GEOCHEMICAL AND HYDROLOGIC CONTROLS ON PRODUCED WATER GEOCHEMISTRY FROM SHALE GAS RESERVOIRS

Numerous efforts are underway to characterize fluids injected to hydraulically fracture shales and other continuous hydrocarbon reservoirs. In contrast, few studies have examined the composition or attempted to identify reactions that control the chemistry of the fluids produced when gas extraction begins. We have explored time series geochemical analyses of produced fluid samples from more than 20 Marcellus Shale gas wells in the northern Appalachian Basin to provide information about the dominant reactions and processes occurring in the reservoir. Results from a battery of compositional multivariate analyses of geochemical data indicate that four primary processes affect the evolution of the water produced during the first 90 days: mixing of injected fluid with formation brines, release of boron and potassium from clay minerals, dissolution of evaporite minerals, and sulfate reduction or sulfate mineral precipitation. The ²²⁸Ra/²²⁶Ra signature of Marcellus shale gas produced fluids is typical for waters that have reacted with uranium-rich shales, suggesting that waters produced from the Marcellus Shale are those already present within the shale, rather than those transported from adjacent sandstone or limestone units. A transition in water source, from injected fluid to formation water, was traced by chemical and δD and δ¹⁸O time series data. This transition may result from imbibition of injected fluid into low permeability zones followed by preferential flow of brines from high permeability zones as production begins. Our findings suggest that both reservoir petrophysical/hydraulic properties and geochemical reactions control the composition and source of fluids produced from hydraulically fractured shales.