In 2001 and 2002,Dartbrook Coal Mine in the Hunter Valley of NSW,encountered lowpermeability coal in Longwalls 107,108 and 109.Standard horizontal in-seam drain holes werefound to require extremely long times to drain the coal to a gas content level below 5 cubic metersper tonne.Hydraulic fracture stimulations,using water and sand,were therefore carried out in twoboreholes in Longwalls 108 at Dartbrook to assess the effectiveness of sand propped fractures instimulating gas drainage from in-seam boreholes.The results from this trial were used to developand carry out hydraulic fracturing treatment in horizontal holes drilled throughout Longwall 109to accelerate gas drainage in that panel.A hydraulic fracturing system,that could be deployed and operated in the panel roadway,wasassembled for the test.A water-inflated straddle packer system was used to isolate sections of theholes for each treatment.The stimulations produced a significant increase in gas drainage ratesfrom the two boreholes with one increasing by a factor of about 180 while the other holeincreased its rate by about 22 times.The stimulated gas rates continuously increased for severalweeks and the higher rates were sustained for the entire period the holes were monitored.Fracture modeling suggests the sand proppant bank may extend to 15m from the borehole.Theunpropped portion of the fracture may extend to more than 40m.A purpose-built fracturingsystem was developed and used at Dartbrook to stimulate holes that covered most of LW109.This full-scale enhancement of gas drainage was successful,producing an average increase in gasdrainage rate of 5,which allowed efficient mining of that panel.Fracture stimulation of horizontal uncased boreholes in coal is limited in application by boreholeconditions.Borehole conditions at Dartbrook were exceptionally good,but this is not generallythe case at other mines.Packer life is severely affected by borehole failure which results inoversize sections of hole and sharp broken out borehole walls.At the same time,the low andvariable fracture initiation pressure along an open borehole in coal limits the applicability ofslurry jet induced fracturing,which relies in part on holding the borehole pressure at a value nearfracture extension pressure.Methods to overcome the problem posed by borehole stability arebeing developed.
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