Proactive interburden fracturing using UIS drilling with validation monitoring

Abstract

A series of gas inrush events occurred during development at Grosvenor Mine resulting in exposure to elevated levels of methane at the production face. A total of 22 gas inrush events occurred, with between 15 and 130 m3 of methane released during each event. The presence of an undrained seam in the immediate floor, geotechnical characteristics of the floor, and the stress environment all contributed to these dynamic floor events, while the geological characteristics of the seam below, such as the seam thickness and ash content of 75%, prevented effective predrainage. However, events only occurred in headings mined parallel to the principal horizontal stress direction. In cut-throughs (C/T) perpendicular to the principal stress direction no events occurred, and higher methane levels were observed at the production face. The solution to preventing the gas inrush events involved creating a conduit in the interburden between the mined seam and the seam in the immediate floor to allow the gas to be drained during the development of the headings, as occurred in the cut-throughs (cut-through and cross-cut are regional terms that are analogous). A series of underground inseam (UIS) holes were drilled using the directional drill rig with the aim of fracturing this interburden ahead of the face and promote floor failure to allow the gas to release consistently. The floor fracturing was conducted using water pressure generated from a longwall salvage pump, with the current UIS drilling equipment retrofitted with a series of subs, packers and a fracturing tool. The packers and the fracturing tool were shifted to desired locations along the drilled UIS borehole to achieve the required fracture. The fractures were monitored using a proving hole and with a HYDAC data logger attached to the salvage pack, with the results analysed on the surface to ensure connectivity to the working seam.