A framework for inclusion of faults in coal seam gas risk assessments

Abstract

Assessments for coal seam gas (CSG) projects may be required to consider the potential hydrological impacts of faults on surface and near-surface groundwater assets. This study presents three distinct end-member geological scenarios and outlines methods for characterising fault-related groundwater flow within a risk assessment context. Scenario 1: a regional aquitard isolates the water assets from the coal seams. There is little risk of leakage across the aquitard because there are no faults, or because the faults have maximum displacements less than the thickness of the aquitard. Scenario 2: a region-wide aquitard is not present, and the seams and the groundwater assets are located within the same groundwater system. In this scenario, CSG development may cause pressure changes to propagate parallel to the strike and dip of the fault in the fault damage zones. Scenario 3: regional aquitard(s) are present, but larger displacement faults breach the aquitards, allowing for possible combinations of across-fault connections between the different aquifers, and between aquifers and the coal seams. In this scenario, potential flow pathways between the groundwater and the CSG field need to be characterised using Allan Maps (fault plane profiles). It is essential to compare calculated flow rates of any new or potentially new flow pathways with the predevelopment flow regime. It is also important to recognise that flow estimates are best made using a Darcy’s law treatment for flow across fault zones and within the aquifers, and a Snow’s law treatment (discrete fracture network) for flow through fractures in fault damage zones.