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Effect of viscosifier additives of fracturing fluids on the mobility of barium in disposal wells of the Arbuckle formation
Flowback and produced water (wastewater) from unconventional gas wells in Oklahoma are injected into deep disposal wells of the Arbuckle formation. Total dissolved solids includes heavy metal ions (e.g., Ba) whose concentrations are in many cases hundreds of time above the US drinking water standards. This poses a huge environmental risk as wastewater disposed into deep wells might leak to shallow groundwater systems due to well integrity flaws or presence of natural fractures.
This work focuses on the effect of viscosifiers on the mobility of heavy metals, specifically we focus on the effect of guar gum on the mobility of Ba2+. Guar gum is used to increase the viscosity of the fracturing fluid to help suspend proppant, and Ba2+ is one of the most abundant heavy metals detected in wastewaters. Although the effect of organic polymers on the mobility of oxidizing reagents in shallow aquifers has been studied before, this is the first work aiming to reveal the transport mechanism of heavy metals under the presence of polymers through Arbuckle dolomite rocks.
Batch and core-flooding experiments were conducted using dolomite rocks (powdered and plugs) prepared from Arbuckle outcrop in Missouri. The porosity and permeability of the plugs were 4-15% and 0.1-15mD. We conducted six batch experiments to study the sorption of Ba2+ on the surface of powdered dolomite under the presence of guar gum. Core-flooding experiments were conducted at a confining pressure of 4000 psi and room temperature at three flow rates (0.01, 0.05 and 0.1 ml/min).
Our results indicate that guar gum increases the sorption of Ba2+ on the surface of dolomite through chelation-like interactions between guar gum and Ba2+, however the mobility of Ba2+ through dolomite plugs does not increase as one might expect. Guar gum readily clogged the pore space of the dolomite plug. This was reflected by a reduction in the hydraulic conductivity of the dolomite plug and the accumulation of Ba2+ within the plug.