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Comparative Study of In-Seam Surveying Technology (Coal Interface Detection Project)

Underground » Geology

Published: February 05Project Number: C12024

Get ReportAuthor: Scott Thomson, Scott Adam, Peter Hatherly | CRCMining

In-seam drilling in the Australian coal industry is essential to reduce gas levels. Drilling is also sometimes used to assist mine exploration planning by identifying geological structures. An inherent weakness in the current system of in-seam drilling is the inability to maintain the bit within the confines of the coal seam. As a result, coal seam boreholes tend to undulate, frequently drilling out of the seam and duplicating drilling meters.

It is estimated that 20-30% extra metres are drilled as a result of unplanned branches. Coal seam gas boreholes that undulate are not desirable from a gas drainage perspective. In addition, branching carries a risk of borehole collapse, and establishing a branch takes time.

This project has been designed to primarily address the coal interface detection (CID) problem, and test suitable CID technologies for in-seam drilling. A secondary objective was to evaluate geophysical technologies as an aid to geological interpretation from in-seam drilling. The specific objectives of this project include:

  • A review of available CID technologies.
  • Field work in a horizontal borehole drilled from a highwall to test potential suitable technologies.
  • Recommendations for the application of these technologies.

Field work was carried out in two stages, after the first stage was prematurely terminated by a severe storm and flash flood that swamped the drill site. The trial involved the testing of the following technologies: directional gamma, omni-directional gamma, radar, dielectric (reactivity / resistivity), spectrometric gamma, density, acoustic caliper and a number of survey instruments.

The work established conclusively the value of gamma and density tools for CID application. In particular, directional gamma provided unequivocal results which are simple to interpret. Density proved a valuable aid in delineating dykes from roof / floor intersections. Extremely encouraging results were also obtained from the dielectric tool and acoustic caliper. Radar and spectrometric gamma produced unconvincing results and the former is unlikely to be useful as a CID tool in its current form. Spectrometric gamma underperformed, but in principle should not be rejected as a potential CID tool, and needs further work.

In current surface to in-seam medium radius drilling, gamma and EM attenuation profiling are providing encouraging results in a real time CID application.

This project has resulted in a clear direction for in-seam logging in Australian coal mining. Our findings indicate the following:

  • CID technology is already working adequately for Surface to In-seam MRD applications. This particularly relates to those boreholes drilled with a gamma sonde and EM attenuation profiler.
  • This technology should be applied underground and needs to be developed to be Intrinsically Safe.
  • There is significant potential to improve the geological exploration information obtained from in-seam boreholes using post-drilling wireline retrievable shuttles. This technology can be advanced in MRD virtually right away and underground subject to the approval process. The shuttle should include, at a minimum; Directional gamma; Density; Acoustic Caliper; Dielectric

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