Geothermal Energy Plants

Addressing scale formation in geothermal plants 

The output of operational and economic sustainability of a geothermal plant depends on the absence of disruptions and consistent performance of the geothermal plant loop, segmenting into surface and subsurface installations.
Encouraging overall sustainability points towards deterring reduction in operational efficiency and output productivity belonging to a geothermal plant. Factors that discourage continuous and consistent operations include unscheduled shutdowns, intense maintenance, extensive cleaning operations, and the associated follow-up costs. 

In encountering the issue of scale formations along the geothermal plant loop, the cost can also increase when it involves disposing of scales that contain heavy metals, and in some cases radionuclides. These are cost, time, and labor-intensive processes which require additional time set aside outside of production to address. For typical scale formations that occur in geothermal plants, solutions will include cleaning or inhibitive procedures, either mechanical or chemical. 

Understanding scale formation under geothermal conditions

The mixture of liquid and gas that is extracted from geothermal production fields and wells can contain various quantities of soluble species, up to 300 g/kg. Under the thermodynamic conditions of high temperature and pressure, the environment allows scaling and corrosion to occur rapidly. 

Deciphering and addressing scaling problems in geothermal plants can be difficult due to the complexity assigned to the composition of the scale formations in the loop. The composition is an accumulation of influence from a variety of factors: temperature range, fluid pressure, geothermal makeup, water-rock interactions, and operational conditions.

Brines of moderate to high temperatures typically yield calcium carbonate scales, with exceptions covering geothermal wells that occur in the Paris Basin. In this niche geothermal makeup with the same thermodynamic conditions, large concentrations of iron and dissolved sulfide result in the formation of iron sulfide deposits.
Silica scales are a resultant formation from fluids with high enthalpy and low salinity, while fluids of a more liquid nature residing under high-temperature conditions coupled with high Total Dissolved Solids (TDS) content yield both siliceous and sulfide scales. 

Looking into common types of geothermal scales

A) Calcium carbonate

Calcium and bicarbonate ions react to form calcium carbonate, a dense crystalline-like formation that adheres strongly to surfaces. This is the most common problem that moderate to high-temperature geothermal systems face and can be a pressing issue in heat pump systems.

On top of the temperature factor, an increase in pH can also favor the deposition of calcium carbonate.  The mixture of gas and liquids that originate from the geothermal production wells contains significant quantities of dissolved carbon dioxide. In the sequential flashing of the vapor phase that follows, the carbon dioxide release cases the increase in pH, and together with the occurrence of supersaturation, leads to scale formation.
The scale of calcium carbonate can exist in three different forms, with calcite being the primary and least soluble by nature.  The effective anti-scaling solution to counter this issue is CrestoPro™ G470, a high-performance calcite inhibitor that specializes in addressing calcium carbonate scale formation under these thermodynamic conditions.  

B) Silica 

The formation of silica scales originates from the deposition of amorphous silica (SiO2) in moderate to high-temperature geothermal fluids. The mechanism that leads to the silica scale formation is also more complex as compared to the mechanism of calcium carbonate and sulfide scale formation.
The geothermal fluids allow both amorphous and crystalline forms of rock minerals to dissolve under conditions of great depth and pressures. At elevated temperatures, silica then exists in its dissolved state of silicic acid being extracted from the rock minerals. This dissolved state of silicic acid can come together to form hard glassy scales through the process of polymerization or combine with various metals found in the earth to form silicates, which further worsens scaling issues.
The polymerization reaction kinetics belonging to silicic acid are slow and allow silica deposits to form from several minutes to hours after the establishment of supersaturation conditions. Similar to calcium carbonate, an increase in pH also favors the polymerization process to increase silica scale formation.
Another defining characteristic of silica deposits in geothermal plants is that they can be found in every part of the loop installations and are not necessarily confined to a short part immediately after the flashing point. This is a significant issue in brine re-injection systems, where the formation of silica in the form of colloids can block the pores and subsequent passage of the re-injection waterway. 

With the use of CrestoPro™ G474, silica deposition can be effectively disrupted, with an efficiency of up to 90% reduction in targeted silica scale formation.