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Organic Chemical Characterization and Mass Balance of a Hydraulically Fractured Well: From Fracturing Fluid to Produced Water Over 405 Days James S. Rosenblum, E. Michael Thurman, Imma Ferrer, George R. Aiken, and Karl G. Linden Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03362 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017
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Organic Chemical Characterization and Mass Balance of a Hydraulically Fractured
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Well: From Fracturing Fluid to Produced Water Over 405 Days
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James Rosenbluma, E. Michael Thurmanb, Imma Ferrerb, George Aikenc, and Karl G.
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Lindena*
6 7
a
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Colorado, UCB 428, Boulder, CO 80309, United States
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b
Department of Civil, Environmental, and Architectural Engineering, University of
Center for Environmental Mass Spectrometry, University of Colorado, UCB 428,
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Boulder, CO 80309, United States
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c
U.S. Geological Survey, Boulder, Colorado, 80309, United States
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*Corresponding author:
[email protected] 14 15
Abstract
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A long-term field study (405 days) of a hydraulically fractured well from the Niobrara
17
Formation in the Denver-Julesburg Basin was completed. Characterization of organic
18
chemicals used in hydraulic fracturing and their changes through time, from the pre-
19
injected fracturing fluid to the produced water, was conducted. The characterization
20
consisted of a mass balance by dissolved organic carbon (DOC), volatile organic analysis
21
by gas chromatography/mass spectrometry, and nonvolatile organic analysis by liquid
22
chromatography/mass spectrometry. DOC decreased from 1500 mg/L in initial flowback
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to 200 mg/L in the final produced water. Only ~11% of the injected DOC returned by the
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end of the study, with this 11% representing a maximum fraction returned since the
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formation itself contributes DOC. Furthermore, the majority of returning DOC was of the
26
hydrophilic fraction (60 - 85%). Volatile organic compound analysis revealed substantial
27
concentrations of individual BTEX compounds (0.1 – 11 mg/l) over the 405-day study.
28
Nonvolatile organic compounds identified were polyethylene glycols (PEGs),
29
polypropylene glycols (PPG), linear alkyl-ethoxylates, and triisopropanolamine (TIPA).
30
The distribution of PEGs, PPGs, and TIPA, and their ubiquitous presence in our samples
31
and the literature, illustrate their potential as organic tracers for treatment operations or in
32
the event of an environmental spill.
33 34
1. Introduction
35
Unconventional oil and gas development and hydraulic fracturing has grown
36
steadily in recent years, with the advent of directional drilling and fracturing fluid
37
mixtures that permit economic recovery of shale resources. This is demonstrated by the
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major U.S. shale basins (lower 48 states) accounting for 92% of the domestic oil
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production growth and all of the domestic natural gas production growth from 2011 to
40
2014.1 The extraction of shale resources takes place by drilling both vertical and
41
horizontal wells, followed by formation perforation, and the injection of fracturing fluids.
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Fracturing fluids are primarily water (~90%) and proppant (~9%), along with various
43
chemical additives (~1%) that enable operators to pump and deliver proppant through the
44
borehole to elongate and prop-open fissures, creating surface area and continued access
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to oil and gas resources.2,3 Although the extraction of shale resources has had significant
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economic impacts, there are numerous environmental and public health concerns, 2
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especially as it relates to water use and chemical additives associated with hydraulic
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fracturing.4–7
49 50
Water plays a key role in unconventional oil and gas development, from drilling
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to hydraulic fracturing. Hydraulic fracturing uses anywhere from 7,000,000 to
52
50,000,000 L of water for a single well, depending on geological characteristics,
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horizontal length, operator, and type of fracturing method (i.e. slick water, gel, or hybrid-
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frac).8–10 The chemicals associated with hydraulic fracturing are also of environmental
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and public health interest, especially considering that 1% can amount to 500,000 L of
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chemicals used per well (vol:vol). The fracturing fluids are injected into the formation
57
and return to the surface along with the hydrocarbons over the life of the well, generating
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a complex wastewater referred to as “flowback” in the initial stages of the well, and
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produced water during the oil and gas production phase (collectively referenced herein as
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“produced water”). Produced waters are a complex mixture of organic and inorganic
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constituents, that originate either from the fracturing fluid (>1,000 chemicals potentially
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used)
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radioactive material [NORM]),13 or as a result of subsurface reactions between the
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two.12,14 It has been estimated that over 3x1012 liters of produced water are generated
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annually in the U.S. alone, creating a substantial waste stream for the oil and gas
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industry.15
9,11,12
, the shale formation (i.e. hydrocarbons, salinity, metals, naturally occurring
67 68
There are environmental concerns related to produced waters and their ability to
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reach the environment through well casing failures, surface spills, and inadequately
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treated produced water.16–18 These concerns also arise from the most common
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management practice, disposal of water through deep-well injection (95% of wells),
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which has recently been linked to seismic activity19 and the introduction of endocrine
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disrupting chemicals to the surrounding environment.20 The potential environmental
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impacts of produced water can be countered by their potential benefits, particularly for
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water stressed regions, if treated and managed properly (e.g. irrigation of crops).
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Therefore, the need to improve our understanding of produced water constituents,
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specifically their organic composition, for environmental release scenarios or reuse
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situations is vitally important.
79 80
Numerous publications have presented data on the inorganic constituents found in
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produced water samples,21–25 while a paucity of literature exists on the organic
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constituents.12,26,27 Review articles have identified and prioritized organic chemicals that
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have been reported in fracturing fluid mixtures9,11,28 and that could return in produced
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water.5,7,12 However, there are only a few studies to date that target organic chemicals in
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produced water samples, with several focused solely on non-polar compounds (i.e.
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hydrocarbons),14,29 and the few studies that do include the polar fraction have analyzed a
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limited number of samples.3,30,31 Furthermore, the majority of this research has been
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conducted on dry formations (e.g. gas producing), with little research as whole on wet
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formations (e.g. oil and gas producing).27
90 91
The objectives of this study were, first, to characterize and perform a mass
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balance of the organic constituents found in the fracturing fluid and the returned
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produced water over a one-year period from a single well using non-specific organic
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analyses (DOC, hydrophobicity, and fluorescence). The second objective was to identify
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and to follow over time specific volatile and nonvolatile organic compounds. Thirdly, and
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finally, we assessed the possible environmental hazards and removal mechanisms of
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identified organic compounds during different stages of production. As such, this work
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presents a unique comprehensive long-term study of the presence of organic compounds
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used and returned in a hydraulically fractured well.
100 101
2. Methods
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2.1. Well Characteristics and Water Samples
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The focus of our study was a well located in Weld County, CO, which produces
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both oil and gas from the Niobrara Formation at a depth of around 2,000 meters. The well
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was completed using a gel-based hydraulic fracturing fluid with around 11,000,000 L of
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water. The fracturing fluid included 10 different proprietary chemical mixtures for a total
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of around 87,000 liters/Kg of chemicals injected (SI Table 1; FracFocus and operator’s
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well report). Water volume returned was from the separator’s water meter, which data
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logs the flow of water over time.
110 111
The water samples collected from this individual well included the water used to
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make the fracturing fluid (groundwater), the pre-injected fracturing fluid prior to
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injection, and nine produced water samples over time. The first produced water sample
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was taken during the first day (day 1) of the well flowing back, after a 30-day shut-in
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period. Samples were collected on days 4, 7, 15, 22, 80, 130, 220, and 405 after flowing
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back the well. The samples taken on days 1 and 4 were drawn from a tank connected
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directly to the well head (emptied every 4 to 6 hours), and all other produced water
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samples were taken directly from the onsite gas/oil/water separator. All samples, except
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for the fracturing fluid (wide mouth glass jar) were collected in pre-combusted (550°C
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for 4 hr) 1-L amber bottles with no headspace. Samples were immediately transported
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back to the lab and stored at 4°C prior to analysis (< 5 days). Further details about the
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well, sampling procedure, and fracturing fluid can be found in Rosenblum et al. (2017).32
123 124
2.2. Dissolved Organic Matter Characterization, Fractionation, and Mass Balance
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The characterization and fractionation of dissolved organic carbon (DOC) was
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conducted at the U.S. Geological Survey laboratory (Boulder, CO). The fractionation was
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done using XAD resins, according to Aiken et al. (1992),33 and included HPI as the
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hydrophilic fraction (not retained by XAD resins), TPIA as transphilic acids (retained by
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XAD-4), and HPOA as hydrophobic organic acids (retained on XAD-8). DOC analysis
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(non-purgeable organic carbon) of unfractionated water and XAD fractions was
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conducted using a TOC-Vcph analyzer (Shimadzu Corp., Japan). In addition to these
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analyses, ultraviolet light absorbance was measured using an Agilent 8453 photodiode
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array UV-Vis spectrophotometer, with specific ultraviolet absorbance (SUVA254)
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calculated (UV absorbance in cm-1 divided by TOC in mg/L), and a spectrofluorometer
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(Horiba FluoroMax-3, NJ; Raman units [RU]) was used to generate an excitation-
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emission matrix; all samples were filtered through a 0.45-µm prior to analysis.34
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The mass balance was performed, and normalized to the mg of carbon in the
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fracturing fluid, using Equation 1, where V is volume (L) and C is DOC concentration
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(mg/L). The index corresponds to the order the samples were taken (i.e., i=2, day 4;
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i=3…i=10, day 405). Values used and calculated are presented in SI Table 2. : =
"
$
+
#
+
∑+ , " &, − , ) $
+ ) * "
142 143
2.3. Anion Determination
144
The Laboratory for Environmental and Geological Studies (LEGS) at the
145
University of Colorado Boulder performed the chloride analyses using ion
146
chromatography (Dionex Series 4500I). Samples were diluted by x 10, x 100, and x 1000,
147
with the average value being calculated from these three dilutions, if the levels were
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above the analytical method detection limit.30
149 150
2.4. Volatile and Polar Organic Compound Analysis
151
Volatile organic compounds (VOCs) were determined from headspace-free 40-
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mL volatile organic analysis (VOA) vials. VOCs were determined by purge and trap (OI
153
Analytical- 4660) gas chromatography mass spectrometry Agilent 6890 and 7890 GC-
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MS) by a NELAC/A2LA accredited lab using EPA method 8260C (Origins Laboratory
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Inc., Denver CO). Polar organic compounds were determined using ultrahigh pressure
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liquid chromatography (1290 Agilent UHPLC; Agilent Technologies, CA) with an
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ultrahigh definition quadrupole time-of-flight mass spectrometer (qTOF-HRMS; 6540
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Agilent qTOF; Agilent Technologies, CA), with a 2-5% error of signal intensity for direct
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injection. All samples were filtered through a 0.2-µm filter (Acrodisc), and 10 µL were
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injected onto a reverse phase C8 analytical column (Zorbax Eclipse XDB-C8). The
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mobile phases were H2O with 0.1% formic acid and acetonitrile. The chromatographic
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method was: 5 minutes (10% H2O with 0.1% formic acid/90% acetonitrile), followed by
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linear gradient to 100% acetonitrile after 30 minutes, with a flow rate of 0.6 mL/min. Ions
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were generated using electrospray Jet Stream Technology, operating in positive ion
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mode. The data were processed using MassHunter software (version 6.1). This method
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and details about the filters used during sample preparation are further described in
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Thurman et al. (2014)3. Triisopropanolamine (98%) was purchased from Acros Organics.
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3. Results and Discussion
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3.1. Dissolved Organic Carbon (DOC) and Mass Balance
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The
non-specific
organic
characterization
(DOC,
hydrophobicity,
and
172
fluorescence) of the groundwater, fracturing fluid, and produced water samples are
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presented in this section. Figure 1A shows the concentrations of DOC and chloride from
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the beginning to the end of the study. DOC concentrations ranged from ~1500 mg/L in
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day 1 flowback water to ~200 mg/L in the final produced water (SI Table 3), which is
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approximately a ten-fold decrease in total DOC. The pre-injected fracturing fluid had a
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DOC of ~2200 mg/L, while the groundwater used to mix the fracturing fluid was ~5
178
mg/L DOC (SI Table 3); thus, the addition of fracturing fluid additives increased the
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DOC by several orders of magnitude. SI Table 1 shows the list of ingredients and their
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volumes from the FracFocus report and Well Report for this well, including 37,000 L of
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guar slurry, 11,000 L of non-emulsifiers (proprietary surfactants), and additional organic
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ingredients that likely contributed to the measured DOC. Chloride concentrations rapidly
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increased from day 1 to day 22, and then appeared to reach a pseudo steady-state over the
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remainder of the study (Figure 1A and SI Table 3).32 Figure 1B shows chloride versus
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DOC concentrations over time.
186 187
B
2000
12000
1500
9000
ClDOC
1000
Cl- (mg/L)
DOC (mg/L)
A
e Tim
1 Day
0
0 0
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6000 3000
500
400 Day
100
200
300
1500
400
1000
500
0
DOC (mg/L)
Time (Days)
189
Figure 1: Concentration of chloride and DOC in the produced water samples over time
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(A), and Cl- versus DOC over time (B). Note, the y-axis for chloride represents both plots
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The observed decline in DOC concentrations with time is consistent with other
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studies that investigated DOC in wells over time.35–37 Hayes, who investigated several
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wells with 3 to 5 samples over 90 days, had similar findings regarding DOC over time for
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the majority of the wells studied,35 whereas Cluff et al. had nearly the same trend for the
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three wells they investigated.36 Kim et al., who investigated two Denver-Julesburg Basin
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(DJ-Basin) samples over 200 days, showed similar total organic carbon (TOC) results to
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those seen here; although they had roughly a 50% decrease in TOC over the 200 days
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compared to our 70% decrease of DOC over 220 days.37 Furthermore, research that has
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denoted the age of a produced water (time since flowback) have generally shown
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declining levels of DOC, chemical oxygen demand (COD), or TOC over time.38–40
202 203
A mass balance was performed for the DOC measured in the produced water
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samples to determine the percentage of DOC returning over the course of the study
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(Figure 2). Approximately 10.5% of the injected DOC returned by day 220, and only an
206
additional 0.6% returned between days 220 and 405 (11.1%). This 11.1% represents a
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maximum fraction returned, because the formation itself contributes DOC, which is not
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discernable from the fracturing fluids DOC. Therefore, these results indicate that a small
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percentage of DOC from the injected fracturing fluids returns to the surface over 405
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days of operation (< 11.1%). This 11.1% does not consider the volatile organic
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compounds (VOCs), because they are removed during non-purgeable organic carbon
212
analysis, so the TOC recovered is likely slightly higher than the 11.1% of the DOC
213
observed. This low level of returning DOC is not seen with the returning water volumes,
214
which had around 34% of the volume injected return by day 405 (Figure 2); more than
215
three times that of the DOC.
216 217
These DOC results could be solely from dilution and mixing with subsurface
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brine, however it is likely the returning fluid is influenced by more than just dilution and
219
mixing, which is often thought to be a key contributor to the total dissolved solids (TDS)
220
of returning produced waters.32 A combination of effects, such as sorption,
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biodegradation, subsurface reactions, or imbibition are likely playing a role. Sorption is
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likely due to the organic-rich mud, shale, and hydrocarbons present within the borehole
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that could sequester DOC from the produced water. Biodegradation is also possible since
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there were high levels of DOC that could be labile (i.e. guar).41 A significant diversity of
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microorganisms have been shown to return in produced water,36,42 even in the presence of
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biocides.43 Imbibition in particular is important to consider, even for oil and gas bearing
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formations,44 since it has been proposed that the majority of injected fluids are retained
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(e.g. imbibed) in shale formations and that the returning waters are primarily natural
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occurring formation brine.45 Thus, the 34% of injected water that returned (Table SI 2;
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4% increase from Day 220 to Day 405), which again represents a maximum fraction
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returned because much of the returning volume is likely influenced by the formation
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itself, suggests that a substantial volume injected was likely imbibed in the formation.
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Furthermore, Kim et al (2016)37 performed a mass balance of returning DJ-Basin
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produced water over time, and saw significant variation in their frac fluid additive
235
recoveries, with recoveries of aluminum at 3%, zirconium at 9%, and potassium at
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33%.37 Therefore, the data and literature suggest that imbibition, along with
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mixing/dilution, could both be playing significant roles, while sorption and
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biodegradation could also be contributing, and that it is additive-specific. Furthermore,
239
this 11.1% returned DOC could be composed of injected, formation, and subsurface
240
reaction based DOC, which make up the ~2,300 kg of DOC returned by day 405,
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including chemicals of environmental concern from either the fracturing fluid (i.e.
242
biocides) or formation (i.e. benzene), validating the need to characterize and identify this
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DOC.
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% DOC and Water Returned
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DOC
80
Water
60 40 20 0 0
244
100
200
300
400
Time (Days)
245
Figure 2: Percent of DOC returned based on a mass balance of ~24,000 Kg of DOC
246
injected. Percent of water returned is also plotted and was calculated using the volume
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returned on that day relative to the volume injected (11,000,000 L).
248 249
3.1.1. Dissolved Organic Carbon Fractionation and Characterization
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The fractionation of produced waters DOC using XAD resins is a valuable
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method for determining the distribution of organic matter class in oil and gas
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wastewater.30 The fractionation results for the produced waters over time indicate the
253
majority of DOC for all of the samples and fracturing fluid was composed of low
254
molecular weight hydrophilic compounds (HPI; Figure 3a). This is in agreement with
255
Lester et al. (2015)30 who investigated a single DJ-Basin flowback sample.30
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The day 1 sample had the highest percentage of hydrophilic acid (HPI; Figure
257
3A). The percentage of HPI steadily decreased over time, similar to that of the DOC
258
(Figure 3A & B). HPI has been linked to simple organic acids (e.g. acetic acid), which
259
can be found at high levels in produced waters.30 HPI and hydrophilic neutral compounds
260
are also likely responsible for much of the polar organic constituents (i.e. guar), and they
261
have been reported to consist of proteins and carbohydrates.46,47 Carbohydrates are of
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particular interest, since they are generally used as gelling agents (i.e., guar) during
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hydraulic fracturing and were used in the well, studied herein (SI Table 1). This result,
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then suggests that the gelling agents, which would be found in the HPI fraction, could be
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responsible for a significant portion of the DOC in these samples. This hypothesis is
266
supported by the concentration of guar slurry injected (SI Table 1) and the trends shown
267
in Figures 1 and 3B, which illustrate that DOC and HPI follow nearly identical trends
268
over time. However, it should be noted that the HPI could also consist of acetone, acetic
269
acid, and other hydrophilic DOC compounds that are known to be present in produced
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waters, either from the formation or the injected fluid.29,30
271 272
The other fractions, transphilic acids (TPIA) and hydrophobic organic acids
273
(HPOA), increase relative to the total measured DOC over time (Figure 1 and Figure 3A).
274
TPIA is thought to consist of intermediately polar compounds and has been linked to
275
aromatic constituents with acidic functional groups, which have been found in produced
276
water.14,48 HPOA is generally thought to consist of the less polar compounds and has
277
been associated with humic material, along with phenols and cresols,49 which have been
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measured in DJ-Basin produced waters.30 Since these fractions increase relative to the
279
total DOC over time (Figure 3A), it is likely that these fractions are mostly from the
280
formation water and not the injected fluid. However, it is important to note that these
281
fractions could also be forming in the subsurface from the degradation of injected
282
compounds. This is illustrated in Figure 3B, which shows TPIA and HPOA
283
concentrations decrease by 43% and 48%, respectively, while the HPI fraction decreased
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by 89%.
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B HPI TPIA
80
HPOA
60 40 20
80
1000
60 40
500
20 0
0 100
200
300
400
Time (Days)
Time (Days)
286
1500
HPI TPIA HPOA
0
80 13 0 22 0 40 5
22
7
15
4
1
0
100
HPI DOC mg/L
Percent of Total DOC
100
TPIA or HPOA DOC (mg/L)
A
287
Figure 3: DOC fractionation of produced waters over time by XAD resins 4 and 8 (HPI,
288
hydrophilic fraction; TPIA, transphilic acids; HPOA, hydrophobic organic acids).
289 290
UV absorbance at 254nm (SUVA254) and fluorescence were measured on all of
291
the samples.50 Results are presented in SI Table 3 and SI Figure 1. The results for UV
292
absorbance at 254nm show decreasing absorbance over time, while SUVA254 increases
293
slightly over the course of the study. As UV absorbance at 254nm is generally linked to
294
aromaticity, it is likely that SUVA254-rich formation-based aromatic constituents (e.g.,
295
benzene) not solely linked to the original fracturing fluid constituents were introduced.
296
That is, the UV 254nm values do not decrease proportionally to the DOC over time
297
(SUVA254), suggesting that a portion of the DOC that absorbs highly at 254nm could be
298
from the formation. Excitation emission matrices were generated from the fluorescence
299
data (SI Figure 1). The discussion of these results is presented in the SI, along with SI
300
Figure 1.
301 302
3.2. Characterization and environmental implications of Volatile Compounds
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The determination of specific volatile compounds and their environmental
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implications are presented in this section. Sixty-five volatile organic compounds (VOCs)
305
were determined in all of the samples from the pre-injected fracturing fluid to produced
306
water recovered on day 405. Selected results are presented in Figure 4, which shows
307
VOC concentrations over time (concentrations of VOCs and their reporting limits are
308
presented in SI Table 4). The groundwater, which was stored on site, contained three
309
VOCs: 2-hexanone (312 µg/L), acetone (44.7 µg/L), and benzene (8.08 µg/L). Their
310
presence could be the result of chemicals (i.e. biocides, preservatives) added to the
311
storage container, produced water previously stored in the tank, the groundwater, or from
312
these volatile chemicals being transported (i.e., air) to the open storage tank during
313
drilling operations.
314 315
The pre-injected fracturing fluid contained several VOCs including 2-butanone
316
(10.7 mg/L), acetone (230 mg/L), bromomethane (0.07 mg/L), and chloromethane (0.03
317
mg/L). Acetone has been detected in flowback and produced water, as well as ambient air
318
around oil and gas wells, and is thought to be a common solvent used in cleaning
319
purposes.30,35,51 2-butanone has also been previously reported in produced water, and is
320
thought to be used as a friction reducer or industrial solvent.35 Chloromethane has been
321
described as a fracturing fluid additive, but has yet to be detected in produced water.9,52
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Neither chloro- or bromomethane were detected in any produced water samples,
323
suggesting that they either transform downhole or sorb to the formation and particles in
324
the borehole.14
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3.2.1. BTEX and Substituted Benzenes
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Benzene, toluene, ethylbenzene, and xylene (BTEX) were found in the mg/L
328
range in every produced water sample analyzed (Figure 4A), and are frequently detected
329
in produced waters and oil and gas impacted waters.14,29,30,35 Ethylbenzene and xylene
330
concentrations appear to decrease overtime, after day 22, while toluene decreases until
331
day 220 then increases on day 405. Benzene concentrations varied over the course of the
332
study with no particular trend. Benzene concentrations ranged from 6-12 mg/L, with the
333
lowest concentration being detected on day 220, while the second highest concentration
334
was detected on day 405 (Figure 4A). The high concentrations of ethylbenzene and
335
xylene in the early samples that peaked at day 22, suggest that BTEX compounds could
336
have been constituents of fracturing fluid additives (e.g. components of petroleum
337
distillates), even though they were not detected in the pre-injected fracturing fluid
338
sample. Nonetheless, the high concentrations observed for BTEX over the course of the
339
study suggest that substantial amounts are introduced from the formation, probably from
340
the oil in this formation, which would be anticipated for water in contact with an oil-
341
bearing shale. These concentrations are an order of magnitude lower than that recently
342
found in the Permian Basin produced waters (average concentration = 107 mg/L, n=8)29,
343
whereas other oil-bearing formations have shown high variations in reported benzene
344
levels from 0.026 to 4 mg/L.53–56
345 346
Benzene concentrations measured for all of the produced waters in the time series
347
studied are of environmental concern in the event of a spill/release, or if the produced
348
waters are to be used for applications other than recycling for future hydraulic
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fracturing.29,57,58 These environmental concerns are the result of benzene concentrations
350
being orders of magnitude higher than both the maximum contaminant level (MCL;
351
0.005 mg/L) and discharge permit levels used in produced water treatment facilities
352
(Pennsylvania [PA], U.S.).17 Toluene was also present in concentrations greater than its
353
MCL (1.0 mg/L) and PA discharge permits for all of the produced waters in the time
354
series studied.17 Ethylbenzene was greater than its MCL (0.7 mg/L) for days 7, 22, and
355
80, while xylenes were never greater than its MCL (10 mg/L total xylenes).
356
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Xylenes, total Toluene Ethylbenzene Benzene
A 15
mg/L
10
5
0 0
100
200
300
400
Time (Days)
B 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene Isopropylbenzene sec-Butylbenzene
2.5
mg/L
2.0 1.5 1.0 0.5 0.0 0
100
200
300
400
Time (Days)
C 2.5
2-Hexanone 2-Butanone 4-Methyl-2-pentanone
mg/L
2.0 1.5 1.0 0.5 0.0 0
100
200
300
400
Time (Days)
357 358
Figure 4: Concentrations of BTEX (A), select methylbenzenes (B), and select ketones (C)
359
in produced water samples over time (day 1 to day 405).
360 361
Substituted benzenes were found at or near mg/L concentrations during the early
362
stages of the well (day 1 to day 80), and were above the limit of detection until day 220 18
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(Figure 4B). These four substituted benzenes all have the same trend, with concentrations
364
increasing until day 22 and then decreasing until day 200 and being below the detection
365
limit by day 405, suggesting they are likely associated with fracturing fluid additives.
366
1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and isopropylbenzene have been
367
described as fracturing fluid additives, and have also been detected at the µg/L range in
368
produced water from both the Barnett and Marcellus shale.9,35,59 1,2,4-trimethylbenzene
369
has been described as a surfactant, corrosion inhibitor, and friction reducer and has also
370
been reported in surface water near treatment facilities that receive oil and gas fluids in
371
Pennsylvania.17 Both 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene were also
372
recently reported in monitoring wells near a hydraulically fractured well in Wyoming.60
373
Substituted benzenes have been shown to be irritants (i.e. eye, skin, and respiratory)61,
374
although no MCLs or oral reference doses exist for these four compounds; however a
375
recent study did quantify their potential toxicity using a quantitative structure-activity
376
relationship model.62 Therefore, the environmental impacts associated with substituted
377
benzenes are not well understood and require further research.
378 379
3.2.2. Ketones
380
Numerous ketones were at the mg/L level for portions of the study. Acetone was
381
the ketone detected at the highest concentration on day one (22 mg/L) and was detected
382
over the course of the study with concentrations ranging from 0.6 mg/L on day 7 to 9
383
mg/L on day 405. The levels of acetone over the course of the study are likely due to its
384
high concentration in the fracturing fluid (230 mg/L), meaning it would take a substantial
385
amount of time to dilute (i.e., formation water), sorb, or biodegrade acetone from the
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returning produced water. Also the polar nature of acetone would support its presence in
387
the water fraction for an extended period of time, whereas non-polar or mid-polar
388
compounds would partition into the oil. Acetone has been detected in numerous produced
389
waters, been found to form during biodegradation of fracturing fluid additives, and has
390
been reported as a fracturing fluid additive.9,30,35,41,52 Acetone has been described as “less
391
toxic” than other industrial solvents,61 and although it does not have an MCL, it has an
392
oral reference dose (RfD) of 0.9 mg/kg/day, which is more than three times that of the
393
total xylenes RfD (0.2 mg/kg/day). Total xylenes has an MCL of 10 mg/L.
394 395
The other ketones detected were 2-hexanone, 2-butanone, and 4-methyl-2-
396
pentanone (Figure 4C). 2-Hexanone was detected at its highest concentration on day 1 (1
397
mg/L) and was last detected on day 130 (0.1 mg/L), suggesting that it could have been
398
formed in the subsurface or from an unknown source. 2-hexanone has not previously
399
been reported in produced water or as a chemical additive used in hydraulic fracturing,
400
thus supporting the notion that it was formed during a subsurface reaction, potentially due
401
to the use of tert-butyl peroxide, which is a strong oxidant. 2-hexanone was historically
402
used in paint, paint thinner, and various chemical processes.63 However, because 2-
403
hexanone was determined to have negative health effects as a neurotoxin, it now has
404
restricted uses in the U.S.63 It has an RfD of 0.005 mg/kg/day, which is the same RfD
405
value as benzene and dichloromethane.64 Thus, 2-hexanone is a chemical of concern
406
because it was found in the produced water samples for at least 130 days.
407
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408
2-butanone was detected at 10 mg/L in the fracturing fluid, but was not detected
409
until day 80 in the produced waters samples. This is different than acetone, which was
410
detected at high levels in the fracturing fluid and was also detected in the day 1 sample.
411
These unexpected results for 2-butanone could reflect subsurface reactions. That is, 2-
412
butanone could be yielding 2-hexanone in the early stages of the well, because 2-
413
butanone was detected in the fracturing fluid and appears to increase over time while 2-
414
hexanone is decreasing (Figure 3c). However, the addition of an ethyl group to 2-
415
butanone seems unlikely, even in the presence of a strong oxidant (i.e. tert-butyl
416
peroxide). Nonetheless, 2-butanone appears to be a fracturing fluid additive, even with
417
its unanticipated levels over the course of the study because it was detected in the
418
fracturing fluid, reported as a fracturing fluid additive,9 and has also been detected in
419
produced water and waters impacted by produced water.30,60
420
pentanone was at or near mg/L levels from day 1 until day 22, but was not detected by
421
day 80. This suggests that it was likely associated with the fracturing fluid and not the
422
formation. 4-methyl-2-pentanone has been described as a fracturing fluid agent, and is a
423
common solvent used in industrial processes.9 2-butanone (RfD of 0.6 mg/kg/day) and 4-
424
methyl-2-pentanone (reference concentration for inhalation exposure of 3 mg/m3) have
425
been shown to effect developmental and musculoskeletal systems.64 This could support
426
monitoring for both chemicals in the event of a spill, particularly 4-methyl-2-pentanone
427
in the early stages (i.e. first few weeks) of the well and 2-butanone at the later stages (i.e.,
428
first year).
429 430
3.3. Semi-Polar and Non-Polar Organic Compounds
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431
The analysis for specific polar organic compounds and their environmental
432
implications are presented in this section. The UHPLC-Q-TOF-HRMS total ion
433
chromatograms (TIC) for the pre-injected fracturing fluid and five produced water
434
samples are shown in Figure 5. The peaks visible in the TIC are dominated by three
435
classes of homologues, which have been previously identified in produced waters (Figure
436
5): polyethylene glycols (PEGs)3, polypropylene glycols (PPGs)65, and linear alkyl
437
ethoxylates (LAEs).3 These three homologous compounds were identified in the
438
fracturing fluid and the produced water samples, and generally decreased in magnitude as
439
the produced waters age increased (Figure 5). During hydraulic fracturing, surfactants are
440
added to aide in the pumping of fracturing fluids and the movement of sand and oily
441
substances.30 The surfactants are thought to be potential fingerprinting compounds, since
442
they are identified so frequently in produced waters and waters impacted by them.3,17,65,66
443
Their substantial signal in both the fracturing fluid and produced waters, along with their
444
presence over the course of the study supports this notion, specifically for PEGs and
445
PPGs. It should also be noted that PEGs were detected in the groundwater samples from
446
3.5 to 11.5 min (SI Figure 2), and could have been the result of biocide addition in the
447
tank, since they were added to prevent microbial growth during storage; although they
448
could have also been from flowback and produced water that were mixed or previously
449
stored in the holding tank.
450
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451 452
Figure 5: Total ion chromatogram (TIC) of five produced water samples over 220 days
453
and the pre-injected fracturing fluid (insert above other TICs). Note the inserted
454
fracturing fluid TIC is compressed to fit the figure and is not to scale on the y-axis
455
(relative abundance). Polyethylene glycols (PEGs), polypropylene glycols (PPGs), and
456
linear alkyl ethoxylates (LAEs) are denoted in the figure, along with triisopropanolamine
457
(TIPA). The x- and y-axes represents the acquisition time and the relative size of the
458
peaks (counts), respectively. Samples were analyzed as received (< 48 hours of
459
sampling). It is important to note, that there are numerous other compounds present in
460
these samples and in the regions identified (as PEGs, PPGs, LAEs, and TIPA), and that
461
these identifications are shown to illustrate the major ions present and not the total
462
compounds present.
463 464
The PEGs were detected from 4.4 min to 14.0 min (more hydrophilic zone). The
465
PEGs showed increasing ethylene oxide (EO) units, from PEG-EO4 to PEG-EO16 with
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accurate masses of 217.1050 m/z (Na+ adduct; retention time 4.4 min; 2.0 ppm error) and
467
740.4645 m/z (NH4+ adduct; retention time 12.4 min; 1.0 ppm error), respectively
468
(sample, day 1). The average mass difference between PEGs was 44.0262 mass units,
469
representing the addition of one ethylene oxide group [-CH2-CH2-O-]. The next class of
470
compounds was PPGs, which were detected from 14.6 min to 22.2 min and show
471
increasing propylene oxide units from PPG-PO4 to PPG-PO12 with accurate masses of
472
273.1673 m/z (Na+ adduct; retention time 14.6 min; 0.2 ppm error) and 732.5470 m/z
473
(NH4+ adduct; retention time 22.2 min; 0.5 ppm error), respectively (sample, day 1). The
474
PPGs had an average mass difference of 58.0419 mass units that is equal to a propylene
475
oxide group [-CH2CH(CH3)-O-].
476 477
The LAEs were detected from 22 min to 26.8 min and show four different classes
478
of LAEs that have varying carbon chain lengths (C9, C10, C11, and C12). This work
479
describes the first putative identification of LAEs in produced water with carbon chain
480
lengths of 9, 10, and 11 (SI Table 5), whereas the 12 carbon chain length LAEs have
481
previously been described.3 The LAEs had an average mass difference of 44.0262 mass
482
units that is equal to an ethylene oxide group [-CH2-CH2-O-]. The LAEs included over 25
483
unique putatively identified compounds, from LAE9-EO13 to LAE12-EO9, which had
484
accurate masses of 734.5266 m/z (NH4 adduct; retention time 22 min; 0.8 ppm error) and
485
600.4688 m/z (NH4 adduct; retention time 26.8; 1.2 ppm error), respectively (samples,
486
day 1 and day 15). It should be noted that individual standards are not available for PEGs,
487
PPGs, and LAEs, with only homologue mixtures available at this time, with these
488
compounds being identified and semi-quantitated.
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489 490
PEGs, PPGs, and LAEs are common products in personal care products,
491
medicines, foods, and industrial processes.65 For instance, PEGs are used in laxatives,
492
PPGs are used in ice cream, and LAEs are often found in detergents.65 Their use in
493
commercial products that humans frequently ingest or are in contact with suggests they
494
are not acutely toxic; however, the extent of polymerization is crucial when considering
495
toxicity.65 PEGs of high molecular weight have been described as inert and non-toxic,67
496
while the monomer ethylene glycol has known human toxicity.68 PPGs have been shown
497
to have a similar relationship to that of PEGs, with polymers or high molecular weight
498
PPGs being considered safe,69 while the monomer propylene glycol is considered toxic.70
499
Similarly, LAEs are considered harmless as a polymer.71,72 However, the monomer
500
ethylene glycol can be formed, because the LAEs studied here can have up to 13
501
repeating ethylene oxide groups that are attached to a carbon chain (SI Table 5).68
502
Therefore, consideration of whether the monomer is being formed is a key research
503
question, especially considering the use of strong oxidants as breakers, high temperatures
504
and pressures in the well, and biological processes in the subsurface that could lead to
505
monomer formation. It should be noted that the monomers discussed here are not
506
detectable in our UHPLC-Q-TOF-HRMS, and should be investigated in future research
507
of produced waters.
508 509
The compound triisopropanolamine (TIPA) was found in every sample from the
510
fracturing fluid to day 405. The peak can be seen in Figure 5 around 2.8 min. TIPA was
511
identified by its accurate mass of 192.1594 m/z, corresponding to the protonated molecule
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512
and elemental formula C9H21NO3. TIPA was further confirmed by retention time,
513
accurate mass, and by performing MS-MS on a pure standard that validated the putative
514
identification, with major fragments at 174.1494 m/z and 156.1383 m/z, (SI Figure 3).
515
The resulting concentration appears to be in the mg/L range for all of the samples
516
analyzed, with relative intensities comparable to the 1 mg/L standard.
517 518
TIPA is a tertiary amine, which is typically used as an emulsifier, stabilizer, and
519
surfactant during industrial processes. It is also frequently used in cements as a hardening
520
accelerator.73 TIPA has been reported as a fracturing fluid additive11,52 but had yet to be
521
reported in a produced water sample. TIPA is an industrial agent that has been described
522
as a moderate irritant for skin and a severe irritant for eyes,61 but has been shown to be
523
non-toxic or genotoxic in in-vitro bioassays.74
524 525
3.3.2. Surfactant Levels Over Time
526
Figure 5 and 6 illustrate the substantial changes in concentration of PEGs, PPGs,
527
LAEs, and TIPA in the produced water samples over time. All three surfactants
528
decreased in line with their hydrophobic nature (octanol-water partition coefficient; Kow),
529
i.e., the most hydrophobic LAEs decreased most rapidly followed by the PPGs, and then
530
PEGs.75 Because a quantitative measure would be challenging due to matrix-induced
531
ionization effects and specific response factors for these types of compounds,17 relative
532
abundance was used as a way to compare these compounds levels over time (i.e., semi-
533
quantitative).
534
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535 536
Figure 6: Relative abundance (concentration at time, t / original concentration; Ct/Co) of
537
the highest detected surfactants from each class (PPG-PO6, PEG-EO6, and LAE11-EO9),
538
and TIPA over the course of the study from the day 1 sample to day 405.
539 540
Hydrophobicity, as expected, is a key indicator for what injected chemicals are
541
present in the produced waters over time. This is demonstrated by the identified
542
compounds’ Kow values and their elution time through the UHPLC, because the least
543
hydrophobic compounds elute from the column first, while the most hydrophobic
544
compounds elute last. The PEGs are the least hydrophobic surfactants with an estimated
545
log Kow of -2.35 earliest elution time and the greatest relative abundance (Figure 5) over
546
the course of the study. The PPGs had the second highest concentrations among the
547
samples analyzed, have an estimated log Kow of -0.25, and eluted after the PEGs. In
548
contrast, LAEs, for which the concentrations decreased the fastest, have an estimated log
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549
Kow of 2.4,5 the highest of the surfactants studied, and also eluted the latest. These results
550
suggest the LAEs are likely partitioning into the oil phase or sorbing to the formation or
551
particles in the wellbore (LAE estimated log Koc= 1.45 [carbon-normalized sorption
552
coefficient]), which could explain the rapid removal from the water phase of the LAEs
553
compared to PPGs and PEGs. Conversely, the PEGs and PPGs, both with lower Kow
554
values, indicating that they will remain in the aqueous phase rather than partitioning into
555
the shale or oil phase, were detected in the produced water throughout the study period
556
(Figure 5). This hypothesis is further supported by TIPA, which remained at the highest
557
concentrations over time and had an estimated log Kow of -1.25 (Figure 5). Although its
558
estimated Kow is slightly higher than the PEGs, TIPA’s hydrophilic nature is illustrated
559
by its early elution time in the TIC (2.8 min) compared to PEG-EO6 (9.5 min) or PEG-
560
EO4 (4.4 min), which further supports the role hydrophobicity plays on returning
561
chemicals.
562 563
These results show how surfactants change over time, in terms of what class of
564
compounds might be present and how their concentrations change. The results also
565
support the importance of a chemical’s hydrophobic nature, and how that dictates when
566
the chemical will return in an oil-bearing formation. This is important when considering
567
the chemicals to target as tracers in the event of a spill or water reuse scenario,
568
particularly for an oil and gas containing formation. That is, the water and oil are
569
constantly mixing in the borehole until they reach the separator, providing substantial
570
time for compounds to partition from the water to the oil. These findings coupled with the
571
presence of PPGs, PEGs, and TIPA over the course of the study, illustrate three indicator
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572
chemicals that are unique to the water phase and that were found in the fracturing fluid
573
used (Figure 5). TIPA in particular could be of importance, due to its hydrophilicity, that
574
would result in TIPA being in produced waters for extended periods of time, along with
575
TIPAs non-ubiquitous nature compared to many surfactants (i.e. PEGs).
576 577
Overall, this study provides a unique organic characterization that fills a
578
substantial knowledge gap; that much of the injected DOC remains in the subsurface (on
579
a mass balance basis), and that the returning DOC is primarily hydrophilic based on the
580
return of the hydrophilic compounds, either volatile or non-volatile, over the 405-day
581
study period. Furthermore, this work newly-identified persistent compounds that remain
582
for long periods of time, and that have been found in produced waters around North
583
America or effluent discharges from a produced-water treatment facility.3,17,30,65,66 Future
584
research should continue to characterize the organic composition of produced waters (e.g.
585
in general and over time), with particular attention on hydrophilic organic compounds
586
and the suspended solids fraction, while work is also needed to characterize chemical or
587
biological degradation products of these specific constituents, and expound on the
588
physical mechanisms that impact the returning compounds.
589 590
Supplemental Information
591
Supplemental information includes tables and figures for the wells FracFocus report,
592
mass balance values, DOC and non-specific characterization, VOCs analyzed and
593
detected, Kendrick masses for LAEs, excitation-emission matrixes, the total ion
594
chromatogram of the groundwater used to make the fracturing fluid, and MS-MS spectra
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595
for Triisopropanolamine. An Excel file includes all data generated by the USGS
596
presented in SI Table 3 and Figure 3. At the time of publication, data other than
597
presented in this paper are not publically available from the University of Colorado.
598 599
Acknowledgements
600
This work was supported by the AirWaterGas Sustainability Research Network, and was
601
funded by the National Science Foundation under Grant No. CBET-1240584. Opinions,
602
findings, recommendations, and conclusions conveyed in this paper are those of the
603
author(s) and do not necessarily reflect the views of the National Science Foundation.
604
Any use of trade, firm, or product names is for descriptive purposes only and does not
605
imply endorsement by the U.S. Government. The researchers would also like to
606
acknowledge Dr. Jessica Rodgers for her review of the manuscript, Dr. Avner Vengosh
607
for his insight on our data set, and our industry collaborators that permitted access and
608
took the time to help us collect the samples. The authors dedicate this paper to the
609
memory of George Aiken, colleague, mentor, and friend.
610 611 612 613
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