Horizontal Post Drainage Design

Coal mine gas has been ranked as the number one critical issue to be addressed to achieve high production rates from longwall (LW) faces and is probably the most significant single factor that will constrain future mining productivity from LW mines in Australia. Increased LW retreat and development rates in a number of Australian coal regions, including the Hunter Valley, Illawarra, and Bowen Basin regions have produced mine gas levels that challenge safe and productive operation and are a serious threat to sustained and efficient coal production.

Gas drainage in Australian LW mining is increasingly challenging because more mines have complex gassy conditions, multi‐seam environments beneath previously extracted goafs, and environments where drilling conventional surface gas drainage boreholes is not practical. In recent years, horizontal post drainage has been trialled at a few sites in Illawarra and Hunter Valley with mixed results. In these trials, horizontal post drainage boreholes were drilled from the surface using advanced surface to inseam drilling technologies to address LW gas emissions. This drainage method has the potential to capture substantially large and consistent volumes of gas and significantly reduce the methane concentration of gas in the mine ventilation air which serves to improve mine safety and productivity, increase mine methane utilisation, and reduce greenhouse gas emissions. Horizontal post drainage provides an alternative approach to LW goaf gas capture which requires substantially less surface disturbance than the conventional surface goaf drainage method using vertical wells. However, there are significant risks involved in this approach:

· Instability of horizontal drainage boreholes during LW retreat;

· Low gas drainage efficiency (high dilution from the goaf and ventilation); and

· Insufficient gas reduction at LW workings.

To assist the development of horizontal post drainage technology and address the risks associated with this approach CSIRO aims, through ACARP Project (C18047), to develop a design methodology and procedures for horizontal post drainage, in conjunction with Glencore Coal (formally Xstrata Coal) and BHP Billiton Illawarra Coal.

The CSIRO research team has taken a comprehensive approach to the project research and successfully completed the following tasks:

· Review of relevant overseas experiences with horizontal post drainage;

· Analyses of recent horizontal post drainage experience and data at West Cliff, Appin and Blakefield South;

· Detailed mine gas emission monitoring and measurements at the LW3 at Blakefield South;

· Numerical modelling of mining induced strata conditions and goaf gas flow dynamics;

· Development of a conceptual horizontal post drainage model; and

· Development of horizontal post drainage design methodology and procedure.

The key findings from the project are significant and have important implications to coal mine methane management in Australia.