Underground » Ventilation, Gas Drainage and Monitoring
This project's purpose was to determine parameters that may be indicative of outbursting from normal gas drainage drilling using down hole motors. The objectives were to measure, on a sample by sample basis, gas content and diffusion coefficient of the coal being drilled. It was also intended to measure the particle size distribution and the ratio of cuttings volume to hole volume on a continuous basis.
A special sieving system was developed for the purpose and was tested at Appin Colliery. While the system generally worked, its labour requirements made it impractical. However it did enable the determination of cuttings volume/length and cuttings size distribution. It revealed that a significant proportion of cuttings remained in the hole forming a cuttings bed. This could be stirred up and moved by rotation and flushing.
The gas content measurement nearly worked. The main problem encountered was in not knowing where the cuttings came from. This problem was associated with the formation of the cuttings bed in the hole. These retained cuttings affected transport in an undetermined manner after the bed became established. The other unexpected finding was that larger size cuttings appear to have lower gas contents. Why this is the case is unknown. It may be a problem of cuttings transport speed, and once again knowing where the cuttings came from and how long they have been in the hole.
The diffusion coefficient for the cuttings can be determined fairly readily.
The operation at Grasstree mine achieved little except for some understanding of how to work in high gas flow conditions.
No assessment of the outburst risk at either of the trials was made. What we achieved in the case of Appin was to provide some idea of the energy available from desorbing cuttings on the basis of their size distribution, diffusion coefficients and gas contents. This assessment was only made at four locations and so the significance of the variation in the potential energy release estimates is statistically uncertain. Having real comparable numbers for energy release is however a very significant step forward in outburst detection development.
It is thought that a straightforward discrete cuttings sampling operation can be used to determine gas content, cuttings size distribution and diffusion coefficient from a normal gas drainage hole. To be able to prove this will require a hole to be sampled for cuttings and cored over a statistically significant sample size.