254 



UNITED STATES MINERAL RESOURCES 



Mountain Arsenal well in Colorado by Healy, Rubey, 

 Griggs, and Raleigh (1968). 



Bowen (1973) correctly pointed out that "To 

 understand properly the impact of the production 

 of electric power on the environment, it is necessary 

 to evaluate more than just the power plant, whether 

 it is geothermal, nuclear, or fossil fueled ; the entire 

 fuel cycle from mining, processing, transportation, 

 and the disposal of spent wastes must be consid- 

 ered." When viewed in this light, the environmental 

 impact of geothermal generation does indeed appear 

 to be minor compared with fossil-fuel or nuclear 

 generation. The environmental impact of geother- 

 mal generation is restricted to the generating site, 

 whereas much of the environmental impact of other 

 modes of generating takes place at other sites 

 (mines, processing plants, disposal sites) and is 

 commonly neglected in the evaluation of environ- 

 mental impact of a power plant. 



GEOLOGIC ENVIRONMENTS 



Geothermal reservoirs are the "hot spots" of 

 larger regions where the flow of heat from depth in 

 the earth is about li/o-S times the worldwide aver- 

 age of 1.5x10"^ calories per square centimeter per 

 second. Such regions of high heat flow commonly 

 are zones of young volcanism and mountain build- 

 ing and are localized along the margins of major 

 crustal plates (Muffler and White, 1972, fig. 1). 

 These margins are zones where either new material 

 from the mantle is being added to the crust or 

 crustal material is being dragged downward and 

 "consumed" in the mantle. In both situations, molten 

 rock is generated at depth and moves buoyantly 

 upward into the crust. The resultant pods of igneous 

 rock provide the heat that is then transferred by 

 conduction to the convecting systems of meteoric 

 water. 



There are two major types of geothermal sys- 

 tems: (1) hot water (White, 1970, 1973) and (2) 

 vapor dominated ("dry steam") (White and others, 

 1971 ; Truesdell and White, 1973 ; White, 1973) . In 

 a hot-water geothermal system, the fluid in the rock 

 at depth is water alone. Steam is produced by boiling 

 as the fluid moves up a well to the surface, and a 

 mixture of steam and water is produced at the sur- 

 face; the water must be removed from the steam 

 before the steam is fed to a turbine. Vapor-domi- 

 nated geothermal systems, on the other hand, con- 

 tain both water and steam in the reservoir at depth. 

 With decrease in pressure upon production, heat 

 contained in the rock dries the fluids first to satu- 

 rated and then to superheated steam, which can be 

 piped directly into a turbine. Among geothermal 



systems discovered to date, hot-water systems are 

 perhaps 20 times as common as vapor-dominated 

 systems (White, 1970). 



Potentially recoverable geothermal resources also 

 occur in some regions where the normal heat flow 

 of the earth is trapped by insulating impermeable 

 clay beds in a rapidly subsiding geosyncline. For 

 example, along the gulf coast of the United States, 

 temperatures of 150°C-273°C are found at depths 

 of 4-7 km in geopressured zones (Jones, 1970). 

 Waters in these geopressured zones are not circulat- 

 ing meteoric water; they are produced by compac- 

 tion and dehydration of the sediments themselves. 



RESOURCES AND PROBLEMS 



Estimates of the geothermal resources of the 

 United States and of the world differ by as much as 

 six orders of magnitude. White (1965, p. 14) stated 

 that "existing worldwide utilization equivalent to 

 about 1 million kw * * * probably can be increased 

 at least 10 times [that is, to 10* Mw] under present 

 economic conditions and maintained for at least 50 

 years." Banwell (1967, p. 155) estimated a poten- 

 tial heat production of 2x10^ kg-cal/sec from geo- 

 thermal energy associated with "Pacific type vol- 

 canism." At 14 percent thermal eflSciency, this rate 

 of heat production could sustain electrical generat- 

 ing capacity of about 10" Mw. Rex (1971a, p. 54) 

 stated that he and his colleagues " * * * are esti- 

 mating the western [conterminous] U.S. geothermal 

 potential from 10^ to 10' megawatts." White (1965) 

 and Muffler and White (1972) estimated that the 

 world geothermal resource to a depth of 3 km for 

 electrical generation by proven techniques is ap- 

 proximately 2 XlO" calories (equivalent to 58,000 

 Mw for 50 yr). Rex (1972a) stated that " * * * the 

 present recoverable [geothermal] resource for the 

 western third of the continental United States, ex- 

 cluding Alaska, is of the order of 10^ megawatt- 

 centuries. This figure could be expanded by another 

 factor of 10 by the inclusion of the eastern two- 

 thirds of the United States and another factor of 

 10 [that is, to 10'° megawatt-centuries] by improve- 

 ments in technology." ' John Banwell and Tsvi Mei- 

 dav (oral presentation, Ann. Mtg. Am. Assoc. Adv. 

 Sci., Philadelphia, 1971; ms. supplied by Tsvi Mei- 

 dav) stated that "The geothermal energy reserves 

 of the world are orders of magnitude greater than 

 the total reserve of any other form of fossil energy." 



The wide variance among these resource estimates 

 reflects several factors — predominantly, the defini- 



^ If one assumes 14 percent themxal efficiency (as in Rex, 1972b), this 

 electricity is produced from 4.93 X 10^'' cal. or approximately eight times the 

 estimate of White (1965, p. 2) for the total heat stored under the 

 United States to a depth of 10 km. 



