Assessment of fault zone properties for CSG development areas

There is an increasing need to understand the influence of faults in both gas production performance and the resulting potential impact on adjacent groundwater resources. Faults can exhibit a wide variety of hydraulic properties. Where resource development induces changes in pore pressure, the effective stress and thus the permeability can be transient. In this study, w explored strategies for characterizing fault zone properties for the initial purpose of evaluating gas production performance. The same fault characterization can then be incorporated into regional groundwater flow models to more accurately represent stress, strain and the resulting transmissivities when assessing the impact of gas development on adjacent aquifers. Conventional fault zone analysis (juxtaposition, fault gouge or shale smear, fault reactivation) is combined with hydrodynamic analysis (distribution of hydraulic head and hydrochemistry) and surface water hydrology and hydrochemistry to evaluate across fault or up fault locations of enhanced hydraulic conductivity at specific locations of complex fault systems. The locations of identified vertical hydraulic communication from the hydraulic analysis are compared with the fault zone architecture derived from the 3D seismic volume overlain with the in-situ stress characterization. This provides an independent method of assessing other potential hydraulic communication locations based on seismic alone where no other hydraulic or hydrochemical information is available. A groundwater model is used in an inverse approach to assess the hydraulic properties required to generate observed hydrochemical anomalies so that a realistic range of rock properties can be assigned throughout the geological model to other faults. We use a case study example of the Gloucester Basin in New South Wales in eastern Australia to demonstrate how some of these techniques can be applied. The Gloucester Basin has been subject to exploration of coal seam gas with some pilot testing but no commercial production. It also contains data from groundwater monitoring bores and surface water features. The methodologies described can be applied elsewhere when faults play a key role in determining gas production performance or characterisation of groundwater flow system hydraulics in gas development areas.