Geochemical and sulfate isotopic evolution of flowback and produced waters reveals water-rock interactions following hydraulic fracturing of a tight hydrocarbon reservoir

• Published in: The Science of the total environment Vol. 687; pp. 1389 - 1400
• Main Authors: Osselin, F., Saad, S., Nightingale, M., Hearn, G., Desaulty, A-M., Gaucher, E.C., Clarkson, C.R., Kloppmann, W., Mayer, B.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 15-10-2019; Elsevier
• Subjects: Flowback geochemistry; Heavy metals; Hydraulic fracturing; Persulfate breaker; Sciences of the Universe; Stable isotopes; Tight gas; Flowback geochemistry; Tight gas; Persulfate breaker; Stable isotopes; Hydraulic fracturing; Heavy metals; hydraulic fracturing; persulfate breaker; stable isotopes; heavy metals; flowback geochemistry; Tight gas Hydraulic fracturing Flowback geochemistry Persulfate breaker Heavy metals Stable isotopes
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2019.07.066

Although multistage hydraulic fracturing is routinely performed for the extraction of hydrocarbon resources from low permeability reservoirs, the downhole geochemical processes linked to the interaction of fracturing fluids with formation brine and reservoir mineralogy remain poorly understood. We present a geochemical dataset of flowback and produced water samples from a hydraulically fractured reservoir in the Montney Formation, Canada, analyzed for major and trace elements and stable isotopes. The dataset consists in 25 samples of flowback and produced waters from a single well, as well as produced water samples from 16 other different producing wells collected in the same field. Additionally, persulfate breaker samples as well as anhydrite and pyrite from cores were also analyzed. The objectives of this study were to understand the geochemical interactions between formation and fracturing fluids and their consequences in the context of tight gas exploitation. The analysis of this dataset allowed for a comprehensive understanding of the coupled downhole geochemical processes, linked in particular to the action of the oxidative breaker. Flowback fluid chemistries were determined to be the result of mixing of formation brine with the hydraulic fracturing fluids as well as coupled geochemical reactions with the reservoir rock such as dissolution of anhydrite and dolomite; pyrite and organic matter oxidation; and calcite, barite, celestite, iron oxides and possibly calcium sulfate scaling. In particular, excess sulfate in the collected samples was found to be mainly derived from anhydrite dissolution, and not from persulfate breaker or pyrite oxidation. The release of heavy metals from the oxidation activity of the breaker was detectable but concentrations of heavy metals in produced fluids remained below the World Health Organization guidelines for drinking water and are therefore of no concern. This is due in part to the co-precipitation of heavy metals with iron oxides and possibly sulfate minerals.
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•Acidification of reservoir fluids by pyrite oxidation triggers dolomite dissolution.•Oxidation of the reservoir rock by persulfate breaker releases heavy metals.•Anhydrite dissolution is responsible for the release of sulfate in flowback water.•Precipitation of barite, celestite, calcite is to be expected during flowback.