Silica is the hardest scale to control in geothermal systems. It forms hard, stubborn layers especially in the heat exchangers of binary (ORC) plants and in reinjection lines, reducing heat transfer and reinjection capacity.
Why does silica precipitate?
High-temperature reservoir water carries a lot of dissolved silica. As steam is separated, silica concentration in the remaining brine rises; as the fluid cools, the solubility of amorphous silica falls. Once saturation is exceeded, silica first forms small polymers, then colloidal particles that stick to surfaces. The polymerisation rate depends on temperature, pH and salinity and generally increases with pH.
Metal silicates
When iron, magnesium or aluminium are present, silica combines with these metals to form metal silicate layers, which are harder and denser than pure amorphous silica. Iron released by corrosion feeds this process, so silica and corrosion programmes should be designed together.
Control methods
- Temperature limit: keeping reinjection temperature above the silica saturation limit.
- pH management: in some fields a controlled pH reduction slows polymerisation; the effect on corrosion is assessed at the same time.
- Inhibitor and dispersant: special polymers that delay silica polymerisation and disperse particles already formed.
- Shorter residence time: long holding times in tanks give silica time to polymerise.
The right combination depends on the field’s silica concentration, temperature profile and plant design.
| Measurement | What it shows |
|---|---|
| Total and monomeric silica | Degree of polymerisation and inhibitor effect |
| Silica saturation index | Size of the scaling risk |
| Heat exchanger approach temperature | Loss of heat transfer |
| Reinjection injectivity | Plugging of line and well |
Frequently asked questions
Can silica scale be prevented completely?
Complete prevention is not possible in every field, but managing temperature, pH and inhibitor together markedly reduces the scaling rate and extends cleaning intervals.
Is a silica inhibitor the same as a calcite inhibitor?
No. The mechanisms differ; silica needs specific inhibitors and dispersants. The two products can be used together if compatible.
Why is the risk higher in binary plants?
The fluid is cooled to lower temperatures, so silica saturation is exceeded more; the cold end of the heat exchanger is the riskiest zone.
Related guides
- Geothermal power plant chemicals (overview)
- Calcite scaling in geothermal wells
- Corrosion in geothermal systems
- Downhole dosing via capillary tubing
- Heat exchanger and ORC fouling
- Scaling in reinjection lines and wells
- Geothermal heating and greenhouse systems
- Sulphide scales (stibnite, iron sulphide)
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