24 / 40
Chemical Technology • July 2015
22
Organic compounds
in produced waters
from shale
gas wells
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,
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.
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