Thermal conductivity of unconsolidated materials for geothermal energy: the importance of petro-physical parameters and their relationships

Le energie rinnovabili stanno diventando sempre più importanti per la vita quotidiana, e una di queste è la geotermia, che produce energia sfruttando il calore naturale proveniente dall'interno della Terra. L'utilizzo di questa fonte di energia è considerato sicuro e sostenibile in quanto non genera direttamente emissioni di gas serra o altri inquinanti atmosferici. La temperatura del sottosuolo rimane piuttosto costante durante tutto l'anno, rendendola una fonte di energia affidabile e continua. Questa tecnologia può generare energia elettrica e calore per riscaldare e raffreddare gli edifici. Quest'ultimo fenomeno è responsabile della maggior parte delle emissioni di CO2 in Europa (Commissione europea, 2016). In questo contesto, l'energia geotermica sembra essere la fonte più promettente di energia rinnovabile. Nonostante questi vantaggi, è comunque ancora poco sfruttata a livello globale. È fondamentale esaminare il sottosuolo a livello idro-termo-petrof

Renewable energies are becoming increasingly paramount in daily life, with geothermal energy emerging as one such form that harnesses natural heat emanating from the Earth's interior. The utilization of this energy source is regarded as secure and sustainable due to its lack of direct greenhouse gas emissions or other atmospheric pollutants. The subsurface temperature remains relatively constant throughout the year, rendering it a dependable and continuous energy source. This technology can generate electricity and provide heating and cooling for buildings. The latter phenomenon is responsible for the majority of CO2 emissions in Europe (European Commission, 2016). In this context, geothermal energy appears to be the most promising source of renewable energy, however, it remains underutilized on a global scale. It is essential to examine the subsurface from a hydro-thermo-petrophysical perspective, particularly within the first two hundred meters, to comprehend how to harness the heat beneath the surface effectively. This study methodology can prove valuable in numerous domains, such as geology for geothermal applications and underground storage of carbon dioxide (CCUS) or heat (UTES). It can also be beneficial in other developmental areas such as engineering, ceramics, and agriculture. The objective of this study is to emphasize the significance of certain parameters and demonstrate their interrelationships. Specifically, thermal conductivity will be considered, thought to be directly correlated with porosity, bulk density, grain size, and water content. Understanding how these factors interact and influence thermal conductivity is fundamental for designing low-enthalpy geothermal systems. To achieve this goal, we have developed a laboratory employing the Guarded Hot Plate (GHP) method to precisely measure the thermal conductivity of unconsolidated materials. This method yields reliable and replicable data, providing a solid foundation for the analysis of geological materials. Although samples ranging from gravel to clay have been employed, the material has been categorized into specific intervals to determine and quantify the influence of grain size on thermal conductivity (Wentworth, 1922). In addition to petro-physical analysis, the samples have been tested and studied to determine the present mineralogical phases. Subsequently, the samples underwent thermal tests at various degrees of saturation. The materials under study were sourced from various regions in the Po Valley (Italy), Greece, and Canada. The results have been processed using statistical techniques and compared with literature data and empirical equations. Identifying trends among these parameters has been a primary research objective to facilitate the exploration of geothermal reservoirs and the extraction of heat from them. Furthermore, this work aims to enhance the use of software for subsurface modeling using specific values for each soil type, as well as to improve the scientific literature.