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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 26 (1976), Pages 65-68

Geothermal Exploration from Deep-Well Data

William F. Tanner (1)

ABSTRACT

Logs from more than 500 deep wells have been studied, covering Florida and adjacent parts of Georgia and Alabama, as part of an effort to assess more accurately the geothermal potential of the area. Within peninsular Florida, two favorable anomalies were confirmed, and other interesting areas were outlined in the Florida panhandle and adjacent states.

Bottom-hole temperatures were used as the basis for several kinds of maps: geothermal Previous HitgradientNext Hit, 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 (but more noise).

Groundwater 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 highly variable numerical values. The deep-well bottom hole temperatures (BHT) are thought not to be equilibrium values, but the errors in BHT measurements appear to be relatively small.

Radioisotope anomalies, from shallow water sources, also confirmed the two anomalies. One of these was explored further by gravity methods, and it 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 transmissability, which can be developed from ordinary procedures, well known in the oil industry, for obtaining porosity values and permeability indications from logs. This will be important in case of 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 the thermal conductivity of deep rocks. This value can be obtained by direct measurement on cores, or it can be computed from the equation:

Kh = kVa^rgrb where V is sonic Previous HitvelocityNext Hit (to be read from the Sonic Log or continuous veolcity log) and ^rgr is mass density (from radioactivity logs). This estimate of thermal conductivity can be used to convert thermal Previous HitgradientTop values to heat flow values.


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