Scaling issue in geothermal brines

Geothermal brines contain varying concentrations of dissolved minerals, which are primarily determined by the lithology of the geothermal reservoir. The geochemical composition of these fluids reflects water–rock interactions occurring at high temperatures and pressures within the subsurface. In general, brines from carbonate-hosted reservoirs tend to be rich in calcium, magnesium, and bicarbonate ions, while those associated with siliceous (e.g., volcanic or granitic) formations typically exhibit elevated concentrations of silica and alkali metals. Siliceous formations remain stable in high-temperature, high-pressure conditions within the reservoir and issues arises cooling brines rich in silica as their solubility decrease as temperature and pressure also decrease. Cooling these brines therefore promotes the silica precipitation and scaling along pipelines, wellbores, or reservoir fractures.

While carbonate scaling has been effectively managed for decades through the application of specific chemical inhibitors, silica precipitation remains a persistent challenge for the geothermal industry. As a result, geothermal fluids with high silica concentrations are often underutilized due to the risk of operational issues and reduced system efficiency.

In several Italian geothermal sites, silica is not the only problematic phase. A significant additional scaling issue arises from the deposition of stibnite (antimony sulfide, Sb₂S₃), a mineral that precipitates at temperatures even higher than those typically associated with silica scaling. The simultaneous occurrence of these phases complicates scaling control strategies and poses a serious limitation to the sustainable exploitation of high-enthalpy geothermal resources. In heat exchangers handling geothermal brines enriched in stibnite (Sb₂S₃) and silica, the cooling process used to transfer heat to secondary fluids leads to a decrease in the solubility of antimony-, sulfur-, and silica-bearing species. As the temperature drops, supersaturation occurs, promoting the nucleation and growth of mineral scales on internal surfaces. This deposition progressively reduces the effective pipe diameter, forms insulating layers on heat exchanger surfaces, diminishes heat transfer efficiency, and significantly increases maintenance requirements and operational costs.

To address these scaling issue the GEOFLEXHeat solution foreseen the injection of chemicals to inhibit stibnite precipitation and to precipitate silica, which is then filtered and collected for further usages.

By addressing the stibnite and silica issues geothermal operators can enhance plant productivity, prolong equipment life, and maintain reservoir performance, ensuring the long-term viability of geothermal energy production.

GEOFLEXHeat System

In our system it is foreseen an heat exchanger up stream to reduce brine temperature from 200°C to 80°C then a scaling reactor down stream to reduce silica concentration. The inhibitors for stibnite issue are supposed to be injected before the heat exchanger while the chemical injection before the scaling reactor; the designed sequence, from literature analysis, make sense and follows logical chemical and operational principles for managing both stibnite scaling and silica removal. This is the rationale: as the inhibitors are injected into the brine at 200°C, upstream of the heat exchanger, this prevents stibnite (Sb₂S₃) scaling, which is triggered by the reduction in temperature in the heat exchanger, where the antimony solubility decreases. After the brine exits the heat exchanger at 80°C, a chemical is injected to initiate silica removal through the scaling reactor. The temperature of 80°C is optimal for silica removal, as the chemical precipitation is highly efficient at this range.

Stibnite inhibitors work by adsorbing onto potential nucleation sites, preventing antimony-sulfide precipitation. Injection before the heat exchanger ensures the inhibitor is fully mixed with the brine and active during the critical cooling phase (200°C → 80°C), where stibnite supersaturation occurs. By preventing stibnite formation in the heat exchanger, we will protect high-value equipment from fouling, maintain thermal efficiency, and avoid operational downtime.

At 80°C, silica concentrations in the brine are high but manageable. The chemical injected for silica treatment promotes silica removal without affecting the antimony inhibitors. The two processes are independent: as stibnite inhibitors target Sb₂S₃ formation by reducing nucleation; the other chemical precipitation targets silica.

It will be necessary to implement continuous monitoring of key parameters (temperature, pH, antimony, silica, sulfide) to adjust chemical dosages dynamically.

The advantages of this sequence can be resumed as follow:

• Stibnite scaling is managed upstream, and silica scaling downstream, without overlapping treatment zones.
• Reduced Risk of Cross-Reaction: separating chemicals injection points minimizes chemical interference.

We will conduct tests to validate the interaction of chemicals in our specific brine chemistry. A pre-treatment will be evaluated to remove excess sulfides (if present) upstream, as they can exacerbate stibnite scaling. This sequence aligns well with best practices in geothermal brine processing and should work effectively.