An Overview of Silica Scaling Reduction Technologies in the Geothermal Market

Abstract

Renewable energy sources play a vital role in the energy mix with geothermal energy providing an opportunity to harness the natural heat coming from the Earth for sustainable power production. As innovative drilling technologies come to market, it is easier to extract heat from various localities across the globe, leading to significant development in the geothermal sector. The economic viability of this resource can be significantly impacted when energy output declines due to scale deposition. Scale formation is a major challenge in the exploitation of geothermal wells, particularly in liquid-dominated geothermal fields. One of the most robust forms of scale build-up common to higher temperature geothermal wellbores and surface equipment for power production is silica scaling. Silica is one of the Earth’s most abundant elements that can precipitate from brine due to various factors. The accumulation of scale deposits significantly impacts the lifespan and efficiency of surface equipment and geothermal wells by restricting fluid flow, thus reducing efficiency and performance. To guarantee the peak performance and longevity of geothermal systems, it is essential to implement a strategic maintenance plan for scaling reduction in geothermal systems. Throughout this review, relevant case studies highlight scaling reduction methods for silica scale in subsurface wellbores and surface facilities.

1. Introduction

Geothermal energy, a sustainable and renewable resource, plays a critical role in the global energy supply. With innovative drilling technologies coming to market, the extraction of heat is more accessible, increasing geothermal development on a global scale.

As the geothermal industry continues to grow, operators seeking to increase production generally have two primary strategies: embark on capital-intensive drilling campaigns to initiate new wells or optimize and extend the productivity of existing wells. The financial outlay and inherent risks associated with drilling new wells often overshadow the benefits, making it critical for operators to implement a strategic maintenance plan for existing wellbores and surface equipment.

A fundamental component of this strategy is to address the formation of scaling attributed to mineral precipitation from the brine during the reinjection and production process. In geothermal systems, the chemistry of the brine can affect the efficiency of power plants through the deposition of scale. Silica scale build-up is common within high-temperature geothermal wellbores and surface equipment [1]. Various factors result in the precipitation of silica, which include changes in temperature and pressure, flow velocity, pH level, the degree of supersaturation, and the presence of ions in solution [2,3,4,5,6].

The deposited scale adheres to the casing and completion equipment of wellbores. In surface equipment, it can lead to problems with the facilities, which results in a decrease in the flow rates leading to a decrease in the extracted energy and performance of the geothermal system [1,7]. Consequently, operations require downtime for the recovery or replacement of damaged parts and to remove and dispose of flow impediments generated by the scale deposition [7,8].

Throughout this review, the formation of silica and precipitation kinetics are documented in geothermal brines. The management of silica in subsurface wellbores will include preventative methods, pressure and temperature management, monitoring and detection techniques, as well as current treatments used to clean scale build-up in the subsurface. Conventional chemical and mechanical solutions that have been used to clean the accumulation of scale build-up are reviewed, along with innovative technologies, such as electro-hydraulic pulsing. Silica scaling reduction strategies will be highlighted through case studies to increase the operational efficiency and longevity of geothermal systems.

In geothermal power plants, silica scales can affect surface equipment, such as heat exchangers, separators, and pipelines, hindering heat transfer, reducing efficiency, and causing significant operational problems [7,8]. In this review, the treatment of silica scales in geothermal power plants will be focused on common methods for scale reduction, along with the importance of continuous monitoring for early detection and the effective management of scale.

A geothermal system producing below its potential can quickly become uneconomical; therefore, an effective treatment that reduces silica scaling provides a strong incentive in terms of cost reduction to operators. The total cost of scale treatment has been estimated at USD 1.4 billion for the oil and gas industry, indicating the importance of scale reduction to optimize production [8]. With the removal of silica from geothermal systems comes an opportunity to recycle waste silica for other essential purposes. From agriculture to the health and wellness industry, there are economically viable opportunities for the silica scale collected from the geothermal system.

The efficient utilization and continued growth of high-temperature geothermal resources are tied to improvements in the monitoring and management of silica scaling in geothermal systems. Research and development have focused on innovative technologies aimed at improving the reduction, efficiency, and sustainability of silica scaling management and monitoring. Key developments are highlighted and include innovative retention systems, fiber optics, and chemicals (inhibitors and anti-scalants).

2. Formation of Silica Scale Use

Geothermal systems utilize the Earth’s heat for power generation and direct-use applications. One of the major challenges during high-temperature geothermal production is the formation of silica scales [9]. Geothermal systems are susceptible to scale deposition due to various factors, which include changes in pressure and temperature, pH level, flow velocity, the degree of supersaturation, and the presence of ions in solution [3,4,5,6]. Silica (SiO₂) is a common constituent of geothermal brines, and its precipitation and deposition can severely impair the efficiency and longevity of geothermal facilities [1]. This section provides an overview of the behavior of silica in geothermal brines and the mechanisms of silica scale formation.

2.1. Silica in Geothermal Brines

Geothermal brines are often rich in dissolved silica due to the dissolution of silicate minerals in the deep subsurface. The silica concentration in geothermal brine is typically between 100 and 300 ppm depending on the geological and thermal conditions of the geothermal reservoir [1]. The solubility of silica in water increases with temperature. As such, geothermal fluids from high-temperature reservoirs typically have higher dissolved silica concentrations.

Silica exists in various forms in geothermal fluids, primarily as monomeric silicic acid [H₄SiO₄ or Si(OH)₄] [10]. As the geothermal fluid ascends to the surface and cools, the solubility of silica decreases, leading to supersaturation and subsequent precipitation. This process can result in the formation of silica scales, which deposit on the surfaces of geothermal equipment, pipelines, wells, and reservoirs. Amorphous silica, a non-crystalline form of SiO₂, is the dominant type of silica precipitated on the surface [2].

2.2. Mechanisms of Silica Scale Formation

2.2.1. Silica Solubility

The Silica Saturation Index (SSI) is a crucial parameter in understanding and predicting silica scale formation in geothermal systems [11]. It provides a measure of the potential for silica precipitation based on the concentration of dissolved silica and its solubility under specific conditions. The SSI is defined as follows:

$$SSI={\displaystyle \frac{{CSiO}_2}{{CSiO}_2 solubility}}$$

where CSiO₂ is the concentration of dissolved silica in the fluid, and CSiO₂ solubility is the solubility of silica at the given temperature and pressure. A general rule is that once SSI nears the value of 2, silica polymerization and precipitation occur rapidly at a neutral to alkaline pH of 6.6–8.5 [11].

The solubility of silica in water is temperature-dependent, with higher temperatures resulting in increased solubility [2,11]. In the geothermal reservoir prior to utilization, the silica concentration is typically in equilibrium with quartz, the crystalline form of silica [2]. As geothermal fluid cools during its ascent to the surface or through heat exchange processes, the solubility of silica decreases, leading to the supersaturation (SSI > 1) and potential precipitation of amorphous silica [12].

Silica solubility at geothermal reservoir temperatures above 185 °C is usually controlled by quartz, while solubility at temperatures <185 °C can be controlled by chalcedony [11]. Examining these two curves, it becomes clear that the “window of opportunity” for operating geothermal plants that are “silica free” lies between the quartz and amorphous curves as shown in Figure 1 [12].

Dissolved salts and pH also affect the solubility of silica in geothermal brines [2]. Based on the geothermal reservoir and geological environment, the presence of calcium, magnesium, iron, aluminum, and manganese can react with Si(OH)₄ to form metal silicates. Aluminum-rich and iron-rich silicates (metal silicates) are the most common and are observed in geothermal fields, such as Salton Sea in California and Kyushu in Japan [11].

Silica is more soluble in highly acidic (pH < 3) and highly alkaline conditions (pH > 10). The solubility of amorphous silica increases with increasing temperature, while solubilities remain relatively constant over the pH range of 5.5–8.5 [11].

2.2.2. Silica Precipitation Kinetics

The kinetics of silica deposition is concerned with the rate at which a supersaturated solution will result in scale deposition [2,11]. Specifically, the rate at which silica polymerizes and precipitates as colloids or through molecular deposition.

Weres et al. [6] indicate that silica precipitation in geothermal systems consists of the following steps:

• The formation of silica polymers of less than nucleus size.
• The nucleation of an amorphous silica phase in the form of colloidal particles.
• The growth of supercritical amorphous silica particles by further chemical deposition of silicic acid on their surfaces.
• The coagulation or flocculation of colloidal particles to give either a precipitate or a semisolid material.
• The cementation of the particles in the deposit by chemical bonding and further molecular deposition.
• The growth of a secondary phase between the amorphous silica particles.

Silica polymerization is a process where monomeric silicic acid molecules react to form oligomers and eventually larger silica polymers. The process results in the formation of colloids and is often referred to as homogeneous nucleation. This is the dominant process at high SSI. Heterogenous nucleation, on the other hand, is the deposition of amorphous silica on pre-existing colloidal amorphous silica and is dominant at low SSI. If colloids are formed, they may adhere to surfaces following steps 4 to 6. On the other hand, molecular deposition involves the direct deposition of Si(OH)₄ onto surfaces without the formation of colloids [2].

Various authors [2,11] have identified several factors affecting the kinetics of silica deposition. The critical factors are as follows:

• The degree of supersaturation (SSI): higher concentrations of dissolved silica increase the likelihood of polymerization and colloid formation (polymerization is slow if SSI is <2 and very fast >3).
• pH: silica polymerization is favored at neutral to slightly alkaline pH values and slows drastically when pH is lower in the acidic values.
• Temperature: a general guideline for silica suggests that it is only possible to cool the water by approximately 100 °C without the risk of scaling [12].
• Other ions in solution/catalysts: researchers [6,13] report that dissolved NaCl or other electrolytes in the geothermal fluid may speed up solubility equilibrium and that fluoride encourages silica polymerization. Silica colloids can be coagulated by cations in the geothermal fluid, particularly calcium and iron. Corrosion accelerates silica deposition due to the release of iron ions.

3. Occurrence of Silica in Subsurface Wellbores

3.1. Preventative Methods

3.1.1. pH Modification

A preventative method that is used to slow down the precipitation of silica is through pH modification. Acid can be injected by setting up a chemical metering pump into the brine pipeline [12]. By decreasing the pH to below 5 through dosing with acid, the supersaturated solution can be reinjected without causing significant scaling issues [1]. An important consideration is that acidifying the brine can increase the corrosion rate of the casing through the addition of sulfuric acid or hydrochloric acid. Modifying the pH level is not as effective at scale prevention in the reservoir since the brine can react with minerals in the rock formation that can neutralize the acidity and raise the pH [14].

3.1.2. Casing Composition

The integrity of steel casings can be compromised with the development of scale build-up and corrosion. Through implementing a more resistant casing material, the deposition of inorganic scale can be significantly reduced. Glass-fiber-reinforced composites may show improved scaling prevention. The inner surface of the pipes contains the polymer matrix of epoxy or polyethylene. These well coatings provide a different surface energy than steel pipes [8]. Implementing a casing material that is naturally more resistant to scaling is a more permanent approach for scaling avoidance, although it requires a higher upfront cost. Scale forms a rough surface in the casing with rippled deposits that lean against the flow. The flow capacity can be significantly reduced, even with the scale being only a few millimeters thick [12]. Therefore, it is important to have a clean pipe to ensure optimal fluid flow.

3.1.3. Chemical Inhibitors

Another proactive method to suppress scale formation involves the use of a specific class of chemicals known as scale inhibitors. Their purpose is to alter the chemistry of the brine to prevent, delay, or reduce scale formation. Only chemicals that can effectively be used in small concentrations are typically considered for cost reasons. Careful brine and scale analysis is critical prior to choosing an appropriate inhibitor and dosage or further blockages can occur. As geothermal brines can contain dissolved CO₂ and H₂S, the interaction with corrosion inhibitors will need to be considered as part of any scale or corrosion management strategy [15].

The procedure for testing inhibitors is as follows:

7.

Inject each inhibitor at a high dosage at the beginning;

8.

Slowly decrease the dose rate while checking the control points;

9.

Determine the inhibitor that is most effective at the lowest dosage rate.

Optimum dosage is crucial for a healthy plant operation. More dosage than needed will not prevent scaling more efficiently; rather, it will lead to the precipitation of by-products of the inhibitor, such as calcium phosphate [1].

Scale inhibitors have been used to reduce the precipitation of scale in wells by encircling and holding certain cations to prevent them from sticking together to form scale. It has been challenging to find commercially viable inhibitors for the troublesome precipitation of silica and heavy-metal sulfides [1]. Dozens of inhibitors were developed in recent years in parallel with the growth of the geothermal energy sector.

The types of geothermal inhibitors can be classified as follows:

• Phosphonic acid inhibitors;
• Inhibitors manufactured by phosphonic acid salts such as Na and K;
• Polymer inhibitors.

Phosphonic acid inhibitors are usually the most effective due to their high concentration of active agents at low dosage rates. With their acidity being a pH level of less than 2, they can cause serious corrosion, specifically in high-temperature wells and must be used with caution [1]. The use of a sodium or potassium-based alkali can neutralize the pH, but will require a higher dosage rate in comparison to their acidic forms. To maintain the wellbore integrity, it is recommended that inhibitors are used as a short-term solution. Polymer inhibitors are typically used in very high-temperature geothermal systems due to their high stability under such conditions [1].

A chemical metering pump with a capillary tube can be used to continuously inject the inhibitors in the well at a deep enough depth to effectively mix with the brine but prior to the onset boiling. Once scale has formed, inhibitors are no longer effective, being that they are a prevention method rather than a treatment method.

3.1.4. CO₂ Injection

Experimental results show that the presence of CO₂ in reinjection fluids can significantly inhibit silica scaling. While silica will still form, it slows down the kinetics and decreases the pH, with less build-up in the wellbore and reservoir. The adoption of the CO₂ injection will rely heavily on the ability to commercialize this method beyond the pilot stage, as well as find a way to make it economically viable for operators. The opportunity to inject the CO₂ from the geothermal brine production leads the way toward a low carbon economy [13].

According to von Hirtz [11], the Puna geothermal power plant has utilized gas injection since its start-up for scale and emissions control. This geothermal field is characterized by low non-condensable gases that allow for 100% gas injection in production wells. A mixture of brine and condensate forms an injectate with a pH of ~4.5 that is highly effective in preventing silica scaling.

3.2. Pressure and Temperature Management

Pressure and temperature surveys are important in the evaluation of the formation pressure and temperature. A frequent location for the significant build-up of metal cations in iron silicates is just above the slotted liner hanger in the lower part of the production casing. This is because the flashing of rising hot water causes the concentration of some components to rise and the pH to change, which makes the insoluble particles precipitate. Manipulations to pressure and flow rate can adjust the scaling deposition depth up to 50m within the wellbore by varying the flash point location. Operators may be able to use wellhead pressure control to ensure scale deposition within the production casing, where it is possible to ream without damaging the slotted liner [16]. This adjustment does not prevent scale formation, but it is possible to adjust the zone of deposition for easier clean-out.

Fluid reinjection is used to increase operational efficiency, prevent reservoir pressure depletion, recharge the aquifer, and address environmental concerns related to excess brine disposal [17]. However, drawbacks such as clogging the fluid flow paths have been observed. Geothermal operators aim to increase the efficiency of energy generation by lowering the temperature of the reinjected fluid. The resulting temperature differences when injecting cold water back into the reservoir are up to 80 °C in low-enthalpy reservoirs and more than 180 °C in high-enthalpy reservoirs [18]. Consequently, various chemical reactions can occur, leading to flow limitations in the reservoir [17].

As mentioned, a general guideline for silica suggests that it is only possible to cool the water by approximately 100 °C without the risk of scaling [12]. Therefore, reservoir water at 220 °C would need to be separated above 120 °C to avoid scaling. As a result, the higher the reservoir temperature, the higher the water temperature for reinjection to ensure thermal efficiency [12].

3.3. Monitoring and Detection Techniques

Thermochemical modeling is the primary predictive tool to predict why, where, and when scaling will occur [19]. The prediction and control of scaling lead to reliable geothermal energy supply. Through understanding the chemical composition of the geothermal fluid, the behavior of materials in the geothermal environment can be anticipated. Effective scale management requires the online monitoring of scaling tendencies, along with the detection and identification of scale deposits under flow conditions.

A thermochemical prediction tool is essential for determining the stability of the geothermal brine and can simulate scale prediction under various pressure and temperature conditions. A simulator can perform what-if analyses and identify the optimal scenario where no scaling occurs under specific conditions of temperature, pressure, and water composition [19]. The precipitation rates for scale with or without the use of inhibitors can also be calculated. Through obtaining measurements on a regular basis, major ion concentrations can be analyzed to enable operators to take corrective actions in a timely manner and effectively maintain their geothermal systems (Figure 2).

Schlumberger’s energy glossary defines flow assurance as the “design, strategies, and principles ensuring the uninterrupted hydrocarbon flow from the reservoir to the point on sale”. In the geothermal space it is focused on the uninterrupted flow of geothermal fluids to and from the power plants. Silica and other geothermal scales play a significant role in interrupting the flow of geothermal fluids. Flow assurance in production systems is critical in ensuring efficient and cost-effective operations of geothermal systems. Advanced technologies, such as remote 3D visualization, 4D seismic, intelligent completions, and smart wells, enable operators to monitor downhole pressure and temperature in real time to detect scale formation [19]. Once the data is collected, it can be integrated into predictive simulators to forecast and assess the likelihood of scale deposition. Coupling real-time data with simulation models can assist in developing a viable approach to alleviate problems in the entire system proactively [19].

The detection of silica build-up in the wellbore at an early stage is an important part in ensuring the longevity of the geothermal system. Gathering data frequently from the wellbores will allow the operator to observe a decrease in the flow rate and wellhead pressure. Once that occurs, the amount of scale inside the casing is probably already significant. When an inhibitor is in use and the capillary tubing is extracted for inspection or to run pressure and temperature surveys, check for the presence of scale on the tubing [20].

Identifying the location of scale deposition within the system is essential before implementing strategic control measures. For the location of scale deposition, calipers and go-devils are commonly used. They are effective tools to calibrate the casing and determine the thickness of scale, demonstrating the variations in the wellbore inner diameter open to flow. Sinker bars can be run to tag the bottom for access purposes in the wellbore. This information is important to make strategic decisions on the timing of clean-outs. Figure 3 shows the various types of go-devil and caliper tools that can be used in downhole surveys to detect silica scaling [21].

10.

A. Go-devils of different diameters, made of copper wire, constructed and used by Eda Renováveis for the detection of scale inside the casing by running in the well on a wireline until it stops at the scale obstruction. There are no temperature constraints.

11.

B. Go-devil on a sinker bar, constructed with copper wire, with interchangeable baskets of different diameters. Suspended by wireline and can be used in high-temperature wells without compromising performance.

12.

C. Electrical caliper logging tools with two, three, and four arms or multi-fingers are used globally. Most of the caliper tools and cables have temperature limitations up to 200 °C. High-temperature versions are available up to 350 °C for a limited time. As a result, the wellbore may require quenching with cold water prior to use.

13.

D. Kinley microscopic multi-finger caliper tool can be used in high-temperature environments up to 315 °C. This circumferential survey from the tool helps to find and measure deformed tubing, rings of corrosion and erosion, isolated pits, lines of pits, and scale.

Downhole restrictions can have a significant impact on wellbore access, well integrity, re-injection, and production flow rates. Downhole visual imaging is another effective method to visualize scale build-up within the wellbore.

After ensuring wellbore access through running a caliper or go-devil, a downhole camera can be deployed to provide a video record of the wellbore, as shown in Figure 4 [22]. Visual diagnostics are combined with computational analytics to provide real-time data on wellbore issues at temperatures up to 200 °C. It is important to ensure the fluid is clear and does not contain solids suspended in solution, as it can affect the visibility of the wellbore.

High-resolution acoustic imaging technology is being deployed as a cutting-edge technology capable of capturing sub-millimetric measurements to assess scale build-up and wellbore integrity. DarkVision’s fluid agnostic technology can be deployed in wellbores up to 150 °C using high-density arrays and cloud-based processing to provide high-quality data. A digital twin of the well is constructed post-scan and uses a 3D point cloud, including the intensity of the signal return and the exact location of the surface from which the return was generated. Through a multifaceted dataset comes an integrated perspective on the surface condition and textural relief of the casing. The capture of this data is fluid agnostic without the need for fluid clarity or a light source, which are required for optical based downhole tools. Real-time imaging is currently underway for this technology [23].

Once we know the location of scale build-up, verification of the scale type is essential. Silica and calcite are white colored in appearance and difficult to distinguish at first glance. Silica scales can appear grey or black due to small amounts of iron sulfide, a corrosion product from the metal casing. A quick method to differentiate the two is by placing a drop of HCl on a sample; if bubbles form, it indicates the presence of calcite [12]. Otherwise, scale analysis can be a tedious process involving X-ray diffraction (XRD) to identify the crystal substance, electron microscopy (SEM) for distributive and qualitative analysis, and wet chemistry analytical methods shown in Figure 5 [12].

3.4. Treatment Strategies and Case Studies

3.4.1. Chemical Solutions

In geothermal systems, the conditions and brine chemistry of each geothermal project can differ significantly. Therefore, selecting a suitable chemical program is essential to maximize effectiveness while minimizing wellbore damage. It is critical to ensure that the chemicals are thermally stable to up to 250 °C, or the required wellbore temperature [15]. While biodegradable chemical solutions are hitting the market for treating scale, they have not yet been effectively adapted for silica deposits. Chemical brine analyses can be used to determine the composition of the deposits and the relative proportions of the chemicals used in the steps of the cleaning process.

Chemicals are commonly used as they can perform many functions from mitigation to the removal of scale deposits through dissolution. The use of chemicals for inhibitors and pH modification, as previously discussed, are preventative methods that can be used prior to scale forming. Once scale deposits have accumulated in the wellbore, acidization through bull heading or acid flushing can be applied. Chemicals can also be used when porous media within the formation becomes obstructed. In this case, acidization can stimulate the formation to enhance the permeability and flow rate of the system.

Injecting chemicals is not always economical to control scaling in a geothermal operation. Given the low efficiency rate of chemicals, a significant amount needs to be injected on a frequent basis in order to increase productivity.

Traditional chemical methods for trying to clean scaling in wells uses hydrochloric acid (HCl), and, while this impacts calcite, silica and silica-based deposits are not soluble in HCl. Mud acid, which is commonly a blend of HCl combined with HF (hydrofluoric acid), has been more effective for treating silica as shown in the case study at Wairakei Field [24]. The addition of HF increases the corrosion within the well, which affects the wellbore integrity. As a result, acid must be used sparingly, as a short-term solution, as to not affect the longevity of the geothermal system.

There is the risk of not having the optimal chemical for the wellbore and reservoir conditions. Incompatibilities between the acid and the brine or reservoir rock can result in the precipitation of additional scale and cause more damage to the system than improvement. Corrosive chemicals, such as HF, can corrode metal surfaces, resulting in pits and holes in the wellbore casing, along with the addition of metal scale deposits. It has also been discovered that hydrophobic layers form on some silicates after acid exposure. The constant flow of acid metal silicate deposits has been ineffective in treating these hydrophobic layers [24].

Certain forms of chemicals can be extremely dangerous to handle and transport. As a result, there are significant health and safety hazards that must be considered. The spent acid must be flowed or lifted with proper disposal. Regulatory restrictions have been discussed for chemical use in the subsurface, with the potential to ban specific chemicals as a result of the well integrity risks, as well as the impact to humans and the environment. New developments in the chemical treatment space are occurring using a food-grade base to provide a safer process [24]. Further experimentation will be required to understand the effectiveness on various types of scale deposits.

With bull heading, the acid will tend to follow the path of least resistance, resulting in uneven acid distribution and the lack of ability to treat targeted feed zones. Acid flushing is another chemical method for wellbore cleaning. A coiled tubing unit is used to propel acid into targeted feed zones. It is critical to understand the inflow and outflows of feed zones, as they can dilute or re-direct acid. Understanding the optimal operating conditions for acidization is important, as there can be a higher risk of fracturing the formation due to stimulation. Monitoring the production increase is also critical in determining the frequency of treatment required when building out a maintenance plan (

Table 1. Advantages and challenges of bull heading and acid flushing for geothermal wellbores.

Chemical Method	Advantages	Challenges
Bull heading	Injection of acid from surface Lower cost than acid flushing Ability to stimulate the reservoir	Acid follows path of least resistance, resulting in uneven acid distribution Lack of ability to target feed zones
Acid flushing	Ability to target specific feed zones More uniform distribution of acid Most effective at stimulating the reservoir	Requires coiled tubing unit Acid follows the path of least resistance and feed zones can dilute or re-direct acid Higher cost than bull heading

).

Chemical treatment case studies are from the Wairakei Field in New Zealand, as well as the Los Azufres and Las Tres Virgenes fields in Mexico, which use acid flushing and bull heading techniques (

Table 2. Chemical treatment case studies in the Wairakei field in New Zealand [24] along with the Los Azufres and Las Tres Virgenes fields in Mexico [25].

Geothermal Field	Method and Chemicals	Wellbore Temperature	Scale Type	Results
Wairakei, New Zealand	Acid flushing and bull heading with 10% HCl and 5% HF	200–260 °C	Silica and calcite	The HCl was effective in treating calcite, but HF was required to treat silica. The addition of HF increased corrosion, along with additional environmental and safety risks. Caustic soda was added for pH modification of the brine on the surface, prior to reinjection, to aid in neutralizing and reducing the corrosion. The production rate doubled from this acidization process.
Los Azufres and Las Tres Virgenes, Mexico	Acid flushing and bull heading with 10% HCl and 5% HF	260 °C	Silica and calcite	Acid flushing was used to treat a majority of the wells, with bull heading deemed less effective. Of the 17 wells treated, 15 were effective. Corrosion inhibitors were also added to the acid blend. The average percentage of improvement ranged from 13 to 540%, with an average of ~176%. Treatment was considered economic in comparison to drilling new wells.

).

3.4.2. Mechanical Solutions

Mechanical solutions are a common form of treatment once scale deposits have formed. Cleaning operations can consist of various mechanical treatments that include brushing, broaching, reaming, and water jetting. The scale type and severity of build-up will determine which treatment method(s) are appropriate for wellbore cleaning. It is not possible to clean slotted liners or the reservoir with mechanical tools, except through the water jetting process.

Table 3. Advantages and challenges of brushing, broaching, reaming, and water jetting for geothermal wellbores.

Mechanical Method	Advantages	Challenges
Brushing	Compact Quick to deploy by wireline/slickline Lower cost compared to using a rig	Unable to treat wellbores that are completely scaled up Most effective for scale types that are easily removed
Broaching	Compact Quick to deploy by wireline/slickline More effective than brushing Lower cost compared to using a rig	High risk of tool getting stuck in hole Success is dependent on skill level of wireline operator Difficult to treat silica scale Difficult to treat thick scale build-up
Reaming	Most effective mechanical method Ability to revive fully scaled-up wellbore Effectively treats robust scale	Unpredictable rig times can lead to high costs Higher carbon footprint and safety risk compared to wireline tools Higher risk of damaging completion equipment compared to wireline tools
Water jetting	Compact Better than standard hot water injection Remove blockages beyond the casing ID	Requires frequent treatment for effective results Unable to treat wellbores that are completely scaled up

demonstrates the advantages and challenges of the mechanical methods described above.

Brushing is a wireline deployed technique that uses bristle wires to scrape the sides of the wellbore to loosen and remove encrusted material. The brushing tool is an abrasive technique that is most effective on scale types that are easy to remove. Brushing is less commonly deployed as it would take significantly longer to remove scale build-up in the wellbore (Table 3).

Broaching is a wireline intervention with a mechanical toothed tool that is used to remove material within the wellbore. Broaching is not meant to be the primary method for scale removal but can complement other treatment techniques to help maintain optimal production. The success of the operation will depend on the tool technology, the well conditions, wireline experience, and scale type. The treatment of silica scale can be a challenge with broaching due to the high compressional strength of the material. It has been proven to be more effective in treating calcite scales. In geothermal wells, the tool is run on wireline or slickline into a known blockage and forced through to develop a hole. The process is then repeated, completing multiple runs with the tool size gradually increasing to open the diameter of the wellbore [26]. The wireline operation provides a cost-effective alternative to full wellbore cleanouts using coiled tubing or a drilling rig (Table 3). Flowing the well is a part of the process to clear out the debris from the wellbore. Specific requirements include the ability to withstand extreme temperatures and corrosive fluids. The ability to reliably fish out the tool in case it gets stuck in the hole is also important to consider. There are improvements being made to the tool design to increase effectiveness through a focus on hardened teeth and cutting impact angles [26].

Reaming (milling) is a mechanical technique with a rotating bit that can be deployed with a coiled tubing unit or with a drilling rig. Various bit sizes are used to improve accessibility and to clean out scale. Cleaning operations using a drill rig are recommended when the deposition zone is in the production casing. When the scale deposits are within the slotted liner, there becomes a higher risk of damaging the completion equipment (Figure 10). There are two methods that have been adopted for reaming that include the following:

14.

Reaming with the well quenched;

15.

Reaming during well discharge.

Reaming the scale deposits with the well quenched, known as killing the well, requires continuous cold water to be injected into the well to keep it under control. This is known as the conventional method for reaming and can take, on average, 10 days to carry out the operations. It must not be rushed, as the injection of cold water can result in casing damage that can lead to wellbore integrity issues. The temperature or flow rate of water must be increased or decreased gradually in order to minimize thermal strain of the casing [21]. Once the well has been controlled, cleaning operation can begin, maintaining the injection of cold water to the end. A challenge with this method is that, as scale deposits are loosened, these sediments fall to the bottom of the well, making the well shallower, and can be also introduced into the reservoir to clog the pore spaces. It is important to continue reaming the sediments at the bottom of the well to break them up further. When the well is flowed after the operation, these deposits can then be brought to surface [21]. As a result of temperature changes in the wellbore from quenching, time is required after the operation to recover and regain the initial conditions of the well.

Reaming the scale deposits during well discharge is a method that is performed without cooling or quenching the well (killing the well). The well is kept hot and flowing to a separator pipe on the surface where cuttings can be gathered rather than left to accumulate at the well bottom. Originally developed in Iceland over a period of 25 years, this more complex method allows the casing to keep the same properties, and the cement bond remains intact since the well does not undergo thermal cycling due to changes of temperature [20]. The total time that the well is out of service is reduced. The increase in production rates is also recognized immediately without requiring the wellbore to heat back up after undergoing quenching.

With reaming, in order to prevent casing damage during scale removal, the outer diameter of the bit needs to be sufficiently smaller than the inner diameter of the casing, resulting in a thin layer of scale being left on the inner wall of the casing after completion. The remaining scale has the ability to prevent water from being injected into the formation through slotted liners and will require other treatment methods to provide reservoir access. One method that has been used to clean out the wellbore and slotted liners is water jetting, where a coiled tubing unit is fitted with a low-speed, self-rotating waterjet nozzle system under high ambient pressure [27].

Case studies in the Dieng geothermal field in Indonesia, Kizildere field in Turkey, and Ahuachapán field in El Salvador are included in

Table 4. Mechanical treatment case studies in the Dieng field in Indonesia [28], Kizildere field in Turkey [29], and Ahuachapán field in El Salvador [21].

Geothermal Field	Mechanical Method	Wellbore Temperature	Scale Type	Results
Dieng, Indonesia	Broaching, reaming, and water jetting	280–330 °C	Silica and calcite	Previous attempts with broaching and water jetting were unsuccessful in increasing the production rate. Reaming was then chosen as the optimal method for this field in treating the sulfide and silica scale.
Kizildere, Turkey	Reaming	242 °C	Silica and calcite	Reaming occurred during well discharge. After mechanical treatment, production returned to previous levels in 12–18 months. Acidization treatment was then used to clean scale in the reservoir fractures to further increase production.
Ahuachapán, El Salvador	Reaming	228–250 °C	Silica and calcite	Reaming occurred with the well quenched. Resistance was detected in the slotted liner of one of the producer wells. Rotation was applied and caused damage to slotted liner and collapse. Numerous fishing operations occurred, but the liner was unable to be fully recovered. A sidetrack was then drilled.

.

3.4.3. Electro-Hydraulic Pulse Solution

Blue Spark Energy [30] has pioneered an innovative technology, eliminating the necessity for mechanical, chemical, or explosive-based solutions for wellbore cleaning. The BLUESPARK^® electro-hydraulic pulsing (EHP) technology removes scale build-up in the near wellbore region to ensure optimal wellbore performance in cased or open-hole completions. EHP tools are run on regular wireline cable with no special power source required. The technology empowers operators with an economical and safe treatment method to substantially reduce the industry’s carbon footprint by up to 90% compared to conventional maintenance methods. There is a low health and safety risk that does not involve the transportation of hazardous goods or require permits to operate.

At the core of EHP lies a straightforward yet powerful equation: (P = E/t) allows power to be harnessed efficiently, where P is power, E is energy, and t is time. A relatively small amount of electrical energy, 1000J, is stored, amplified, and then released over an extremely short period of time. By compressing the time frame, a large amount of power can be generated and released, creating a shockwave and an electro-hydraulic pulse. It is through this release that the pulse treats flow impediments within a targeted section of the wellbore. Each hydraulic pulse is a high-power shockwave travelling at the speed of sound, which radiates laterally away from the tool, producing a compressional force. When the shockwave interacts with a material possessing a different acoustic impedance than the liquid through which the wave is propagating, such as steel casing, it reflects and travels back inwards towards the tool as a tensile force. The stresses generated through this interaction are significant enough to exceed the tensile strength of scale material, thereby delaminating the scale, but are much less than the yield strength of steel, protecting the integrity of the casing and cement.

Each BLUESPARK^® tool is comprised of five integral components [30], all housing proprietary technology responsible for generating the pulse (Figure 6).

A distinguishing feature of the tool is its ability to replicate these pulses hundreds or thousands of times at 5 s increments. The higher the tensile strength of the scale material, the more cumulative pulses deployed to ensure an efficient break-up of the material to be flowed to the surface. The pulses can be repeated up to 12,000 times per run, treating up to 120 m. Once the maximum pulse count is reached, the tool is brought to the surface to refurbish the electrode and promptly sent back downhole for further pulsing. With the successful treatment of 39 types of scale to date, through over 1000 operations globally, it provides an effective solution to treat the most challenging scale deposits including silicates, along with iron-based corrosion products commonly present in geothermal wellbores (Figure 7). The EHP will travel through the scale in the slotted liners and effectively clean the slots to restore flow (Figure 7). There is no depth restriction for tool deployment.

A challenge with conventional treatment methods is the ability to ensure wellbore integrity without the significant risk of damaging the wellbore completion equipment. These subsurface risks are eliminated when using electro-hydraulic pulsing (EHP) technology, as it provides a sustainable solution that ensures the integrity of the wellbore. With over 1000 operations to date, the technology is proven [30]. Cement bond logs are commonly used to demonstrate the wellbore integrity of the cement behind casing after EHP operations (Figure 8).

A non-compressible fluid is required in the borehole to conduct the shock wave created, but it only needs to cover the interval being treated (

Table 5. Advantages and challenges of using the electro-hydraulic pulse technology in geothermal wellbores [30].

Treatment Method	Advantages	Challenges
Electro-hydraulic pulse	Eco-friendly, safe, and low carbon footprint Rigless, run on wireline to deploy Most effective method to clean cased, open hole, or slotted liners Ability to remove hard scale, successfully treating 39 scale variations, including silica Targeted approach and immediate results	Able to treat in wellbores up to 130 °C and requires wells to be cooled prior to treating higher-temperature wells. Requires non-compressible fluid in treatment interval.

). Therefore, the targeted treatment section will need to be in the liquid phase. The EHP tool, similar to other wireline tools, has temperature constraints of up to 130 °C. Case studies demonstrate the opportunity to deploy the tool for heating systems and power systems, with injector or producer wellbores >130 °C needing to be actively cooled during deployment or quenched prior to treatment.

Case studies that have used the EHP technology have been documented in

Table 6. Case studies that have deployed EHP technology to remove flow impediments and increase flow rates [30].

Geothermal Field	Wellbore Type and Completion	Scale Type	Results
Tongonan, Philippines	Injector well, slotted liner completion	Silica	The wellbore was cooled to 100 °C. A five-meter interval was treated for silica scaling and resulted in a 50% increase to the injection rates.
Hatchōbaru, Japan	Injector well, open hole completion	Silica	The wellbore was cooled to 120 °C. A 40 m interval was treated with an operating time of 13 h. The wellbore cleaning of silica scale resulted in the injection rate to increase by 1200%. The final stable rate was within 5% of the original rate after completion 6 years prior.

. These case studies demonstrate the simple and efficient process that increased the flow rate of the wellbore. Through maintaining wellbore integrity with EHP technology, operators can extend the lifespan of geothermal wells and the economic viability of projects.

3.4.4. Optimize Efficiencies through Combining Cleaning Solutions

Strategic considerations should be considered to optimize efficiencies through combining cleaning solutions. The fluid chemistry, reservoir conditions, drilling regulations, and economics will all play an important factor for the treatment(s) chosen. Below are examples of how to combine different methods when building a maintenance program that take into consideration optimal performance and wellbore longevity.

• Combine mechanical and electro-hydraulic treatment: Mechanically ream scaled-up well until 2.75-inch electro-hydraulic tool can fit into wellbore to efficiently clean by pulsing.
• Combine chemical inhibitor and electro-hydraulic treatment: Use inhibitor to decrease the amount of scale build-up. When extensive cleaning is required, use electro-hydraulic pulsing rather than full wellbore acid treatment.
• Combine chemical acidization and electro-hydraulic treatment: Use electro-hydraulic pulsing to clean the near wellbore impediments and create a pathway for acid flushing to propagate laterally into the reservoir feed zones.
• Combine mechanical and chemical treatment: Mechanically clean out wellbore casing ID, followed by acid flushing to propagate laterally into the reservoir feed zones.

4. Occurrence of Silica in Surface Facilities

4.1. Silica Scaling in Surface Equipment

Silica scaling is a common issue in high-temperature geothermal surface facilities, where silica scaling forms in various parts of the geothermal surface system shown in Figure 9. The authors of [16] identified key areas for the formation of silica scale within the surface facilities, which include the following:

• Pipelines: transporting geothermal fluid from wells to surface facilities, through to downhole re-injection (Figure 10);
• Separators;
• Water-collecting tanks;
• Effluent disposal pond;
• Heat exchangers (Figure 9);
• Turbines: including turbine condensers, nozzles, and blades.

Silica scale deposition in surface facilities can have several detrimental effects:

• Reduce heat transfer efficiency: scale deposition in turbine nozzles causes loss of power output due to restricted flow. Similarly, scale depositions in the condenser result in a loss of vacuum efficiency and power. In Organic Rankine Cycle and combined-cycle geothermal power plants, silica scaling in heat exchangers significantly lowers heat transfer, resulting in a significant pressure drop that reduces power generation.
• Flow restriction: scale build-up can narrow the cross-sectional area of pipes, leading to a significant drop in pressure and reduced fluid flow.
• Equipment damage: The abrasive nature of silica particles can cause wear and tear on mechanical components.
• Increase operational costs: frequent cleaning and the replacement of scaled components, such as filters, incur additional costs and downtime.

4.2. Methods for Monitoring Geothermal Scale

Monitoring geothermal scale is important for several reasons:

• Operational efficiency: scaling can reduce the heat transfer efficiency in heat exchangers and clog pipelines, leading to increased operational costs.
• Maintenance and downtime: regular monitoring helps in planning maintenance activities and reducing unplanned downtime.
• Safety: accumulated scale can lead to blockages and potential safety hazards in high-pressure systems.
• Longevity of equipment: continuous monitoring and timely descaling can prolong the life of expensive equipment.

4.2.1. Visual Inspection

Visual inspection is the simplest form of monitoring, where operators regularly inspect equipment for visible signs of scaling. While straightforward, visual inspection is limited by its subjectivity and inability to detect early-stage scaling within pipelines and internal components.

4.2.2. Pressure and Flow Measurements

Monitoring pressure drops and flow rates in pipelines can indicate the presence of scaling. A significant pressure drop or reduced flow rate can suggest that scale build-up is restricting fluid movement. This indication provides indirect evidence of scaling but does not offer specific information about the composition or location of the scales.

4.2.3. Chemical Analysis

Regular chemical analysis of the geothermal brine can help predict scaling tendencies. By monitoring the concentration of scaling precursors, operators can assess the potential for scale accumulation and implement preventive measures. Chemical analysis requires sampling and laboratory testing, which can be time-consuming and may not provide real-time data.

4.2.4. Scaling Plates and Coupons

Scaling plates and coupons are widely used tools for the direct and quantitative monitoring of geothermal scale.

Scaling plates: flat metal or non-metal plates that are placed in the geothermal fluid flow path. Scale plate dimensions utilized are 5.4 × 2 cm [32] but are not limited to this size. Over time, scale deposits on the plates, mimicking the scaling process in the actual equipment. Operators periodically remove and analyze the plates to assess the scale type and amount formed. Scaling plates provide valuable information on the rate of scale accumulation and allow for the testing of various materials to determine their susceptibility to scaling.

Coupons: small, standardized metal samples inserted into the geothermal fluid stream. Coupon dimensions utilized are 2 × 1.3 cm [32] but are not limited to this size. They are usually composed of the same materials that are used in the actual plant equipment. Similar to scaling plates, coupons accumulate scale over time. They can be removed and analyzed through techniques such as gravimetric analysis (measuring weight change) and surface microscopy to evaluate the thickness, composition, and morphology of the scale. Coupons are often used in conjunction with corrosion monitoring to assess the combined effects of scaling and corrosion on plant materials.

4.3. Treatment Strategies and Case Studies

4.3.1. Preventative Methods

To prevent silica scaling, geochemists use thermodynamic modeling to apply appropriate treatment methods through the adjustment of temperature and/or pH. Most methods used to prevent and mitigate the formation of silica scales in geothermal surface facilities have been described by Gallup and von Hirtz [33]. Common methods specific to brine chemistry and process conditions include the following:

• pH modification;
• Aging or pond retention/evaporation/percolation ponds;
• Brine and condensate mixing;
• Crystallization reactor-clarification technology.

pH Modification

The solubility of silica is dependent on the pH of the geothermal fluid as mentioned in Section 2. Silica is more soluble in highly acidic (pH < 3) and highly alkaline (pH > 10) conditions. By modifying the pH of geothermal fluids, it is possible to manipulate the solubility of silica and prevent its precipitation.

Ph modification, often referred to as pH mod, is one of the most used methods employed across the world to mitigate silica scaling. The process involves acid being added to the brine for silica scale control. The focus is on retarding the polymerization and molecular deposition kinetics of Si(OH)₄. The variety of chemicals used for pH mod are shown in

Table 7. Common pH mod methods compiled from authors [11,33,34].

Additive Groups	pH Mod Additives
Acid additives	Hydrochloric acid (HCl) Sulfuric acid (H2SO4) Sulfurous acid (H2SO3) Hydrofluoric acid (HF) Organic acids—acetic or formic (may require larger volumes) Carbon dioxide
Alkali additives	Sodium hydroxide (NaOH): used to increase the pH Ammonia (NH3): can also raise the pH, but handling and safety considerations are important. Sodium carbonate (Na2CO3)
Buffer solutions	Buffers can be used to maintain the pH within a specific range, thus controlling the solubility of silica. These solutions often consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

[3,27,28].

Effective pH modification requires precise monitoring and control systems to ensure that the pH remains within the desired range. Geothermal surface facilities employ automated pH monitoring systems equipped with sensors that provide real-time data. These monitoring systems can be integrated with dosing pumps that adjust the addition of acids or alkalis to maintain the target pH. The selection of pH mod additives must take into account the geothermal fluid composition to avoid a chemical reaction and the formation of scales as byproducts [35].

A case study in Olkaria, a geothermal field in Kenya, uses pH modification. In order to neutralize the acidic pH of the condensate (pH 2.5) to prevent silica precipitation, sodium carbonate is added to raise the pH to 6.0. The sodium carbonate treatment method was first introduced in Olkaria II in 2001 by Sinclair Knight Merz [35]. The suspected major concern with this method is side reactions from the impurities in the chemical such as silicates and sulfides.

The Salton Sea Geothermal Field in the United States also uses pH modification. The field is characterized by significantly high salinity and total dissolved solids within the brine. Following the successful development and the utilization of the crystallization reactor-clarification technology (CRC), pH mod was carried out due to the high costs associated with the CRC technology. The process included the addition of HCl to the spent geothermal fluid to keep silica and associated elements in solution. The brine was then reinjected into the reservoir [36].

Cooling and Aging of the Produced Water

Once the heat has been harnessed from the geothermal fluid, the spent fluid is sent to a retention pond to store the fluid prior to reinjection. The spent geothermal fluid is held at a generally neutral pH, where it is cooled naturally with increasing residence times (aging) from hours to days. The process allows for the polymerization of silica, which allows the formation of colloids, which will settle to the bottom of the pond. To improve the formation of silica colloids, polymers and flocculants can be added to the fluid. Deposits from aged geothermal fluid are soft and easy to remove. von Hirtz [11] spoke of an example of these deposits from the Botong Field in the Philippines, where colloidal silica formed a “gelatinous fluffy precipitate” that did not settle in the ponds and was re-injected.

Brine and Condensate Mixing

Silica scaling from geothermal fluid being supersaturated with amorphous silica can be controlled through dilution using varying sources. Brine and condensate from the geothermal powerplant are often used for dilution [33]. They are mixed in a variety of ratios including 90% condensate and 10% brine to 50% condensate and 50% brine [35]. This mixing can occur in pipelines, ponds, or other suitable points in the geothermal system. Experiments in Hellisheiði, Iceland [37], diluted separated water with 35 °C condensate, successfully lowering silica supersaturation and decreasing silica precipitation. Currently, condensate dilution is utilized at the Hellisheiði and Nesjavellir geothermal power plants in Iceland [38].

Crystallizer Reactor-Clarification Technology

Aging and cooling geothermal brine in ponds exposed to oxygen can be corrosive to piping and injection well tubulars. This is particularly true in geothermal systems that have significantly high salinity and total dissolved solids, such as the Salton Sea Geothermal Field in the United States. To combat this, a closed system utilizing crystallization and clarification has been developed, known as crystallization reactor-clarifier (CRC) technology. CRC technology focuses on iron-rich amorphous silica. As the brine is flashed to steam in the crystallizer, silica seed crystals are injected, leading to silica precipitation to form large amounts of sludge and minimal traces of silica and iron in the geothermal fluid. Polymer treatment and pH mod using lime are often performed to trigger silica precipitation. The sludge is removed from the geothermal fluid in clarifier tanks, treated, and then disposed of in landfills. Additional details on CRC technology are documented in [36,39].

4.3.2. Mechanical Cleaning

The most basic form of silica scale removal in geothermal surface facilities is mechanical cleaning. Mechanical cleaning is often used to clean heat exchangers and piping with the most common methods as follows:

• Water jetting/blasting (hydroblasting) method: high-pressured water jet through a special nozzle capable of breaking up scale. This is the most common method used in geothermal to remove scaling in surface facilities [7].
• Poli-Pig: a shell-shaped plastic foam sponge mass that is pressed in the pipelines. Various shaped Poli-Pigs can be deployed, armed with steel spikes, to strip scales from the surface. They are most effective in scale thicknesses of <20 mm [40].
• Impact-cutter method: various shaped steel cutter blocks are attached to a flexible shaft and are rotated to remove geothermal scales from pipelines. Impact cutters are effective for various thicknesses [40].

5. Utilization of Silica for a Circular Economy

Processes to address silica scaling in geothermal fluids often lead to the precipitation of silica, such as the aging of brine and the retention systems discussed in Section 4 and Section 6, respectively. Silica scale has long since been seen as just as another waste stream. To reduce waste and increase the economics of high-temperature geothermal power plants, research and development has been focused on converting this waste stream into a marketable commodity.

The industrial applications of silica are numerous and include refractory bricks, cement, health and wellness, pharmaceuticals, and agricultural products. A review of three key examples from the geothermal industry are below:

5.1. GeoSilica, Iceland

• GeoSilica focuses on the development of health and wellness products from a geothermal source [41]. They produce liquid dietary supplements that contain purified silica, which is believed to have various health benefits that include supporting the immune system, improving skin health, and promoting joint function [41].
• Source of silica: Hellisheiði geothermal power plant [41].
• Extraction process: GeoSilica uses a proprietary process to extract silica from the geothermal fluid produced by the power plant [41].

5.2. Geo40, New Zealand

• Geo40 produces colloidal silica, which is a versatile product used in various industries [41]. Applications of colloidal silica include use in agriculture as a soil conditioner, construction for concrete enhancement, in high-tech industries for polishing silicon wafers, and in electronics [42].
• Source of silica: The silica is sourced from geothermal power plants operated by Contact Energy, particularly the Wairakei and Ohaaki power stations [42].
• Extraction process: Geo40 employs a unique process that involves extracting silica from the geothermal fluid before it is reinjected back into the geothermal reservoir. This process not only extracts valuable silica but also helps in reducing scaling issues in the power plant infrastructure [42].

5.3. Sulasih-Sulanjana, Indonesia

• Indonesian researchers (from Gadjah Mada University (UGM) and PT Geo Dipa Energi) have developed a novel fertilizer using silica scale waste known as Sulasih-Sulanjana [43]. The use of silica as a byproduct addresses the waste management issue in geothermal energy while also offering a sustainable alternative for the agriculture sector. By converting waste silica into fertilizer, researchers aim to improve soil quality and crop yields, along with contributing to more sustainable agricultural practices. The project showcases the potential for integrating geothermal energy and agricultural development to promote environmental sustainability and resource efficiency.
• Source of silica: Dieng geothermal power plant in Central Java [43].
• Extraction process: uses nano particulate technology to process silica [43].

An additional benefit of silica removal from the geothermal system is that it allows the geothermal brine to be used as a source of enhanced evaporative cooling. This technique significantly improves the power output during warmer months from binary power plants using air cooling for condensing the working fluid [44].

6. Future Trends and Research Directions

Silica is one of the most challenging scale types to treat in geothermal systems. Research to unlock innovative processes to remove scale and advancements to current methods have been ongoing in order to reduce costs, increase productivity, and improve the sustainability of silica scale treatment. The following section explores current research and development trends in silica scale prevention, treatment, and monitoring in geothermal systems. The focus is on innovative technologies such as combining retention tanks with scaling reactors, nanobubbles, chemical inhibitors, the True Fluidics Oscillator (TFO) Pulsating Waves Method, and fiber optics for monitoring scale formation, as well as paint and coatings.

6.1. GeoSmart Innovative Retention Systems

The GeoSmart project, through the European Horizon 2020 program, has developed an innovative retention system to optimally manage geothermal scaling to reduce the negative effects on the reinjection well and associated systems shown in Figure 11 [45].

The retention/retaining tank method aims to lower silica saturation in geothermal brine by converting monomeric silica to amorphous silica through polymerization. The system is installed after the heat exchanger to avoid scaling during the diminution of the temperature to 50 °C. The two major components of the system include the following:

• The scaling reactor where molecular deposition on surfaces is promoted.
• The retention tank where monomeric silica polymerization to form silica polymers is promoted.

The scaling reactor is the first part of the system where the fluid is maintained in the induction pipe, allowing additives to have the desired effects on the fluid to optimize the pH (maintaining a pH of 8.5) and salinity through pH control and the addition of silica seeds. This modification will prioritize the heterogenous silica precipitation pathway in the induction pipe. It is within the scaling reactor that the deposition of silica on the surface of the tank is promoted. The system aims to reduce the monomeric silica concentration before the fluid enters the retention tank. To maximize the deposit on the surfaces, the geometry of the reactor is designed with a large contact area between the surfaces and the silica particles with the help of a mechanism that enhances the number of collisions between them.

After most of the silica is collected from the fluid, it travels to the retention tank. The retention tank has a horizontal geometry encouraging laminar flow and results in less contact with the surface to avoid scaling on the inside of the tank. A special coating (two-part epoxy: fluoropolymer-based protective coatings and amorphous sol–gel material) was applied to allow the surface to be kept free of silica. pH control is utilized to keep the pH acidic to increase monomeric silica polymerization out of the reactor. Once the fluid is out of the retention tank, chemical inhibitors are added along with brine/condensate mixing to further prevent the risk of silica deposition in the reinjection lines and wellbores [45].

The retention system provides two key economic opportunities alongside the treatment of silica scaling:

• Recover additional waste heat through the coupling of a geothermal power plant with a district heating system and/or with a low-temperature Organic Rankine Cycle (ORC).
• Sale of silica scale through brine mining.

This new technology will be tested in one of the project’s demonstration sites, the Kizildere Power station in Turkey, operated by Zorlu Enerji in fall 2024. The site’s geothermal energy is not fully exploited as it reinjects the brine above the amorphous silica saturation temperature to avoid silica scaling. The GeoSmart project aims to lower the reinjection temperature of the brine from 104 °C to 50 °C to extract more heat from the fluid without creating too much scaling in the system. This approach will thereby increase the plant efficiency and recover 936 GWh of thermal energy (considering a flow rate of 1700 t/h). The recovered energy would be sufficient to supply the thermal demand for the local district heating. The proposed solution offers additional advantages by reducing greenhouse emissions by 755,000 TOE per year in comparison with the district heating, which uses gas as a heat source [45].

Case Studies of Similar Retention Tanks That Are in Operation Today: Hellisheiði and Nesjavellir Power Plants (Iceland)

The focus of the case study is on two Icelandic geothermal power stations located in the Hengill Central Volcano in southwestern Iceland: Nesjavellir and Hellisheiði. The geothermal waters in this area are 250–320 °C with a low salinity fluid. Both power plants are aging the separated waters with different techniques prior to re-injection to allow the monomeric silica in excess of amorphous silica to polymerize.

The Nesjavellir power station started producing power in 1990 with the production of 100 MWt for Reykjavík district heating. Today, the thermal energy production has increased to 360 MWt with plans for further increase in production capacity based on extracting even more heat from the geothermal water. The power plant has been producing electricity since 1998, with two 30 MWe turbines and, over the years, has gained an additional two turbines, resulting in a total production of 120 MWe.

Before 2004, the separated waters from the power plants poured into the lava fields located near Nesjavellir and mixed with the local underground water that was flowing into Lake Þingvallavatn. As only the condensed steam was reinjected at that time, more interest was put into reinjecting the wastewater. As a result, there was an increase in drilling new injection wells with experiments conducted to prevent silica scaling. This led to the construction of a retention tank to age the water before being reinjected shown in Figure 12 [46].

The retention tank is a 649 m³ horizontal pipe with a length of 144 m and a diameter of 2.39 m. It was designed to retain 90 L/s of water for 2 h with a temperature of 80 °C. In 2006, the separated water was mixed with condensed steam as it left the retention tank to lower the concentration of the monomeric silica being dissolved in the water [46].

The Hellisheiði plant’s electricity production started in 2006 with the installation of two 45 MWe turbines. Additional turbines were installed in 2007 and 2008 for a total of 303 MWe capacity today. Most of the power plant’s effluent waters are reinjected into the ground at 180 L/s in the Gráuhnúkar area, where the water is transported by pipes. In this scenario, the pipes act as retention tanks as they were constructed on purpose with larger diameters than needed to slow down the flow and increase retention time in the pipe while the waters are transported 3 km away from the power station [36]. At the Hellisheiði power plant, a combination of pH modification brine/condensate mixing is also used to manage silica scaling. During experiments in 2011 [36], hydrochloric acid was added to lower the pH of the geothermal fluids, preventing silica precipitation. HCl was only used during experiments. The ongoing acidification relies on H₂S and CO₂ dissolution. This approach has proven effective in maintaining the efficiency of the power plant and reducing maintenance costs [37,46]. An analysis of these scale reduction methods was conducted to show their efficiency with the help of the WATCH program [46,47].

For the Nesjavellir power plant, the data shows that the retention tank reduces the concentration of monomeric silica, going from 730 ppm to 519 ppm for a flow of 163 L/s and from 722 ppm to 487 ppm for a flow of 106 L/s (

Table 8. Concentration (in ppm) of monomeric, polymeric, and total silica separated from the Nesjavellir and Hellisheiði power stations in Iceland [46].

Nesjavellir Power Plant	SiO2, m (Monomeric Silica)	SiO2, t (Total Silica)	SiO2,p (Polymeric Silica)
Before retention tank (flow: 106 L/s)	722	726	4
After retention tank (flow: 106 L/s)	487	738	251
After mixing with condensate (flow: 106 L/s)	321	509	188
Before retention tank (flow: 163 L/s)	730	745	15
After retention tank (flow: 163 L/s)	519	732	212
After mixing with condensate (flow: 163 L/s)	376	517	141
Hellisheiði Power Plant	SiO2, m (Monomeric Silica)	SiO2, t (Total Silica)	SiO2, p (Polymeric Silica)
After low-pressure boiler (flow: 175 L/s)	766	811	45
Pipe to Gráuhnúkar 1 km (flow: 175 L/s)	720	789	69
Pipe to Gráuhnúkar 3 km (flow: 175 L/s)	731	784	53

). Additionally, mixing the separated water with the condensate, after being aged, lowers the concentration even further. In Hellisheiði, the concentration reduced from 766 ppm to 720 ppm at 175 L/s (Table 8). The diminution of concentration is less evident for this power plant. It can be explained by the fact that the temperature of the water is higher, as well as the pH value. However, currently, the temperature in that pipe is not higher, it is colder, and, because of the lower acidity after pH modification, the polymerization process has slowed down. The reduced polymerization of monomeric silica observed in the separated waters from Hellisheiði was due to the higher temperature and pH of the water. The water in the pipe is typically 122 °C but can reach up to 174 °C. While silica polymerization has not been analyzed above 122 °C, supersaturation with respect to amorphous silica occurs at 145 °C.

In September 2004 and 2008, the retention tank was opened to examine the internal surfaces. They were coated with <1 to 3 mm layers of amorphous silica. It is, therefore, evident that minor deposition of silica occurs inside the retention tank, but generally the tank can be considered as a successful method to reduce the silica scaling potential of the separated water from Nesjavellir power plant.

6.2. Nanobubbles

Nanobubbles have been identified since 1994, with many studies focused on understanding their behavior in aqueous solutions [48]. Nanobubbles are gas bubbles with diameters in the nanometer range that have unique properties due to their high surface area-to-volume ratio and surface charge. These properties allow nanobubbles to interact with dissolved ions and particles in novel ways, potentially preventing the nucleation and growth of silica scale. They also exhibit longevity in aqueous solutions and stability at high temperatures [48].

Recent research has focused on understanding how nanobubbles can inhibit silica scale formation [48]. The theoretical research proposes the application of nanobubbles as an environmentally friendly and cost-effective alternative to chemical inhibitors for scaling and corrosion.

6.3. Chemical Inhibitors

For many years, water treatment companies have been searching for an effective silica inhibitor that helps to prevent scale deposits at a cost-effective dosage. Chemical inhibitors/anti-scalants have been proved to be effective in reducing the occurrence of silica scaling [49,50]. Even though proven effective as far back as 1982, research is still ongoing by various institutions testing proprietary chemicals such as Nalco and University of Indonesia [51].

• Sinaga and Tobing [51] tested chemical inhibitors consisting of phosphinocarboxylic acid copolymers.
• Dual active silica inhibitors are being developed by chemical companies: copolymer of acrylic acid and hydoxypolyethoxy ally ether that show potential to inhibit/retard the rate of silica polymerization and dispersing polymerized silica [52].
• Alternative solutions, such as chemical anti-scalants, have proven effective in reducing silica deposition [49,50].

6.4. True Fluidics Oscillator (TFO) Pulsating Waves Method

The True Fluidics Oscillator (TFO) Pulsating Waves Method is an innovative approach to treating geothermal scale in the wellbore. The TFO method utilizes pulsating waves generated by a fluidics oscillator to create oscillatory pressure waves that physically disrupt scales in wells [53]. This technology can use chemicals or fresh water pumped through the TFO tool to dissolve or remove scale in the wellbore. TFO tools can be operated using coiled tubing or conventional tubing.

Kushonggo et al. [53] used the TFO method to successfully treat a lost circulation zone in a geothermal production well in the Dieng Geothermal Field in Indonesia that contained galena, siderite, calcite, and dolomite scaling. The tests that followed the treatment showed an increase in power generation from 3.5 MW up to 11 MW.

6.5. Fiber Optics for Monitoring Geothermal Scale

6.5.1. Technology Overview

Fiber optic sensing technology is a powerful tool for monitoring scale formation in real time. Fiber optic sensors can detect changes in temperature, strain, and other parameters to provide detailed information about the conditions within geothermal systems.

6.5.2. Applications in Geothermal Systems

Fiber optic sensors have been successfully deployed as permanently installed features in certain geothermal wells. These sensors provide continuous real-time data to allow operators to monitor downhole conditions and take corrective actions before the significant accumulation of scale occurs. However, in relation to silica scale, there has been recent research and development focused on further developing the following aspects of fiber optic tools, which includes real-time remote monitoring, heat resistance, high sensitivity, compactness, easy setup, and cost-effectiveness [54,55]. These developments are being carried out by exposing the core sensing area of the fiber optic cable to detect the scale-formation-induced refractive index change. Tests have been carried out in high-temperature geothermal settings in Sumikawa, Japan, that have a silica-fused fiber optic cable core connected to a white light source and a visible near infrared spectroscopy detector or a portable spectroscopy [55]. The results of the tests showed that an exposed silica-fused fiber optic core can be used at 350 °C.

6.6. Paint and Coatings

The geothermal industry has experienced a growth in interest in the use of protective painting and coatings on heat exchangers and downhole equipment. While most are focused on corrosion prevention, a few are used to prevent silica scale adherence to enhance equipment lifespan and improve thermal efficiency.

Fanicchia and Karlsdottir [56] utilized simplified descriptions for paint and coating, such as the following:

• Paint: single or multi-layer, made of a polymer material;
• Coating: single or multi-layer, made of metals, ceramics, or their combination.

The major types applied to mitigate silica scaling include the following [56]:

• Silicon carbide (SiC), known as a filled polymer;
• Polytetrafluoroethylene (PTFE)-blended polyphenylene sulfide (PPS);
• Phenolic-based.

These coatings exhibit robust corrosion protection and have low bond strength with geothermal scale deposits [57]. For more details, refer to Sugama [58].

The advancements described above represent significant progress in addressing the challenges posed by silica scaling in geothermal systems. Continued research and development will further enhance the sustainability and efficiency of geothermal energy production, contributing to the global transition to renewable energy sources.

7. Conclusions

Silica scaling reduction technologies were reviewed within the geothermal market to allow for increased operational efficiency and longevity of geothermal systems. Silica is present in various forms within geothermal fluids, and as these fluids travel to the surface and cool, the solubility of silica decreases, leading to supersaturation and precipitation. The formation process can result in silica scale depositing on the surfaces of geothermal equipment, pipelines, wells, and reservoirs. Amorphous silica is the dominant type of silica scale, which is a non-crystalline form of SiO₂ and silicates.

Geothermal fluids contain impurities that can accumulate in the reservoir, clogging the downhole and surface equipment. The ability to predict, control, and mitigate these operational issues is essential to maintaining a consistent energy supply. The Silica Saturation Index can be used to provide a measure of the potential for silica precipitation based on the concentration of dissolved silica and its solubility under specific conditions. The solubility of silica is dependent on the pH of the geothermal fluid, where silica is more soluble in highly acidic (pH < 3) and highly alkaline (pH > 10) conditions. By modifying the pH of geothermal fluids, it is possible to manipulate the solubility of silica and prevent its precipitation. Dissolved salts also affect the solubility of silica in geothermal brines. The presence of calcium, magnesium, iron, aluminum, and manganese can react with Si(OH)₄ to form metal silicates depending on the reservoir conditions.

For scale occurrence in subsurface wellbores, there are various techniques that are being strategically used to maintain and operate these geothermal systems. Preventative methods in the subsurface are critical to limit the precipitation of silica. Examples of preventative methods include pH modification, casing composition, chemical inhibitors, and CO₂ injection. Additionally, pressure and temperature management plays a critical role in the amount of scale deposited, along with the location of deposits. Thermochemical monitoring, along with detection techniques, such as caliper, go-devils, and visual imaging can be effective in understanding the current wellbore conditions. There are various conventional solutions used to clean scale build-up, including mechanical and chemical methods, as well as innovative technologies, such as electro-hydraulic pulsing that have been highlighted in case studies.

Silica scales are a prevalent issue in geothermal power plant facilities, affecting heat exchangers, separators, and pipelines. These scales can hinder heat transfer, reduce efficiency, and cause significant operational problems. The treatment of silica scales involves various methods, with the most common being pH modification, brine/condensate mixing, and mechanical removal techniques, each aiming to prevent or mitigate the formation of scale deposits. The innovative retention system has also been documented through GeoSmart’s demonstration projects in Iceland. Continuous monitoring through using sensors, such as coupons and analytical techniques, is critical for early detection and effective management to ensure the longevity and efficiency of geothermal power plants.

Industrial applications of silica provide an economical solution to manage scale waste. The innovative uses consist of refractory bricks, cement, health and wellness, pharmaceuticals, and agricultural products. Incorporating an industrial use for silica scale from geothermal projects allows operators to work towards a circular economy while building sustainable products.

Even though current methods allow for the successful mitigation and reduction of silica scales in geothermal systems, research and development is still ongoing. The focus of improvement has been on reducing costs, increasing productivity and developing sustainable solutions for silica scale treatment. Several innovative methods are being developed and tested. Innovative retention systems consist of dual active chemicals retarding silica polymerization and dispersing polymerized silica. Nanobubbles are recognized for their stability at high temperatures and longevity in aqueous solutions. Fiber optics are being used with exposed core for monitoring scale formation. Coatings are being used for heat exchangers to provide corrosion protection and low bond strength with geothermal scale deposits.

To optimize efficiencies, strategic considerations should be considered through combining cleaning solutions. The fluid chemistry, reservoir conditions, drilling regulations, and economics will all play an important factor for the treatment(s) used. By having a strategic maintenance plan in place, geothermal projects can guarantee the peak performance and longevity of geothermal wells as important sources of renewable energy in building the future energy system.