Chemical Technology July 2015

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 Please contact the editor for this section or go to www.dx.doi.org/10.1039/C4EM00376D to read the original full paper online. 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 10 4 more 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]. 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

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- 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

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.

24

Chemical Technology • July 2015