Metal Associations in Marcellus Shale and Fate of Synthetic Hydraulic Fracturing Fluids Reacted at High Pressure and Temperature

Chemistry of flowback and produced waters from unconventional gas wells is controlled by reactions between fluids injected for well stimulation and the target geologic formation. Release of salts and metals from the formations are well documented, however, the fate of organic additives under down-hole conditions is not. One outcrop and six core samples of Marcellus Shale were obtained from locations throughout Pennsylvania and sequentially extracted to determine metal associations within various mineral matrices. Metal concentrations in different matrices varied significantly across the shale samples analyzed. In comparison to core samples, lower metal and salt concentrations were detected in the shale outcrop likely due to carbonate and salt dissolution during oxidative weathering processes. The shale outcrop was reacted with synthetic hydraulic fracturing fluids (SFFs) of high or low organic content to study the fate of organic additives and their influence on metal mobilization. The organic content of the fluid and high pressure and temperature (HPT; 1200 PSI and 80°C) conditions had little effect on metal dissolution. However, the organic composition changed significantly depending on the reaction conditions (pH and temperature effects). Major findings of this work include the following: (1) addition of oxidants to hydraulic fracturing fluids promotes degradation of organic additives in hydraulic fracturing fluids; (2) mineral composition of shale will effect solution pH, and low pH conditions will promote organic degradation and metal mobilization during hydraulic fracturing; and (3) surfactant additives are persistent under HPT conditions and will return in flowback waters.