From the bottom to the top: Assessing the potential subsurface migration of fracking-related fluids along natural geological pathways

Abstract

Over the last century humans have increasingly gone from just extracting fluids from the subsurface to also injecting them, for example hydraulic fracturing (fracking) for shale gas. Concurrently, there has been increased environmental interest in how injected fluids might interact with natural groundwater systems. One potential environmental impact of fracking is the contamination of shallow groundwater from the upward migration of injected and formation fluids along natural geological pathways. High-permeability fault and fracture zones have generally been regarded as the highest-risk pathway. Therefore, the first aim of this thesis was to determine a horizontal respect distance between fluid injection locations and known faults. Using published microseismic data a horizontal respect distance of ~900 m was proposed. The second aim of this thesis was to consider complex geological structure in numerical models to investigate hydrogeological factors which might increase the vulnerability of shallow groundwater resources to the potential upward migration of fracking-related fluids. Using 2D and 3D numerical models four hydrogeological factors increased the vulnerability of a shallow aquifer to upward fluid migration from an underlying shale reservoir: increased stimulated fracture extent; greater amounts of overpressure; the presence of low-permeability faults; and the absence of deep high-permeability formations. Low-permeability faults acted to compartmentalise groundwater, thereby discouraging regional horizontal flow whilst encouraging upward flow through strata. The integration of groundwater quality and seismic reflection data provided a novel method for identifying compartmentalisation, but compartmentalisation could not be identified from surface water quality data in the studied basin. However, analysis of surface water quality data could demonstrate regional-scale groundwater-surface water interactions. Importantly, this thesis highlights the need to understand and model interactions between deep groundwater, shallow groundwater, and surface waters by integrating data from a variety of industry and environmental regulatory sources.