Evaluation of Temperature Effects on Silica and Barium Sulphate Scale Formation in Geothermal Systems Using Geochemical Modelling and Experimental Analysis

This thesis investigates the temperature-dependent scaling behavior of silica (SiO¿) and barium sulphate (BaSO¿) in geothermal systems using an integrated approach combining PHREEQC geochemical modelling, static precipitation experiments, dynamic core-flood tests, tube-blocking experiments, and solid-phase characterization. The primary objective was to evaluate the predictive capability and limitations of thermodynamic modelling by comparing PHREEQC predictions with experimentally observed precipitation kinetics, aqueous-phase depletion, recovered scale mass, deposition morphology, and hydraulic impairment under geothermal-relevant conditions. PHREEQsilica phase. Instead, the recovered material likely included amorphous or poorly crystalline silica-rich aggregates together with residual brine-derived comC modelling successfully identified the thermodynamic precipitation potential of the investigated brines. For the silica system, the model predicted silica-phase equilibrium precipitation potentials of approximately 0.488 g as SiO¿ at 25 °C and 0.412 g as SiO¿ at 85 °C. These values were lower than the experimentally recovered dried masses of 1.5049 g and 1.275 g, respectively, indicating that the recovered solids were not composed only of the modelled equilibrium ponents. The ICP-OES-derived aqueous mass balance further confirmed substantial dissolved silicon removal, corresponding to approximately 1.24 g as SiO¿ at 25 °C and 1.068 g as SiO¿ at 85 °C. The time-dependent experimental results showed that silica precipitation was governed by complex kinetic and colloidal processes rather than equilibrium alone. At 85 °C, silica removal was rapid during the early stage but stabilized quickly, whereas at 25 °C, precipitation was slower but more sustained, resulting in a higher final recovered mass and a stronger apparent first-order fit (R² = 0.9675). Increasing the magnesium concentration at 25 °C further increased the recovered mass to 1.61 g, suggesting that Mg²¿ influenced silica polymerization, colloid destabilization, or aggregation. Dynamic core-flood experiments revealed that temperature controlled both plugging kinetics and deposition morphology. While the 80 °C tests showed faster plugging onset,the 25 °C tests produced a more compact inlet filter cake and severe permeability reduction of 99.02%. This indicates that hydraulic damage was controlled not only by the amount of scale formed, but also by deposition structure and location. For the barium sulphate system, PHREEQC predicted strong supersaturation with respect to barite (SI = 3.31), and static tests confirmed rapid precipitation consistent with this thermodynamic prediction. Dynamic tube-blocking tests further showed that barite deposition rates increased from 7.37 × 10¿¿ g/min at 25 °C to 1.48 × 10¿³ g/min at 60 °C, indicating enhanced ion mobility, mass transfer, crystal growth, and surface attachment under flowing conditions. Overall, this study demonstrates that PHREEQC is an essential screening tool for evaluating geothermal scaling potential, but reliable scale prediction requires coupling thermodynamic modelling with experimental validation of time-dependent precipitation, aqueous-phase depletion, deposition morphology, and transport-controlled hydraulic impairment.