Mining of Chemistry Data Explains the Fate of Radium in Hydraulic Fracturing Wastewater

Hydraulic fracturing for oil and gas extraction produces large volumes of wastewater with high concentrations of radium (Ra) and other contaminants. The hydraulic fracturing (flacking) process involves drilling followed by injection of highly pressurized fluid into a black shale formation in order to cause fractures and extract oil and gas. In the process, potential sources of Ra include: bedrock, deep brine trapped within the shale formation, and recycled drilling and injection fluids. Recent studies recognized the origin of Ra from black shale rock, however, more evidence is needed in order to fully understand the sources and fate of Ra. We aim to reveal the sources of major contaminants, particularly Ra, in wastewater (e.g. flowback, produced water), and determine their release pathways. This work processed the data from wastewater reports of three major oil and gas operators in Pennsylvania in the past five years provided by North Central Pennsylvania Department of Environmental Protection (PADEP). Combined with data from literature, we acquired 828 wastewater chemistry data with the following types: drill fluid, injection fluid, brine, fracking fluid, flowback water and produced water. The chemical distribution indicated that all the chemicals are location sensitive. For each waste type, we chose three nearest locations to obtain average solution chemistry weighted by distance, and used conservative elements Br and Cl to separate fluid sources. The sources of cations were then obtained via mass balance calculations. Our results suggested that approximately 50%-100% of the Ra in the flowback or produced water was from brine, the rest of Ra was released from bedrock through cation exchange with Ca and/or Na. The findings of this work have implications to the evolution of deep brine in shale formation and wastewater treatment strategies.