Background for this classification summary
- Visit the Classification Index to see other groups.
- The summary on this page follows the classification scheme of Elsner and Hoelzer, 2016. Quantitative Survey and Structural Classification of Hydraulic Fracturing Chemicals Reported in Unconventional Gas Production. Environmental Science and Technology. 3290-3314. DOI: 10.1021/acs.est.5b02818.
- Elsner and Hoelzer’s thorough and influential article provides a wealth of chemical context about fracking materials including health and environmental implications and suggestions for deeper analysis. However, the work was based partially on a early set of FracFocus (through 2013). Subsequent years have added new materials and millions more records. To extend their analysis to the rest of the FracFocus materials, we used Google’s Gemini to assist us in assigning classifications in line with EH’s methodology.
- Analysis from a FracFocus download, Jan 2025.
- Masses are not available for all records, so reported masses below are sums of only the available masses. See documentation.
- To provide useful technical information about these chemical groups, we use generative AI tools to assist with summaries (Google’s Gemini). These tools are prompted with details of the classification scheme (by Elsner and Hoelzer, 2016) and information about reported chemicals in the group. To limit inaccuracies, we feed the resulting output to competing models (OpenAI’s ChatGPT) to check for accuracy and ambiguity. In addition, we consulted Fink, Johannes, “Petroleum Engineer’s Guide to Oil Field Chemicals and Fluids” Third Edition. (2021) Gulf Professional Publishing. 1028pp.
- Functions reported here are summarized from the FracFocus field “Purpose.” There are limitations to this data: not all chemical records have a reported purpose (the “systems approach” explicitly excludes “Purpose” values from chemical records). Furthermore, “Purpose” typically describes the product, not individual chemicals within the product. The individual ingredient may serve a chemical role in the product that is different from the reported “purpose” (for example, synthetic polymers might be labeled “proppant”, but they are not primary proppants, but rather coatings for sand grains to enhance their performance.) In general, consider these “functions” as the larger purpose played by the product that contains the ingredient.
Inorganic Chemicals comprise a substantial portion of the chemicals utilized in hydraulic fracturing, as evidenced by the 1,210,000 records, totaling 13.3 trillion pounds, reported to FracFocus between 2014 and 2024. (See also Insoluble inorganic materials.) This broad category encompasses a diverse range of primarily non-carbon-based compounds, including acids, bases, salts, and various other inorganic substances, which play critical roles in different aspects of fracking operations. The dominant mass of this group is driven by the most abundant member, water.1
FracFocus data reveals that inorganic chemicals can be categorized into several subgroups:
- Inorganic acids (19 reported chemicals): Includes strong acids such as hydrochloric acid and sulfuric acid, commonly used in acidizing treatments to dissolve formation rock and improve permeability.
- Inorganic salts (65 reported chemicals): Includes various salts such as sodium chloride, potassium chloride, and calcium chloride, which can be used to adjust the salinity and density of fracking fluids.
- Inorganic bases (17 reported chemicals): Includes bases such as sodium hydroxide and potassium hydroxide, which serve as pH adjusters or acid neutralizers.
- Inorganic oxidizers (23 reported chemicals): Includes oxidizing agents such as peroxides and persulfates, primarily used as gel breakers to degrade polymers in fracking fluids.
- Inorganic reducers (8 reported chemicals): Includes reducing agents such as sodium sulfite and sodium bisulfite, which function as oxygen scavengers to prevent corrosion in wellbores.
- Borates (23 reported chemicals): Includes borate compounds such as boric acid and borax, which function as crosslinkers in high-viscosity gel formulations and pH buffers.
- Ammonia and its derivatives (12 reported chemicals): Includes ammonium chloride and ammonium sulfate, which are used as pH buffers and for precipitation control, rather than as microbial growth nutrients.
- Zirconium-based compounds (10 reported chemicals): Includes zirconium oxychloride and zirconium carbonate, which function as crosslinkers in polymer gels and corrosion inhibitors.
- Other metal salts (22 reported chemicals): Includes various transition metal salts, such as those of magnesium, zinc, and copper, which serve as crosslinkers, biocides, or corrosion inhibitors.
- Other inorganic chemicals (10 reported chemicals): A broad category including various inorganic chemicals not classified elsewhere. Water is in this group and therefore, the most abundant of all subgroups.
- Radioactive tracers (5 reported chemicals): Includes radioactive isotopes such as iodine-131 and technetium-99m, used for tracing fluid movement and evaluating formation properties. They are reported very infrequently.
While these inorganic chemicals themselves may not directly serve all reported functions, they are crucial components of fracking products. FracFocus data indicates that products containing inorganic chemicals are commonly used as:
- Carriers: Delivering other additives to the target formation.
- Acidizers: Enhancing the effectiveness of acid treatments.
- Breakers: Facilitating the breakdown of gels or polymers used in fracking fluids.
- Crosslinkers: Facilitating the formation of crosslinked polymeric gels.
- Biocides: Controlling microbial growth.
- Scale control agents: Preventing the formation of mineral scales.
- Friction reducers: Reducing friction in the wellbore.
- Iron control agents: Controlling the concentration of iron ions in fracking fluids.
Subclass: inorganic acids
Number of reported materials: 19
The ‘inorganic acids’ subgroup within inorganic chemicals consists of acidic compounds that are primarily mineral-derived and do not contain carbon in an organic framework. These acids play crucial roles in hydraulic fracturing, primarily in acidizing treatments to dissolve minerals, remove formation damage, and enhance permeability.
Common examples reported to FracFocus include strong mineral acids such as hydrochloric acid (HCl, CASRN 7647-01-0) and sulfuric acid (H₂SO₄, CASRN 7664-93-9), which are widely used in acid stimulation. Weaker acids in this category include phosphoric acid (H₃PO₄, CASRN 7664-38-2) and phosphorous acid (H₃PO₃, CASRN 13598-36-2), which are sometimes used as pH adjusters, scale inhibitors, or corrosion control agents. Hydrochloric acid is present in most disclosures, often at very high quantities.
Most common inorganic acids:
| Material | Name | Records and mass |
|---|---|---|
7647-01-0 | Hydrochloric acid | 133,000 records — 8,800,000,000 pounds |
7664-38-2 | Phosphoric acid | 8,850 records — 3,790,000 pounds |
7664-93-9 | Sulfuric acid | 3,620 records — 796,000 pounds |
13598-36-2 | Phosphonic acid | 3,250 records — 2,040,000 pounds |
Subclass: inorganic salts
Number of reported materials: 65
The ‘inorganic salt’ subgroup within inorganic chemicals consists of ionic compounds composed of cations (positively charged ions) and anions (negatively charged ions). These salts play diverse roles in hydraulic fracturing, primarily in adjusting the salinity and density of fracking fluids.
Common examples reported to FracFocus include sodium chloride (NaCl, CASRN 7647-14-5), potassium chloride (KCl, CASRN 7447-40-7), calcium chloride (CaCl₂, CASRN 10043-52-4), sodium sulfate (Na₂SO₄, CASRN 7757-82-6), and sodium iodide (NaI, CASRN 7681-82-5).
Beyond modifying fluid salinity and density, these salts serve additional functions:
- Clay stabilization (e.g., KCl replaces exchangeable cations in swelling clays, preventing fluid-induced expansion).
- Scale inhibition (e.g., calcium and sulfate salts influence precipitation of scale-forming minerals).
- Corrosion control (e.g., chloride salts can modify ionic strength, influencing corrosion rates).
The presence and concentration of inorganic salts in fracking fluids must be carefully managed to optimize performance and minimize operational issues.
Most common inorganic salts:
| Material | Name | Records and mass |
|---|---|---|
7647-14-5 | Sodium chloride | 77,600 records — 1,680,000,000 pounds |
10043-52-4 | Calcium chloride | 11,200 records — 711,000,000 pounds |
7757-82-6 | Sodium sulfate anhydrous | 10,600 records — 10,200,000 pounds |
7447-40-7 | Potassium chloride | 7,150 records — 1,700,000,000 pounds |
7681-82-5 | Sodium iodide | 6,690 records — 103,000 pounds |
Subclass: inorganic bases
Number of reported materials: 17
The ‘inorganic base’ subgroup within inorganic chemicals consists of compounds that increase pH by dissociating to release hydroxide ions (OH⁻) or by neutralizing acidic components. In hydraulic fracturing, these bases are primarily used for pH adjustment of fracking fluids, either to maintain optimal conditions for other additives or to neutralize acidic components.
Common examples reported to FracFocus include strong bases such as sodium hydroxide (NaOH, CASRN 1310-73-2) and potassium hydroxide (KOH, CASRN 1310-58-3), which are highly effective in increasing pH. Additionally, weaker alkaline compounds, including potassium carbonate (K₂CO₃, CASRN 584-08-7), sodium bicarbonate (NaHCO₃, CASRN 144-55-8), and sodium carbonate (Na₂CO₃, CASRN 497-19-8), function primarily as buffering agents to help maintain stable pH conditions.
Beyond pH control, inorganic bases play roles in:
- Buffering (stabilizing pH fluctuations in fracking fluids).
- Gel breaking (assisting in the degradation of crosslinked gel structures).
- Scale control (carbonates interacting with dissolved ions to prevent scale formation).
The selection of inorganic bases depends on the specific formulation and operational requirements of the hydraulic fracturing process.
Most common inorganic bases:
| Material | Name | Records and mass |
|---|---|---|
1310-73-2 | Sodium hydroxide | 64,800 records — 246,000,000 pounds |
1310-58-3 | Potassium hydroxide | 26,200 records — 54,700,000 pounds |
584-08-7 | Potassium carbonate | 5,630 records — 56,700,000 pounds |
144-55-8 | Sodium bicarbonate | 4,600 records — 4,460,000 pounds |
497-19-8 | Carbonic acid sodium salt (1:2) | 3,640 records — 9,800,000 pounds |
Subclass: inorganic oxidizing
Number of reported materials: 23
The ‘inorganic oxidizing’ subgroup within inorganic chemicals comprises compounds that act as oxidizing agents, accepting electrons from other substances to facilitate chemical reactions. In hydraulic fracturing, these oxidizers serve multiple functions, including:
- Breaking down organic polymers or gels (e.g., oxidative degradation of crosslinked guar gels).
- Controlling microbial growth (acting as biocides by disrupting microbial cell structures).
- Iron control (oxidizing Fe²⁺ to Fe³⁺ to prevent iron-related scale formation).
- Removing sulfides (oxidizing hydrogen sulfide (H₂S) to prevent corrosion and toxicity).
Common examples reported to FracFocus include:
- Peroxydisulfates (e.g., diammonium peroxydisulfate, (NH₄)₂S₂O₈, CASRN 7727-54-0), which act as strong oxidizers for breaking down gels and organic compounds.
- Hypochlorites (e.g., sodium hypochlorite, NaOCl, CASRN 7681-52-9), widely used as a disinfectant and microbial control agent.
- Chlorites (e.g., sodium chlorite, NaClO₂, CASRN 7758-19-2), which generate chlorine dioxide (ClO₂), a potent biocide and oxidizer.
- Perborates (e.g., sodium perborate tetrahydrate, NaBO₃·4H₂O, CASRN 10486-00-7), which serve as oxygen-releasing oxidizers for gel-breaking applications.
- Hydrogen peroxide (H₂O₂, CASRN 7722-84-1), a versatile oxidizer used for biocidal activity, gel breaking, and iron control.
These oxidizing agents play essential roles in maintaining the efficiency and safety of hydraulic fracturing operations.
Most common inorganic oxidizing :
| Material | Name | Records and mass |
|---|---|---|
7727-54-0 | Diammonium peroxydisulfate | 53,500 records — 199,000,000 pounds |
7681-52-9 | Sodium hypochlorite | 20,800 records — 61,400,000 pounds |
7758-19-2 | Sodium chlorite | 18,900 records — 71,300,000 pounds |
10486-00-7 | Sodium perborate tetrahydrate | 17,500 records — 118,000,000 pounds |
7722-84-1 | Hydrogen peroxide | 12,000 records — 36,000,000 pounds |
Subclass: inorganic reducing¶
Number of reported materials: 8
The “inorganic reducing” subgroup within inorganic chemicals consists of compounds that act as reducing agents, capable of donating electrons to other substances. In hydraulic fracturing, these reducing agents are primarily used for oxygen scavenging to prevent corrosion or for controlling redox reactions in the fracking fluid. Common examples reported to FracFocus include sulfites like sodium sulfite (Na2SO3, CASRN 7757-83-7) and bisulfites like sodium bisulfite (NaHSO3, CASRN 7631-90-5) and ammonium bisulfite (NH4HSO3, CASRN 10192-30-0), as well as thiosulfates like sodium thiosulfate (Na2S2O3, CASRN 7772-98-7) and phosphonates like ammonium hydrogen phosphonate (NH4H2PO3, CASRN 13446-12-3).
Most common inorganic reducing :
| Material | Name | Records and mass |
|---|---|---|
7772-98-7 | Sodium thiosulfate | 3,680 records — 120,000,000 pounds |
7631-90-5 | Sodium bisulfite | 2,230 records — 1,020,000 pounds |
7757-83-7 | Sodium sulfite | 1,020 records — 681,000 pounds |
10192-30-0 | Ammonium bisulfite | 429 records — 991,000 pounds |
13446-12-3 | Ammonium hydrogen phosphonate | 311 records — 3,040 pounds |
Subclass: borates¶
Number of reported materials: 23
Borates are a class of inorganic compounds commonly used in hydraulic fracturing fluids for their ability to crosslink guar-based gels, modify pH, and aid in fluid recovery.
Their primary function is as crosslinkers for guar gum, a gelling agent that increases the viscosity of fracking fluids. At high pH (alkaline conditions), borates form reversible bonds with hydroxyl groups (-OH) on polysaccharides, strengthening the gel and improving its ability to suspend and transport proppants into fractures. This gel structure is pH-sensitive—when the pH decreases, crosslinking weakens, allowing the gel to break down for easier flowback of the fluid.
Borates also serve as pH buffers, helping to maintain an alkaline environment that enhances crosslinking efficiency and reduces clay swelling and corrosion. While borates can contribute to gel breakdown under acidic conditions, dedicated breakers (oxidizers or enzymes) are typically used for viscosity reduction.
Most common borates:
| Material | Name | Records and mass |
|---|---|---|
13709-94-9 | Potassium metaborate | 12,300 records — 42,700,000 pounds |
| 1319-33-1 | Ulexite | 10,800 records — 195,000,000 pounds |
| 1303-96-4 | Borax (B4Na2O7.10H2O) | 9,330 records — 23,800,000 pounds |
10043-35-3 | Boric acid (H3BO3) | 4,820 records — 10,800,000 pounds |
16481-66-6 | Potassium metaborate (KBO2) hydrate(3:4) | 3,850 records — 8,600,000 pounds |
Subclass: inorganic – ammonia and derivatives
Number of reported materials: 12
Inorganic ammonia and its derivatives serve various functions in hydraulic fracturing fluids, primarily in clay stabilization, pH control, and additive stabilization.
- Ammonium chloride (NH₄Cl, CASRN 12125-02-9) is commonly used as a clay stabilizer, preventing swelling of water-sensitive clays that could impede hydrocarbon flow. It achieves this by modifying cation exchange interactions, reducing clay expansion and migration.
- Ammonium sulfate ((NH₄)₂SO₄, CASRN 7783-20-2) is sometimes used as a breaker to assist in the controlled degradation of guar-based gels.
- Ammonium phosphate salts, such as monoammonium phosphate (NH₄H₂PO₄, CASRN 7722-76-1) and diammonium phosphate ((NH₄)₂HPO₄), may be present in some formulations for scale control or corrosion inhibition.
- Ammonium hydroxide (NH₄OH, CASRN 1336-21-6) is used for pH adjustment, helping to maintain optimal fluid conditions and prevent unwanted precipitation of additives.
- Urea (CO(NH₂)₂, CASRN 57-13-6), while not an ammonia derivative, is sometimes included in this group due to its nitrogen content and its role as a stabilizer in certain fluid formulations.
These ammonia-based compounds contribute to the stability and effectiveness of hydraulic fracturing fluids by controlling clay behavior, maintaining pH, and facilitating the performance of other additives.
Most common inorganic – ammonia and derivatives :
| Material | Name | Records and mass |
|---|---|---|
12125-02-9 | Ammonium chloride | 56,700 records — 158,000,000 pounds |
7722-76-1 | Ammonium phosphate | 6,630 records — 97,700 pounds |
1336-21-6 | Ammonium hydroxide | 3,580 records — 142,000 pounds |
7783-20-2 | Ammonium sulfate | 2,940 records — 43,500,000 pounds |
57-13-6 | Urea | 2,940 records — 3,190,000 pounds |
Subclass: Zirconium-based
Number of reported materials: 10
Zirconium-based compounds are widely utilized in hydraulic fracturing fluids as crosslinking agents for guar gum and other polymeric gelling agents. These zirconium complexes react with hydroxyl groups in guar-based fluids, forming a three-dimensional crosslinked gel structure capable of suspending and transporting proppants into fractures.
- Zirconium acetate lactate oxo ammonium complexes (CASRN 68909-34-2) and Tetrakis[2-[bis(2-hydroxyethyl)amino-κN]ethanolato-κO]zirconium (CASRN 101033-44-7) function as crosslinking agents, enhancing gel strength and thermal stability.
- Zirconium oxychloride (CASRN 7699-43-6) serves as a precursor to active zirconium species that participate in crosslinking reactions.
- Chloro hydroxy lactate oxo sodium complex (CASRN 174206-15-6) acts as a stabilized zirconium complex, ensuring controlled crosslinking under specific pH and temperature conditions.
Zirconium-based crosslinked gels are often designed to be reversible, allowing for controlled gel breakdown via chelating agents or reducing agents after the fracturing process. This reversibility enables efficient fluid recovery while ensuring optimal proppant placement within the fracture network.
Most common Zirconium-based:
| Material | Name | Records and mass |
|---|---|---|
| 68909-34-2 | Zirconium, acetate lactate oxo ammonium complexes | 1,840 records — 15,400,000 pounds |
101033-44-7 | Tetrakis[2-[bis(2-hydroxyethyl)amino-kappaN]ethanolato-kappaO]zirconium | 785 records — 1,990,000 pounds |
7699-43-6 | Zirconium oxychloride | 97 records — 366,000 pounds |
| 174206-15-6 | Zirconium, chloro hydroxy lactate oxo sodium complexes | 88 records — 666,000 pounds |
23519-77-9 | 1-Propanol, zirconium(4+) salt | 42 records — 159,000 pounds |
Subclass: “other metal salts”¶
Number of reported materials: 22
“Other metal salts” encompass a diverse range of inorganic compounds used in hydraulic fracturing fluids for various functional roles. Their applications depend on fluid composition and downhole conditions.
- Copper salts, such as copper(II) chloride (CASRN 7447-39-4), cupric chloride dihydrate (CASRN 10125-13-0), copper sulfate (CASRN 7758-98-7), and copper(I) chloride (CASRN 7758-89-6), may be used as biocides, disrupting microbial activity that could lead to equipment fouling or formation damage. However, their solubility and potential for precipitation must be carefully managed in fracturing fluids.
- Magnesium stearate (CASRN 557-04-0) functions as a lubricant, anti-caking agent, or flow improver, reducing friction and improving fluid consistency.
The specific function of each metal salt depends on overall fluid composition and the targeted downhole environment, ensuring optimal performance in hydraulic fracturing operations.
Most common “other metal salts”:
| Material | Name | Records and mass |
|---|---|---|
7447-39-4 | Copper(II) chloride | 3,040 records — 130,000 pounds |
10125-13-0 | Cupric chloride dihydrate | 2,720 records — 361,000 pounds |
557-04-0 | Magnesium stearate | 1,120 records — 4,330 pounds |
7758-98-7 | Copper sulfate | 1,020 records — 58,900 pounds |
7758-89-6 | Copper(I) chloride | 640 records — 28,000 pounds |
Subclass: Others¶
Number of reported materials: 10
The “Others” subgroup within inorganic fracking chemicals encompasses a variety of compounds with diverse functions. Water (7732-18-5) is present in virtually all disclosures as the base fluid for most fracking operations and/or as an ingredient in many fracking products. The other members of this miscellaneous group are uncommon. Potassium antimonate (29638-69-5) may be used as a stabilizer or as a catalyst. Arsenic (7440-38-2), though potentially present as an impurity or as a component of other materials, has no known direct, intentional use in fracking fluids and its presence is likely undesirable due to its toxicity. Iron(II) sulfate heptahydrate (7782-63-0) can be used as an oxygen scavenger to prevent corrosion. Cobalt(II) acetate (71-48-7) may have applications as a catalyst or as a component in certain crosslinking systems, although its use is less common.
Most common Others:
| Material | Name | Records and mass |
|---|---|---|
7732-18-5 | Water | 482,000 records — 13,300,000,000,000 pounds |
29638-69-5 | Potassium antimonate | 315 records — 203,000 pounds |
7440-38-2 | Arsenic | 287 records — 600 pounds |
7782-63-0 | Iron(II) sulfate heptahydrate | 235 records — 10,100 pounds |
71-48-7 | Cobalt(II) acetate | 207 records — 37,400 pounds |
Note: 7732-18-5 (water) is excluded from the following graph; the material is present in virtually all disclosures throughout the FracFocus years
Subclass: radioactive tracers¶
Number of reported materials: 5
Radioactive tracers used in hydraulic fracturing are primarily radioisotopes incorporated into the fracturing fluid in minute quantities to track subsurface fluid movement and proppant placement. Common examples include Scandium-46 (⁴⁶Sc, CASRN 13967-63-0), Iridium-192 (¹⁹²Ir, CASRN 14694-69-0), and Antimony-124 (¹²⁴Sb, CASRN 14683-10-4). These isotopes may be incorporated into solid proppant materials (such as ceramic proppants) or introduced in soluble chemical forms.
In some cases, tracers exist as oxides, such as Iridium oxide (IrO₂, CASRN 12030-49-8) or Scandium oxide (Sc₂O₃, CASRN 12060-08-1), though their specific formulation depends on the tracking objective. Other radioisotopes, such as Iodine-131 (¹³¹I) and Technetium-99m (⁹⁹ᵐTc), are also used for tracking fluid flow due to their high solubility.
Once introduced into the subsurface, these tracers emit gamma radiation, which can be detected using gamma-ray spectroscopy or neutron logging tools. This data helps assess fracture propagation, proppant distribution, and the effectiveness of the fracturing operation.
They are reported very infrequently.
Reported radioactive tracers:
| Material | Name | Records and mass |
|---|---|---|
12030-49-8 | Iridium oxide | 1 records — 83,300 pounds |
12060-08-1 | Scandium oxide | 1 records — 36,000 pounds |
13967-63-0 | (~46~Sc)Scandium | 1 records — 1 pounds |
14683-10-4 | Antimony-124 | 1 records — 1 pounds |
14694-69-0 | (~192~Ir)Iridium | 1 records — 1 pounds |
- All molecular structure images courtesy of the US EPA, except where noted. ↩︎