Clearly only a small part of this heat can be extracted at present.
In general the deeper the well, the hotter the rock, although the thermal gradient may not be linear.
The principle of enhanced geothermal systems is simply:
- To drill multiple wells deep enough to find hot rock
- To pump pressurised water (mildly acidic brine) so as to stimulate rock fracturing and increasing permeability
- To create and maintain a hot water reservoir and heat exchanger in the permeable and fractured hot rock
- To pump up hot pressurised water from production wells and use it for power production as well as direct uses.