Report: APP CMHS Project 1

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3.1.5. Surface to In-seam Drilling

Surface to inseam (SIS) drilling is utilised for two different purposes: pre-drainage of the seam with extended lead time, where there is good surface access; and post drainage of longwall goafs.


The use of Medium Radius Drilling (MRD) technique that drain gas ahead of mining through drainage holes drilled from the surface is one viable means of achieving the required drainage ahead of development mining process. The MRD method compares favourably against cross panel drilling techniques in terms of cost due to fact that it is independent of the underground workings and can drain gas over a longer period of time. Additionally the gas drained from the target areas can be used commercially for power generation or transported via pipeline to a PowerStation for large scale drainage programs over a longer period of time.

The MRD technique is being used successfully in the Bowen basin to pre drain gas ahead of development and longwall mining at several mine sites. How ever this technique will not be applicable to all mines due to depth and surface constraints and the requirement for significant lead time (18 months to 3 years) to allow for water draw down. Such as the conditions found in the Illawarra coalfields where depth surface constraints and low permeability make the method unsuitable. Such long lead times allow for a wide spacing of holes to be adopted but require up front capital expenditure and detailed planning. Mines using MRD surface to inseam technology include Moranbah North Coal Mine and Grasstree Coal Mine.

Tight radius drilling (TRD) has been developed and implemented (Figure 26) for areas such as the Illawarra Coalfields where there is limited surface real estate and significant gas reservoirs that must be drained prior to mining. The technology has also benefited from the inclusion of water jet assistance.

Figure 26 Tight radius drilling

An extension of tight radius drilling that is being developed by CRC Mining from the petroleum industry is that of coiled tube drilling (Figure 27). Instead of using straight drill rods that are threaded together, a coiled drill string wound on a large diameter drum is used with a down the hole motor. More detailed information about the drilling technology can be found in the website

Figure 27 Coiled Tube Drilling (Technicoil website)

Goaf Drainage

As for pre drainage techniques, post gas drainage is performed by the placement of drill holes into the target areas, in this case the mined out areas (goafs) to allow the gas generated during mining to be captured before migrating to, and contaminating the working face. Post drainage techniques are much more difficult to plan and execute as the gas quantities present in the goafs may fluctuate with mining conditions. The drill holes themselves are also exposed to dynamic ground conditions affecting the stability and performance of the holes. Even with a high concentration of goaf holes at close spacing, methane concentration at the return end of the face can exceed the permitted operational gas levels, leading to a significant number of face stoppages. The main difficulty in goaf gas control optimisation is the lack of understanding of gas flow patterns in the longwall goafs and the influence of various mining parameters on this goaf gas flow behaviour.

Post drainage of gas from longwall goafs is typically carried by either drilling boreholes from the surface as shown in Figure 28 into the goaf at regular intervals and extracting the gas as with a suction fan arrangement. These extraction systems can draw anywhere from 500 to 5000 l/s of gas from the goaf depending on the size of the installation and operate at pressures around 1 to 10 kPa and up to 40 kPa at some mines. This system shows goaf holes that vent to the surface, but drainage can also be conducted, in some circumstances, through panel seals and piped to a return shaft or borehole to the surface.

Figure 28 Typical goaf drainage, hole layout

The following guidelines are recommended for optimum goaf gas drainage strategy:

  • Surface goaf holes for gas drainage provide the highest capacity and lowest cost option for goaf gas drainage under most circumstances.

  • Goaf holes should be drilled on return side of the goaf, preferably at 30 to 70 m from gateroad depending on the longwall caving conditions.

  • Goaf gas drainage holes diameter should be in the range of 250 mm to 400 mm for optimum flow rates and the goaf holes may be drilled at 100 to 300 m spacing depending on the goaf gas emissions and other conditions.

  • The total capacity of the goaf gas drainage plants should be around 2 to 3 times the expected goaf gas emissions in the panel to achieve optimum performance of the gas drainage system and provide better gas control on the face.

  • The goaf gas drainage system should include a combination of goaf holes near the face and deep goaf holes in the panel in order to improve the overall gas drainage efficiency and to reduce the effects of barometric pressure changes on tailgate gas levels.

  • The strategy of continuous operation of deep goaf holes at moderate capacity should be implemented. i.e., intermittent operation of deep goaf holes at high capacity may not improve the overall efficiency and may lead to problems.

  • The ventilation system in the panel should be designed to minimise oxygen ingress into the goaf, including immediate sealing-off all the cut-throughs behind the face, in order to improve overall gas drainage efficiency.

  • Oxygen concentration level in the goaf holes flow should be less than 5 % to reduce spon com risk in the longwall goafs.

  • Gas drainage from adjacent old goafs should also be carried out wherever possible, depending on the goaf gas emission flow rates.

  • Goaf gas drainage should be carried out from more goaf holes at optimum capacity. For example, gas drainage from 3 to 4 holes, instead of 1 to 2 holes would reduce oxygen ingress into the goaf.

    Sites where surface goaf drainage installations are used include Appin Coal Mine, Moranbah North Coal Mine and Austar Coal Mine.

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