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Chemical Technology • July 2015
received (neutral) extraction, while any asphaltenes are
more likely to be extracted from acid solution.
Hetero compound: natural versus additives.
The presence in
each of the produced water samples of a wide range of
saturated and aromatic compounds is expected given the
nature and composition of oil. In addition, it is expected that
substituted compounds such as carboxylic acids and other
oxidation products be observed given their relationship to
materials such as humic and fulvic acids that is produced
by biodegradation of dead organic matter [23]. Some of
the compounds observed are man-made in origin and are
related to drilling fluids, frac fluids, or tracers. Conversely,
some of the hetero substituted compounds observed must
be the result of secondary reaction chemistry due to water
treatment rather than their presence in either the connate
waters or the frac fluid.
The presence of the certain fluorinated compounds are
commonly employed as flow tracers.
Relative distribution of organic compounds.
Neutral versus acid
extraction should provide an indication of the relative polar-
ity of the hydrocarbons. That is, extraction fromneutral water
should advantage non-polar hydrocarbons as they have the
lowest solubility in water. Acid extraction should enhance
the removal (and therefore analysis) of polar hydrocarbons
with higher solubility in non-acidic water.
Figure 3 shows the normalised saturate, aromatic,
resin and asphaltene (SARA) composition for each of the
produced waters. In each case the majority of organics are
saturated, and only a small fraction comes under the other
three categories. This is in contrast to the prior analysis
of coalbed produced water [15] which showed significant
concentrations of polyaromatic hydrocarbons (PAH). Given
that the source of PAHs in coalbed methane produced
water is thought to be due to their leaching from the coal,
it is unsurprising that low levels should be seen in shale
derived produced water.
Further analysis shows that the aromatic compounds
are exclusively substituted benzene derivatives, rather
than PAHs. The lack of hazardous (carcinogenic) PAHs, and
generally low aromatic content, in shale produced water is
a positive result and significantly lowers the toxic effects of
the water compared to coalbed methane produced water
[15] and off-shore produced waters from conventional oil
and gas production [10,11]. The low levels of resins and
asphaltenes are also consistent with the ‘mature’ nature of
a gas reservoir as compared to coal (and to a lesser extent
oil) formations since condensates are virtually devoid of
asphaltenes.
An alternative differentiation of the organic compounds
identified is by carbon content (ie, C
n
). An observation of
higher C
n
is consistent with lower volatility of higher mo-
lecular weight hydrocarbons.
Acid extraction (Figure 9b) enhances the detection of
the other halogenated (chloro and bromo) organics. It is
important to note that unlike fluorocarbons, chlorocarbons
or organobromides are not used in drilling or frac fluid or
as tracers. This indicates that these chemicals are sourced
from the reservoir material rather as a result of man-made
pollution. So where do these compounds come from?
A consideration of the chlorocarbons and organobro-
mides observed shows they are related to the alkene,
alcohol or carboxylic acid also present.
Anti
-Markovnikov
addition to 1-octadecene (present in each water sample)
would allow for 1-bromooctadecane. However, 1-bromoocta-
decane or 1-chlorooctadecane can be prepared from
alcohols or the carboxylic acids (via halodecarboxylation)
directly by the reaction of bromide salts. Both sodium and
calcium bromide are both used as a drilling fluid additive
[26, 27]. In addition, 1-bromooctadecane could be formed
from 1-chlorooctadecane using LiBr or NaBr (especially in
the case of a phase transfer reaction that would occur at
an oil–water interface) [28]. It has been previously reported
that chlorocarbons may be formed during oxidative chlorina-
tion of waste waters, and that subsequent interaction with
bromide salts results in organobromide formation [29]. The
presence of chlorocarbons could cause a potential issue
The presence
of chloro-
carbons and
organobro-
mides formed
as a conse-
quence of
using chlor-
ine-containing
oxidants
suggests
that industry
should con-
centrate on
non-chemical
treatments of
frac and pro-
duced waters.
WATER TREATMENT
Figure 1 Representative GC trace for Marcellus produced water
after CHCl
3
extraction.
Figure 2 Percentage of peaks in the GC/MS of the produced
waters with a quality range based upon the peak number and
peak area.