Pulse Infusion Shotfiring
Inducer shotfiring is considered to be the oldest and most widely used method to control outbursts in mines, and it was initially developed in France by the end of 19th or beginning of the 20th Centuary in the Gard coalfield. Blast holes are drilled into the face and loaded with charges. The charge delivers a short intense shock wave to the strata, which may trigger outburst, Stress relief is accomplished whether an outburst occurs or not by the fracturing of the blast area, transferring high stress concentrations to an area farther in advance of the face. Danger from outburst occurring under such circumstances is minimal since mine personnel and equipment are evacuated from the area.
The Inducer shot firing can be very effective in reducing outburst hazards, and since its development the methods found applications in various European coalfields, Canada, Japan and also in Australia. The application of inducer shotfiring has been practiced in a number of Australian mines, and in particular, there is several documented reporting of the practice by various researchers from Metropolitan Colliery, NSW and Collinsville, Queensland (Chap 5, Lama and Bodziony, 1996) (Click).
A major concern with this technique as used throughout Europe and as was used at the Metropolitan Colliery is the apparent increase in the number of outbursts in a particular mine over and above what was previously or usually the case, and irrespective of the gas type. Such reporting is in agreement with the perceptions held various researchers that inducer shotfiring has negative impact on coal permeability outside the immediate fractured boundaries of the coal seam
Two of the key elements associated with outbursts are mine gases and tectonics. Just as gas drainage involves the prevention of outbursts in the seam and or associated rock strata to prevent outbursts. Currently, there are four different mechanisms used to de-stress a seam outburst prevention, these being stress relief drilling, stress relief mining, and inducer shotfiring and water infusion.
Stress relief techniques serve to reduce outburst potential, by reducing the ability of the immediate face to store the energy necessary for outbursts. The fracturing of the immediate face insures that high gas pressures or structural stress conditions do not exist in the next cut. Structural stress is shifted to the solid coal inbye the face. Gas pressure is reduced by coalbed gas flow through the induced fracture systems. Stress relief boreholes of about 100 to 200 mm in diameter are drilled into the face to mechanically destress the coalbed by removing large volume of drill cuttings. Stress relief slot cutting on longwall faces is reported to be highly successful for reducing outbursts. Relaxation slots are systematically cut from 2.0 m to 1.0 m deep into the face, destressing the immediate face area.
Water infusion, the injection of water at high pressure (>7 Mpa), has been explored as a stress relief mechanism to prevent outbursts. The water pressure fractures the coal in advance of the face and provides for slow relaxation of stress through lateral movement enhanced by the presence of the water. However, the effectiveness of water infusion decreases as the depth of cover increases, owing to the reduction in coalbed permeability with depth. Other methods using the same principle include pulsed infusion shotfiring and hydrofraccing.