The Influence of Water-Rock Interaction on Trace Element Mobilization during Shale Gas Production

Abstract

The extraction of natural gas from shale reservoirs has generated a substantial increase in the volume of produced brine. In addition to being highly saline, these brines often contain elevated concentrations of naturally-occurring radionuclides and toxic metals. These characteristics present many challenges with regard to effective treatment and disposal. This dissertation investigated the mobilization of Ba, As, U, and Ra from shale in contact with hydraulic fracturing fluids under typical reservoir pressure and temperature conditions through a series of batch and flow-through experiments as well as geochemical simulations. Comparison of experimental data with flowback samples collected from a shale gas well in Michigan demonstrated that a majority of toxic elements present in production wastewaters likely originate from connate brines and are not substantially enhanced by well completion activities. X-ray computed tomography and scanning electron microscopy analysis demonstrated the co-occurrence of calcite-depleted regions and exposed pyrite at the fracture face in the core-flooding experiments. Following this observation, a 2D reactive transport model was developed to further study the effect of fast calcite depletion on pyrite dissolution and associated arsenic leaching. The relative importance of advection, diffusion, and reaction rate in controlling mineral dissolution was evaluated through analysis of model domain Péclet (Pe) and Damköhler (Da) numbers. Calcite dissolution was shown to be mass transport rate limited, while the dissolution of pyrite embedded within the calcite-depleted shale matrix was controlled by a combination of surface reaction and mass transport. Additionally, the mechanism and controls for Ra mobilization in produced brines were investigated and used to develop an empirical relationship for predicting Ra activity in shale gas wastewaters. It was shown that adsorption/desorption is the primary process controlling Ra mobilization. Ra activity can be estimated prior to drilling activity if the U and Th content of the shale and the TDS of produced water is known. This knowledge can be used to guide optimal wastewater treatment and disposal strategies prior to any drilling activity, thereby reducing hazards associated with elevated Ra activity in shale gas wastewaters.