Mine and coal seam gas groundwater extractions as drain boundary condition

In order to model the extraction of groundwater from mine workings and CSG development wells, the MODFLOW Drain (DRN) package is used. The DRN package uses a head-dependent flux boundary condition that removes water from the model cells that represent the coal mines and the CSG wells. In the case of open-cut coal mines, the drain boundary condition is applied to all model cells of all layers in and above the target coal seam that are within the mine footprints. In the case of longwall mines, the drain boundary condition is applied only to the model layer corresponding to the Hoskissons Coal which is the target formation for the three longwall-mining projects in the Namoi subregion. However, the hydraulic properties of the overburden material above the longwall panels are progressively changed over the simulation period to reflect hydraulic conductivity enhancement as reported in Section In the case of CSG wells, the drain boundary condition is applied to the model cells within which a proposed CSG well of the Narrabri Gas Project is present. A specified flow rate is not used in defining the mine and CSG extractions because of the large uncertainty in those flow rates. By using the DRN package for representing the mine and CSG water extractions it is possible to parameterise the drain conductance using a wide range of values over multiple model runs and thus explore the uncertainty in the mine water makes and produced water and the resulting impact on drawdown distribution. This approach also enables constraining the groundwater model using the historical mine water makes and produced water during uncertainty analysis rather than forcing the mine water make and produced water as a specified input.

The flow rate time series generated for baseline and CRDP mines are provided in Section 2.1.6 of companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a).

Last updated:
6 December 2018
Thumbnail of the Namoi subregion

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