"normal" tors (Gutenberg, 1951; Birch, 1955; Jacobs, geothermal gradient and conducted heat flow. 1956; Verhoogen, 1960; Wyllie and Tuttle, 1960; Clark, 1961; Ringwood, 1962). Never­ 3. Hot spring areas, characterized in their theless, the general order of magnitude of upper parts by convective transport of most thermal energy within the earth at depthE' of the total heat flow in circulating water or now accessible by drilling, or that may be­ steam. come accessible, is of some value to the purpose of this report. 4. Composite hydrothermal systems in­ volving in their upper parts convective and The global average heat flow is about conductive heat transfer of types 2 and 3. 1.5 x 10-6 cal/cm2 /sec (hereafter referred Near the surface heat is transferred largely to as 1. 5 heat-flow units); the calculated by conduction through rocks of low mass continental average from sparse data ir permeability that permit little or no dis­ slightly higher (1.65 units) (Lee and Mac­ charge of water or steam. Temperatures Donald, 1963). immediately below the insulating layers are too high to be explained by conductive heat The heat stored above surface tempera­ flow alone; circulating fluids are the indicated tures in the outer 100 km (kilometers) of the major agents of heat transfer. earth is about 2 x 1028 cal.!/ This amount o:V energy is equivalent to the heat lost by con­ Types 3 and 4, grouped together as the hy­ duction at the surface of the earth for nearl~ .. drothermal types, are particularly important 100 million years, to solar radiation re~ for geothermal energy because they provide ceived by the earth for 10,000 years, to heat reservoirs relatively near the surface 2 x 1022 kwhr (kilowatt hours); or to the heat and because they also insure a natural fluid content of 3 x 1ol8 short tons of coal. for transferring heat from the reservoir to the powerplant. The importance of this nat­ Wells have been drilled to depths of abou~ ural fluid is considered in a later section of 8 km and drilling can now attain a maximun1 this report. of about 10 km. Heat stored under the sur­ face of the earth to a depth of 10 km is about ACKNOWLEDGMENTS 3 x 1026 cal. Heat stored under the United States to the same depth is about 6 x 1024 cal. The author is indebted to many individuals. which is equivalent to conductive heat trans­ Particularly noteworthy have been contribu­ fer from the United States for nearly 1 i mil­ tions of Gunnar Bodvarsson of Iceland and lion years, or to solar radiation received by James Healy and John Banwell of New Zea­ the United States for approximately 200 years; this heat is also equivalent to about land. Arthur H. Lachenbruch of the U.S. 14 Geological Survey has been very helpful in 5 x 1018 kwhr or the heat content of 9 x 10 many ways, specifically suggesting the con­ short tons of coal. cept of volumetric specific heat. The author's opportunities to study relationships at depth The heat content of rocks in areas of in many different geothermal systems were "normal• geothermal gradient is an ex­ made possible through generous cooperation tremely low-grade source of energy. In th~ of domestic companies and individuals, but preceding calculation the heat is assumed to the author takes responsibility for the inter­ 1 See section on • Assumptions, statistics, and conversion pretations contained in this report. factors•. LARGE AREAS OF HIGHER-THAN-•NoRMAL• GEOTHERMAL GRADIENT 3 be dispersed through about 100 million cubic this mean is about 2 times the global average.
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