Recent discussions on potential Australian shale gas developments have raised public concerns regarding groundwater protection. This paper presents the results of modelling the movement of the hydraulic fracturing (HF) fluid after injection in north Perth Basin shale targets (Carynginia Formation). The model is also used to predict the migration of HF fluid along an imaginary adjacent fault. Similar studies are rare, both in terms of the technique applied and the geographical area covered.
A 3D model of a part of the North Perth Basin was constructed extending 100 km (East-West) by 10 km (North-South) by 5 km (in depth) with local grid refinement in the target zone to represent the fractured zone having higher hydraulic conductivity and higher specific storage. The fracture extent was determined by 2D fracture propagation modelling (PKN/GDK models). The model was run using the multispecies, density and temperature dependant MODFLOW-SEAWAT code, which simulates HF fluid injection and migration and the post-injection flow-back. The injection rate and the concentration of the additives in the HF fluid are based on field data, and the modelled flow-back rate is calibrated to the observed flow-back data of about 62% of the injected volume.
The simulation indicated that the HF fluid plume was contained in close proximity to the injection well and was reduced to a very small fraction of the original injection concentration (0.1 mg/L) within a distance of less than 250 m. Simulation also showed that injecting near the fault did not influence the flow of the HF fluid along the fault, with concentrations decreasing rapidly with height. A model sensitivity analysis was carried out, which showed that the plume extent is mostly sensitive to the rate of HF fluid injection.
The model shows that the HF fluid is contained within the target formation and that fluid intersecting a fault of high transmissivity results in the concentrations dissipating faster. The maximum concentration of the HF fluid declined to well below the human risk-based concentration of the critical constituents (within <1 year), based on several guidelines such as US EPA, European Chemicals Agency and Institute of Medicine.
This study indicates low impacts of hydraulic fracture stimulation during shale and tight gas development on potable groundwater sources in the north Perth Basin.