Neame, T., Smith, T., Townley, L., Narayan, K., and Toogood, G. (2013), Assessing the cumulative impacts of coal seam gas development on water resources in the Gunnedah Basin, northern NSW - using a regional groundwater flow model., IAH Congress, Perth, September.

Extraction of coal seam gas (CSG) is a relatively new and emerging industry in Australia. Fields are developing in the Surat and Bowen Basins of Queensland and in the Gunnedah, Gloucester, Clarence-Morton and Sydney Basins in New South Wales. Considerable public interest currently surrounds the development of CSG resources. One focus of this interest is the potential effect that CSG development will have on groundwater and surface water resources and Matters of National Environmental Significance (MNES).

Commonly, target coal seams are held under pressure by the static head of water and this water must be removed by pumping to lower the hydrostatic pressure in the coal seam ‘reservoir’ and to stimulate desorption of methane from the coal seams. Extraction of water is commonly achieved using bores penetrating the coal seam, which initially remove water and subsequently gas and water as the hydrostatic head is drawn down and gas desorption increases. Consequently, CSG exploration and extraction has potential for groundwater drawdown and depletion of overlying or adjacent water supply aquifers.

In this paper, a regional groundwater flow model developed for the Gunnedah Basin is presented. The complex multi-layered stratigraphy has been represented as 24 layers. The model domain extends from Muswellbrook in the south east to Walgett in the northwest and from Gunnedah in the east to Coonamble in the west and comprises approximately 720,000 model cells. Calibration was achieved using a variety of techniques including head and flux matching.

CSG water extraction was simulated by wells distributed across the proposed development areas extracting in proportion to the initial head and seam depth, and all extraction commenced simultaneously as a conservative measure. Simulated extraction commences in 2011 and ceases in 2055 with a simulation period of 1495 years. A range of model output styles illustrate the outcomes of the simulation in terms of the magnitude and timing of groundwater impact and flux quantification in relation to groundwater management areas.

The key findings include that maximum drawdown predicted in the Pilliga Sandstone (GAB southern recharge beds) is less than half a metre. The minimal impact on the Pilliga Sandstone and the slow recovery are considered to be due to the effective hydraulic isolation of the target coal seams by successive overlying aquitards and aquicludes including the Benelebri Member, basal Napperby shale and Purlawaugh Formation. The maximum predicted drawdown at the water table is less than three metres at one location in the Gunnedah area outside of the Namoi Alluvium. Drawdown at the water table is less than one metre at all other locations. The isolated incidence of drawdown greater than one metre is due to the outcropping of the coal seam at surface. The numerical model assumes a uniform distribution of CSG extraction wells across the project area as a conservative measure; however, this is likely to be modified to reflect the lower CSG potential towards the outcrop and to minimise the incidence of impact on groundwater. This numerical groundwater modelling indicates that potential impacts on regional groundwater resources from CSG development activity are likely to be minimal but to persist for extended periods of time. Improved model confidence may be obtained through the measurement of hydraulic parameters and by collection of temporal head data throughout the model domain. A programme for the acquisition of field and laboratory data to improve the selection of hydraulic parameters, and to gather temporal groundwater head data throughout the model domain to improve model calibration, has already been initiated to enable greater confidence to be achieved in future modelling.