A Framework for Inclusion of Faults in Coal Seam Gas Risk Assessments, Narrabri Project Case Study

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 paper 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 pre-development 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.

In the talk we will use the Narrabri region of northern NSW (Australia) as a case study. This is a Senario-1 case where the CSG development is overlain by the thick and laterally continuous Napperby Shale and Digby Formation. We will present a 3D model based on a digital elevation model, outcrop geology (Pratt, 1998), and CSG drilling data (NSW Geologic Survey). A set of field analogues will be presented for the aquitards in the region. Model suggests that faulting observed by Santos (see DPIE, 2020) at the seam level does not propagate to the base of the Napperby/Digby regional aquitards. Further, if faults were to be material, they would result in elements of the aquitards being exposed at the surface in the development area. Finally, it should be noted that the deeper coal seams are overpressured illustrating that, the deeper seam is sealed from the regional hydrostatic groundwater system.