Effect of geothermal brine properties on silica scaling in enhanced geothermal systems

Abstract

Silica scaling in Enhanced Geothermal Systems (EGS) is currently a concern affecting the economic viability of geothermal energy extraction. Knowledge of the effect of brine properties, such as pH and salinity, on silica scaling is essential for the effective design and operation of EGS reservoirs. This paper presents a laboratory study of silica scaling under variable environmental conditions. A set of silica scaling experiments was conducted in Teflonlined stainless steel autoclaves at 200°C and saturated vapour pressure in order to simulate the conditions of a geothermal well in the Cooper Basin in South Australia. Tests were conducted in solutions supersaturated with respect to amorphous silica and in solutions supersaturated with respect to a granite sample extracted from the Habanero 3 well. Samples of stainless steel wire mesh were used as the platform for scale deposition. Colloidal silica concentrations of the reactant solutions were used to indicate the relative extent of silica scale deposition; these were calculated by the difference between the measured total dissolved and reactive silica concentrations. A scanning electron microscope was used. (i) to compare the relative surface coverage of the silica scale deposits and thus verify results from the reactant solution analysis; and (ii) to differentiate between different scale morphologies. The rate of silica scale deposition was found to increase when Na ions were added to solution. Results from experiments used to determine the effect of Cl ions on the rate of silica scale deposition were inconclusive due to the dissolution of the wire mesh sheets. The extent of silica scaling was found to increase with increasing alkalinity. This trend contradicted the literature where a reduction in the extent of silica scaling at values of pH greater than 10 has been reported. This discrepancy could be due to a result of the high levels of silica supersaturation during autoclave quenching.