Identified problems linked to geothermal operations. Problems to be considered (and solved)
Suggested solutions – and methods developed for potential solutions through the PERFORM study work
Sites tested and additional comments
Operational problems as consequence of improper design and dimensioning of the sub-surface facilities (like gravel packs, screens and casings/tubings).
- Conduct workover operations.
- Replace gravel packs and screens, if possible.
Clogging of pores, perforations etc. by particles.
- Use a self-cleaning particle filter or another type of particle filter.
Grünwald, Oberlaa and Insheim.
Particle filters have been tested at these sites
Calcite scaling. Precipitation of calcite is a common problem. Calcite scaling is primarily related to CO2 de-gassing or temperature increase.
The solubility of calcite decreases as the temperature increases.
Supersaturation with calcite indicates a potential risk of calcite scaling. A high downhole Saturation Index (SI) for calcite (SI > c. 0.3) points to potential scaling problems.
- Avoid CO2 de-gassing by maintaining a high operation (injection) pressure that exceeds the bubbling point.
- Use inhibitors to keep Ca in solution.
- Use cation filters to remove Ca2+. Such filters could e.g. be based on seeded crystallization (FACT filter*). The formed carbonate crystals are to be removed by filtration.
- Add CO2 to the brine to prevent de-gassing (CO2 control).
Generally a challenge in plants with high CO2 and Ca2+ content, for example Pijnacker-Nootdorp, Insheim, and Ammerlaan.
- Further lab. and field tests are needed for examining the FACT filter performance.
- Usually, the operators can handle this problem by pressurizing the system.
Barite scaling in the injection wells, the plant components or in the pores of the reservoir rocks. Pronounced barite scaling is observed at sites producing from hot, saline brines. Scaling with respect to barite is particularly a problem if the thermal water is super-saturated with barite, i.e. if the Saturation Index is high (> c. 0.5). The greatest risk occurs at the surface due to cooling (lower temp.)
- Use scaling inhibitors.
- Use cation filters with adsorption materials for barium removal (e.g. chitosan or zeolite) prior to re-injecting cooled water.
- Avoid site-locations with Ba-rich brines in the system, if possible.
- Avoid sites with CaSO4-rich brines, as the stability of the CaSO4 ion pair decreases significantly when the temp. is lowered, leading to Ba2+ + SO42- → BaSO4.
Margretheholm, Insheim, Horstberg, Den Haag, and Groß Schönebeck.
- Further lab. and field tests are needed for examining the effect and performance of the cation removal filters.
Corrosion due to oxygen ingress. May cause destruction of materials and formation of Fe-oxides that again could lead to clogging of screens and gravel packs in the injection wells.
- Avoid (or limit) the amount of oxygen ingress, e.g. by maintaining a high operation pressure.
- Use casings of composite material.
Corrosion due to oxygen ingress observed at Sønderborg, Lund and other plants.
H2S-induced corrosion. H2S in the geothermal fluids – or formed by reduction of sulphates caused by bacterial activity – may lead to formation of corrosion products such as sulphides (e.g. FeS). The sulphides may clog sub-surface facilities, cause material corrosion and/or result in bad smell.
- Remove H2S by adding iron-based additives (iron hydroxide (granulate) or FeCl3 (liquid)).
- Usually, this process leads to precipitation of Fe-sulphide particles. The generated particles can be removed by filtering (cf. Regenspurg et al., 2020).
Sønderborg, Pyrzyce and Oberlaa.
- Especially a problem at strongly reducing conditions. May also be a problem in presence of high concentrations of sulphates in the brine.
Galvanic corrosion due to dissolved Pb2+ and Cu2+ in the formation brine. Especially pronounced if the chloride concentration > 100,000 mg/L. May lead to generation of metallic lead (Pb(0)) and copper (Cu(0)) in the geothermal wells.
- Use particle filters for removal of metallic Pb and Cu.
- Use cation filters with adsorption materials for removing Pb2+ and Cu2+ from the geothermal water (materials could be chitosan, Fe-oxide, and zeolite).
- Use corrosion inhibitors.
- Use tubings and casings made of corrosion-resistant material, e.g. casings of composite material.
- Use corrosion-resistant alloys to prevent (or limit) galvanic corrosion, e.g. stainless steels.
Margretheholm (Pb), Sønderborg (Pb) and Gross Schönebeck (Cu).
- Cation filters have been successfully tested in the laboratory – a field test is still needed.
- Carbon steel and several higher alloyed steels were tested in the lab. in order to examine/control the corrosion processes.
- Use corrosion-resistant alloys to prevent (or limit) CO2 -induced corrosion.
- Or use a suitable corrosion inhibitor.
The selection of right materials for both the surface and downhole installations is essential. To be considered at an early stage in plant history. Alternatively, injection of an inhibitor should be considered.
Temperature optimization is used to soft-stimulate the reservoir and/or to determine the best flow properties of the fluid with respect to the operational cost and energy output.
*) FACT: Filtration Assisted Crystallization Technology