CuAP. UJ GEOPHYSICAL WELL TESTING 847 



As was stated above, the normal reciprocal gradient varies from about 

 27.5 meters per degree C. (or 50 feet per degree F.) to about 35 meters 

 per degree C. (or 64 feet per degree F.) in normal areas. Regionally, the 

 observed reciprocal gradients may be quite different from these values, 

 depending on geologic structure.^ In wells where the drill has penetrated 

 uplifts of basement rocks, the temperature-depth curve is concave toward 

 the depth axis; that is, the reciprocal gradient increases with depth. This 

 is because of the better heat conductivity of the basement rocks (see 

 Fig. 11-19). Conversely, in areas of sedimentary beds of great thickness 

 the reciprocal gradient decreases with depth (Fig. 11-19). This may be 

 caused by the decrease of porosity' (hence, water content'^ and thermal 

 conductivity) with depth, or by the compression of the isogeothermal 

 surfaces as the basement rocks are approached. However, when the base- 

 ment is entered, these types of curves turn again toward the depth axis. 

 According to Van Orstrand,^" only 5 per cent of 400 wells investigated up 

 to 1932, had a linear depth-temperature curve; 26 per cent were concave, 

 and 59 per cent were convex to the depth axis. 



C. Thermal Properties of Rocks 



The thermal behavior of rocks and formations is characterized by three 

 properties. They are: (1) the thermal conductivity, or the heat current 

 traversing the unit section for the unit of heat gradient, expressed in cal. 

 cm.~' see."' centigrade"'; (2) the specific heat, which is the quantity of 

 heat required to raise the unit of mass by the unit of temperature, in 

 cal. gram"' centigrade"', or B.t.u. per pound per degree F. ; and (3) 

 the diffusivity K, a property derived from the first two, and given by 



K = ^^, (11-4) 



where c is thermal conductivity, s is specific heat, and 5 is density. A 

 fourth property, the heat given off in certain chemical reactions, is like- 

 wise of importance. However, it is difficult to define and is discussed in 

 the next section. 



Since diffusivity is a derived property, this discussion will be confined to 

 specific heat and thermal conductivity. The specific heat of most minerals 



"Van Orstrand, A.A.P.G. Bull., 19(1), 78-115 (Jan., 1935). 



18 For correlations of depth-temperature with depth-porosity curves see Van 

 Orstrand, ihid., 18(1), 19 (Jan., 1934). 



'^ Depths to boiling point of water are 7000 feet or less in one-third of the loca- 

 tions investigated by Spicer (A.A.P.G. Bull., 20(3), 279 [March, 1936]). In the 

 remaining two-thirds, they are 10,000 feet or less. 



2» Physics, 2(3), 139 (March, 1932). 



