Analysis of Ions in Hydraulic Fracturing Wastewaters Using Ion

Chapter 6

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Analysis of Ions in Hydraulic Fracturing Wastewaters Using Ion Chromatography C. A. Fisher*,1 and R. F. Jack2 1Thermo

Fisher Scientific, 1214 Oakmead Parkway, Sunnyvale, California 94085, United States 2Thermo Fisher Scientific, 490 Lakeside Dr., Sunnyvale, California 94085, United States *E-mail: [email protected]

The quantity of water required for hydraulic fracturing puts considerable stress on this often scarce resource. As a result, well operators are increasingly recycling wastewater for additional fracturing cycles. The ion content can be used to guide the treatment strategy and indicate adjustments needed to the mix of fracturing fluid additives to optimize recovery. Organic acids and inorganic anions modulate pH, induce corrosion, and can be precursors to disinfection by products. Cations are primarily indicative of the propensity for scale formation, which can be countered by additional treatment or an increase in anti-scaling agents. Ion chromatography (IC) is the primary technique for ion analysis and uses ion-exchange chromatography to separate ionic species, which are then detected by conductivity. A challenge of wastewater analysis is the high salt content, which is overcome by sample dilution. In flowback, chloride was the predominant anion, followed by bromide, with a steady increase in concentration as more fluid was recovered. There was a similar increase in cations with sodium dominating, followed by calcium, strontium, and magnesium. Produced water had a comparable pattern of relative ion concentrations. Significant variability resulted from analyzing samples from different locations, indicating that geology plays a prominent role in determining ionic content.

© 2015 American Chemical Society In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Introduction Hydraulic fracturing has been used for decades in the U.S., although it has increased considerably in the last decade spurring rapid growth in the extraction of oil and gas from shale deposits. This process consists of drilling a well vertically down several thousand feet (thousands of meters) to a layer of hydrocarbon-rich shale and then horizontally for a mile (1.6 km) or more. Fracturing fluid is then injected under high pressure through perforations in the horizontal well casing to fracture the adjacent shale, releasing the natural gas and oil trapped there. The liquid portion of hydraulic fracturing fluid is composed of approximately 99% water with the remainder consisting of chemical additives. Sand is added to this fluid as a proppant to keep open the cracks that are formed, thereby facilitating oil and gas recovery. To optimize recovery, additives, such as friction reducers and scale inhibitors (1), are tailored to the site’s geology and the chemical characteristics of the water used (2). Following the release of pressure, the fluid that returns to the surface is referred to as flowback water, which is then pumped into lined storage ponds or tanks prior to recycling or disposal. In Pennsylvania (the state with some of the most extensive hydraulic fracturing activity) an average of 10% of the fluid injected is recovered as flowback (3), although, in some cases, recovery is as low as 4%. One concern of this low recovery is the possibility of leakage from sites of injection into the aquifer. Despite this low recovery, it is unlikely that residual fracturing fluid will eventually migrate to and contaminate overlying groundwater (4). Once gas or oil appears, marking the start of production, the water recovered is referred to as produced. This water contains some residual fracturing fluid, but will consist primarily of water that was present within the shale layer prior to fracturing (formation water). While hydraulic fracturing wastewater can be disposed of by injection into disposal wells, this practice is expensive because water typically is transported to locations that are distant from the hydraulic fracturing site and there is evidence that deep well injection may cause earthquakes (5), raising public fears and opposition to this activity. To address these concerns, wastewaters are increasingly being desalinated and treated for reuse, with the additional benefit of a reduction in the demand for local water (6). At each hydraulic fracturing location, a comprehensive water management plan should be established to optimize usage and minimize the impact on local water quality (7, 8). There are several points within the water use cycle at which an assessment of water quality would be beneficial (Figure 1). Regular monitoring of the quality of ground/well water in the vicinity of the hydraulic fracturing site could act as an early warning if contamination occurs as a result of hydraulic fracturing activities and can provide some assurance to local residents of the safety of this water. Assessing local water quality prior to the start of any hydraulic fracturing activities will also provide a baseline to which any values obtained post-hydraulic fracturing can be compared. In addition to liquids, there are also solid (e.g. sediments) and gaseous samples that are produced at a hydraulic fracturing site, which require multiple instruments to provide a comprehensive analysis of constituents, measuring properties such as salinity, radioactivity, and hydrocarbon composition (Figure 2). 136 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 1. Water use cycle in hydraulic fracturing. (see color insert)

Figure 2. Hydraulic fracturing workflow monitoring. AAS, atomic absorption spectroscopy; ICP-OES, Inductively Coupled Plasma Optical Emission Spectroscopy; MS, Mass Spectrometry; HR, high resolution; IRMS, Isotope Ratio Mass Spectrometry; MC, multicollector; LC, liquid chromatography; MS/MS, tandem MS; CAD, charged aerosol detector; GC, Gas Chromatography; GM, Geiger-Müller; NaI, Sodium Iodide detector; TDS, total dissolved solids; DO, dissolved oxygen ; HF, hydraulic fracturing. 137 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Not only is it a best practice in support of responsible hydraulic fracturing, as noted above, but ongoing site monitoring can be mandated by local regulations. Such monitoring can include samples collected from the recovered liquids as they emerge from the well and any settling ponds or tanks used for wastewater storage, in addition to adjacent surface and ground waters. Natural gas (primarily methane, a greenhouse gas) can be inadvertently released at hydraulic fracturing sites and should be monitored to ensure minimal impact on the environment (9). In the event of an inadvertent wastewater spill, the concentration of metals, cations, anions, surfactants and the amount of radiation present can be used to draft an effective response plan to minimize impact on the surroundings. To track the origins of released wastewater the ratio of isotopes, such as 87Sr/86Sr (characteristic of geological formations) (10), or identification of specific surfactants (11) can be used as tracers. Another potential hazard to human health is radiation derived from naturally occurring radioactive material (NORM), which can be monitored by measuring 226Ra, a major NORM component and a proxy for this radioactivity (12). In addition to preparedness in the event of any accidental release during storage or transport, a comprehensive analysis of wastewater composition can be used to determine its suitability for immediate reuse in additional hydraulic fracturing events (adjusting the additive blend to optimize gas and oil recovery) or to devise an efficient treatment strategy prior to other beneficial uses (such as crop irrigation or road deicing) (13). Sediments and brines resulting from treatment can also be evaluated prior to disposal as part of a responsible management plan. This chapter will focus on those analytes that can be measured using IC, which include inorganic anions, cations, and organic acids. Ion Chromatography The primary analytical technique used to determine ionic species in solution is IC. In this technique, a sample is injected onto a column containing resin modified with ion exchange groups for which the ions in solution have varying affinities based on their physical properties (charge, shape, hydrophobicity, size) (Figure 3) (14). Eluent at either a steady (isocratic) or increasing (gradient) concentration displaces bound ions. Following separation, the eluent passes through a suppressor module in which the background conductance of the eluent is reduced, while the electrical conductance of the analyte ions is enhanced. The conductivity of the solution is then measured by a conductivity detector (CD) and plotted versus time to produce a chromatogram of the analytes present in the sample. The conductivity is directly proportional to the analyte concentration. A standard curve is created from the peak areas of a mix of ion standards at several concentrations that is then used to quantify the analytes present in a sample. Eluents can be prepared manually or, as shown in Figure 3, automatically using an eluent generation cartridge in which a specified concentration of eluent is produced by electrolysis of water. Immediately following this cartridge is a continuously regenerated trap column (CR-TC) that removes any ionic contaminants in the eluent stream. Additional IC innovations that have facilitated 138 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


analysis include: the development of 4 µm resin particles that deliver higher resolution than previously available particles (~7 µm) (15); higher pressure systems, which allow the use of faster flow rates for shorter run times; and Capillary IC, which scales down injection volumes, column size, and flow rates so that less sample and reagent are used and less waste is generated (16).

Figure 3. Overview of a Reagent-free Ion Chromatography system. CR-TC, continuously regenerated trap column. (see color insert)

IC with suppressed CD has several advantages (14): in contrast to methods that rely on UV/Vis detection, there is no need for a chromophore to be present on the molecule of interest nor is there interference due to absorption of matrix components in the UV/Vis spectrum; multiple analytes can be detected in a single run; sample preparation is typically minimal, requiring only filtration and dilution; the column chemistry chosen dictates what species are retained, allowing separation to be optimized for a specific set of analytes and elimination of other matrix components in the void volume; large disparities in analyte concentrations can be tolerated (e.g. 10,000:1 sodium:ammonium); relatively high salt samples can be directly injected due to the availability of high capacity columns; additional data can be collected with downstream detectors because CD is non-destructive. Some of the limitations of IC with CD include: only charged (ionic) molecules are directly detected. Post-column reactions in combination with UV/Vis detection can be used to extend the range of analytes detected although each of these reactions is for a single, specific analyte; runs range from 10–30 min, although shorter run times can be achieved if the conditions are altered to focus on a smaller subset of ions; because column separation is optimized for singly and doubly charged molecules, highly charged molecules (≥ 3) are tightly retained 139 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

under standard conditions resulting in long retention times. Reaction conditions or column chemistry can be modified to favor these species, but at the expense of less tightly bound ions; definitive analyte identification requires the use of an additional detection technology, such as mass spectrometry.


Challenge of Wastewater Analysis Low analyte concentration measurements often push the limits of detection using IC, while at the other extreme, samples such as hydraulic fracturing wastewater, can have levels that exceed 150,000 mg/L (2). Such high concentrations necessitate loading less sample volume or diluting prior to injection to avoid exceeding column capacity and to ensure that the concentrations determined fall within the calibration range. The latter is particularly important when regulated methods are used, which typically prescribe a concentration range for each analyte. There are several ways to determine sample concentration and thus the need for dilution or otherwise reducing the amount of sample injected: • •

•

Manual conductivity measurement followed by dilution. This can be tedious, labor intensive, and prone to errors. Automation using chromatography data system software that allows samples to be run undiluted and then, if the peak height or area of analytes in the resultant chromatogram exceeds a predefined limit, the amount of sample injected is reduced before re-analysis by using 1) a partial loop injection, 2) a smaller sample injection loop, or 3) dilution using an autosampler (Figure 4). An advantage of this approach is that the analysis is analyte-specific since each will produce a characteristic peak. A disadvantage is that the dilution is performed post-run, which means that the column is subjected to high concentration samples that can diminish its life. An alternative approach, that saves column life by determining potential overloading before injection, is the use of in-line conductivity measurement. With this approach the sample conductivity is determined prior to injection and, if the conductivity measured exceeds a specified value, the sample is either automatically diluted or less of it is injected.

One of the consequences of sample dilution is that, as samples are diluted, the limits of quantification for the analytes of interest become higher. For all of the analytes discussed here, the levels measured are sufficiently high that any adverse effects of their presence can be adequately assessed.

140 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 4. Software-enabled sample analysis options for determining need for sample dilution (17). (see color insert)

Ion Analysis Anions in Flowback Hydraulic fracturing flowback from the Marcellus shale (Pennsylvania) was obtained from barrels of wastewater that were successively recovered in 500 barrel batches from the first 5000 barrels. i.e. F1 = barrels 1-500, F10= barrels 4500-5000. For IC analysis, the eluent concentration was adjusted to optimize peak separation and then the linear calibration ranges were determined by measuring the peak responses to concentration using triplicate injections of calibration standards. Plotting peak area versus concentration demonstrated linearity for the concentration ranges used with coefficients of determination (r2) > 0.999. Samples were diluted as required so that the concentrations measured fell within the linear range. As exemplified by the chromatogram in Figure 5, chloride was the predominant anion present in all of the flowback aliquots analyzed (and at its highest concentration in this fraction (F10)), while bromide was the second most abundant at ~100-fold lower concentration. The bottom portion of this figure displays an expanded view of the upper chromatogram and shows that low levels of sulfate and the organic acids, acetate and formate, were also detected.



Figure 5. Determination of inorganic anions and organic acids in hydraulic fracturing flowback (F10) (17).

Over time, as the hydraulic fracturing process continues, the anionic content of flowback solutions changes. As can be seen in Figures 6 and 7, chloride and bromide concentrations increased ~10-fold from the first to the second fraction and then, in subsequent fractions, showed a slower, but steady increase. The observed increase in ionic content suggests that the longer fracturing fluid is in contact with the shale layer, the more salt that is mobilized into the flowback and the closer the wastewater comes to having a composition that is virtually identical to that of produced water. i.e., essentially no residual fracturing fluid, with the constituents being dictated by the formation water and the mineral content of the specific shale formation. These elevated concentrations would increase the amount of processing required for water treatment. Conversely, the concentration of acetate decreased sharply and then remained at low levels from fraction two onwards. To address the possibility that the drop in acetate levels was due to a concomitant increase in chloride levels, which may have overloaded the column and reduced its ability to bind other analytes, acetate was added to 100-fold dilutions of fractions five and ten at a concentration 50% of that already present. The recovery of acetate was 114 and 103%, for fractions 5 and 10, respectively. If high chloride levels had decreased the binding of acetate to the column, a lower recovery would have been expected, particularly for fraction 10, which had the highest chloride concentration. Since this was not the case, the large drop in acetate measured indicates a decrease in the amount present in hydraulic fracturing flowback water samples after the first fraction. Organic acids, such as acetate and formate, are added to fracturing fluid to adjust the pH, which is maintained within a narrow range to ensure the effectiveness of fracturing fluid additives (2). In contrast to the other analytes quantified, the level of sulfate remained relatively constant, averaging 13 mg/L. 142 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 6. Concentration of chloride in flowback (17).

Figure 7. Concentration of inorganic anions and organic acids in flowback (17). (see color insert) Anions in Produced Water Produced water was obtained from several locations within the U.S. and were diluted 50-, 200-, and 500-fold for the Texas (TX), California (CA), and North Dakota (ND) samples, respectively, to be within the calibration range and to ensure that the column was not overloaded (Figure 8). Note that the earlier elution times and differing peak order obtained for produced water when compared to flowback (Figure 5) was due to use of an isocratic method for the latter and gradient for the former. The gradient, which started at 15 mM KOH, was used to increase the separation of the earlier eluting organic acids, which facilitated peak integration, and was then increased to 29 mM to ensure that all peaks eluted in less than 12 minutes. Similar to the results for flowback, the predominant anion present in produced water was chloride, followed by bromide at ~200-fold lower 143 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


concentration, and then sulfate. Acetate and formate were present at less than 50 mg/L, while low, but detectable amounts of fluoride were only present in the TX sample.

Figure 8. Determination of anions in produced water (18). (see color insert) The anion concentrations in produced water varied considerably depending on its source with ND having the highest overall values, followed by CA, and then TX. Large differences are also evident when the produced water samples are compared to the ion concentrations that were obtained from Marcellus Shale flowback. For this comparison, fraction 10 was used because it had the highest ion concentrations for the majority of the anions quantified. Chloride was highest in ND, followed by the Marcellus Shale flowback, CA, and TX (Figure 9). It was anticipated that produced water samples would have considerably higher ion concentrations than flowback water because produced water contains a high proportion of formation brine (i.e. water naturally residing within the shale layer). In contrast, flowback can contain a significant amount of fracturing fluid, which is typically low in salts. Consistent with this expectation, sulfate was significantly higher in produced water compared to flowback. While the chloride concentration from the ND sample was almost twice as high as for the Marcellus Shale flowback, the chloride concentrations of the other produced water samples were much lower. The reason for this discrepancy is likely due to comparing flowback and produced water from different hydraulic fracturing locations. It 144 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


has been reported that total dissolved solids from different shale formations can vary by as much as an order of magnitude (19), with considerable variation being found even within shale formations (20). Unfortunately, a direct comparison of flowback and produced water from the same site was not possible due to the lack of appropriate samples. Another factor that can influence results is the initial quality of the water used in the fracturing fluid. Moderately salty (brackish) waters, such as recycled wastewater, are increasingly being used as a starting component of this fluid and, as a consequence, the resultant flowback would likely have significantly higher salt concentrations compared to that obtained if fresh water was used.

Figure 9. Comparison of anions in produced water and flowback (FB) (18). (see color insert) The high chloride concentration of the ND sample indicates that this water would need considerable treatment and/or dilution if it is to be used for additional hydraulic fracturing, whereas the TX water would require much less treatment. As with Marcellus shale flowback, the relatively high bromide concentration of the ND sample points to the need for monitoring of wastewater that is treated for surface water discharge, particularly if this water will be a source of drinking water production, due to the potential for formation of bromate (a regulated carcinogen) (21) during ozonation or by surface water exposure to sunlight. Cations in Flowback As was done for anion analysis, to prevent column overloading and ensure that the analyte concentrations were in their linear range, samples were diluted 100-fold prior to cation analysis. The predominant cation present in flowback was sodium, while calcium was the second most abundant at just under one third that concentration (Figure 10). These were followed in concentration by magnesium, strontium, and potassium. The lower portion of Figure 10 displays a zoomed in view of the chromatogram in the upper portion, which shows that the concentrations of barium, ammonium, and lithium were less than 250 mg/L. In contrast to the conditions used in figures 5 and 8 (4 and 2 mm i.d. columns, respectively), the data shown in Figure 10 was produced using a capillary setup 145 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


(0.5 mm i.d. columns). A capillary IC system uses considerably less water (~100-fold less) and, consequently, generates much less waste, while obtaining data using a sample injection of only 0.4 μL (compared to the 25 µL used with a 4 mm i.d. system).

Figure 10. Determination of cations in hydraulic fracturing flowback (Fraction 4) (22).

Consistent with the results obtained for anions, the concentration of the majority of cations increased approximately 10-fold from the first to the second fraction and then, in subsequent fractions, showed a slower, but steady increase (Figure 11). While most showed a gradual increase, barium had a more dramatic change, more than doubling (from 160 mg/L (F2) to 360 mg/L (F10)). If this wastewater is to be reused for additional hydraulic fracturing events, knowledge of the cations present can be used to optimize the fracturing fluid mixture. The propensity of cations, such as calcium, strontium, and barium to form scale would gradually occlude the cracks that are formed in the shale or build up in pipes used to transport hydraulic fracturing-derived fluids reducing the efficiency of oil or gas recovery. To minimize scale formation, the amount of anti-scaling additive used would need to be increased and/or additional dilution with fresh water would be required.



Figure 11. Concentration of cations in hydraulic fracturing flowback fractions (22). (see color insert)



Cations in Produced Water As with anion analysis, the TX and CA samples were diluted 50- and 200-fold, respectively, while the ND sample was diluted further from the 500-fold dilution to 1000-fold prior to analysis because the ammonium concentrations were higher than the linear range. Samples were analyzed using conditions as shown in Figure 10. Consistent with the trend in anion concentrations, the concentration of cations was highest in the ND sample with sodium being the most abundant, followed by calcium, potassium, ammonium, and magnesium (Figure 12). Low concentrations of strontium and lithium were measured, while barium was not detected. A similar concentration trend was observed for the CA and TX samples, with sodium the highest, followed by calcium and magnesium. There was more than ~2x higher concentration of sodium (the most abundant cation) in the produced water from ND than in the other samples, including the flowback. Marcellus flowback water had considerably higher concentrations of strontium and barium, comparable levels of lithium, magnesium, and calcium, and much lower amounts of ammonium and potassium when compared to the ND produced water sample. While strontium and barium were low or absent in produced water, the relatively high calcium concentrations in the ND sample make recycling more challenging due to its propensity to form scale. An additional concern for any ND produced water treatment plan is the relatively high concentration of ammonium (~3,500 mg/L), particularly if this water is to be treated and discharged to surface water. Ammonium can damage aquatic and terrestrial life directly, in addition to increasing the risk of the formation of toxic disinfection byproducts in drinking water utilities that may be downstream of any treatment discharge. Brine treatment facilities may not adequately remove ammonium as evidenced by the elevated levels (up to 100 mg/L) still present in the effluent from such facilities in Pennsylvania (23) suggesting that particular care should be taken when handling high ammonium wastewaters to minimize the environmental impact. As noted for anions, the differences in cation concentrations of samples obtained from different sites were likely due to the differing geology of the shale formations in which the hydraulic fracturing occurred.

Figure 12. Comparison of cation concentrations in produced water and flowback (18). (see color insert) 148 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Conclusion The concentration of both high and low abundance ions in hydraulic fracturing flowback and produced water can be accurately determined using standard bore or capillary systems with dilutions performed either manually or automatically, with the option of software enabled pre- or post-run analysis. The most abundant anions were chloride, sulfate, and bromide, while sodium, calcium, magnesium, potassium, and ammonium were the most prevalent cations. There was significant variation in ion concentrations when produced and flowback wastewater were compared, although the bulk of the differences seen were attributed to sampling from different geologies. For both types of wastewater, planning treatment and reuse strategies should consider the high concentration of anions, such as chloride, and the scale-forming cations that will dictate the precise mix of additives needed for optimal oil and gas recovery. Additionally, if discharge to surface water occurs following treatment, the potential impact on downstream processes, such as the formation of bromate during drinking water purification, should be assessed.

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Analysis of Ions in Hydraulic Fracturing Wastewaters Using Ion

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