Accessing Scotland's geothermal resource: regulatory guidance

Geothermal heat explained

Geothermal energy is the natural heat that exists within our planet. The Scottish Government is targeting three geological settings within Scotland for exploration in respect of their geothermal potential:

Water is held naturally in rocks as groundwater, and flows continuously into active mine workings, requiring them to be pumped out. However, when mining and pumping ceases, the abandoned mine workings become flooded. Mines can extend to relatively deep levels so, in some cases, abandoned mine workings can provide easy access to warm water. This warm mine water can be accessed by means of a borehole, and the heat can then be made available for space heating or domestic hot water heating.

Aquifers are bodies of permeable rock that can conduct significant quantities of groundwater. The largest and most conductive of these generally occur in sedimentary strata. Those that are deep enough to hold hot water can be classed as hot sedimentary aquifers. The hot water can be accessed and abstracted from the aquifer by means of a borehole and the heat can then be made available for space heating or domestic hot water heating.

Crystalline rocks at several kilometres depth can be hot enough to be used to generate electricity. Such rocks usually lack open fractures and consequently have very low permeability. Essentially dry, they are known as hot dry rock (HDR) resources. Some granite intrusions generate their own heat, so they can be hotter than other rocks at the same depth. Exploiting HDR resources typically relies on creating an Engineered Geothermal System (EGS), in which a network of open fractures is created to hydraulically connect boreholes drilled some distance apart into a hot rock zone. Cold water injected into the open fractures through one borehole passes through the fractures, and the resulting superheated water, or steam, is then extracted through another borehole. The thermal energy stored in the water can be converted into electricity at the surface.

Some crystalline rocks are naturally permeable. These rocks can contain significant amounts of hot water and are sometimes known as hot wet rock (HWR) resources. The water can be abstracted by means of a borehole and an EGS is not required. In the case of both HDR and HWR resources, following the extraction of the heat, the water can be re-injected at the site, maintaining the level of groundwater available for future abstraction.