Results of leaching experiments on 12 energy-related shales from the United States

Inorganic compositions of flowback and co-produced waters from hydrocarbon extraction have been studied directly and through laboratory experiments that seek to replicate subsurface water-rock interaction. Here a broad analysis is made of compositions from the U.S. Geological Survey Produced Waters Database (v2.3) and leachates (water, hydrochloric acid, artificial brine) for 12 energy-resource related shales from across the United States. The database illustrates common ranges for 26 elements in 4 produced water types and enhanced solubility with increasing ionic strength is observed for Al, Ba, Fe, Li, Mn, Rb, Sr, and possibly 11 other elements. Differences are observed between laboratory leachates and produced waters. Reasons for those differences include: (1) ionic strength effects and low ionic strength of some leachates relative to subsurface brines, (2) pH-sensitivity of constituents and leachate offsets from produced water pH values, (3) oxidative conditions and enhanced pyrite oxidation in leachates compared to subsurface conditions, (4) kinetic controls on reaction rates and insufficient durations for leaching experiments, (5) leachate water-rock ratios that are commonly 3 or 4 orders of magnitude greater than in the subsurface. The findings provide important context for interpreting produced water compositions and efforts to simulate them in the laboratory.