GEOTHERMAL RESOURCES 



255 



tion of the term "resources" as used by the respec- 

 tive authors. Schurr and Netschert (1960, p. 297) 

 stated : 



The "resources" of a mineral raw material may be defined 

 according to any technical and economic criteria that may 

 be considered relevant and appropriate. The resources con- 

 sist of that part of the resource base (including reserves) 

 which seem likely to become available given certain tech- 

 nologic and economic conditions. * * * Whether any particu- 

 lar definition of resources is useful depends on the context 

 in which it is to be used and the possibility and need for 

 expressing it quantitatively. It is essential in any event 

 that an explicit definition always be given in conjunction with 

 the use of the term. 



Particularly for geothermal resources, it accord- 

 ingly is incumbent upon any author to define his 

 usage of "resources" and to state explicitly his as- 

 sumptions. Most authors are seriously remiss in this 

 regard, and their resource estimates must therefore 

 be viewed with skepticism. 



The "resource base" was defined by Schurr and 

 Netschert (1960, p. 297) as all of a given material 

 present in the earth's crust, whether its existence 

 is known or unknown and regardless of cost con- 

 siderations. For geothermal resources, the "resource 

 base" is therefore all of the heat in the earth's crust, 

 presumably measured from 0°K (^ — 273.15°C). 

 For obvious practical reasons, heat below mean sur- 

 face temperature of the earth (about 15°C) is ex- 

 cluded from consideration, and the geothermal re- 

 source base may be considered as all heat above 

 15°C in the earth's crust. Quantification of the geo- 

 thermal resource base depends on the definition of 

 the term "crust," the estimate of the dimensions 

 of the crust, the three-dimensional distribution of 

 temperature within the crust at any particular 

 moment, and the heat flux between the mantle and 

 the crust and between the crust and the atmosphere. 



What proportion of the geothermal resource base 

 should be considered as a resource? This proportion 

 depends on evaluation of the following factors, many 

 of which are interdependent : 



1. Depth of extraction, which is dependent on the 



technology and economics assumed. White 

 (1965) gives estimates of geothermal re- 

 sources to depths of 3 km, the deepest geother- 

 mal well drilled to date, and 10 km, the deepest 

 well likely to be drilled in the foreseeable 

 future. 



2. Three-dimensional temperature distribution to 



the depth assumed. 



3. Specific heat distribution; a convenient approxi- 



mation presented by White (1965) is based 

 on the fact that most earth materials have 

 nearly equal volumetric specific heats. 



4. Porosity, effective porosity, specific yield, and 



permeability. 



5. Physical state of the fluid in the reservoir (wa- 



ter or steam). 



6. Available technology. 



7. Economics of various possible uses. 



8. Governmental policy with respect to societal 



needs, environmental constraints, and avail- 

 ability of other forms of energy. 

 Many physical factors that control the distribu- 

 tion of heat to any given depth can be evaluated, 

 at least rudely (White, 1965, 1973 ; Banwell, 1967 ; 

 Rex, 1971b; Muffler and White, 1972). More tenu- 

 ous are the assumptions of technology, economics, 

 and governmental policy. These assumptions are 

 critical to geothermal resource estimation, and dif- 

 ferences (explicit or imphcit) among them are in 

 great part responsible for the vast range in magni- 

 tude among geothermal resource estimates. 



Geothermal reserves (defined as those resources 

 recoverable at present at costs competitive with 

 alternative forms of energy) are clearly limited if 

 one considers only generation of electricity. For the 

 generation of electricity using proved and demon- 

 strated technology, the geothermal reservoir must 

 have a temperature of at least 180°C; at lower 

 temperatures the quantity of flash steam is inade- 

 quate (fig. 28). Reservoirs at this or higher tem- 

 perature, at recoverable depths, and with adequate 



< 250 



10 20 30 40 50 60 70 



STEAM, IN WEIGHT PERCENT 



Figure 28. — Weight percent of steam produced by flashing 

 water at various temperatures to a turbine inlet pressure 

 of 50 lb in"^ gauge (James, in press). Graph calculated 

 from data given in tables 1 and 2 of Keenan and others 

 (1969). 



