Chemical Technology July 2015

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.

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

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

The presence of chloro- carbons and organobro- mides formed as a conse- quence of using chlor- ine-containing that industry should con- centrate on non-chemical treatments of frac and pro- duced waters. oxidants suggests

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Chemical Technology • July 2015