Chemical Technology July 2015

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Chemical Technology • July 2015 24
of shale reservoirs.
As part of our studies into the treatment of oil–water
mixtures [16] and understanding the challenges in treating
frac and produced water to make it ideal for re-use, we are
interested in the composition and compositional variation
between various produced waters. Such information will
assist in understanding of whether a particular treatment
process can be used generally or which treatment processes
should be applied to different produced waters. Our study
is presented herein along with suggestions for future treat-
ment protocols based upon the results.
Experimental
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Results and discussion
Conductivity, pH and salt content
Before considering the organic content, we wanted to deter-
mine the inorganic content of the produced waters under
study. Other studies have shown the conductivity and pH
of the as-collected water. There is no direct relationship
between the conductivity and pH for the samples indicating
that the conductivity is a function of salt content and identity
rather than acidity (see below).
The chemistry of a shale reservoir is unlike that of a con-
ventional oil or gas reservoir that is flushed with hundreds of
pore volumes of transient water resulting in leaching of the
rock and other components to an equilibrium level. Shale
has a very low permeability (concrete is 10² to 104more per-
meable) and there has been little or no movement of fresh
water (or waters of a different mineral content) since the
rock was formed. Furthermore, shales are under-saturated
to water and the levels of salt in the connate waters within
the shales are often at salinity equal to the seawater the
shale was deposited from. In other words, shale is a reac-
tor waiting for an influx of fresh ingredients, and thus when
under-saturated fresh water or evenmoderate salinity water,
is introduced during a frac, salts, some organics, and other
minerals that were in equilibrium with the connate waters
are solubilized.
The ion content for each of the produced water samples
was determined by ICP-OES. The results are summarized
in Table 1. High alkali metal levels are not an issue with
regard to the re-use of the produced water in subsequent
hydraulic fracturing. In contrast to the alkali metals, alkaline
earth (Group 2) metals are associated with scale formation
[19, 20]. In particular, when calcium and barium levels are
above ca. 20 000 mg L-1 scale inhibitors must be employed
and or the salt content lowered before the water can be
re-used down hole [4].
Carbon content
The total carbon (TC), non-purgeable organic carbon (NPOC),
also known as total organic content (TOC), and total inor-
ganic carbon (TIC) for each produced water sample was
measured (Table 2) and the results are shown in Figure 3.
For all of the produced water samples the NPOC is signifi-
cantly higher than the TIC.
Identification of organic compounds
Figure 1 shows a representative GC for Marcellus produced
water. The peak assignment is provided based upon the fit-
ting of the integrated mass spectrum for each peak. While
all the peaks could be assigned a suitable compound, there
is a quality parameter (Q) that provides a goodness of fit of
the data, ie, a confidence level in the assignment. Figure 2
shows the percentage of peaks in the GC of the produced
waters within a particular quality range of the assignment
by mass spectrometry.
We note that in the work of Orem et al less than 20 % of
potential organic compounds were actually identified [15].
For simplicity in giving a representative example of the
types of organic compound found in each water sample, we
have limited the contents of our Tables to those compounds
that are assigned with confidence in more than one well
sample. Aromatics are defined as molecules containing one
or more aromatic rings, and are slightly more polarisable.
Resins and asphaltenes have polar (heteroatom) substitu-
ents. The distinction between the two is that asphaltenes
are insoluble in heptane whereas resins are miscible with
heptane. As such the resins should be observed by as-
Unlike coalbed
methane pro-
duced water,
shale oil/
gas produced
water appears
not to contain
significant
quantities of
polyaromatic
hydrocarbons,
reducing the
potential
health hazard.
Table 2 Chemical analysis (mg L1) of the produced water samplesa
Element Marcellus (PA) Eagle Ford (TX) Barnett (NM)
Na 523.6 45.9 5548.9
K 2605.8 17043.3 4566.5
Li 0.0 1200.6 84407.4
Rb 47.0 0.0 0.0
Mg 289.7 28.2 5747.2
Ca 1387.5 111.2 33971.8
Sr 92.9 34.5 2461.8
Ba 0.0 4.7 17.2
Ti 0.0 16.2 15.1
V 4.2 16.2 14.6
Cr 11.0 13.6 11.5
Mn 0.0 11.5 9.4
Fe 8.4 1246.5 75.7
Ni 0.0 36.5 12.0
Cu 2.6 0.0 0.0
Zn 65.8 0.0 684.9
Hg 14.6 0.0 0.0
B 0.0 40.2 70.5
Si 727.1 4416.6 0.0
Sn 206.2 3.1 124.2
P 0.0 29.2 1177.1
As 26.1 0.0 2.1
Sb 0.5 2.1 9.9
Bi 36.5 50.1 161.3
S 189.0 413.9 290.8
Table 1 Conductivity and pH of as collected produced water samples
Water Conductivity (mS) pH
Marcellus (PA) 28.5 6.85
Eagle Ford (TX) 31.1 5.95
Barnett (NM) 52.8 7.43