Characterising organic and inorganic chemical signatures of water associated with shale gas extraction operations

Abstract

During hydraulic fracturing, hydrocarbons trapped in formations flow back to the surface with water used during operations, also called “produced water”. The potential for degraded water quality in the vicinity of extraction operations thereby increases risks to public health via ingestion of contaminated groundwater. Chemical characterisation of produced water provides valuable information to manage water and preserve resources, and to treat and mitigate wastewater in future extraction operations. Currently, the lack of standard methods to quantitatively analyse produced water provides challenges for comparisons of inter-laboratory and inter-basin data. This work addresses some of the knowledge gaps associated with the characterisation of water arising from shale gas extractions operations. A combined protocol for the universal preparation of samples with organic and inorganic constituents in aqueous matrices was achieved using a solid phase extraction (SPE) C18 cartridge. The products of various shale-fluid experiments using this method indicated that in the case of produced water contamination by hydrocarbons, compounds will more likely originate from operations on site rather than from contact with the lithology. Maturation of shale is a prerequisite for hydrocarbon generation, and associated changes in shale mineral composition and the mobilisation of its trace elements could create challenges for future shale gas extraction operations, more so where acidic injection fluids are used. Therefore, careful water management practices during shale gas extraction operations are critical to avoid episodic contamination incidents. The use of fluorescence excitation emission matrix was demonstrated to be a useful portable ex-situ analysis technique to discriminate anthropogenic sources of contamination from natural signatures of water surrounding extraction operations, thereby providing a cost-efficient tool to better characterise contamination incidents during monitoring. Understanding produced water chemistry related to shale gas extraction operations is therefore essential to effectively assess and mitigate environmental and public health risks.