Hydrogeological role of Narrabeen Group fractures following coal seam gas extraction in the Southern Coalfields of NSW.

Coal seam gas extraction is a relatively young and rapidly developing industry. Many members of Government and the general public have expressed concern about the potential negative environmental impacts of coal seam gas extraction and its associated processes. Gaps in knowledge, such as the role of fractures in facilitating fluid flow, and their potential to impact groundwater, need to be further investigated in order to properly assess the risks associated with coal seam gas extraction. With this in mind, this study was conducted to determine the ability of fractures within the Narrabeen Group in the Southern Coalfields of New South Wales, to facilitate fluid flow following coal seam gas extraction. The upper limit of fracture hydraulic conductivity in the modelled conditions was determined, and limitations of this potential were identified. The ability of Narrabeen Group fractures to contribute to environmental degradation following coal seam gas extraction was also assessed. Outcrops of Narrabeen Group units were studied in order to constrain the density and extent of systematic fracturing. The micro scale characteristics of these fracture planes were recorded by examining core samples from the Camden Region, and literature was consulted in order to identify the parameters of a 'worst case scenario' following coal seam gas extraction. The observed fractures have the potential to conduct environmentally significant quantities of water, up to 38.9 litres per day over a 25m 2 area, which equates to 199% of the 'Long-term average annual extraction limit' set by the NSW Office of Water. This potential is limited by the micro-scale characteristics of fracture planes, including the effects of infill minerals, multiple interruptions to laminar flow, and the potential for rheological sealing of fracture planes in three claystone aquitards present in the group. The results suggest that while there is potential for the existent fractures to facilitate fluid flow and the associated environmental impacts, the likelihood that these impacts will be realised to any significant extent is limited. It is suggested that further research continue with the micro scale characteristics of fracture planes, the potential of Narrabeen Group claystones to seal fresh fractures via rheological deformation, and the degree of subsidence following coal seam gas extraction.