Impact of Calcium on Inhibition Mechanism and Performance of Two Polymer-Based Silicate Scale Inhibitors in Low Enthalpy Geothermal Brine Systems

Silica scaling is an operational issue in geothermal energy production. In this work, a static bottle test methodology was developed to assist in identification of silicate inhibitors, demonstrating 80-90% inhibition performance, for application in low-enthalpy geothermal systems. To identify effective products for the control of amorphous and magnesium silicate scaling, the inhibition mechanisms and inhibition efficiency (IE) of two sulphonated polymer-based scale inhibitors, denoted A5 and SI B, were studied. This was performed for a brine containing [Mg]initial = 120ppm and [Si]initial = 1880ppm at 95ºC and pH8.5. The impact of adding 500ppmmix calcium was subsequently assessed.In the absence of calcium, a minimum inhibitor concentration (MIC) of ~50ppm for SI B and ~100ppm for A5 were required to control amorphous and magnesium silicate scale effectively at ~60-90% IE. In the system containing calcium, although two approaches were used to adjust the pH of the brine to 8.5, to reduce the tendency for Ca(OH)2 precipitation, and thereby improve scale inhibitor effectiveness, it was found that neither SI B nor A5 prevented precipitation of silicate-based scale.To identify the inhibition mechanisms, two complementary methods were compared to track SI consumption. The first measured sulphur content, unique to the structure of SI B and A5, using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The second employed a matrix-matching Hyamine technique, targeting the functional groups along the polymer backbone to detect the inhibitor directly.