Chemical Technology July 2015

Organic compounds in produced waters from shale gas wells by Samuel J Maguire-Boyle, Department of Chemistry, Rice University, Houston, Texas and Andrew R Barron, Department of Chemistry, Department of Materials Science and Nanoengineering, Rice University, Houston and Energy Safety Research Institute, College of Engineering, Swansea University, Wales, UK

The quality of shale gas produced waters, as well as frac flowback waters, is a current environmental concern and disposal problem for producers. Re-use of produced water for hydraulic fracturing is being encouraged; however, knowledge of the organic impurities is important in determining the method of treatment.

T he fastest growing trend for US domestic energy generation is overwhelmingly un-conventional natu- ral gas [1] and the environmental benefits of energy generation from gas compared to coal is obvious [2,3]. The vast amount of hydrocarbons potentially available in the continental United States is in the form of onshore shale gas as seen from the developments currently under way, including: the Marcellus, Haynesville, Fayetteville, Barnett, Eagle Ford, Bakken, Antrim, Utica, Niobara, New Albany, Woodford, and Bossier plays. Unconventional natural gas production has increased tenfold between 2001 and 2011, [4,5] and shale gas is projected to increase from 23 % of US natural gas production in 2009 to 47 % by 2035, offsetting declining production from conventional sources. The ability to extract shale gas in an economic and timely manner has been achieved by the development and use of hydraulic fracturing and horizontal drilling techniques. The fracturing fluid chemistry is tailored on a case-by-case basis for each geographical area and sometimes even on a well-by-well basis.

Although typical fracturing fluid contains predominantly water (ca. 90 %) as well as sand or a ceramic proppant (8–9 %), the fluid also contains all or some of the follow- ing classes of chemicals: salts, friction reducers, scale inhibitors, biocides, gelling agents, gel breakers, and organic and inorganic acids [6,7]. There have been wide-spread concerns over the use of these additives; however, the large volumes of water used during hydraulic fracturing is also a concern. After the hydraulic fracture the pressure is released and the frac fluid is allowed to return to the surface. The chemi- cally engineered water that returns to the surface is called ‘flowback water’, and is mixed with water that is already in the source rock, ‘connate fluids’. Generally the flow rate of the returning water is very high with very little hydrocarbon. The amount of load water (the amount actually injected during the frac) recovered after a fracture ranges from 5–15 % in the Eagle Ford and Haynesville, to 50–60 % in the Barnett and some parts of the Marcellus. Once this water has returned to the surface, however,

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Chemical Technology • July 2015