3 Hot dry rock (HDR) fields
Heat flow through some parts of the continental crust can be well above normal locally because the underlying rocks contain abnormally high concentrations of uranium, thorium and potassium, which generate considerable heat. To add significantly to surface heat flow and thereby create high-temperature anomalies at shallow depths requires a large volume of such radioactive rocks. This condition is satisfied by some, but not all, granitic igneous intrusions, whose original magma became charged with heat-producing elements because of geochemical fractionation (Sheldon, 2005).
Would such intrusions contain enough water to constitute geothermal resources?
They are crystalline rocks and therefore have negligible porosity and permeability, so they are not natural geothermal resources.
Notwithstanding their lack of fluids to transfer heat toward the surface, thermal anomalies in such intrusions do constitute significant thermal resources, if they are shallow enough to be reached by drilling. Hot dry rock (HDR) systems require such rocks to be artificially fractured at depth so that water can be pumped into them, heated above boiling temperature and then returned to the surface to flash to steam in an electricity generating plant. Figure 6 shows in schematic form how hot, dry rock can become a high enthalpy geothermal resource. A well that penetrates the intrusion is used to inject water at very high pressures into zones of natural fracturing in the rock. Water injected through the well not only heats up but enhances any fractures and creates more, by a process of hydrofracturing. This creates paths along which water can move, effectively creating an artificial heat exchange zone that feeds superheated water to another well that takes it back to the surface. The practical difficulties of extracting heat from such deep rocks can be formidable. Drilling through crystalline rocks is far more costly than through sedimentary strata. For an HDR field to be viable, water has to circulate through large volumes of hydrofractured rock. The HDR approach is being pursued because of its potential in areas well away from active plate margins. Steam generation that cools 1 km3 of a fractured hot dry rock by only 1 °C will provide the same amount of energy as 7 × 104 t of coal.
A typical artificial HDR field would be at a depth of 3-6 km. During the 1970s and 1980s granitic areas of the USA, the UK, France and Germany became the focus of considerable research. The US experiment was conducted in an area with abnormally high heat flow (60 K km−1) in New Mexico and proved that HDR-heated water was sufficient to power a 60 kW turbine for one month. The UK project was based at Rosemanowes in Cornwall and focused on the crucial techniques needed to hydrofracture crystalline rock.
Cornwall is underlain by several large, highly radioactive granite intrusions, parts of which are exposed at the surface (Figure 7). This creates a large zone where surface heat flow is higher than in surrounding areas, beneath which there are no granitic rocks. Rosemanowes (R on Figure 7) is not located over the area of highest heat flow associated with the Lands End granite, but where an abandoned quarry in another granite outcrop became available for the experiment. To be viable for geothermal power generation, the experimental HDR field would have to achieve at least 150 °C at depth.
If the geothermal gradient at Rosemanowes is 37 K km−1, and the surface temperature averages 10 °C, what is the minimum depth to which drilling must penetrate to be viable for geothermal power generation?
The geothermal gradient must produce a temperature difference of 150 - 10 = 140°C. So the minimum depth of drilling needs to be 140°C/37°C km−1 = 3.78 km.
The Rosemanowes experiment showed that it was more difficult to get a closed circulation path than had been hoped. At high pressures (100 to 1000 times atmospheric pressure) runaway hydrofracturing of the rocks occurred and the water leaked away; water losses are sustainable only if they remain less than 10%. The Rosemanowes site was abandoned. The findings at Rosemanowes indicated a need for real-time monitoring of the progress of hydrofracturing and paved the way for more elaborate projects, such as the EU funded programme at Soultz-sous-Forêts in France, near the upper Rhine Valley. Soultz was chosen because its higher geothermal gradient allowed high temperatures to be exploited at shallower depths than at Rosemanowes. Initially two wells were drilled, to 3.5 and 5 km, with ancillary wells containing geophones to monitor microseismic activity as the fractures formed — this approach can also locate the newly developed fractures. Careful injection of water allowed the two main wells to be connected efficiently with no loss of fluid. A four-month test showed that 10 MW of energy could be extracted from a fluid flow rate of only 25 kg s−1. The project was a success and confirmed HDR technology as a major potential source of clean, renewable energy, provided it produced electricity at an economically competitive price.