Quantitative Survey and Structural Classification of Hydraulic Fracturing Chemicals Reported in Unconventional Gas Production

Much
interest is directed at the chemical structure of hydraulic
fracturing (HF) additives in unconventional gas exploitation. To bridge
the gap between existing alphabetical disclosures by function/CAS
number and emerging scientific contributions on fate and toxicity,
we review the structural properties which motivate HF applications,
and which determine environmental fate and toxicity. Our quantitative
overview relied on voluntary U.S. disclosures evaluated from the FracFocus
registry by different sources and on a House of Representatives (“Waxman”)
list. Out of over 1000 reported substances, classification by chemistry
yielded succinct subsets able to illustrate the rationale of their
use, and physicochemical properties relevant for environmental fate,
toxicity and chemical analysis. While many substances were nontoxic,
frequent disclosures also included notorious groundwater contaminants
like petroleum hydrocarbons (solvents), precursors of endocrine disruptors
like nonylphenols (nonemulsifiers), toxic propargyl alcohol (corrosion
inhibitor), tetramethylammonium (clay stabilizer), biocides or strong
oxidants. Application of highly oxidizing chemicals, together with
occasional disclosures of putative delayed acids and complexing agents
(i.e., compounds designed to react in the subsurface)
suggests that relevant transformation products may be formed. To adequately
investigate such reactions, available information is not sufficient,
but instead a full disclosure of HF additives is necessary.