Inhibition Mechanism and Performance of Two Polymer-Based Silicate Scale Inhibitors in Low Temperature Geothermal Brines: Impact of pH

Silica scaling is acknowledged as a production problem in geothermal fields. Previous research has stated that "brine acidification" potentially inhibits amorphous silica scaling up to 97% (Gallup, 2002). However, the industry is still trying to identify scale inhibitor products that are able to match this performance under such low pH conditions. Hence, this work has developed a static bottle test methodology to assist in identifying silicate inhibitors with 80-90% inhibition performance, that are applicable for low enthalpy geothermal brines to prevent amorphous silica and magnesium silicate scale formation. Various products were evaluated to assess their inhibition efficiency (IE) and the mechanism through which they operate.

The inhibition mechanism was evaluated by monitoring SI consumption using two methods: (i) by determining the concentration of sulphur (S) contained within the SI B and A5 structure by Inductively Coupled Plasma-Optical Emissions Spectrometer (ICP-OES), and (ii) by using a matrix-matching Hyamine technique, to detect SI B and A5 via the functional groups on the polymer backbone.

The silicate scaling regimes were defined over a pH range of 4, 6 and 8.5. PHREEQC geochemical modelling was used to confirm the formation of magnesium silicate and amorphous silica scale under these conditions. Results are presented for two sulphonated polymer- based scale inhibitors (SI), denoted SI B and A5, studied at 95°C, pH8.5/6/4 and their active concentration range 20-200ppm. At pH8.5, SI B and A5 record 80-90% and 55-85% IE, respectively, against silicate scale, however at lower pH levels of 6 and 4, no IE was observed.

Although commercial scale inhibitors show limited inhibition performance against amorphous silica scale (~75% IE), industry is keen to compare their effectiveness against the "brine acidification" process, which has previously been found to be highly effective. Combining our static bottle performance tests over a range of pH values with both PHREEQC geochemical modelling and inhibitor consumption tests (by ICP-OES and Hyamine) has provided some insights into silicate scale inhibition and control.