Reducing the Impact of Longwall Extraction on Groundwater Systems

The Australian coal mining industry is under increasingly intense public and political scrutiny because its license to operate is linked with unprecedented social and environmental accountability. Much of this accountability is associated with water management to ensure that the mine operations do not have an adverse impact on the environment or water resource.

Longwall mining under or adjacent to surface water, sub-surface aquifers, water reservoirs and flooded workings has been undertaken and is planned at a number of mine sites in Australia. Due to the rock mass deformations associated with longwall mining, unexpected adverse interaction with these water bodies may result if mining is not properly managed. In addition, aquifer interference is the subject of legislative controls and community concern in Australia, particularly in Queensland and New South Wales. Thus the ability to predict and manage mining induced water inflows and aquifer interference can significantly affect mining safety and economics.

The aim of the project was to better understand rock deformation during mining and allow more accurate predictions of the hydrogeological response. Project outcomes are anticipated to assist the coalmines reduce risk and environmental impact, positively influencing mining safety, productivity and coal resource recovery.

The project investigated the effects of longwall mining on hydrogeology and water inflow into the mine workings.

Within this project, systematic underground hydrogeological monitoring and measurements were carried out including piezometer, extensometer and water inflow monitoring during mining. On the basis of field monitoring and measurement results site hydrogeological characterisation was undertaken and in situ hydrogeological models were established. Groundwater flow mechanisms were investigated using a large scale CSIRO coupled mechanical and fluid flow computer code, called COSFLOW. The flow estimates obtained from 3D numerical simulations were compared with the monitoring data.

Finally, a series of 2D coupled numerical simulations of the hydrogeological response during mining were conducted, using COSFLOW, to examine the impact of aquitard thickness and distance from the mining seam on mine water inflow into mine workings.

The key project results are summarised in the following discussion area:

· New COSFLOW permeability change formulation;

· Hydrogeological characterisation and in situ hydrogeological models;

· 3D COSFLOW calibration and simulation;

· Conceptual hydrogeological response models;

· Impact of aquitard thickness and distance on mine water inflow; and

· Monitoring procedures and assessment methods.