GEL DIFFUSION STUDIES ON FACTORS AFFECTING BARITE SCALING IN MARCELLUS SHALE GAS WELLS

The formation of barite (BaSO₄) scale can reduce production in oil and gas fields by blocking fractures and reducing the porosity of reservoir rock. Due to the low solubility of barite, chemical approaches to scale prevention involving chelation or inhibition of crystal growth often have limited success in oil field wells. Saline produced waters from Marcellus shale gas wells contain elevated levels of barium, and geochemical equilibrium modeling of these waters indicates that barite is supersaturated throughout the system. Anti-scalant compounds are routinely added to injected waters at Marcellus gas wells as a precautionary measure. Previous laboratory flow-through experiments conducted with recycled produced water, fracturing fluid and Marcellus shale demonstrated barite precipitation on fracture surfaces. Specific factors that may affect barite precipitation in the field, such as pH, solute concentration, the presence of anti-scalant compounds, diffusion gradients, and nucleation sites, are difficult to study based solely on water quality analyses and flow-through laboratory experiments. In this study, we examined physical and chemical factors affecting barite precipitation in Marcellus shale gas waters using two-dimensional diffusion gradient agarose and silica gels. This method allows the counter diffusion of selected ions or chemicals towards each other under conditions of controlled gel permeability and temperature. Using time-lapse photography, the formation of mineral precipitates can be visualized, and changes in precipitation rates due to changes in reactant mixtures and concentrations can be determined. Primary reactant distribution in gels was determined by slicing gels and analyzing the contents, and by microelectrode profiling. Variables potentially influencing barite precipitation in Marcellus waters were tested, including pH, salinity, temperature, major cations in production water (Ba, Sr, Ca), sulfate, and anti-scalant chemicals used to minimize barite precipitation.