Evaluation of the scaling and corrosion in Tai'an geothermal water, China

Abstract

Tai'an city, located in Shandong Province, China, is rich in geothermal resources, characterized by shallow burial, high water temperature, and abundant water supply, making them high value for exploitation. However, corrosion and scaling are main challenges that hinder the widespread application and effective utilization of geothermal energy. This study focuses on the typical geothermal fields in Tai'an, employing qualitative evaluations of the geochemical saturation index with temperature, combined with the corrosion coefficient, Ryznar index, boiler scale, and hard scale assessment, to predict corrosion and scaling trends in the geothermal water of the study area. The results show that the hydrochemical types of geothermal water in the study area are predominantly Na-Ca-SO₄ and Ca-Na-SO₄-HCO₃, with the water being weakly alkaline. Simulations of saturation index changes with temperature reveal that calcium carbonate scaling is dominant scaling type in the area, with no evidence of calcium sulfate scaling. In the Daiyue Qiaogou geothermal field, the water exhibited corrosive bubble water properties, moderate calcium carbonate scaling, and abundant boiler scaling. Feicheng Anjiazhuang geothermal field showed non-corrosive bubble water, moderate calcium carbonate scaling, and significant boiler scaling. The Daidao'an geothermal field presented corrosive semi-bubble water, moderate calcium carbonate scaling, and abundant boiler scaling. The findings provide a foundation for the efficient exploitation of geothermal resources in the region. Implementing anti-corrosion and scale prevention measures can significantly enhance the utilization of geothermal energy.

Graphical abstract

Keywords

Geothermal / Tai'an / Scaling / Corrosion / Central Shandong Uplift Area

Introduction

Corrosion and scaling are key challenges in the exploitation and utilization of geothermal resources (Zhu, 2006; Gallup, 2009; Pazheri et al. 2014). These issues arise from the complex interactions between chemical components and materials under varying conditions of temperature, flow rate, and pressure. Whether in geothermal power generation, heating, or other direct applications, corrosion frequently affects wellhead devices, pipes, and heat exchangers (Arteaga et al. 2013; Zarrouk et al. 2014; Liu, 2015). These challenges severely limit the widespread application and efficient utilization of geothermal energy (Stahl et al. 2000; Lv, 2020).

Understanding the chemical composition of geothermal water and predicting its corrosion and scaling tendencies are crucial for effectively managing geothermal water systems. While many studies have focused on scaling prediction, most emphasize carbonate and silicate scaling. Predictive indices, such as the Ryznar Index (RI) (Ryznar et al. 1944), Larson Index (LI) (Larson et al. 1967), Langelier Saturation Index (LSI) (Langelier, 2011), and Riddick Index (Riddick, 1944), are commonly used to evaluate scaling tendencies (Pátzay et al. 1998). Arnorsson et al. (1982) utilized the WATCH water chemistry program to predict scaling trends in Icelandic geothermal water. With advances in computational tools, an increasing number of scholars have used simulation software such as PHREEQC to calculate mineral saturation index, coupled with these above indices to predict scaling tendencies (Florent et al. 2000; Appelo et al. 2005; Marimuthu et al. 2005; Delalande et al. 2011; Manan et al. 2023). In China, calcium carbonate scaling is predominant in geothermal systems. Researchers utilize complementary approaches combining the Ryznar and Larson indices for more accurate predictions (Zhu et al. 2004; Wei et al. 2012; Wang et al. 2015). Recent methodologies include hydrogeochemical simulations to predict corrosion and scaling tendencies (Liang et al. 2020; He et al. 2022; Pang et al. 2023). These methodologies encompass utilizing index calculation along with the WATCH simulation outcomes to ascertain scaling tendencies (Yu et al. 2007; Wei et al. 2012), integrating index calculations with mineral saturation indices to identify scaling types (Zhang et al. 2016; Song et al. 2020), and establishing quantitative models for calcium carbonate scaling in wellbores (Cen et al. 2022; Lin, 2022; Lei et al. 2023).

Shandong Province, hosts a significant distribution of geothermal resources, divided into three major geothermal anomaly zones: Eastern Shandong, South Central Shandong, and Southwestern Shangdong. Tai'an city lies within the South Central Shandong geothermal area, encompassing Taishan-Daidao'an, Feicheng-Anjiazhuang, and Daiyue geothermal fields (Tan et al. 2019；Cao et al. 2018；Kang et al. 2022；Zheng et al. 2023). This region is characterized by convective belt-like geothermal reservoir wtih shallow burial depths, fault-controlled structures, and higher geothermal gradients beneath surface layers (Cai et al. 2015; Tan et al. 2019; Mao et al. 2019; Liu et al. 2020). Geothermal resources in Tai'an offer rich potential due to their high water temperature and are abundant water supply. Current exploitation models focus on bathing, recuperation, agriculture, and aquaculture (Li et al. 2020; Meng et al. 2021; Li et al. 2021). The city's 14th Five-Year Plan for Energy Development emphasizes optimizing hydrothermal geothermal use, including water-source heat pump systems leveraging sewage and mine water resources. Despite growing utilization of the geothermal resources, scaling and corrosion hinder efficient exploitation. This study targets geothermal water in Tai'an main fields (Taishan-Daidao'an, Feicheng-Anjiazhuang, and Daiyue), employing geochemical simulations, Ryznar Index, and other evaluations to predict the corrosion and scaling trends and propose solutions to mitigate scaling and corrosion, enhancing resource efficiency.

1 Geological conditions

The geothermal fields in Tai'an City comprise the Daidao'an geothermal field, Feicheng-Anjiazhuang geothermal field, and Daiyue-Qiaogou geothermal field, all located within the central uplift in Shandong Province. As shown in Fig. 1(a) and Fig. 1(b), these fields are predominantly located at the intersections of uplifts and depressions, where the influence of faults is notable. Deep and extensive faults act as conduits for heat from deep sources, while atmospheric precipitation percolates throughrock fissures and heat-controlling faults. This enables deep circulation and thermal energy exchange under the influence of regional hydraulic head pressures. The resulting ascends to shallow depths and accumulated within structural faults to form geothermal reservoirs. These geothermal fields are characterized by relatively small distribution areas, shallow burial depths, high geothermal gradients beneath the covering layers, typical of convective belt-like geothermal reservoir (Mou, 2017; Wang et al. 2020; Gao et al. 2023).

The Taishan Daidao'an geothermal field, located in Daidao'an Village, Tai'an City, is a deep-circulating convective, belt-like geothermal reservoir positioned at the convergence of the Tailai Depression and the Mount Tai Uplift. The field features highly developed fault structures, prominently the Mount Tai Fault and the Daidao'an Fault. The reservoir layers predominantly consist of oolitic limestone of the Zhangxia Formation and chert nodule limestone of the Cambrian Zhushadong Formation, buried at depths of 200–800 m. The Feicheng Anjiazhuang geothermal field, located in Anjiazhuang, Feicheng, Tai'an City, lies at the intersection of the Wenkou Depression and the Bushan Uplift. Its reservoir layers, within the Cambrian Zhushadong Formation, are predominantly composed of dolomite, dolomitic limestone and marl. These layers are buried at depths of 10 m–400 m, with a geothermal gradient of 3.0°C/100 m–8.58°C/100 m. The Qiaogou geothermal field, situated in Qiaogou Village, Zoulai Town, Tai'an City, lies at the intersection of the Tailai Depression and the Xinfu Mountain Uplift. Its main reservoir layer consists of Neoarchean granodiorite, with a top plate buried at a depth of approximately 10 m. The reservoir temperature is 20°C–41.4°C, and its water inflow capacity is 120 m³/d–480 m³/d.

2 Sample collection and calculation methods

2.1 Sample collection

In this study, a total of five sets of geothermal water samples were collected: One from the Qiaogou geothermal field (TADR-004), one from the Anjiazhuang geothermal field (TADR-005), and one from the Daidao'an geothermal field (TADR-006). The remaining two sets were obtained from geothermal fields situated at the foot of Mount Tai in the north of Tai'an City (TADR-002 and TADR-003). See Fig. 1(a) for the sampling locations.

Field measurements of pH and temperature were conducted using handheld testing devices. For comprehensive water quality analysis, 2.5 L polyethylene plastic buckets were used to collect the samples. These samples were dispatched to the Key Laboratory of Groundwater Sciences and Engineering, Ministry of Natural Resources, for water quality analysis. This analysis adhered to the GB/T 8538—2008 National Standard for Testing Natural Mineral Water. During the test process, cations such as K⁺, Na⁺, Ca²⁺, Mg²⁺ were quantified using the Inductively Coupled Plasma (ICP) method, while anions like HCO₃⁻, Cl⁻, SO₄²⁻, and F⁻ were analyzed through ion chromatography. The balance error between anions and cations was controlled within 3%.

2.2 Calculation method

The hydrochemical data analysis in this study employed the Schukalev classification method to determine hydrochemical types. Software tools including SPSS, Aquachem (for hydrochemical type analysis), PHREEQC (for saturation index calculation), and Excel were utilized for analyzing the hydrochemical data. To quantitatively assess the corrosion and scaling trends of geothermal water, a range of index methods were employed, including the corrosion coefficient, Ryznar Index, Larson Index, total amount of boiler scale evaluation, and hard fouling evaluation.

1) Total boiler scale evaluation (H₀)

The total amount of boiler scale (H₀) was calculated using:

(1)$ \begin{split} H_0 =& S + \rho + 36r({\text{F}}{{\text{e}}^{{\text{2}} + }}) + 17r({\text{A}}{{\text{l}}^{{\text{3}} + }}) + \\& 20r({\text{M}}{{\text{g}}^{{\text{2}} + }}) + 59r({\text{C}}{{\text{a}}^{2 + }})\end{split} $

Where: S is suspended matter content in geothermal water (mg/L); ρ is C (colloid content) = SiO₂ + Fe₂O₃ + Al₂O₃ (mg/L).

Judgment criteria:

H₀< 125: Geothermal water with little scaling;

125≤H₀<250: Geothermal water with a little scaling;

250≤H₀<500: Geothermal water with abundant scaling;

H₀≥500: Geothermal water with significant scaling.

2) Calcium carbonate scaling

a) For chloride ion content ≥ 25 mol%, the Larson Index (LI) can be used to assess scaling tendency and corrosion of calcium carbonate:

(2)$ \mathit{\mathit{ }\text{LI}}=\frac{\rho(\text{C}\text{l}^-)+\rho(\text{SO}_{\text{4}}^{\text{2}-})}{\mathit{ALK}} $

Where: $ \rho\left(\mathrm{Cl}^{-}\right) $ is chlorides concentration (mg/L); $ \rho\left(\mathrm{SO}_{4}^{2-}\right) $ is sulfate concentration (mg/L); ALK is total alkalinity as CaCO₃ (mg/L).

Judgment criteria:

LI≥0.5: Non-scaling, corrosive;

0.5≤LI<3.0: Slightly corrosive;

3.0≤LI<10.0: Highly corrosive;

LI<0.5: Scaling potential, no corrosive.

For this study, chloride ion content was below 25 mol%, so this method was not applied.

b) For chloride ion content <25 mol%, the Ryznar Index (RI) was used:

(3)$ {{RI}}=\text{2p}\text{H}_{\text{s}}-\text{p}\text{H}_{\text{a}} $

(4)$ \text{p}\text{H}_{\text{a}}=-lgc(\text{C}\text{a}^{\text{2}+})-lg(\mathit{\text{ALK}\mathrm{)}}+K_{\text{e}} $

Where: pH_S is calculated pH; pH_a—measured pH; c(Ca²⁺) is calcium concentration; ALK is total alkalinity as HCO₃⁻; K_e: is a constant (1.8–2.6, dependent on total dissolved solids and water temperature).

Judgment criteria:

RI<4.0: Very severe scaling;

4.0≤RI<5.0: Severe scaling;

5.0≤RI<6.0: Moderate scaling;

6.0≤RI<7.0: Slight scaling;

RI>7.0: Non-scaling.

3) Calcium sulfate scaling

For water temperature < 100°C, calcium sulfate precipitates as dihydrate gypsum (CaSO₄·2H₂O). The CaSO₄ scaling is assessed using the R.S index:

(5)$ {{R}}{{.S}} = {10^{\left( {{\text{log }}ppm{\text{C}}{{\text{a}}^{{\text{2}} + }} + {\text{log }}ppm{\text{S}}{{\text{O}}_{\text{4}}}^{2 - } - \log {{\text{K}}^{}}} \right)}} $

Where: K is a constant related to temperature and total dissolved solids.

Judgment criteria:

R.S<1: No scaling;

R.S>1: Scaling potential.

4) Hard fouling evaluation

The hard fouling coefficient K_n was calculated:

(6)$ {K_{\text{n}}}{\text{ = }}{H_n}/{H_o} $

(7)$ \begin{split} {H_{\text{n}}} =& {\text{Si}}{{\text{O}}_{\text{2}}}{{ + 20}}r{\text{(M}}{{\text{g}}^{{{2 + }}}}{\text{) + 68 [}}r({\text{C}}{{\text{l}}^ - }) +\\& r{\text{(S}}{{\text{O}}_{\text{4}}}^{{{2 - }}}) - r{\text{(N}}{{\text{a}}^{{ + }}}) - r{\text{(}}{{\text{K}}^{{ + }}}{\text{)}}]\end{split} $

Where: H_o is total amount of boiler scale (mg/L); r is milligram equivalents per liter of ion content

Judgment Criteria:

K_n<0.25: Soft sediment;

0.25≤K_n≤0.5: Moderate sediment;

K_n>0.5: Hard sediment.

5) Corrosion evaluation

Corrosion coefficient (K_k) can be used to evaluate the corrosivity of geothermal water.

For acidic water:

(8)$ \begin{split} {K_{\text{K}}} =& 1.008[r({{\text{H}}^ + }) + r({\text{A}}{{\text{l}}^{{\text{3}} + }}) + r({\text{F}}{{\text{e}}^{{\text{2}} + }}) + r({\text{M}}{{\text{g}}^{{\text{2}} + }}) -\\& r({\text{HCO}}_{\text{3}}^ - ) - r({\text{CO}}_{\text{3}}^{{\text{2}} - })]\end{split} $

For alkaline water:

(9)$ {K_{\text{K}}} = 1.008[r({\text{M}}{{\text{g}}^{{\text{2}} + }}) - r({\text{HCO}}_{\text{3}}^ - )] $

Where: r is milligram equivalent concentration of ions (meq/L).

Judgment Criteria:

K_k>0: Corrosive;

K_k<0 and K_k+0.0503Ca²⁺>0: Semi-corrosive;

K_k<0 and K_k+0.0503Ca²⁺<0: Non-corrosive.

6) Foaming evaluation

The foaming coefficient F was calculated:

(10)$ F{\text{ = 62}}r{\text{(N}}{{\text{a}}^{\text{ + }}}{\text{) + 78}}r{\text{(}}{{\text{K}}^{\text{ + }}}{\text{)}} $

Where: r(Na⁺), r(K⁺): Ion content (mg/L).

Judgement Criteria:

F<60: Non-foaming;

60≤F≤200: Semi-foaming;

F>200: Foaming.

3 Results and discussion

3.1 Hydrochemical characteristics of geothermal water

In the study area, the geothermal water temperatures ranged from 25°C to 69°C, with pH values between 7.13 and 8.1 (Table 1). Notably, the Total Dissolved Solids (TDS) value of the geothermal water sample from Anjiazhuang, Feicheng reached 2,240 mg/L, while samples from other areas exhibited TDS values ranging from 455 mg/L to 940 mg/L. Hydrochemical analyses were performed using a Piper diagram (Fig. 2), Schoeller diagram (Fig. 3), and major ion percentages (Fig. 4).

The findings reveal that Na⁺ and Ca²⁺ are the primary cations, while SO₄²⁻ and HCO₃⁻ are the main anions. Hydrochemical types identified include Na-Ca-SO_4, Ca-Na-SO₄-HCO₃, Na-Ca-SO₄-Cl-HCO₃, Na-Ca-SO₄-Cl, and Ca-Na-Mg-SO₄-HCO₃. Concentrations of Na⁺, K⁺, Ca²⁺, Cl⁻, and SO₄²⁻ in geothermal water surpass those in surface water and shallow groundwater due to prolonged pathways, enhanced water-rock interactions, and higher geothermal reservoir temperatures. Conversely, Mg²⁺ level is generally lower in geothermal water than in shallow groundwater, likely due to cation exchange and adsorption processes at elevated temperatures. The concentrations of bicarbonate ions in geothermal water exhibit minimal variation compared to shallow groundwater and surface water and show weak correlation with other ions.

The SPSS analysis revealed a significant positive correlation between Cl⁻ and K⁺, Na⁺, Ca²⁺, SO₄²⁻, and TDS (Table 2). A strong correlation between Na⁺ and Cl⁻ suggests a shared origin, possibly the dissolution of rock salt or other deep-seated minerals. In the study of groundwater hydrochemistry evolution, the equivalent ratio of cations r(Na⁺)/r(Cl⁻) provides a better indicator of cation exchange process. As shown in Fig. 5, the equivalent ratios of geothermal water are greater than 1, indicating relatively strong cation exchange reactions.

The chlor-alkali indices CAI1 and CAI2 further illustrate the likelihood of these cation exchange reactions (Fig. 6). When Na⁺ and K⁺ in the water exchange with adsorbed Ca²⁺ and Mg²⁺, CAI1 and CAI2 values are positive. Conversely, when Ca²⁺ and Mg²⁺ exchange with adsorbed Na⁺ and K⁺, CAI1 and CAI2 values become negative. The magnitude of the absolute values of CAI1 and CAI2 reflects the intensity of the exchange reactions. It can be seen from the figure that the indices for the geothermal water are negative with large absolute values, consistent with the earlier analysis, confirming that cation exchange reactions are indeed relatively strong.

The hydrogeochemical processes governing the formation of major ions in hot water predominantly involve the dissolution and precipitation of carbonate, sulphate, and silicate minerals. As illustrated in the Fig. 7, geothermal water in the study area closely aligns with the 2[Ca²⁺] = [HCO₃⁻] line, with a slight inclination towards the [Ca²⁺] side. This suggests that the Ca²⁺ and HCO₃⁻ ions in geothermal water predominantly originate from the dissolution of carbonate minerals like calcite and dolomite. The tendency towards the Ca²⁺ side suggests additional dissolution of silicate minerals like feldspar. The [Mg²⁺]/[Ca²⁺] ratio in geothermal water is consistently below 1, signifying a limited influence of magnesium-deficient minerals like calcite on the hydrochemical composition of deep geothermal water. Similarly, the [Na⁺]/[Ca²⁺] ratio remains relatively low, approximating [Na⁺]/[Ca²⁺] = 1, further indicating a minimal contribution of silicate minerals dissolution to the water's composition. Additionally, the [Ca²⁺+ Mg²⁺]-[HCO₃⁻+SO₄²⁻] diagram provides insight into the hydrogeochemical processes affecting the primary chemical components of geothermal water. The observed ratio of [Ca²⁺+ Mg²⁺]/[HCO₃⁻+ SO₄²⁻] approximates 1 (Fig. 8), indicating that the precipitation-dissolution equilibrium of carbonate, silicate, and sulfate minerals collectively influences the composition of geothermal water. However, the relatively minor role of silicate mineral dissolution supports that the reservoirs are predominantly affected by the precipitation-dissolution dynamics of carbonate and sulfate minerals.

3.2 Hydrogeochemical simulation of scaling mechanism and the saturation index

To further analyze the predominant scaling type and its tendencies, hydrogeochemical modeling (Aquachem) was conducted to simulate the saturation state of geothermal water. The mineral Saturation Index (SI) (Oddo et al. 2021; Stahl et al. 2000) was used, which is defined as:

(11)$ SI{\text{ = lg }}\frac{{IAP}}{K} $

Where：K is the equilibrium constant for the mineral dissolution reaction; IAP represents the activity product of the relevant ions in the mineral dissolution reaction. Positive SI values indicate supersaturation (precipitation tendency), while negative SI values suggest undersaturation (dissolution tendency).

The geochemical simulation results (Figs. 9 and 10) reveal that carbonate scaling is the dominant type in the study area. Sulfate scaling is minimal, and appearing only at sampling points TADR-A002 and TADR-A003, and silicate scaling occurs marginally at 100°C. The moderate to low-temperature nature of the geothermal water supports the predominance of carbonate scaling.

3.3 Corrosion and scaling analysis

Given that chloride ion concentrations remain below 25%, the Larson Index (LI) is inapplicable for calcium carbonate scaling and corrosion assessment. Instead, the corrosion coefficient (K_k) and Ryznar Index (RI) were used, alongside evaluations for hard fouling and foaming (Table 3 and Fig. 11). The findings indicate no calcium sulfate scaling, consistent with the saturation index analysis. However, all samples exhibit calcium carbonate scaling. Specifically, geothermal water samples from Nanmatao Village and Qiaogou Wuyue Wenquan in Daiyue District are classified as non-corrosive, while those from Shila Village shows moderate corrosive property. Samples from Anjiazhuang in Feicheng and Wenquan Road in Taishan District are deemed corrosive. Based on the RI assessment, calcium carbonate scaling varies, with slight scaling in Nanmatao Village and Shila Village, while the total amount of scaling is higher. The remaining three geothermal water samples exhibit moderate scaling. According to the evaluation of total amount of boiler scale, geothermal water sample from Anjiazhuang in Feicheng has a significant amount of scaling at 724.26 mg/L, while others range from 256.66 mg/L to 380.27 mg/L; Hard sediments are present in all geothermal water samples. Water samples from Nantao Village and Wenquan Road in Taishan District are classified as semi-foaming, whereas those from other areas are identified as foaming water. Remarkably, the geothermal water from Anjiazhuang in Feicheng demonstrates the highest foaming coefficient, significantly surpassing other geothermal water samples. Overall, the geothermal water sample from Anjiazhuang in Feicheng demonstrate the highest level of corrosion, scaling, and foaming, significantly exceeding those of the other regions.

4 Conclusions

(1) The geothermal fields in Tai'an, including the Taishan Daidao'an, the Feicheng Anjiazhuang, and the Daiyue geothermal fields, are classified as fracture geothermal reservoirs. The predominant hydrochemical types are Na-Ca-SO₄ and Ca-Na-SO₄-HCO₃, with pH levels ranging from 7.13 to 8.1, indicating weakly alkaline water. Simulations reveal that scaling tendencies are predominantly influenced by calcium carbonate.

(2) Due to low chloride levels (<25%) in all geothermal water samples, the Larson index (LI) is not suitable for evaluating corrosion and scaling trends. Corrosion is instead evaluated using the Corrosion coefficient (Kk), while scaling tendencies are assessed with the total amount of boiler scale evaluation, the Ryznar Index (RI) and the hard scale coefficient (K_n). The results reveal that the geothermal water of Daiyue Qiaogou geothermal field is corrosive bubble water, moderate calcium carbonate scaling, and abundant boiler scaling. The geothermal water of Feicheng Anjiazhuang geothermal field is non-corrosive bubble water, moderate calcium carbonate scaling, and significant boiler scaling. The geothermal water of Daidao'an geothermal field is corrosive semi-bubble water, moderate calcium carbonate scaling, and abundant boiler scaling.

(3) Corrosion and scaling are widespread in geothermal water across the study area. the Feicheng Anjiazhuang geothermal field exhibits the highest levels of corrosion, scaling, and foaming compared to other regions. Therefore, anti-corrosion measures, scale inhibitors, and passivation agents should be implemented to enhance the efficiency of geothermal development and utilization.

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