Unlocking the deep geothermal energy potential of the Carboniferous Limestone Supergroup

The direct use of geothermal heat from the Carboniferous Limestone Supergroup (‘Carboniferous Limestone’) could make a significant contribution to delivering affordable, low-carbon heating to thousands of buildings, homes and offices through a district-heating system, as well as providing heating for agriculture and horticulture. Geothermal heat from the Carboniferous Limestone offers a clean alternative energy resource to help in the UK to achieve carbon net zero emissions by 2050.

Traditionally, the use of geothermal energy has focused on generating power using conventional steam-turbines in areas where high-temperatures (more than 160°C) occur at shallow depths (less than 500 m). Such settings are usually associated with plate boundaries and areas of active volcanism, as found in Iceland and Kenya. Now, due to technological advances in geothermal drilling, low-moderate temperature (over 60°C) deep geothermal energy can be utilised for industrial or residential (direct use) heating.

Deep geothermal energy is defined as the heat resource found at depths of between 500 m and 5000 m, where heat is transported by groundwater that is circulating within the pores of rocks in deep aquifers. These resources are exploited by doublet systems, which involve two deep boreholes, one for extracting the warm water and another one for reinjecting the cooled water back into the subsurface (Figure 1). As these fluids are pumped to surface they pass through a heat exchanger that transfers the heat into a district heating system (Figure 2). Such schemes have been developed in Belgium, the Netherlands and Germany.

In the UK, the Carboniferous Limestone is proven as a productive aquifer, hosting the circulation of deep hydrothermal fluids responsible for the thermal springs in Bath (Figure 3), Bristol, the Taff Valley and Buxton. The advantage of the Carboniferous Limestone as a resource is that it covers large areas of the UK subsurface and coincides with many large towns and cities. A number of schemes have successfully developed the Carboniferous Limestone geothermal resources for heating purposes, such as those in Belgium and the Netherlands.

The geothermal team at BGS has started a research programme looking at the utilisation of the deep geothermal energy held in the Carboniferous Limestone as a renewable heat resource for the UK.

During the Early Carboniferous period, the UK was situated near the equator experiencing a similar climate to that of modern-day tropics. Carbonate reefs developed across the northern and southern parts of the UK which, at that time, were separated by a strip of landmass called the Wales–Anglo–Brabant Massif (Figure 4). As a result of climate change 359–326 million years ago, sea levels dropped and these carbonate reefs became exposed to the rain (which acts as a very mild acid), causing the limestone to dissolve and creating underground drainage systems with sinkholes and caves. These features are preserved in the geological record as so-called palaeokarst.

These ‘karstified’ surfaces, along with solution-enhanced fractures and faults, provide connectivity in the rocks through which geothermal fluids can flow. BGS research focuses on identifying areas in the UK where we may find such deep flow systems (aquifers) and on assessing their potential for producing geothermal heat.

Sustainable development goals

The BGS research into the utilisation of deep geothermal energy in the Carboniferous Limestone as an energy resource for the UK directly addresses a number of the UN Sustainable Development Goals (SDGs).

About the author

Darren Jones

Petroleum geologist

BGS Edinburgh

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Contact Darren Jones or visit geothermal energy for more information.

References