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Conference Paper

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Publication Details

This conference paper was originally published as Heritage, Y & Gale, W, Using Helium as a Tracer Gas to Measure Vertical Overburden Conductivity Above Extraction Panels, in Aziz, N (ed), Coal 2009: Coal Operators' Conference, University of Wollongong & the Australasian Institute of Mining and Metallurgy, 2009, 146-154.


The potential of helium injection into goaf and overburden strata as a tool to determine and measure goaf to surface connectivity is discussed. Laboratory studies investigated the flow mechanics and flow velocities of injecting helium through fractures by goaf injection technique and applied the laboratory findings to the field. From the laboratory studies, it was found that the mechanics of helium flow through fractures is by bubble flow. A relationship between gas velocity and fracture aperture was found allowing the determination of fracture conductivity through helium injection, which was comparable with previous works. Field trials of helium injection into the goaf were successfully conducted to determine whether a connection exists between the surface and the goaf. Helium injection process was carried out in two stages; the measurement of background helium, and injected helium. The average fracture aperture was determined from the arrival time of the first injected helium pulse, which takes the most direct path to the surface. The equivalent average conductivity was calculated from the average fracture aperture. Another technique of borehole helium injection was used to determine connection in the fracture network of the overburden. The borehole helium injection technique is a more direct approach of injecting helium into the fracture network of the overburden. With a borehole drilled into the highly permeable caved zone of the goaf, then borehole helium injection can demonstrate more quickly if a connection to the surface exists. A repeatable technique of helium injection into the goaf or borehole has successfully been developed and demonstrated to prove connectivity between the goaf and surface of a longwall coal mine. These techniques will prove an effective tool for monitoring of environmental and hydrological problems.

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