2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 161-2
Presentation Time: 1:35 PM


PRATT, Dyan L.1, HENDRY, Jim2, BARBOUR, Lee3 and SCHMELING, Erin2, (1)Civil & Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N5A9, Canada, (2)Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada, (3)Department of Civil, Geological and Environmental Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada

Methane and other gaseous hydrocarbons cause interference in the spectroscopic measurement of stable water isotopes. A study has been undertaken to assess and quantify the use of various pre-treatment options for stripping C1-C6 gases in samples and include: 1) catalytic oxidation using CuO and high temperature via a tube oven; 2) coupling a Nafion gas drying system to remove/separate water vapour from hydrocarbon gas; 3) activated carbon system to absorb C1-C6 hydrocarbons. Initial testing was undertaken using a tube oven operated at three temperatures (850, 925, 1000° C) with samples containing 5 concentrations of methane (0 to 4.4%) as well as 5 concentrations of C1-C6 mixture (0 to 5%-CH4) which were vapour equilibrated with two isotopic reference standards (SK-T and BIR). Water isotope measurements (δ2H and δ18O) were recorded on a Picarro L2120i. Preliminary results show a 99-100% reduction of methane and all C1-C6 hydrocarbons at 925° C and 1000° C. The used of the CuO catalyst does not provide reliable δ18O values due to additional oxygen entering into the vapour stream. C1-C6 mixtures appear to fluctuate more than just CH4 in vapour in the less depleted waters. CH4 only vapour appears to plot closer to the standard water values for δ2H, for SK-T specifically, isotope measurements are approximately 16‰ more depleted, and for BIR standard waters, about 10 ‰ more enriched. This worsens with the addition of C1-C6 gasses, with SK-T measuring 20-60 ‰ more depleted and BIR measuring 20 ‰ more depleted after concentrations exceed 2.5% (CH4). Further refinement of this in in progress. Pretreatment of vapour was also performed with a PermaPure Nafion gas drying tube at four temperatures (70, 80, 90 & 100° C). The Nafion tube removed 98-100% of the methane but only provided a peak concentration of water vapour of approximately 800 ppmv, far below the requirement for a stable isotope measurement on Picarro. Measured isotope values plotted inconsistently with no correlation to the actual standard water value and consequently this method was considered unacceptable. The final pre-treatment option consisted of an activated carbon inline filter. The activated carbon was successful in absorbing methane, but also absorbed all water vapour as well, causing a null measurement on the Picarro and due to an unacceptably low H2O-ppmv of 300.