Determination of the Mineralization Degree (Total Dissolved Solids) of Geothermal Fluids

The mineralization degree, expressed as total dissolved solids (TDS), is one of the most important chemical indicators for characterizing geothermal fluids and underpins resource evaluation, plant design, and environmental compliance. Determination of geothermal fluid mineralization degree typically relies on gravimetric drying, ion summation, and electrical conductivity methods performed in accredited third-party laboratories in accordance with standards such as DZ/T 0064, GB/T 8538, HJ/T 51, and ASTM D5907. This article explains the testing principle, sample types, analytical items, methods, quality control, and typical applications of geothermal fluid mineralization measurement.

Technical Overview

The mineralization degree of geothermal fluid refers to the total mass of dissolved solids per unit volume of water, comprising the sum of all major cations, major anions, and dissolved minor components such as silica and boron. It is conventionally reported in mg/L or g/L and is often used interchangeably with total dissolved solids (TDS) or total soluble salts in geothermal engineering practice. Because geothermal fluids circulate through deep reservoirs at elevated temperature and pressure, they dissolve substantial quantities of mineral species, and mineralization values can range from a few hundred mg/L for shallow fresh geothermal water to more than 300 g/L for sedimentary basin brines.

Mineralization degree is a primary classification criterion for geothermal water. A common classification used in geothermal exploration is: fresh water (below 1 g/L), slightly brackish water (1–3 g/L), brackish water (3–10 g/L), saline water (10–50 g/L), and brine (above 50 g/L). The parameter directly influences scaling and corrosion tendency, reinjection suitability, heat exchange equipment selection, discharge permitting, and the balneological value of thermal springs. Accurate determination therefore requires strict control of sampling, filtration, preservation, drying temperature, and analytical balance conditions, following recognized standards such as DZ/T 0064 (groundwater quality examination methods), GB/T 8538, HJ/T 51, ASTM D5907, and ISO 5667 for sample preservation and transport.

Testing Samples

Geothermal Well Water

Water collected from production wells during exploration drilling, well testing, or long-term operation. Representative samples are taken after the well has been purged and physicochemical parameters (temperature, pH, conductivity) have stabilized, so that the collected water reflects undegassed reservoir conditions as closely as practicable.

Hot Spring Water

Naturally discharging thermal spring water collected at the orifice or weir. Special attention is paid to sampling before atmospheric exposure causes carbon dioxide loss, carbonate precipitation, or oxidation of reduced species such as ferrous iron and sulfide.

Geothermal Brine and Deep Formation Water

High-salinity fluids from sedimentary basins, often rich in sodium, chloride, bromide, iodide, lithium, boron, and potassium. These samples typically require dilution, additional preservation measures, and analytical techniques calibrated for high dissolved salt matrices.

Reinjection Water and Tail Water

Treated geothermal effluent destined for reinjection or surface discharge. Mineralization measurement of these streams is essential for monitoring reservoir chemistry evolution, verifying reinjection compatibility, and confirming compliance with discharge limits.

Testing Items

A complete mineralization assessment of geothermal fluid generally includes the following determinations:

• Total dissolved solids (TDS) by gravimetric drying at 105 ± 2 °C and, where chloride-rich, at 180 ± 2 °C
• Major cations: potassium, sodium, calcium, magnesium
• Major anions: chloride, sulfate, bicarbonate, carbonate, fluoride, nitrate
• Characteristic geothermal components: dissolved silica (H₂SiO₃ / SiO₂), boron, lithium, strontium, bromide, iodide, arsenic, and iron species
• Supporting parameters: pH, electrical conductivity, total hardness, total alkalinity, total suspended solids, and free carbon dioxide
• Anion–cation balance verification, with the relative deviation between total cations and total anions (in milliequivalents) normally required to be within ±5%

Testing Methods

Gravimetric Method

The reference method for mineralization determination. A measured volume of filtered geothermal water (typically 0.45 µm membrane filtration) is evaporated in a pre-weighed porcelain or platinum dish and dried to constant mass at 105 ± 2 °C. For waters in which calcium and magnesium chloride or sulfate retain crystallization water, and where magnesium chloride may hydrolyze, drying at 180 ± 2 °C is applied; for highly mineralized chloride waters, a small excess of sodium carbonate solution may be added before evaporation so that hygroscopic salts are converted to stable carbonates. The residue mass divided by the sample volume yields the mineralization degree in mg/L. This approach conforms to DZ/T 0064, HJ/T 51, and ASTM D5907 practice.

Ion Summation Method

Mineralization is calculated as the sum of the individually determined concentrations of the major dissolved constituents (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, CO₃²⁻, NO₃⁻, F⁻, and dissolved SiO₂), with bicarbonate conventionally counted as one half of its ion concentration to reflect its loss of carbon dioxide during evaporation. This method provides internal quality checking through the anion–cation balance and is standard practice in geothermal hydrochemical reports.

Electrical Conductivity Method

A rapid estimation technique in which TDS is derived from electrical conductivity using an empirical conversion factor, typically between 0.55 and 0.75 depending on the ionic composition of the geothermal water. It is suitable for field screening, in-situ monitoring of production wells, and continuous online surveillance of reinjection systems, but requires site-specific calibration against gravimetric results for quantitative reporting.

Supporting Instrumental and Classical Analyses

Individual ions supporting the mineralization calculation are determined by ion chromatography (chloride, sulfate, fluoride, nitrate), inductively coupled plasma optical emission spectrometry (potassium, sodium, calcium, magnesium, strontium, lithium, boron), flame atomic absorption or flame photometry (sodium, potassium), EDTA complexometric titration (calcium, magnesium, hardness), silver nitrate precipitation titration (chloride), acid–base titration (bicarbonate, carbonate, alkalinity), and molybdate blue spectrophotometry (dissolved silica). Dissolved silica is particularly significant in geothermal fluids because polymerization during evaporation can bias gravimetric TDS if not properly accounted for.

Applications

Determination of geothermal fluid mineralization degree serves a wide range of engineering and regulatory purposes:

• Geothermal resource exploration and reservoir assessment, including water chemistry typing, geothermometry, and evaluation of reservoir connectivity
• Feasibility studies and design of geothermal heating systems, including selection of plate heat exchangers, titanium or corrosion-resistant piping, and direct-use versus indirect-use circuit configurations
• Scaling and corrosion prediction based on mineralization-dependent saturation indices (such as calcite, amorphous silica, and anhydrite) used to guide chemical dosing and operating temperature windows
• Reinjection scheme design and tracer monitoring, where compatibility between reinjected fluid and reservoir brine prevents formation damage and clogging
• Geothermal power plant water chemistry management for flash and binary cycle installations, including cooling water circuits and blowdown control
• Balneological and spa water certification, medical mineral water evaluation, and development of bottled mineral water sources
• Environmental impact assessment and discharge compliance for geothermal tail water released to surface water bodies or used in irrigation and aquaculture

Quality Control and Reporting

Reliable mineralization data depend on rigorous quality assurance. Samples are collected in clean HDPE or glass containers, filtered at 0.45 µm where dissolved solids are targeted, and preserved and transported according to ISO 5667 guidelines, with analysis completed within the specified holding time. Laboratories apply blank controls, duplicate determinations, certified reference materials, spike recoveries, and anion–cation balance checks. Reports typically state the drying temperature, filtration conditions, ion-by-ion results in mg/L, the calculated ion sum, the gravimetric TDS, and the balance deviation, enabling reviewers to judge data consistency.

FAQ

What is the difference between mineralization degree and salinity?

Mineralization degree and TDS refer to the total mass of all dissolved solids, including silica and bicarbonate, while salinity is sometimes used more narrowly for chloride-dominated dissolved salts. In geothermal practice, mineralization degree is the standard reporting parameter and is expressed in mg/L or g/L.

Why are two drying temperatures, 105 °C and 180 °C, specified?

Drying at 105 °C removes free water but leaves crystallization water in calcium and magnesium sulfates and can cause magnesium chloride hydrolysis. Drying at 180 °C drives off most bound water and yields more reproducible results for chloride- and sulfate-rich geothermal waters, so the appropriate temperature is selected based on the water type.

How are highly mineralized geothermal brines analyzed?

Brines are diluted accurately before instrumental analysis, and gravimetric determination is performed with sodium carbonate addition to stabilize hygroscopic chlorides. High-matrix calibration standards and standard addition verification are used to control salt interference.

How is mineralization degree used in reinjection evaluation?

Comparing the mineralization and ionic composition of reinjected water with native reservoir fluid indicates the risk of precipitation, clay swelling, and formation clogging, which is critical for sustaining reservoir pressure and long-term project economics.

What sample volume and containers are required?

For a full hydrochemical suite including TDS, typically 1 to 2 liters of filtered sample distributed among appropriately preserved containers is sufficient; containers are rinsed with sample water before filling and filled with minimal headspace.

How long does testing take?

In accredited third-party laboratories, routine mineralization and major ion determinations are generally completed within 5 to 10 working days after sample receipt, while extended suites including trace elements may require additional time.