Solutions
General comments
The PERFORM project aims to increase and improve our knowledge about geological and geochemical causes of typical operational problems in geothermal plants and to test potential solutions to handle and mitigate problems. From the PERFORM database, we have a good knowledge of the expected chemical composition of the geothermal waters at individual sites. Knowledge of the chemical composition of the formation waters and the geology is useful in planning actions that can help solving site-specific problems and challenges. Reference is made to short text below and the ‘summary problems-learnings table’ accessible from this website.
Challenges
Corrosion problems and precipitations of calcite, barite, pyrite, and other minerals in the infrastructure facilities are commonly known difficulties when operating geothermal plants. To solve these problems, a number of possibilities exist depending on the specific problem. Below, these possibilities are described briefly.
Preventing scaling
Calcium carbonate scaling can be largely avoided by maintaining an operational pressure exceeding the bubbling point. The operators of the plants are for most part able to do so. However, geothermal systems with high bubble points or large high temperature geothermal plants might require additional calcite scale prevention measures.
Inhibitors
Our results indicate that particularly if waters are calcium rich, barite scale formation may occur after water cooling. In such case, the traditional approach would be to add scaling inhibitors. Adding inhibitors may also be a relevant solution to scaling problems, when dealing with scaling minerals other than barite.
Particle and cation filters
As an alternative to adding inhibitors, improved particle and cation filters have been developed in PERFORM. An innovative particle filter, the HydroGeoFilt system with a self-cleaning function, has recently been developed. The HydroGeoFilt system can effectively remove particles, added adsorption materials, and flocculates from the thermal water (PERFORM Report D3.1). Clogged filter candles can be cleaned mechanically by using an ultrasonic device placed in the centre of the filter system and next, particles can be removed by a back-flushing filter. The HydroGeoFilt system has been tested successfully in the laboratory. Long-time onsite tests are, however, still missing.
Besides particle filters, cation filters may be used for removing dissolved calcite and barium from the thermal water with the purpose of avoiding scaling. An improved cation filter, the so-called FACT filter, is under development in PERFORM – the FACT filter technology is based on seeded crystallization (FACT equals Filtration Assisted Crystallization Technology). The FACT filter is designed for removing calcium, and carbonate crystals that are formed during the crystallization process, are to be removed by filtration as outlined above. Barium may be removed by a cation filter. Special cation filters characterized by high adsorption capacities are under development in PERFORM. These filters use adsorption materials like iron hydroxide, zeolite, and chitosan for removing cations prior to forming scaling minerals. Especially zeolite and chitosan are relevant for barium removal.
Preventing corrosion
Cation filters are also developed with the objective of preventing corrosion. Especially galvanic corrosion can be a problem if high concentrations of dissolved lead and copper occur in saline thermal waters. In PERFORM, special cation filter systems for removing Pb2+, Cu2+ and Fe2+ are currently being developed, so that galvanic corrosion and the subsequent precipitation of metallic lead (Pb), copper (Cu) and iron (Fe) can be avoided or at least limited. Our data analysis indicates that galvanic corrosion is particularly pronounced if the chloride concentration > 100000 mg/L. Thus, especially at geothermal sites with elevated chloride concentrations, precautions to avoid galvanic corrosion should be taken, either by removing Pb2+ or Cu2+ from the produced water or by ensuring that the plant infrastructure is constructed of material not prudent to galvanic corrosion. It is recommended to use composite materials and corrosion-resistant alloys to prevent (or limit) galvanic corrosion and hence, stainless steels may be used.
Corrosion by oxygen ingress occurs at high rates and may cause the formation of substantial amounts of iron-oxides that can potentially cause clogging of sand screens in injection wells. Therefore, introduction of oxygen in the geothermal water stream should be avoided. Most operators of the plants are, in general, able to do so by maintaining an increased operation pressure.
Avoid improperly cleaned tubings
Use of improperly cleaned tubings at installation of geothermal wells should be avoided, since e.g., the release of relict mill scale may take place during the operation of the geothermal plant. The release of relict mill scale may potentially be promoted by corrosion.
Problems related to H2S
Hydrogen sulphide (H2S) is toxic and corrosive and the presence of hydrogen sulphide gas in the geothermal water causes problems at several geothermal sites. Hydrogen sulphide may be removed by adding granulated iron-hydroxide (FGH) or iron chloride (FeCl3 solution). Commonly, this process results in formation of iron sulphide (FeS) that can be removed using a particle filter (e.g., HydroGeoFilt). The H2S aspect and the flocculation process is described in Regenspurg et al. (2020).
References
Regenspurg, S., Iannotta, J., Feldbusch, E., Zimmermann, F.J., 2020: Hydrogen sulfide removal from geothermal fuids by Fe(III)‑based additives. Geothermal Energy (2020) 8:21.
Iannotta, J., 2020: Report on stability and effectivity of particle filters in lab. and field. Hydroisotop (HI). PERFORM report, Deliverable D3.1.