Surat Basin (QGC, 2009) MODFLOW Regional groundwater model – coal seam gas region divided into three subdomains (hydraulic compartmentalisation assumed) Time-varying constant head boundary condition used to represent well field (rather than pumping/flow rates) Dual-phase flow, coal dual porosity and anisotropy, and geomechanical effects not included Coal seams not modelled independently of coal measures No consideration of rainfall recharge Simplified geology, homogeneous isotropic conditions Cumulative impacts not assessed Apparently no accounting of groundwater–surface water interaction None Queensland Water Resources (CWC, 2012) MODFLOW Regional groundwater model (300,000 km2 model domain) Modelling of historical coal seam gas operation, included simulation of multiple coal seam gas operations, was used to develop and assess cumulative impacts and aggregate groundwater extraction associated with coal seam gas extraction Uncertainty analysis was carried out using multiple simulations incorporating changes to the model. The results of this analysis were used to assess uncertainty in the predicted impacts Namoi Catchment (Schlumberger Water Services, 2012) Regional groundwater model (30,000 km2 model domain) coal seam gas well fields modelled using a specified extraction rate over each well field modelled Cumulative impacts assessed (existing and proposed developments) Separate model of surface water system Multi-layered model to address future coal seam gas and coal mine development. Modelling of existing, planned and possible development Cumulative effects assessed through multiple model analyses by comparing the results for a range of alternate development scenarios with a base case of limited development Groundwater impacts on surface water obtained using nominated head boundaries to represent permanent Water courses Sensitivity analyses carried out to assess uncertainty associated with rock permeability and recharge values adopted Powder River Basin, Montana, US (Myers, 2009) MODFLOW Regional groundwater model (1240 km2 model domain) Subregional constant head boundary condition used to represent coal seam gas well field Dual-phase flow, coal dual porosity and anisotropy not included. Geomechanical effects assumed to have no impact Coal seams not modelled independently of coal measures Cumulative impacts not assessed Implicit (uncoupled) groundwater–surface water interaction None
Table AD3: Evaluation of modelling approaches. (Source: IESC, 2014.) Modelling approach/ purpose Advantages Disadvantages Appropriate application Analytical Efficient and simplified analysis of all potential impacts to groundwater resources Useful when data is limited and/or geological and hydraulic conditions are relatively simple Unable to capture complex geologic geometries (e.g. non-uniformly layered geology) or hydraulic conditions (e.g. coal anisotropy) May oversimplify hydraulic processes Screening or preliminary assessment (particularly where data is severely limited) Can be a valuable tool for modelling flow in the vicinity of individual wells Axisymmetric Useful for modelling relatively symmetric conditions (e.g. in vicinity of coal seam gas wells where geological conditions are axisymmetric) Not suitable for regional scale assessment Available tools do not consider gas desorption and migration, dual phase flow or coal dual porosity, which may pose inaccuracies in predicting impacts Not capable of assessing cumulative impacts Assessment of impacts in the near-well (or near-field) under axisymmetric conditions Can be a valuable tool for modelling flow in the vicinity of individual wells Reservoir assessment Designed (and therefore best suited) to predict produced water volumes and depressurisation and in the near-field. Can model near-field produced water Tools do not consider water interaction Most tools account for geomechanical processes, gas desorption and migration, dual phase flow and coal dual porosity, as well as complex geological conditions Reservoir assessment Assessment of impacts to groundwater (not surface water) in the near-field (but not water quality) Used for design of coal seam gas well networks Regional Groundwater Impact Assessment Tools practicable for regional scale impact assessment Capable of representing complex geology, assessing cumulative impacts and changes to groundwater quality Generally ignores geomechanical processes, gas desorption and migration, dual phase flow and coal dual porosity; this may create inaccuracies Regional-scale assessment of impacts, water quality, and cumulative impacts