Hydrothermal and Hot Dry Rock

This document characterizes electric power generation technology for two distinct categories of geothermal resources .

Hydrothermal resources are the "here-and-now" resources for commercial geothermal electricity production. They are relatively shallow (from a few hundred to about 3,000 meters). They contain hot water, steam, or a combination of the two. They are inherently permeable, which means that fluids can flow from one part of the reservoir to other parts o f the reservoir, and into and from wells that penetrate the reservoir. In hydrothermal reservoirs, water descends t o considerable depth in the crust, becomes heated and then rises buoyantly until it either becomes trapped beneat h impermeable strata, forming a bounded reservoir, or reaches the surface as hot springs or steam vents. The water convects substantial amounts of heat from depths to relatively near the surface.

Hot Dry Rock (HDR) resources, on the other hand, are relatively deep masses of rock that contain little or no steam or water, and are not very permeable. They exist where geothermal gradients (the vertical profile of changin g temperature) are well above average (>50oC/km). The rock temperature reaches commercial usefulness at depths o f about 4,000 meters or more. To exploit hot dry rock, a permeable reservoir must be created by hydraulic fracturing , and water from the surface must be pumped through the fractures to extract heat from the rock.

There are both strong similarities and large differences between hydrothermal and HDR geothermal resources an d exploitation systems. Most of the component technologies, i.e., the power plant and well drilling methods, are ver y similar for both systems. The most important differences are that: (a) Hydrothermal systems are commercial today , while HDR systems are not, where as (b) HDR resources are enormously larger (between 3,170,000 EJ and 17,940,000 EJ of accessible energy in the U.S.) than hydrothermal resources (on the order of 1,060 EJ to 5,300 EJ of accessibl e energy) [2]. By way of comparison, in 1995 the U.S. used about 95 EJ of primary energy. U.S. hydrothermal sources could supply that amount for 10 to 50 years. But U.S. Hot Dry Rock resources could supply that amount for somewhere between 30,000 and 500,000 years.

Because of these differences, the general strategic approach of national geothermal R&D programs (including that o f the U.S.) has been to try to lower costs in the hydrothermal commercial arena today and, by so doing, to improv e generic "geothermal" technology enough to make HDR exploitation economically feasible in the not-too-distant future.

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