Prediction of groundwater impacts from coal seam gas extraction in the Surat Basin

Abstract

The Queensland regulatory framework recognises that the impacts of groundwater extraction activities can overlap in areas of concentrated development. Such areas of overlapping impacts can be declared ‘cumulative management areas’ (CMAs). When a CMA is established, the Office of Groundwater Impact Assessment (OGIA) becomes responsible for carrying out a cumulative impact assessment and preparing an Underground Water Impact Report (UWIR). The Surat CMA was declared in March 2011, and since this time, two iterations of the UWIR have been published in 2012 and 2016, underpinned by a gradually evolving regional groundwater flow model. This case study presentation will examine a number novel features of the regional groundwater flow model, resulting from OGIA’s ongoing research and development program, including the development of an adapted version of the MODFLOW-USG groundwater flow modelling code and approximation of coal desaturation and dual-phase flow effects using a modified van Genuchten function. The presentation will also look at the simulation of CSG extraction using a ‘descending drain’ methodology that recognises the gas-filled nature of CSG wells and generating up-scaled properties of highly heterogenous sedimentary material by first generating stochastic realisations of fragments of these layers, and then using ‘numerical permeameters’ to determine both the expected value and stochasticity of these properties, at the regional scale.