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.
Non-functionalized hydrocarbons are a class of organic compounds composed solely of carbon and hydrogen atoms, lacking heteroatoms (such as oxygen, nitrogen, sulfur, or halogens) and polar functional groups. These hydrocarbons exist as alkanes (single-bonded hydrocarbons like methane and ethane), alkenes (double-bonded hydrocarbons like ethene), alkynes (triple-bonded hydrocarbons like acetylene), and aromatic compounds (such as benzene and toluene). The dominant subgroup in this class are the petroleum distillates. We detail the subgroups and provide statistics on their use in fracking operations.1
Petroleum distillates and related products, which are complex mixtures derived from crude oil (such as light distillates and mineral oil), also fall within this category. Some non-functionalized hydrocarbons and highly inert fluorocarbons reported to FracFocus are not individually specified and are categorized as “unspecified or other.” Examples include aliphatic hydrocarbons, generic hydrocarbon listings, hydrotreated light petroleum distillates, and petroleum distillates.
This broad category encompasses multiple subgroups, including alkanes (33 reported chemicals), alkenes (23), aromatic compounds (24), petroleum distillates/products (38), and unspecified/other (7). Additional details on these subgroups will be provided separately.
Between 2014 and 2024, FracFocus—a national hydraulic fracturing chemical registry—published 287,000 records involving non-functionalized hydrocarbons. However, reporting is not always mandatory and may vary by state. These records represent a total reported mass of 4,350,000,000 pounds.
These hydrocarbons are reported to be used in hydraulic fracturing primarily as:
- Friction reducers (to decrease resistance in pipes),
- Gelling agents (to increase fluid viscosity for proppant transport),
- Surfactants and emulsifier controls (to reduce surface tension and stabilize mixtures),
- Corrosion inhibitors,
- Crosslinkers (to modify fluid viscosity), and
- Scale control agents (to prevent mineral buildup).
Subclass: alkanes¶
Number of reported materials: 33
Alkanes, within the context of FracFocus-reported non-functionalized hydrocarbons, are fully saturated hydrocarbons characterized by the general formula CnH2n+2. These compounds consist exclusively of carbon-carbon and carbon-hydrogen single bonds, resulting in low chemical reactivity under standard conditions, as they lack electrophilic or nucleophilic sites.
Commonly reported alkanes in this subgroup include straight-chain hydrocarbons such as undecane (1120-21-4), dodecane (112-40-3), tridecane (629-50-5), and tetradecane (629-59-4). Others, such as ‘Alkanes, C16-20-iso-‘ (90622-59-6), represent mixtures of branched isomers within a specified carbon number range.
Both linear and branched alkanes primarily function as lubricants and viscosity modifiers in fracturing fluids, helping to reduce friction and optimize flow properties. Depending on chain length and molecular structure, alkanes in fracturing fluids range from lighter, volatile components that enhance fluid mobility to heavier, more viscous liquids that influence rheology and stability.
Most common alkanes:
| Material | Name | Records and mass |
|---|---|---|
1120-21-4 | Undecane | 2,930 records — 13,500,000 pounds |
112-40-3 | Dodecane | 2,020 records — 8,810,000 pounds |
629-59-4 | Tetradecane | 2,000 records — 8,800,000 pounds |
629-50-5 | Tridecane | 1,970 records — 8,760,000 pounds |
90622-59-6 | Alkanes, C16-20-iso- | 699 records — 22,100,000 pounds |
Subclass: alkenes¶
Number of reported materials: 23
Alkenes reported as non-functionalized hydrocarbons in FracFocus are unsaturated hydrocarbons containing at least one carbon-carbon double bond. This double bond increases reactivity compared to alkanes, making alkenes more susceptible to polymerization, oxidation, and addition reactions, which may influence their behavior in fracturing fluid formulations.
Commonly reported alkenes in this subgroup include terminal (α-olefin) alkenes, such as 1-hexadecene (629-73-2) and ‘Alkenes, C>10 alpha-‘ (64743-02-8), representing a mixture of long-chain alpha-olefins.
Also frequently reported are cyclic and polycyclic terpenes, including D-limonene (5989-27-5) and the broader category ‘Terpenes and Terpenoids, sweet orange-oil’ (68647-72-3), which are often derived from natural sources such as citrus extract (94266-47-4).
The presence of these compounds suggests they are not included for crosslinking purposes. Instead, they may occur naturally as constituents of certain gelling agents or be intentionally added to enhance fluid recovery and modify interfacial properties.
Most common alkenes:
| Material | Name | Records and mass |
|---|---|---|
629-73-2 | 1-Hexadecene | 7,230 records — 401,000 pounds |
5989-27-5 | D-Limonene | 4,430 records — 7,190,000 pounds |
| 68647-72-3 | Terpenes and Terpenoids, sweet Orange-oil | 3,750 records — 21,700,000 pounds |
| 64743-02-8 | Alkenes, C>10 alpha- | 3,660 records — 405,000 pounds |
| 94266-47-4 | Citrus extract | 3,270 records — 11,300,000 pounds |
Subclass: aromatic compounds¶
Number of reported materials: 24
Aromatic compounds, within the context of FracFocus’s non-functionalized hydrocarbons, are characterized by the presence of at least one aromatic ring, typically a benzene ring, stabilized by delocalized π-electrons, which distribute electron density across the ring, reducing reactivity compared to typical alkenes.
Commonly reported monocyclic aromatics include ethylbenzene (100-41-4), the three xylene isomers (1330-20-7), and 1,2,4-trimethylbenzene (95-63-6), all of which are alkyl-substituted benzene derivatives.
Polycyclic aromatic hydrocarbons (PAHs), such as naphthalene (91-20-3), are also reported. Additionally, heavier alkyl-substituted benzenes, including ‘Benzene, C10-16-alkyl derivatives’ (68648-87-3), contribute to the complex mixture of petroleum-based hydrocarbons in fracturing fluids.
These aromatic hydrocarbons contribute to the solvency of fracturing fluids, aiding in the dissolution of other organic components. However, their presence also raises considerations regarding environmental fate, persistence, and potential bioaccumulation, particularly for PAHs. They are generally present as components of petroleum distillates.
Most common aromatic compounds:
| Material | Name | Records and mass |
|---|---|---|
91-20-3 | Naphthalene | 18,000 records — 11,700,000 pounds |
95-63-6 | 1,2,4-Trimethylbenzene | 14,600 records — 2,910,000 pounds |
| 68648-87-3 | Benzene, C10-16-alkyl derivatives | 3,320 records — 645,000 pounds |
1330-20-7 | Xylenes | 3,070 records — 3,850,000 pounds |
100-41-4 | Ethylbenzene | 2,220 records — 985,000 pounds |
Subclass: petroleum distillates/products¶
Number of reported materials: 38
The “petroleum distillates/products” subgroup within FracFocus’s non-functionalized hydrocarbons represents complex mixtures derived from crude oil refining. These mixtures are classified based on their boiling point range and refining process rather than a specific molecular structure. This subgroup is by far the largest among the non-functionalized hydrocarbons, both by number of records and total mass reported. Because of their nature as mixtures, they typically contain many representatives from other subgroups but are not explicitly identified.
Common examples include “Distillates (petroleum), hydrotreated light” (64742-47-8), which are lower-boiling fractions subjected to hydrotreating—a process that removes sulfur, nitrogen, and other impurities while hydrogenating unsaturated hydrocarbons to improve stability.
“Solvent naphtha, petroleum, heavy arom.” (64742-94-5) and “Solvent naphtha, petroleum, heavy aliph.” (64742-96-7) refer to higher-boiling petroleum fractions, with the former being rich in aromatic hydrocarbons and the latter predominantly composed of aliphatic hydrocarbons. Their differing compositions influence their solvency, volatility, and function in fracturing fluids.
“White mineral oil, petroleum” (8042-47-5) is an ultra-refined, colorless, and odorless mixture of saturated hydrocarbons, valued for its lubricating and hydrophobic properties in industrial applications, including fracturing fluids.
“Distillates, petroleum, solvent-dewaxed heavy paraffinic” (64742-65-0) is a high-boiling fraction that has undergone solvent dewaxing to remove solid paraffins, yielding a more fluid mixture dominated by long-chain alkanes.
Due to their variable composition and physical properties, these petroleum distillates serve diverse functions in fracturing fluids, acting as carriers for additives, friction reducers, viscosity modifiers in gelling agents, and sometimes as solvents to aid in fluid recovery.
Most common petroleum distillates/products:
| Material | Name | Records and mass |
|---|---|---|
| 64742-47-8 | Distillates (petroleum), hydrotreated light | 159,000 records — 3,630,000,000 pounds |
| 64742-94-5 | Solvent naphtha, petroleum, heavy arom. | 21,900 records — 61,700,000 pounds |
| 8042-47-5 | White mineral oil, petroleum | 5,190 records — 83,200,000 pounds |
| 64742-96-7 | Solvent naphtha, petroleum, heavy aliph. | 3,280 records — 90,900,000 pounds |
| 64742-65-0 | Distillates, petroleum, solvent-dewaxed heavy paraffinic | 2,970 records — 8,640,000 pounds |
Subclass: unspecified or other¶
Number of reported materials: 7
The “unspecified or other” subgroup within FracFocus’s non-functionalized hydrocarbons includes materials that do not clearly belong to the alkane, alkene, aromatic, or petroleum distillate subgroups or lack sufficient chemical identification for precise classification.
Examples in this category include complex mixtures such as “Petrolatum, petroleum, oxidized” (64743-01-7) and even unrefined “Petroleum” (8002-05-9), which represents either crude oil itself or minimally processed fractions.
Some highly specialized fully fluorinated compounds also appear in this category, including Perfluoro-1,2-dimethylcyclobutane (2994-71-0), Perfluoro-1,3,5-trimethylcyclohexane (374-76-5), and Perfluoro-2-methyl-3-ethylpentane (354-97-2). These perfluorocarbons are notable for their extreme chemical inertness, high thermal stability, and unique physical properties, which may be relevant in specialized fracturing fluid applications.
The presence of such chemically diverse materials highlights both the challenges of broad classification and the use of niche or less well-defined additives in fracturing fluid formulations.
Most common unspecified or other:
| Material | Name | Records and mass |
|---|---|---|
| 64743-01-7 | Petrolatum, petroleum, oxidized | 405 records — 265,000 pounds |
| 8002-05-9 | Petroleum | 146 records — 546,000 pounds |
2994-71-0 | Perfluoro-1,2-dimethylcyclobutane | 17 records — 20 pounds |
374-76-5 | Perfluoro-1,3,5-trimethylcyclohexane | 15 records — 19 pounds |
354-97-2 | Perfluoro-2-methyl-3-ethylpentane | 14 records — 19 pounds |
- All molecular structure images courtesy of the US EPA, except where noted. ↩︎