Moonee’s windblast conditions are caused by goaf hanging up for around 300m with subsequent falls generating windblast with speeds of 123m per second. (See related story published in Australia’s Longwalls, March 2001).
Ken Mills of Strata Control Technology (SCT) and Rob Jeffrey of CSIRO Petroleum have been helping Moonee use hydraulic fracturing (hydrofraccing) at Moonee since 1999 to generate goaf falls. The technique, used in the petroleum industry, applies high pressure fluid injected into a borehole to initiate a fracture around the borehole. By breaking up the massive strata, goaf falls and the associated windblasts, are made more manageable. While hydrofraccing is successful in bringing the goaf down, it has not yet been able to generate a fall immediately during longwall startup.
Over the last six longwall blocks at Moonee, the first goaf fall in a new panel has been roughly twice as big as any subsequent falls. Prior to the trial using gel, three longwall startup fracture treatments had been performed at Moonee using water, none of which produced a fall during pumping. Insufficient pump rate and high loss into the conglomerate were thought to be the main reasons a fall could not be generated immediately.
It was believed the crosslinked gel would provide a better chance of successfully inducing the first goaf fall. The gel’s high viscosity allows the fluid to bridge across fractures, where water simply leaks away.
Generally at Moonee, the hydro-frac process is undertaken underground because of surface access restrictions. The technique was not tried previously at Moonee because of the lack of surface access due to a national park. Surface access is important because of the requirement for high capacity pumps and the infrastructure (water truck and tanks) to mix the 50,000 litres of gel.
The mine saw a window of opportunity when permission was granted for surface access because of the existence of a track immediately above the area to be drilled.
A vertical H-size borehole was drilled into the centre of a cut-through on the maingate side of LW6 to provide access for injecting the gel to fracture the holes completed in the startup goaf for the panel. The vertical hole was drilled to intersect the maingate roadway.
The gel was mixed in batch tanks and crosslinked at the point of introduction into the pump and was then pumped into the borehole at 700 litres/minute. The auto frac box on the longwall face allowed monitoring of the up-hole pressure which allowed selection of any of the three boreholes to be selected for treatment. Roughly 50,000 litres of crosslinked gel fluid were mixed and pumped into the centre, maingate and tailgate boreholes.
Moonee’s project coordinator, Col Macdonald, witnessed the goaf fall which occurred six hours after initiation. Macdonald said this was the first time the goaf was caved on demand after a new longwall block startup.
According to SCT’s Mills, hydrofraccing had potential applicability in other longwall mines which suffered from periodic weighting problems or would benefit from inducing caving at startup.
“Hydraulic fracturing offers a way of cheaply controlling the caving behaviour of strata,” he said, but added that the technique required a thorough understanding of the stress environment that characterise the ground conditions.
SCT and CSIRO have formed a joint venture, Hydraulic Fracturing Technology, to offer further commercial applications to coal mining and other industries.