Isotopic Analysis of Dissolved Organic Carbon in Produced Water Brines by Wet Chemical Oxidation and Cavity Ring-Down Spectroscopy

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1 Isotopic Analysis of Dissolved Organic Carbon in Produced Water Brines by Wet Chemical Oxidation and Cavity Ring-Down Spectroscopy Randal Thomas 1, Christopher Conaway 1, Nabil Saad 2, and Yousif Kharaka 1 1 USGS Menlo Park, CA 2 Picarro, Inc., Santa Clara, CA

2 Hydrofracturing Fracking Background Fracking is the intentional creation of a fracture network within rocks by an increase in the hydrostatic pressure of the subsurface. Powerful pumps and mixing tanks are used to combine fluids and solid proppants as a means to both fracture the rocks or sediment and leave behind a conductive pathway through which hydrocarbon are to be produced.

3 Hydrofracturing Fracking Background The expansion of this technique into relatively shallow and relatively undeveloped hydrocarbon regions has led to conflict between stakeholders of the hydrocarbon resource and the nearby groundwater. Identification of fluid escape from intentionally fracked hydrocarbon systems is an important issue for local, state, and federal regulators across the USA.

4 Hydrofracturing Fracking Background The minor components in the fracking fluids are of greater contaminant concern esp. due to proprietary chemical compositions One consistent concern is that once fracked, the fluid used might be lost into the formation following the fracture or that it might be lost through the well bore during the pressurization process. Since biological processes in the mesothermal subsurface can rapidly alter the organic composition of a fluid, stable carbon isotopes of the dissolved organic compounds are a relatively robust means to identify differences in the origin of two fluids, especially when coupled with inorganic compound analyses Our goal is to develop a CRDS-based analytical method to use δ 13 C of bulk dissolved organic carbon in groundwater as a marker for fracking components in natural groundwater.

5 IEA Report on Unconventional Gas

6 Golden Rules Prediction

7 Las er Cavity ring-down spectrometer (Picarro G2101-i) 50 ml/min Wavele ngth monitor Measurement Cavity Photodetector Outlet gas flow Open split 25 ml/min Wet chemical oxidation TIC/TOC analyzer (OI Aurora 1030) Ultrapure water NDIR Phosphoric acid Persulfate Valve Permeation tubing Sample Syringe Reaction chamber Scrubber Drier N2 Figure 1 Drain

8 TOC-CRDS CRDS

9 Analytical Method Development KHP standards (Potassium Hydrogen Phthalate) were prepared by calibrated pipette and gravimetry to test the optimal threshold of quantification for the TOC-CRDS. A baseline threshold at 350 ppm was selected below which isotopic values in a transient pulse are not integrated. Low detection levels exhibit substantial negative bias; therefore, we chose a threshold that minimizes the chance of biased results. Moreover, the signal-to-noise is a strong function of the sample size. At samples > 50 micrograms of carbon and a threshold of 350 ppmv CO 2 we obtained an accuracy and precision similar to IRMS analyses.

10 CRDS Peak Detection and Integration

11 A B Figure 2

12 A B C Figure 3

13 A B C Figure 4

14 Optimization for brine analysis The wet chemical oxidation method of CO 2 evolution has important drawbacks for brines. In particular, there is a pronounced isotopic effect that increases at higher salinities. At low salinities (1% or below), increased oxidant compositions and reaction times can minimize these effects. For higher salinity samples, brines are passed through a series of AgCl exchange cartridges that lower the chloride to un-inhibitory levels. There is no observed isotope effect of using these cartridges. Using a software patch for the OI TOC analyzer we increased the reaction times with no benefit to the Cl - effect. In general, the isotope effect of the Cl - interference is a function of the reduced recovery of CO 2 following oxidation. We instead analyze our high Cl - samples following an anion exchange column treatment depicted below.

15 A B Figure 5

16 Las er Cavity ring-down spectrometer (Picarro G2101-i) Wavele ngth monitor Measurement Cavity Photodetector Outlet gas flow 25 ml/min Open split 50 ml/min Combustion Module 980 o C Wet chemical oxidation TIC/TOC analyzer (OI Aurora 1030) Ultrapure water NDIR Phosphoric acid Persulfate Valve Permeation tubing Sample Syringe Reaction chamber Scrubbe r Drier Figure 6 Drain N2

17 Wet Oxidation-Combustion Combustion-CRDSCRDS

18 Wet Oxidation-Combustion Combustion-CRDSCRDS Additional % Recovery using Aurora-CM-CRDS vs. Increased [Cl-] baseline adjusted % additional recovery with high temp combustion_corrected_baselineadjust y = x R 2 = wt % Cl-

19 Using 250ppm CO 2 in N 2 Carrier Gas Plot δ 13 C mix vs.1/[co 2 ] mix

20 CRDS Peak Detection and Integration

21 Conclusions LOQ with TOC-CRDS is 50 ug of C Using anion exchange sample clean up gets rid of Cl - interference. However, it s time-consuming and pricey. CRDS signal-to-noise is a strong function of the sample size and the baseline Increasing the baseline CO 2 up into CRDS linear dynamic range will improve sensitivity Wet Oxidation-Combustion-CRDS is a viable alternative to improve % recovery More work planned to evaluate

22 Acknowledgments OI analytical team for software patch Steve Silva USGS Menlo Park Gilles St-Jean University of Ottawa Funding from USGS National Research Program. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.