Acknowledgements This report has been prepared by Jacobs (Australia) Pty Ltd for the Department of Environment, Land, Water and Planning in collaboration with the Geological Survey of Victoria (part the Department of Economic Development, Jobs, Transport and Resources). Independent review of this report was provided by: Professor Rae Mackay, Director, Geotechnical and Hydrogeological Engineering Group, Federation University Australia Professor Dayanthi Nugegoda, Ecotoxicology, School of Applied Science, RMIT University Professor Simmons, Director, National Centre for Groundwater Research and Training, Flinders University.
the purposes of this assessment the DELWP Resource Share Guidance approach is recommended as the basis of assessing likelihood, as shown in Table 11. Table 11: Proposed likelihood scale for subsidence caused by drawdown in aquifers. Consolidation response / Drawdown Range LOW Less than 1 m drawdown over 30 years MODERATE Between 1 m and 10 m drawdown over 30 years HIGH Greater than 10 m drawdown over 30 HIGH Normally sediments Moderate High High MODERATE Over-consolidated sediments with >20% clay Low Moderate Moderate LOW Over-consolidated sediments with little clay Low Low Low Table 12: Proposed likelihood assessment criteria for subsidence as a result of onshore gas development. Water Resource (Asset) Group Low Consequence Moderate Consequence High Consequence Groundwater users (aquifers) Impact of subsidence is within annual variability in function and operating costs of current and future users. Without modification current pumping regime cannot meet water demand due to subsidence Aquifer, without modification of current pumping infrastructure such as deepening bores, cannot meet demand requirements as a result of subsidence. Surface water users (rivers) Subsidence results in either no change or a material change in flow, with no measurable impairment of users’ ability to access entitlement. Subsidence results in a material change in river flow, with measurable impairment of users’ ability to access entitlement. Subsidence results in a material change in river flow, with significant impairment of users’ ability to access entitlement. Surface ecosystems Subsidence results in a material change in ecosystem condition, with no measurable impairment of ecosystem function. Subsidence results in material change in ecosystem condition, with measurable impairment of ecosystem function. Subsidence results in material change in ecosystem condition, with significant impairment of ecosystem function. Considering the range of values for clay content and the potential range of drawdown that may result from gas exploration or development, the expected range of subsidence as a result is likely to be in keeping with that experienced from current licensed groundwater extraction. That is, land subsidence would be expected to be in the range of millimetres to tens of millimetres. Give the indicative consequence table and the sediment types in the Otway region, there is a low risk of subsidence. Primarily this assessment results from the estimation of low drawdown as a result of gas exploration or development. 6.5 Summary
Modelling regional impacts Table AD2: Modelling Approaches, Simplifications and Uncertainty Analysis at a Project Level. (Source: Commonwealth of Australia, 2014.) Coal seam gas Project /Author Modelling tool used Modelling approach Processes involved Simplifications/ Assumptions Uncertainty Analysis Arrow Energy Surat Gas Project, Australia (Arrow Energy Pty Ltd, 2012) MODFLOW Regional groundwater model (120,000 km2 model domain) Well field represented by individual abstraction wells Cumulative impacts assessed (including other coal seam gas developments) and unsaturated flow, geomechanical effects, and dual porosity nature of coal all assumed insignificant Assessed to be limited groundwater–surface water interaction – simple (non-coupled) approach to groundwater–surface water interaction adopted Coal horizontal anisotropy not modelled Coal not modelled independently of coal measures Hydraulic connectivity of geologic structural features ignored (Hydraulic fracturing not proposed to be undertaken by Arrow, thus its potential impact was not required to be assessed) Deterministic uncertainty analysis only: Sensitivity analysis for specific aquifer parameters and multiple aquifers. Effect of sensitivity-adopted parameter values on calibration performance discussed Indicated significance of hydraulic parameters and range of drawdown magnitudes Australia Pacific LNG Project, Australia (Australia Pacific LNG, 2010; Geoscience Australia and Habermehl, 2010) FEFLOW Regional groundwater Model (172,740 km2 model domain) Finite element method (FEFLOW) allows improved definition of complex geology Dual-phase flow implicitly accounted for by reducing coal seam permeability Cumulative impacts assessed (including other coal seam gas developments) Geomechanical effects, and dual porosity nature of coal all assumed insignificant Simple (non-coupled) approach to groundwater–surface water interaction adopted Coal horizontal anisotropy not modelled Coal seams not modelled independently of coal improved definition of complex geology Dual-phase flow implicitly accounted for by reducing coal seam permeability Cumulative impacts assessed (including coal seam gas developments) Deterministic uncertainty analysis only: Sensitivity analysis (two extreme cases only) for aquifer, recharge and stream conductance parameters. Effect of sensitivity-adopted parameter values on calibration performance discussed Indicated significance of hydraulic parameters and range of drawdown magnitudes