Underground » Ventilation, Gas Drainage and Monitoring
Gas drainage is a major issue currently affecting underground coal mines. The difficulties associated with underground gas drainage are partly responsible for the poor development rates for longwall coal blocks; this in turn limits coal production from underground operations. In an effort to improve longwall productivity and address current safety issues associated with methane drainage the CMTE has been investigating the applicability of high pressure (HP) water (20 - 40 MPa) for assisting conventional rotary drilling at both Appin (BHPC) and Dartbrook (Shell) mines. ACARP (Australian Coal Association Research Program) under Project Numbers C5028 & C6028 have supported the technology. Dunn et al (1997) has presented the results from the C5028 work previously. This report details the subsequent findings from the C6028 supported project.
The aim of the waterjet rotary drilling technology is to produce straighter, longer and faster in-seam gas drainage boreholes than those currently produced by conventional rotary and Down Hole Motor (DHM) drilling technology. A successful outcome would be of considerable benefit to underground coal mines as the use of DHM drilling to gain drainage hole accuracy has significant cost penalties.
From the trials conducted in C5028 it was concluded that the CMTE waterjet rotary drilling project had demonstrated improved hole accuracy and penetration rates for in-seam cross panel drilling. With additional benefits such as reduced cuttings size and reduced feed and torque forces on the rig, this indicated the potential for waterjet rotary longhole drilling (> 1km).
The C6028 ACARP project has allowed the development of the cross panel waterjet rotary drilling technology to be finalised. Waterjet rotary drilling has the potential to drill acceptably straight holes at a significantly faster rate than conventional rotary or DHM drilling. In addition, the problem of holes working their way to the floor (as experienced during the C5028 project) has been rectified by the use of stabilisers. The down hole drilling assembly can now be designed to give a slightly positive build angle (vertical angle of trajectory) and then water pressure can be used to regulate the vertical trajectory by controlling hole size. A larger hole size allows the drill string to work its way downward due to gravity. For hole lengths up to 250 metres, the work to date indicates that waterjet rotary drilling could be commercialised as a high speed cross panel drilling system.
The project objective was to produce a long hole drilling system that has the accuracy of DHM drilling and the productivity of rotary drilling, while minimising the loss of expensive equipment down hole. The ultimate aim of the project was to drill holes of up to 1 km and beyond. During the C6028 project it was not possible to fully demonstrate horizontal azimuth control of the borehole trajectory, using high pressure waterjets to erode the coal preferentially. Problems with drill rod failures and those associated with conducting trials underground, where test time is limited due to production requirements, meant that insufficient testing over long hole lengths (> 300 metres) was conducted. Although a long demonstration hole was not achieved during the project, the authors believe that the technology has now been developed to a stage that a long hole drilling system is possible. The successful demonstration of a system which has a horizontal steerage method for longhole waterjet rotary drilling will lead to a drilling system which has the accuracy of DHM drilling and the productivity of rotary drilling. This technology should then be able to achieve hole lengths in excess of 1 km. Further testing will be required before rotary waterjet drilling can be extended to long holes. Waterjet drilling field trials into a highwall on surface would allow the required testing and development of the horizontal azimuth control before demonstrating the technology underground. Additional funds of around $100,000 would be required to finalise the waterjet rotary longhole drilling technology development.