Current State of Research
There are 3 systems which allow usage of geothermal energy:
- Hydro-thermal systems: uses the energy which is contained in the water of the deep subsurface,
- Petro-thermal systems: uses the energy, which is contained in the rocks of the deep subsurface Hot-Dry-Rock (HDR), Hot-Wet-Rock (HWR), Hot-Fractured-Rock (HFR), Enhanced Geothermal System (EGS)),
- Deep heat probe (borehole heat exchanger): uses wells in the depths up to 3000 m in which heat transfer liquid circulates in a closed system for heat production.
All three systems require deep boreholes, excepting the areas affected by volcanic activities. To achieve a high production rate of hydro-thermal or petro-thermal systems, the wells are targeted at connecting naturally existing fracture systems in the subsurface or fracture systems are generated in hydraulic treatments after the well has been drilled. To generate electrical power, only a few of the above mentioned hydro-thermal systems are relevant, and the petro-thermal systems.
- Systems for Geothermal Energy Useage
The currently available technology for deep wells is adapted for the needs of the oil and natural gas industry. Oil and natural gas wells are constructed to provide a reliable connection between the hydrocarbon bearing reservoir in the subsurface and the surface installations for the duration of exploitation. This is also valid for geothermal wells, especially for hydro-thermal wells with final depths of 3.000 – 4.000 m. For petro-thermal systems, aiming at recovering the heat stored in the dry and hard rock formations, well requirements are quite different from those in typical oil and natural gas wells:
- the average temperature is higher
- the target is not a relatively soft reservoir rock but hard rock, for example, volcanic rocks
- thermal operation requires large contact areas, sustainable subsurface heat exchangers, either naturally existing or artificially created
- minimization of the hydraulic resistance in the wells during production and injection requires large cross sectional areas for flow
- the average depth is larger.
All these less favourable conditions require new solutions. At the same time, the system costs must be decreased: approximate cost of deep boreholes in the oil and natural gas industry for depth ranges of 5.000 m are ca. 2,5 – 3,0 million Euros per 1000 m, which is not economically efficient for deep geothermal systems. The main goal of the gebo research association is therefore to make solid contributions to
- the decrease of drilling costs
- improve drilling technologies for hard and hot rock drilling
- improving the success rate for achieving economic rates
