Current methods of draining gas from coal ahead of longwall mining are sometimes ineffective in parts of New South Wales’ Bulli Seam mines in the southern coalfield. When this happens, roadway development rates are impacted, leading to delays to the real customer, the advancing longwall.
To overcome these delays, some mines have in the past resorted to such methods as remote mining and more recently, face grunching – a form of roadway development using explosives.
Recent studies at the University of Wollongong aimed to relate the changes in the ground geological conditions with permeability, and coal volume changes with the type of gases present. Test sites with different geotechnical settings were used, which were defined by their structural complexity and hence, difficulty in releasing gases. Other parameters such as coal petrography and ground stress regime were also included in the characterisation process.
Disturbed coal is very complex: desorption of gas results in shrinkage, and changes in stress affects permeability and consequently the way the material fails during an outburst event.
With this in mind, researchers focused on combining coal structure with as many of the physical aspects to influence the outburst mechanism. Researchers tested samples for shrinkage, permeability and petrography, with the aim of establishing a database for coal properties.
Coal samples originated from two unique geological areas, characterised as “normal” or “disturbed”. Correspondingly, two different rates of inseam drainage resulted and the disturbed sample was exceedingly difficult to develop roadway headings through.
Results for coal shrinkage showed shrinkage of the 800 “normal” area was twice that of the 900 “structured” area for carbon dioxide. Therefore in practical terms, the 800 area was shown to have greater capacity for inseam gas drainage.
To measure coal permeability, parameters that were monitored included stress, strain, gas flow rate, constant circumferential gas pressure, and constant suction. Gas was charged into the sealed pressure chamber at 3 megapascals for one week to allow sufficient saturation and the strain to be recorded. Once the sample was fully saturated, the release valve was opened and released gas passed through various flow meters. Information from load cells, strain gauges and flow meters was monitored in a data logger connected to a PC.
Permeability testing for each sample used a variety of mine gases under differing axial load gas pressures. The gases used were methane, carbon dioxide, and a 50:50 mixture of both gases. The permeability of each sample was then calculated using the universal Darcy flow equation.
The results showed a marked difference in the resultant permeability between the 800 and 900 panel coals. The difference in permeability (in millidarcy) between the two panels for each of carbon dioxide and methane was quite significant, with 800 panel coals showing three times greater permeability compared to 900 panel coals.
In coal petrography, there was a marked difference in the mineral matter and carbonate content for the samples originating from 900 panel coals. Petrographically, the three samples had similar organic components, with similar vitrinite, liptinite and inertinite contents. However, the mineral contents of the samples were quite different.
One of the 900 panel coals had a much higher mineral content, including much higher carbonate (calcite) and another, although not in sufficient quantities to show in the point count, showed some carbonate. In both samples, the carbonate infilled cleats and also some of the pores with inertinite macerals. If the mineral content and type is common for the coal as a whole in the 900 panel, the permeability and degassing problems associated within the panel can also be explained in terms of petrography.
The permeability tests for both carbon dioxide and methane showed 900 panel coals had much lower permeability than the 800 panel coals. Since permeability is a function of a number of parameters including size, distribution and frequency of cleats, any phenomenon that reduces cleat porosity will decrease permeability. Given that 900 panel coals contain much higher carbonate contents than 800 panel coals, and also have the lowest permeability, it suggests the reduced porosity of the 900 panel coals is due to the infilling of the cleats with carbonate.
When viewed on a mini scale, the reduced permeability also explains why the 900 panel area is much harder to degas. The carbonate infilled cleats restrict the movement of gases from the surrounding coal to the gas drainage holes.
Permeability and petrographic data as well as the shrinkage results, confirmed that the coals from the 900 and 800 panel area were markedly different. This has had consequential results for the efficiency of the mining cycle in these two areas of the mine.
Further work is required to investigate the relationship between mineral matter and the gas regime. Incorporating other mines in this study is seen as vital in continuing coal outburst research to new levels. Support from industry and major industry-funded sponsorship is needed to establish these coal mine outburst relationships, which in turn may further identify the relationship between the desorption rate and shrinkage that leads to unfavourable gas pressure/effective stress gradients in typical underground settings.
This new information can be used to re-evaluate outburst assessment methods in view of geological and geotechnical parameters, which ultimately could lead to enhanced development rates.