Calcite (calcium carbonate) is the most common type of scale in geothermal production wells. Within months it can narrow the wellbore, cut flow and force unplanned shutdowns. The good news: with the right scale inhibitor and the right injection point it can largely be prevented.
Why does calcite scale form?
Reservoir fluid contains significant dissolved CO₂ and calcium under high pressure. As the fluid rises in the well, pressure drops and boiling (flashing) starts at a certain depth. At this point CO₂ moves into the gas phase, the pH of the fluid rises and calcium carbonate becomes supersaturated. Calcite deposits rapidly in a narrow zone just above the flash point.
The location and rate of scaling depend on reservoir temperature, CO₂ and calcium content, flow rate and wellhead pressure. Lowering wellhead pressure or raising flow moves the flash point deeper and can shift the scale zone with it.
Symptoms: spotting scaling early
- Flow rate falling over time at the same valve opening
- Unexpected changes in wellhead pressure
- Calcium measured at the wellhead and separator falling below the reservoir value
- Reduced diameter in downhole calliper (go-devil) runs
How to prevent it
The core method of calcite control is continuous injection of a scale inhibitor below the flash depth through capillary tubing. The inhibitor blocks the growth of crystal nuclei and their adhesion to surfaces and works at very low concentrations. Selection takes into account stability at high temperature, the calcium and alkalinity level of the fluid and compatibility with the corrosion inhibitor.
Biyopol’s BiyoGeo CS is a polymer developed for Turkish geothermal fluids in a TÜBİTAK 1507-supported project. The dose is set from fluid analysis and a field trial.
Chemical prevention or mechanical cleaning?
Reaming or acid cleaning removes scale but does not stop it from forming again; every clean-out means lost production, equipment and labour costs. A continuous inhibitor programme greatly reduces the need for cleaning and protects flow. In most fields prevention is more economical than cleaning.
| Measurement | When | Why |
|---|---|---|
| Calcium and alkalinity (wellhead, separator) | Weekly / daily during trials | Confirms calcium stays in solution |
| Flow rate and wellhead pressure | Continuous (SCADA) | Shows bore narrowing |
| Residual inhibitor concentration | Monthly | Checks dose adequacy |
| Downhole calliper run | Yearly / when suspected | Confirms scale location and thickness |
Frequently asked questions
Where does calcite scale form most?
Usually inside the well in a narrow zone just above the flash point; it can also form at the wellhead, separator and surface lines.
How is the scale inhibitor dose determined?
A starting dose is calculated from fluid analysis, the temperature–pressure profile and flow rate, then optimised during a field trial using calcium and pressure data.
Can existing scale be removed with the inhibitor?
Scale inhibitors do not dissolve thick existing scale; mechanical or chemical cleaning comes first, followed by a continuous prevention programme.
Related guides
- Geothermal power plant chemicals (overview)
- Silica and metal silicate scaling
- 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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