Repository for Oil and Gas Energy Research (ROGER)

Abstract:
Multivariate compositional data analysis methods were used to investigate geochemical data for water injected during hydraulic fracturing and for water produced from 19 Marcellus Shale gas wells in the northern Appalachian Basin. The data were originally published as part of an industry report. The analysis was adapted to consider the compositional nature of the data and avoid potentially spurious correlations present in raw concentration data through the application of log-ratio transformations. Techniques such as robust variation arrays, robust principal component analysis, and relative variation plots were applied to log-ratio transformed data. Results from this battery of multivariate tools indicate that two primary processes affect the chemical evolution of the water returned to the surface during the first 90 days of production: mixing of injected water with formation brines of evaporated paleoseawater origin and injection of sulfate-rich water during hydraulic fracturing may stimulate sulfate reduction at some sites. Spatial variability in sulfate/alkalinity ratios appears to influence variations in geochemical controls on strontium versus barium with elevated proportions of strontium being found in more bicarbonate-poor environments, while barium is a larger proportion in sulfate-poor areas. Comparison of results using a log-ratio approach versus the more common analysis of concentration data reveals both similarities and some marked differences in the resulting interpretations. Results from this work are important in terms of both demonstrating methods to avoid mathematical inconsistencies from using raw brine geochemical data and to further investigate the geochemical controls on produced waters generated from shale gas reservoirs.

Abstract:
Trace levels of arsenic and selenium can be toxic to living organisms yet their quantitation in high ionic strength or high salinity aqueous media is difficult due to the matrix interferences which can either suppress or enhance the analyte signal. A modified thiol cotton fiber (TCF) method employing lower flow rates and centrifugation has been used to remove the analyte from complex aqueous media and minimize the matrix interferences. This method has been tested using a USGS (SGR-1b) certified reference shale. It has been used to analyze Marcellus shale samples following microwave digestion as well as spiked samples of high salinity water (HSW) and flow back wastewater (WRF6) obtained from an actual gas well drilling operation. Quantitation of arsenic and selenium is carried out by graphite furnace atomic spectroscopy (GFAAS). Extraction of arsenic and selenium from Marcellus shale exposed to HSW and WRF6 for varying lengths of time is also reported.

Abstract: