Analysis of produced waters (PW) from hydraulically-fractured shales can provide insight into processes that occur within unconventional gas reservoirs based on changes in aqueous chemistry during the course of flowback and gas production. Mixing between in-situ brine and injected fracturing fluids are the primary cause for an increase in total dissolved solids (TDS) over the course of flowback and production, with contributions from interactions between injected fluids and shale minerals. Although the increase in TDS with time is observed with hydraulically-fractured wells across the Marcellus Shale formation, the heterogeneity of formation waters across the region affects the application of natural geochemical tracers towards interpreting subsurface mixing and fluid-rock reactions that occur during hydraulic fracturing. Laboratory flow-through experiments emulating bore-hole conditions and sequential extraction of shale rocks suggest that hydraulic fracturing fluid and shale mineral interactions negligibly affect the temporal change of PW chemistry observed from the field. In this study, we examined time-series PW samples from two research wells at the Marcellus Shale Energy and Environment Laboratory (MSEEL) field site in Morgantown, West Virginia, southwest of the Appalachian Basin. A spike of elevated Sr and other major ions with distinctly lower 87Sr/86Sr was observed in PW collected between 2 and 3 days after water return from the first well. This observation indicates a delayed mixing of fracturing fluid with in-situ brine caused by an initial isolation of injected fracturing fluid in shale fractures that later became exposed to in-situ brine. Overall, the concentrations of major and minor ions and strontium isotopic composition (87Sr/86Sr) temporarily increase towards the composition of in-situ formation water. In contrast, the TDS in the PW from the second well was at the greatest value as returned on the first day, decreased to the lowest value on day 4, and then increased gradually toward the composition of formation water. Trends observed in the MSEEL wells are similar to results from other wells within the Marcellus Shale. This presentation will discuss in details the geochemical and isotopic data (δ7Li and 87Sr/86Sr) from the two MSEEL research wells and compare with the geochemical evolution of PW in other regions of Marcellus Shale.

Factors controlling the temporal change in chemical composition of Marcellus Shale produced water: Insights from laboratory experiments to field sites

253rd National Meeting (2017)

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