Geochemical element mobilisation by interaction of Bowland shale with acidic fluids

Hydraulic fracturing is widely used to exploit unconventional hydrocarbon sources, to enhance exploitation of geothermal energy and to aid in carbon sequestration through underground storage of captured CO2. The hydraulic fracturing fluids, which are commonly acidic, cause dissolution of minerals and desorption of elements which can lead to groundwater contamination. Batch reactor experiments were conducted to explore the interaction of simulated fracturing fluids with two end member compositions of basinal shales of the Bowland-Hodder unit (Carboniferous, UK) whereby the impact of temperature, fluid acidity, and rock/fluid ratio conditions were investigated. The results demonstrate that the fluid acidity is mainly controlled by the oxidative dissolution of pyrite and the dissolution of calcite, impacting mobilisation and fate of major and trace elements. The dissolution of calcite and pyrite significantly dominates the leaching of Sr and As, respectively. Generally, increased fluid acidity and temperature facilitate element mobilisation due to enhanced mineral dissolution and ion desorption, whereas higher rock/fluid ratio (higher mass of carbonate minerals) raises the buffering capacity and may promote the immobilisation of some metal ions by adsorption and precipitation (e.g. Ba, Pb, Fe, Al, and Mn). Moreover, the surface topography of different minerals in polished shale sample sections after fluid-rock interaction indicates that mineralogical compositions may play an important role in determining the pore structure. This research identifies chemical reaction pathways of geochemical elements (including contaminants) in fracturing fluids over a range of fluid chemistries and environmental conditions, and helps to evaluate element mobilisation from shale reservoirs with differing mineralogies.