Underground » Ventilation, Gas Drainage and Monitoring
Hydraulic fracturing was attempted as a way of stimulating horizontal gas drainage holes in coal at German Creek Mine, Central Colliery. Work was carried out in one horizontal hole drilled into the 307 longwall panel.
Prior to the fracturing trial, the German Creek seam (at 275.5 m depth) was cored from the surface and desorption isotherms were obtained from the core. The minimum horizontal stress in the roof and floor rock was measured at 6.0 to 7.7 MPa. This stress component was measured at between 2.5 to 3.0 MPa in the coal seam. Permeability in the range of 1 md and a seam pressure of 0.47 MPa were measured.
The hydraulic fracturing equipment was set up and operated on the surface. A vertical borehole drilled from the surface into 14 cut-through provided access to the collar of the horizontal hole.
A straddle packer was run into the horizontal hole, inside a string of HQ rods that were reamed into position prior to running the packers. In each attempt to fracture the borehole, the straddle packers failed to seal. Bypass of water occurred under the pressure of the hydrostatic head of water in the injection string. Previous work in the nearby vertical site characterisation hole (DD 316) measured a hydraulic fracture extension pressure of 3 to 3.5 MPa and, because the pressures reached in these trials were generally less than 2.7 MPa, no significant hydraulic fracture growth is thought to have occurred.
The budget for this project was expended during this unsuccessful trail. The report provides details of the successful and unsuccessful operations carried out as part of this trial.