Development of in-seam directional drilling in the Australian coal industry

The presence of seam gas in the form of methane and carbon dioxide presents a hazard to underground coal mining operations. Mining depths have progressively increased with the resultant increases in seam gas content and pressure while improvements in technology have allowed increases in mining advance rates. This has resulted in an increased risk of outburst during mining development and higher gas volumes released during mining. The drilling of in-seam boreholes to drain gas from the coal seam before mining was identified as the key means to mitigate the risk of outburst. In-seam drilling has been undertaken for the past thirty six years to reduce the seam gas content below defined safe threshold levels. This gas drainage was designed to mitigate the risk of gas outburst and lower the concentrations of seam gas in the underground ventilation. The drilling practices have reflected the standards of the times and have evolved with the development of technology and equipment and the needs to provide a safe mining environment underground. Early practice was to adapt equipment from other fields, with rotary drilling being the only form of drilling available. This drilling was utilised successfully but with limitations due to the inability to steer the boreholes. This led to areas not having adequate drainage coverage. Directional drilling was introduced through a research and development but limited by the standard of the technology available. It was not readily accepted as a viable alternative to rotary drilling due to the high cost and relatively low drilling rates compared to that of rotary drilling. Drill rigs from hard rock drilling industries were modified for use underground and combined with high pressure pumps to allow directional drilling with proto-type Polycrystalline Diamond (PCD) drill bits, low powered Down-hole Motors (DHMs) and Eastman single-shot surveying.