Abstract
Knowledge of geochemical processes that occur during hydraulic fracturing of shale resources for natural gas recovery can provide insight into in situ mineral precipitation and dissolution reactions that affect shale pore and fracture networks, and ultimately the ability to recover natural gas. Measurement of dissolved chemical species in produced waters, which serve as indicators of water-rock reactions in the reservoir, is one approach for monitoring subsurface geochemistry. However, an ability to distinguish effects of water-rock reactions versus reservoir fluid mixing on chemical concentration and speciation in produced waters requires experimental investigation to better understand natural geochemical tracer behavior.
In this study, we explored Li, Sr, and U as geochemical tracers of ion−exchange and mineral dissolution reactions occurring during a series of flow-through laboratory experiments designed to evaluate mineral reactions occurring in shale reacted with hydraulic fracturing fluid (HFF). The experiments, reported in detail in a separate study (Paukert Vankeuren et al., 2017), were designed to emulate shut-in conditions (period after hydraulic fracturing but before production when HFF remains in the reservoir to allow for opening of new fractures for hydrocarbon recovery) for Marcellus Shale undergoing hydraulic fracturing in southwestern Pennsylvania, USA. In the companion study presented here, effluent samples from the core flood experiments were analyzed for changes in Li isotopes (δ7Li), radiogenic Sr isotopes (εSrSW), and U concentration as possible unique geochemical tracers to indicate water-rock reaction. Experimental and reactive transport modeling results demonstrated that the peak of U in experimental effluents is derived from dissolution of U-containing calcite, which may explain the pulse of U observed in produced water collected from hydraulically fractured Marcellus Shale on the first day of flowback. Experimental results also showed that changes in δ7Li values and εSrSW from the flow-through experiments differ from values measured in a time series of produced waters from Marcellus Shale gas wells. Thus, although ion−exchange and mineral dissolution and precipitation reactions result in an observable change in Sr isotope signals in laboratory experiments, this change may not be identifiable in produced waters in the field due to complications associated with geologic heterogeneity and overprinting of other signals such as fluid mixing and transport. While U shows some promise as a tracer of HFF-shale reactions, particularly carbonate dissolution, it may be necessary to continue searching for other tracers present in produced waters to aid in elucidating in situ water-rock reactions of interest.
•Roles of hydraulic fracturing fluid-shale interactions in controlling geochemical tracer behavior are evaluated•Hydraulic fracturing fluid-shale interactions negligibly affect the temporal variation in produced water chemistry•Dissolution of carbonate cement in Marcellus Shale releases uranium•Most total dissolved solids in produced water are derived from in-situ formation water•Water-rock interactions in hydraulically fractured shale may be challenging to identify through monitoring produced waters