Barite scaling during the low-flow, shut-in period in hydraulic fracturing operations during shale gas production has been intensively studied, but scaling during the stimulation periods when large volumes of water are injected at high flow rates has been mostly overlooked. Due to the variable nature of the injections, the kinetics of precipitation and the morphology of precipitated scale minerals vary due to different solute concentrations and hydraulic influences on fluid-rock interactions. Barite scaling at different stages of hydraulic fracturing was studied using flow-through experiments with fractured Marcellus shale cores. The experiments were conducted to mimic active stimulation injection at 0.3 mL/min for 4 days and then at low-flow or stagnant shut-in conditions at an injection rate of 0.01 mL/min or zero for 21 days. Monongahela River water, commonly used in fields in Southwestern Pennsylvania and West Virginia, has high sulfate concentrations and was used as the base water for synthetic hydraulic fracturing fluid (HFF). More than 80% of the barite formed inside the cores was from the stimulation period. Small round barite crystals (similar to 5 mu m) were observed in the HFF before injection. Large euhedral barite (10-50 mu m) covered the shale fracture surface in regions with apparent higher flow during the simulated injection period. Smaller (similar to 5 mu m) barite precipitates were found over a larger area of the fracture surface in the shut-in, low-flow stage of the experiments. These findings indicate that mineral scale-related problems with gas production from hydraulically fractured wells likely relate to scaling minerals precipitating during the early stages of the process when fluid is being actively injected. Pretreatment of the HFF to reduce barite seed crystals before injection can be an important step for scaling control.