A vast store of thermal energy is contained within hot – but essentially dry and impervious crystalline basement rocks found almost everywhere deep beneath Earth's surface.
[2] This technology has been tested extensively with multiple deep wells drilled in several field areas around world including the US, Japan, Australia, France, and the UK and investment of billions of research funds.
However no commercial projects are ongoing or likely due to the high cost and limited capacity of the engineered reservoirs, associated wells, and pumping systems.
For this technology to successfully compete with other energy sources, drilling costs would have to drop drastically or new approaches that result in much more extensive, complex, and higher rate flow paths through actual fracture networks would have to be established.
[3] Whereas hydrothermal energy production can exploit hot fluids already in place in Earth's crust, an HDR system (consisting of the pressurized HDR reservoir, the boreholes drilled from the surface, and the surface injection pumps and associated plumbing) recovers Earth's heat from hot but dry regions via the closed-loop circulation of pressurized fluid.
This fluid, injected from the surface under high pressure, opens pre-existing joints in the basement rock, creating a man-made reservoir which can be as much as a cubic kilometer in size.
The idea of deep hot dry rocks heat mining was described by Konstantin Tsiolkovsky (1898), Charles Parsons (1904), and Vladimir Obruchev (1920).
As described by Brown,[5] an HDR geothermal energy system is developed, first, by using conventional drilling to access a region of deep, hot basement rock.
The feasibility of mining heat from the deep Earth was proven in two separate HDR reservoir flow demonstrations—each involving about one year of circulation—conducted by the Los Alamos National Laboratory between 1978 and 1995.
This initial reservoir was shown to essentially consist of a single pressure-dilated, near-vertical joint, with a vanishingly small flow impedance of 0.5 psi/US gal/min (0.91 kPa/L/min).
[8] Of greatest importance, this flow test confirmed that the enlarged reservoir was also confined, and exhibited a low water loss rate of 6 gpm.
[citation needed] A deeper and hotter HDR reservoir (Phase II) was created during a massive hydraulic fracturing (MHF) operation in late 1983.
The Fenton Hill tests clearly demonstrated advantages of a fully engineered HDR reservoir over naturally occurring hydrothermal resources, including EGS.
On the other hand the less confined, more complex, lower pressure, and more pervasively fractured natural systems support much higher well flow rates and low cost development of energy generation.
[citation needed] The experiments at Fenton Hill have clearly demonstrated that HDR technology is unique, not only with respect to how the pressurized reservoir is created and then circulated, but also because of the management flexibility it offers.
[citation needed] There have been numerous reports of the testing of unconfined geothermal systems pressure-stimulated in crystalline basement rock: for instance at the Rosemanowes quarry in Cornwall, England;[19] at the Hijiori[20] and Ogachi[21] calderas in Japan; and in the Cooper Basin, Australia.
[22] However, all these “engineered” geothermal systems, while developed under programs directed toward the investigation of HDR technologies, have proven to be open—as evidenced by the high water losses observed during pressurized circulation.
The concomitant HDR economic variable is the cost of drilling to depths at which rock temperatures are sufficiently high to permit the development of a suitable reservoir.
As noted above, in the late 1990s the DOE began referring to all attempts to extract geothermal energy from basement rock as "EGS," which has led to both biographical and technical confusion.
Biographically, a large number of publications exist that discuss work to extract energy from HDR without any mention of the term EGS.
A more appropriate view would be to consider impermeable HDR rock as a separate state from that of the continuum of permeable rock—just as one would consider a completely closed faucet as distinct from one that is open to any degree, whether the flow be a trickle or a flood.
[citation needed] A definitive book on HDR development, including a full account of the experiments at Fenton Hill, was published by Springer-Verlag in April 2012.