Open Cut Mine Gas Drainage

This study examines the drivers of feasibility of coal seam pre-drainage ahead of open cut mining, and whether it could materially reduce fugitive methane emissions. While well-established underground, pre-drainage is harder to transfer to open cut mining because it is multi-seam, spatially dynamic, and schedule sensitive.

This study considers two representative settings: the Hunter Valley coal measures in New South Wales, with emphasis on the Jerrys Plains Subgroup, and the Moranbah Coal Measures in Central Queensland, including the Goonyella Middle Seam. Differences in seam thickness, rank, gas content, gas composition, structure and seam multiplicity strongly influence feasibility.

Surface-to-in-seam (SIS) drainage is the main reference method considered. Conventional SIS is the most credible basis for further work because it is the most developed directional drainage method and has underground mining precedent. Reverse SIS may be workable in some seam geometries but is more constrained. Skewed SIS remains conceptual. Highwall-to-in-seam (HIS) drainage may offer access advantages, but it creates greater interaction with active open cut mining and is less attractive as a general solution.

Pre-drainage will not remove all gas from a coal seam. Some residual gas will remain, and the time needed to reduce gas content depends strongly on permeability, gas composition and saturation. Furthermore, draining selected seams will not necessarily address emissions from other coal seams and interburden within the mining sequence.

The principal conclusion is that pre-drainage feasibility will vary widely, and (after detailed site-specific studies) may be identified as a potential abatement method for selected sites and seams, not as a ready-to-use or whole-of-mine solution. The strongest cases are likely where seams are thick enough to drill reliably, gas-rich, sufficiently permeable, and supported lead time, and backed by a stable mine plan.

Supporting documentation is also available to download here, including the ACARPMatters E-Newsletter and the Project Synthesis Report.