Abstract: Hot, Dry Rock (HDR) Geothermal Energy--Exploitation and Exploration

A. W. Laughlin, F. G. West

Most of the earth's geothermal energy amenable to recovery is stored in the solid phases of the crust. Reservoir permeability and the presence or absence of natural fluids dictate the extraction technique for recovering this energy and thus the economics. Where permeabilities are high and vapor or water-dominated systems are present, conventional extraction is applicable. As the permeability decreases, stimulation may be required and in the extreme case--HDR--both stimulation and fluid injection are necessary.

The Los Alamos Scientific Laboratory (LASL) method of extracting geothermal energy from HDR requires two drill holes connected at depth by a large, vertical, hydraulic fracture. Water, introduced through one drill hole, is heated as it passes along the fracture toward the second drill hole, through which it is returned to the surface. This method of energy extraction is being tested at Fenton Hill near the Valles Caldera in north-central New Mexico where suitable reservoir conditions have been demonstrated (200°C, essentially impermeable rock) at a depth of 3 km.

With continuing success in developing the extraction technique and characterizing the first proven HDR site, it is now appropriate to examine and evaluate exploration techniques for this type of geothermal deposit.

Geologic, geochemical, and geophysical techniques have been used to identify and characterize the HDR part of the total geothermal system related to the Valles Caldera. As is usual in exploration, a combination of techniques has proved most useful. Geologic mapping, geochronology, and heat-flow measurements were used to evaluate the nature, age, and magnitude of the heat source. A variety of geologic and geophysical techniques provided information on the permeability of the potential reservoir. Much of the information gained and tested by drilling at Fenton Hill now can be applied to the search for other HDR sites.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC