Heat Production from Geothermal Doublet in Caledonian Rocks

Geothermal energy is a renewable resource that can be exploited for greater energy security and clean heat. Typically, open-loop systems rely on natural and artificially enhanced permeability in either pore space, or in faults and fractures. Targeting deeper resources can provide access to more energy due to higher temperatures; however, there is usually diminishing pore space and poorer well productivity. Thus, secondary porosity in the form of fractures and fracture networks, either naturally occurring or through enhanced geothermal systems, is essential to maintaining a permeable geothermal reservoir for heat extraction. Fluid flow and heat transfer are greatly influenced by fracture properties such as aperture, roughness, and orientation. In this work, a subsurface model is developed with a particular focus on Caledonian rocks due to their wide geospatial continuity across the UK and northern Europe, and their high-quality thermo-physical properties, such as thermal conductivity and radiogenic heat production. A typical doublet system is modelled in Cairngorm granite, consisting of one production and one reinjection well connected through a long channel fracture network within the model. Pre-processing to generate the geometry and finite element mesh is achieved using GMSH, while the doublet system was simulated using coupled thermo-hydraulic (TH) processes in OpenGeoSys. The effect of flow rate, injection temperature and fracture aperture on heat production is evaluated. The study aims to provide preliminary insights into the performance of open-loop fracture-dominated geothermal systems in Caledonian rocks.