Print this pageAbstract: Geothermal Exploration from Deep-Well Data
Logs from more than 500 deep wells have been studied, covering Florida and adjacent parts of Georgia and Alabama, as part of a massive effort to assess the geothermal potential of the area. Within peninsular Florida, two favorable anomalies were confirmed, and other attractive areas were outlined--but not studied in detail--in the Florida panhandle and adjacent states. Bottom-hole temperatures were used as the basis for several kinds of maps: geothermal gradient, temperature at 1,000 m, temperature at 2,000 m, and depth to 100°C (typically within 4,000 m of the surface, in the anomalous areas). Unlike earlier maps which used county averages, the present work was done on the basis of single-well readings, thereby providing more detail, as well as more noise.
Ground-water movement at shallow depths distorts the shallow-data field, so that measured heat-flow values taken from water wells, although confirming the general results from deep wells, provided lower numeric values. The deep-well bottom-hole temperatures are known not to be equilibrium values, but the errors in BHT measurements tend to reduce the actual gradients (increases in temperatures near the surface, decreases in temperatures near the bottom) and, therefore, the map anomalies are conservative approximations.
Radioisotope anomalies, from shallow-water sources, also confirmed the two anomalies. One of these was explored further by gravity methods, and may be related to deep structural control.
Two types of information, missing from most geothermal studies based on existing well logs, can be supplied in most instances. One is an estimate of fluid transmissibility, which can be based on ordinary procedures, well-known in the oil industry, for obtaining porosity values and permeability indications from logs. This will be important in exploitation of relatively low-temperature geothermal sources, such as those in the southeastern states, where circulation of water in large quantities may be necessary. The other is an estimate of thermal conductivity of deep rocks. This last value can be obtained by direct measurement on cores, or can be computed from the equation
Kh = kVap^rgrb
where Vp is sonic velocity (to be read from the CVL), and ^rgr is mass density. This estimate of thermal conductivity can be used to convert thermal gradient values to heat-flow values.
AAPG Search and Discovery Article #90975©1976 GCAGS- GC Section SEPM Annual Meeting Shreveport, Louisiana