Risk Minimisation in Longwall Operations in Massive Goaf Conditions Using Microseismic and Hydraulic Fracturing Techniques

Moonee Colliery operates a longwall extraction system in the Great Northern seam of the Newcastle Coal Measures, approximately 100km north of Sydney, NSW. In the initial mine area of longwall blocks1-4a, the Great Northern seam is overlain by a massive conglomerate approximately 35m in thickness at an average depth of cover of 130m. Unanticipated sudden, blocky caving of this material has produced windblasts with associated velocities of 100m/sec and over-pressures of 35kPa. Although these events have not resulted in fatality, the potential safety risk to face personnel was such that the mine was threatened with closure. The development of a Windblast Management Plan to minimise the exposure to windblasts, a Micro-seismic Management Plan to predict caving events, and the use of hydraulic fracturing to generate caving on demand, have all contributed to the saviour of the mine. Prior to the introduction of hydraulic fracturing, falls were highly variable and ranged in goaf area from 2000m2 to 30,000m2 with an average of around 7,000m2. Hydraulic fracturing has introduced consistent and far less variable falls, planned as averaging around 5000m2 and all accompanied by microseismic warning. Moonee Colliery is believed to be the only operation in the world using real-time microseismic warning and using hydrofracturing as a day-to-day operational process. These processes have enabled it to return to financial viability in an environment of strict procedural control, whilst ongoing efforts are made to optimise and enhance these systems. Moonee’s story is a case study of the successful application of research and development work to the economics of a mine