In his Master Thesis, Samuele Frigo performed the characterisation of the geothermal system in fractured crystalline rocks at United Downs (Cornwall, UK) by means of Thermal-Hydraulic-Chemical modelling. Samuele is a Master student from the University of Greifswald, where he studied Earth Sciences. For his Master thesis, he joined the GFZ Potsdam and the CRM-geothermal project.
Samuele’s study aims to contribute to the development of a pilot plant for the co-production of lithium (Li) and geothermal energy from a low-salinity crystalline reservoir in SW England, operated by Cornish Lithium Ldt. The objectives of his study are to characterise the geothermal system under analysis in terms of hydraulics and geochemistry and evaluate the long-term sustainability of the potential co-production operations on site. The multifaceted approach adopted in his research combines traditional hydrogeological and geochemical fieldwork, laboratory and data analysis, and advanced numerical methods.
The field campaign took place between June and July 2023. The fieldwork encompassed hydrogeological testing, chemical-physical fluid monitoring, and sampling fluids and scales from two relatively shallow boreholes (~ 800 and 1100 m depth). The boreholes, 214 meters away at surface, intersect two highly permeable fracture zones between 600 and 700 m depth known to be highly conductive to hot and Li-enriched fluids. Figure 1 shows the pressure drawdown in the production well GWDD_001 during the step-rate airlift test, as well as the relative calculated Productivity Index (PI)—a useful parameter to evaluate the hydraulic performance of the system.
Figure 1: Discharge Q, pressure drawdown Δp and Productivity index PI estimation during the step-rate airlift test. The high estimated PI suggest a good response of the system to fluid production (Frigo, unpublished).
During these tests, Samuele and several CRM-geothermal partners also took gas, fluid and scale samples for subsequent chemical analysis. The analysis results were then used by Samuele for a simple sequential thermodynamic equilibrium modelling with PHREEQC to investigate and quantify the scaling processes potentially occurring during geothermal operation. Precipitation of several mineral phases – mainly including Fe and Al bearing phases, silica, calcite, and fluorite – seems to occur upon production to the surface. Importantly, the aeration of fluids results in a 7-fold increase in scaling magnitude with respect to the operation in anoxic conditions.
Making use of MOOSE/GOLEM, a 3D model simulating coupled thermal, hydraulic and conservative mass transport processes (THC) occurring within the fractured geothermal system (Figure 2) was developed. The THC model was used to evaluate the thermohydraulic and chemical evolution of the system under different production scenarios with varying discharge and injection rates. All THC simulations display positive results, suggesting the modelled pump rate of 25 kg/s as the most suitable for commercial developments, whether the focus is to maximise both power generation and Li extraction.
Figure 2: Structural model of the shallow geothermal site at United Downs used to perform the 3D THC simulations with MOOE/GOLEM. The model includes two porous matrix lithologies – granite and metasediments – two steeply dipping fracture zones, and two wells. The structural model was adapted from a previous version provided by Cornish Lithium Ltd. (Frigo, unpublished).
The outcomes of Samuele’s study, which will be presented in detail in an upcoming publication, suggest that the reservoir at the site of the CRM-geothermal partner Cornish Lithium Ltd. is suitable for long-term heat and mineral extraction. They also highlight scaling potential, such that adequate countermeasures could be applied for the future operation of the system.
Samuele conducted his Master study under supervision of Prof. Dr. M.-Th. Schafmeister (University of Greifswald) and PD Dr.-Ing. M. G. Blöcher and PD Dr. Simona Regenspurg (GFZ).
