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The exact compounds and their proportions are currently rigorously guarded industry secrets and are tailored for each particular geological site. The website of Energy In Depth, a group representing U.S. oil and natural gas producers, makes public a general list of hydraulic fracturing fluid ingredients.
A recent review of the challenges of drilling at one of the U.S.’s most important natural gas reservoirs, the Marcellus Shale in Pennsylvania, also describes some of the components of these fluids (Environ. Sci. Technol., DOI: 10.1021/es903811p).
“This is a hodgepodge chemical soup,” notes Ronald G. Wilhelm, environmental scientist with the Environmental Protection Agency’s Office of Radiation & Indoor Air, in Washington, D.C., and one of the study’s coauthors.
Water makes up the lion’s share—98 to 99%—of most hydraulic fracturing fluid mixtures, according to various sources. But once the fluid forces open a crack in the rock, something else in the fluid needs to keep it from collapsing. For that purpose, engineers use ordinary silica, or sand. Sand, with its fine grains, allows natural gas to seep through to the surface while holding the crack open.
To keep the sand suspended in the fluid, engineers add a variety of substances that increase the fluid’s viscosity; gels such as guar gum or hydroxyethyl cellulose are one option. Isopropyl alcohol, which acts as a surfactant, is also used to increase viscosity.
This thick fluid has to move smoothly through pipes without clinging to the sides. Polyacrylamide, mineral oil, and even diesel oil add the needed slipperiness.
Various acids, such as sulfuric, hydrochloric, and citric, help dissolve minerals and protect the pipes from scale formation. The common antifreeze ethylene glycol, as well as dimethylformamide (CH3)2NC(O)H, helps prevent pipe corrosion, and ammonium bisulfite scavenges oxygen from the fluid.
Engineers must also contend with microbes because some organisms thrive in the warm, watery environment generated during fracturing, producing slimy masses that gum up and corrode pipes. Engineers therefore treat the fracturing fluids with “biocides,” generally toxic compounds that are registered with EPA as antimicrobial pesticides. One commonly used compound, glutaraldehyde, CH2(CH2CHO)2, a medical and dental disinfectant, kills microbes by cross-linking their proteins.
Researchers are looking for more environmentally friendly replacements for some of these chemicals, according to the recent review. For example, relatively nontoxic long-chain oily paraffins can be substituted for diesel oil, the authors of the Environmental Science & Technology paper say.
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