The HDR resource base is generally defined to included crustal rock that is hotter than 150℃, is at depths less than ten kilometers, and can be drilled with presently available equipment. Although wells deeper than ten kilometers are technically feasible, prevailing economic factors will obviously determine the commercial feasibility of wells at such depths. Rock temperatures as low as 100℃ may be useful for space heating with the heater either within the space or external to it); however, for producing electricity, temperatures greater than 200℃ are desirable.
The geothermal gradient, which specifically determines the depth of drilling required to reach a desired temperature, is a major factor in the recoverability of geothermal resources. Temperature gradient maps generated from oil and gas well temperature-depth records kept by the American Association of Petroleum Geologists suggest that tappable high-temperature gradients are distributed all across the United States.
Indications are that the HDR resource base is very large. If an average geothermal temperature gradient of 22℃ per kilometer of depth is used, a staggering 13,000,000 quadrillion B.T.U.s of total energy are calculated to be contained in crustal rock to a ten-kilometer depth in the United States. If we conservatively estimate that only about 0.2 percent is recoverable, we find a total of all the coal remaining in the United States. The remaining problem is to balance the economics of deeper, hotter, more costly wells and shallower, cooler, less expensive wells against the value of the final product, electricity and/or heat.
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