Permeable UK Granite Is Good News for Geothermal Energy

In mid-February, the Geological Society of London raised the hopes of those promoting geothermal energy when results of exploratory drilling in Weardale, County Durham, revealed record levels of permeability in granite. Although the results are promising for the development of geothermal energy, they may have less welcome implications for the safe disposal of radioactive waste in deep repositories.

Scientists from Newcastle University were investigating potential sources of geothermal energy, which is becoming increasingly popular in the search for low-carbon energy resources. Granite can be particularly useful, as it can be rich in radioactive elements that generate heat as they decay. The permeability of the rock is important, as heat is extracted by pumping a "working fluid" such as water into the rock and drawing it back up again.

Professor Paul Younger, who led the research, said that "Hydrogeologists have traditionally viewed granite as poorly permeable," but his team decided to challenge that assumption and found the "highest permeability ever recorded for a granite anywhere in the world."

The results were obtained by pumping naturally occurring saline groundwater from an exploratory borehole and monitoring the change in water levels. A permeability of almost 200 darcies — a unit of permeability — was recorded, far higher than most prolific oil and gas reservoirs.

The scientists believe the find is not unique to the Weardale granite, as there are similar granites worldwide that may display equally high levels of permeability. High natural permeability means less cost for hydraulic stimulation.

However, the research also means that caution needs to be taken when selecting sites for nuclear waste disposal. Granite is a popular rock in which to site repositories, and the higher-than-expected permeability of this rock suggests that safety estimates previously made may have to be reconsidered.