THERMOphysical properties of CALedonian rock materials to de-risk geothermal development (THERMOCAL)

Geothermal technology epitomises the challenges of effective interfacing between engineering, physical and natural sciences. A geothermal facility involves inducing heat flow through two thermal resistances in series. For shallow and deep closed-loop systems, one thermal resistance is represented by the geothermal borehole and its infrastructure (casing, grout/cement, heat exchangers and transfer fluid), which can be specified and engineered with high precision. The other thermal resistance is the ground itself, whose properties are variable and controlled by nature. Heat transfer fluid does not have direct hydraulic contact with the surrounding formation, only transferring heat through conduction, thus reducing geological and reservoir risk. In predicting the performance of a geothermal system, the variability of natural properties must be characterised empirically. Modelling techniques must be employed that propagate uncertainty to outcomes (i.e., available thermal power) in a transparent and statistically defensible manner. The important thermal properties in conduction-dominated settings for subsurface heat extraction and storage are thermal conductivity, volumetric heat capacity and internal heat production. However, databases of rock thermo-physical properties for geothermal engineers are limited and often do not specify measurement conditions. Furthermore, the various multi-scale datasets require mutual calibration against each other and empirical analysis of their relationships. Government funded studies have therefore recommended systematic data acquisition and compilation.

All participating nations in this proposal have large ambitions for geothermal technology and share fundamental similarities: i) continuous shared geological "terrane" from Northern Ireland, through Scotland to Norway in the form of the Caledonian orogenic belt, ii) extensive areas of "hard rock" lithology: exposure is good and representative intact rock samples of uniform size can readily be retrieved and tested, and iii) generation of large quantities of renewable electricity that can be used in geothermal heat pumps (GHP).

Caledonian rocks have high geothermal potential due to their high thermal conductivities and radiogenic heat production values. The THERMOCAL vision is to carry out a systematic measurement programme - with unified sampling and analytical methodologies - of lithologies in the Caledonian terrane and underlying Precambrian basement across northern UK and Norway. Bespoke modelling of closed-loop systems will allow Investigation on the impact of data on real-world geothermal engineering applications, such as GHPs - both shallow and deep.

This will result in: i) a public, representative and quality-assured database of the thermal and radiogenic properties of lithologies of the Caledonian orogenic belt, ii) improved quality assurance for the determination of these properties via interlaboratory ring testing and method harmonisation, iii) the comparison between different types of data, and iv) calibrated modelling approaches to representatively incorporate uncertainty in rock properties into engineering heat transfer models of geothermal infrastructure, ultimately contributing to the de-risking of further geothermal developments.

These outcomes will benefit geothermal engineers designing future sustainable geothermal systems, and households who require efficient and reliable clean heat. The outputs will also be of importance to fields including: underground energy storage, underground structures (e.g., tunnels) and storage of thermogenic wastes, while the gamma spectrometry data may provide background for evaluation of potential national uranium and thorium reserves.