Development of Functionalized Proppant for the Control of NORM in Marcellus Shale Produced Water

One of the major environmental concerns with the recovery of unconventional gas resource from Marcellus Shale is the presence of naturally occurring radioactive material (NORM) in produced water. Ra-226 is the major component of NORM with a half-life of 1600 years that is present at concentrations as high as several thousand pCi/L. Most of the studies on NORM management are focused on above-ground scenarios. The main focus of this study was on functionalizing the proppant (i.e., quartz sand) that is used in hydraulic fracturing to prevent the closure of induced fractures formed during this process and allow release of natural gas so that it can also sequester NORM from the produced water before it reaches the surface. Five different sulfates and carbonates were tested for their ability to capture Ra-226 from aqueous solution and celestite (SrSO₄) was identified as the best choice because of its affinity for Ra-226 sequestration even in the presence of very high total dissolved solids that are characteristic of Marcellus Shale produced water. Among possible ways of coating the proppant with celestite, precipitating celestite directly on the sand surface was found to be the best option as it provided a uniform distribution of celestite and high uptake of Ra-226. Although quartz sand can adsorb some radium from the solution due to electrostatic interactions, adding a small amount of celestite on the sand surface (20-30 mg/g) increased radium removal from the solution containing 5000 pCi/L of Ra-226 to more than 80% in dilute solution and to more than 50% in high-salinity solution even in the presence of very high concentrations of competing divalent cations. The results of this study indicate the potential of coated proppant to sequester NORM in the subsurface and prevent adverse environmental impacts when radiogenic produced water is brought to the surface.