Microbial corrosion in geothermal systems

Geothermal energy systems draw on fluids that have been isolated underground for extended periods, under conditions of high temperature, high pressure, and low oxygen. These are exactly the conditions under which anaerobic and thermophilic microorganisms thrive. Microbiologically influenced corrosion (MIC) is a recognized risk in oil and gas operations, where fluids share many of these same characteristics. In geothermal systems, the same mechanisms apply, and the sustained high temperatures and long fluid residence times can make the risk more pronounced.

To understand where this risk comes from, it helps to follow the fluid itself.

Temperature zones in a geothermal well

Geothermal fluid passes through several distinct temperature zones between the reservoir and the surface, and the microbial community shifts at each stage.

In the deep reservoir,  biological temperature limit is 126°C. Fluid at this depth is hot and anaerobic. As fluid moves up through the production well, temperature drops to a range of 80 to 200°C. This intermediate zone is cooler than the reservoir but still far from ambient. It falls within the range of thermophiles and hyperthermophiles, the organisms adapted to grow at these temperatures.

At the surface equipment stage, temperature falls further, to 20 to 80°C. This range allows an even broader spectrum of microorganisms to remain active, adding to whatever microbial load the fluid has already picked up on its way up.

The fluid is then reinjected into the subsurface, carrying forward the microbial and chemical changes accumulated across the entire cycle.

In other words, MIC risk is not confined to one point in the system. It is distributed across the full path the fluid takes, from reservoir to surface and back down again, with colonization and corrosion able to develop at any stage.

Photograph of a geothermal wellhead and surface piping, showing the industrial infrastructure fluid passes through en route to surface equipment (AI-generated).

MIC risk by system type

Because the underlying driver is temperature and fluid chemistry, not every geothermal application faces the same risk in the same way. The type of system determines which part of that temperature range dominates operations, and therefore which microbial communities are most relevant:

• High-temperature geothermal power plants operate within the range where thermophiles and hyperthermophiles are active, closer to the reservoir end of the spectrum. Above 130 °C, temperatures rise beyond what any known organism can tolerate. Microbial issues are not a concern here.

• Enhanced geothermal systems (EGS) introduce new fracture networks and flow paths, creating fresh surfaces for microbial colonization regardless of temperature.

  
  

• Geothermal heat networks run at the lower end of the temperature range over extended pipe distances, conditions suited to a broader range of organisms than high-temperature systems allow.

  
  

• Deep subsurface energy systems combine high pressure, high temperature, and anaerobic conditions that support specialized microbial communities throughout.

  
  

• CO₂-rich geothermal fluids add a chemical dimension on top of the temperature profile, shifting local pH and supplying an additional carbon source that affects which organisms dominate.

• Geothermal–hydrogen interfaces represent a newer combination, raising the question of whether hydrogen introduced into these systems will interact with the same microbial communities already linked to hydrogen loss in other subsurface storage contexts.

Relevant microorganisms

Across all of these system types, two microbial groups are consistently implicated. Sulfate-reducing bacteria (SRB) are a primary driver of MIC, with metabolic byproducts that directly contribute to corrosion and souring. Iron-reducing bacteria (IRB) affect the chemistry at the metal-fluid interface and contribute to corrosion through a different pathway.

Their activity does not stop at corrosion. The same microbial processes commonly drive mineral scaling, the buildup of deposits that restricts flow and reduces system efficiency. Because corrosion and scaling frequently occur together and share underlying microbial causes, distinguishing their individual contributions to equipment degradation requires direct microbial data rather than inference from physical damage alone.

Close-up of pitting corrosion on a metal surface, showing the localized pits characteristic of microbially influenced corrosion.

Detection

By the time corrosion or scaling becomes visible, the microbial activity driving it has usually been underway for some time, at some point along the well-to-surface-to-reinjection path already described.

Microbify offers MPN (most probable number) tests for sulfate-reducing bacteria and iron-reducing bacteria, providing quantitative data on active microbial populations at any given point in a geothermal well. These results allow MIC risk to be identified before it develops into equipment failure, closing the gap between where the risk originates and when it would otherwise be detected.