Experimental Studies of Deep-Geothermal Fluids and Amorphous Mg-Sio2 Scales Up to 350oC

Abstract

Hydrothermal experiments at static and dynamic conditions up to 350oC and pH range from 1 to 7 were conducted to investigate the chemical compositions of deep geothermal fluids and the various types of scales (eg. Mg-SiO2 and silica) that may deposit from the extraction of these fluids. In the static experiments, a HCl-H2SO4 acid fluid (pH 1 to 3) were interacted with tonalite collected from well WD-1 at 350oC for 15 days. The reacted fluids were found to be enriched in SiO2, Fe and Mg similar to the deep geothermal fluids observed in the world. The deposited components found are amorphous silica and Mg-silicate, and anhydrite. To study the amorphous Mg-SiO2 components, MgCl2 fluid (1 mMol/kg) and Na2SiO3 fluid (1500 mg/kgSiO2) were mixed and sealed in a quartz glass tube after adjusting each pH from 5 to 9. The tube was packed into a SUS reaction vessel with 4NH2SO4 solution and heated up to 300oC for one day. The reaction product was found to be in amorphous form and the chemical composition is mainly Mg and SiO2. Thermodynamic calculations showed that the solubility of the amorphous Mg-SiO2 scale decreases with increasing temperature up to 300oC similar to the results (at 120oC) obtained by Hauksson et al. (1991). In the dynamic experiments, HCl-H2SO4 solution with pH 3 was passed through a 100ml reaction vessel containing tonalite, at 1ml/min for 2 days at 350oC. The concentration of SiO2 is around 400mg/kg and the pH ranged from 3.1 to 3.5 during the reaction. The total volume of SiO2 dissolved from the rock is calculated to be 2%.