Underground » Ventilation, Gas Drainage and Monitoring
1. The basic underlying mechanism for outburst initiation involves the expulsion of coal at a pressure gradient above a critical value which is directly related to the strength and porosity of the coal at the current state. Coal strength, porosity, stress, gas pressure and pressure gradient are important for outburst initiation.
2. The severity of an outburst depends on gas pressure, the hydrodynamic force, the strength and toughness of the coal, and the amount of free gas. For the same pressure gradient, the degree of violence is greater for weaker and more friable coal. Permeability can be important for outburst evolution by controlling the amount of gas that would become available to drive an outburst. Outburst propensity can be changed by changing the method of mining, mine geometry, and the preventive and control measures adopted by the mines.
3. It is necessary to distinguish between outburst initiation and post-initiation outburst evolution for outburst control and management.
4. Preventive measures are designed to directly impact the factors that drive the outburst process. The most effective preventive measure has been proven to be gas drainage. This is simply because gas is the main driving force for initiating and sustaining an outburst, and outburst will not occur without gas. Gas is required for dislodgement and transportation of outburst coal, and the hydrodynamic force generated by the flowing free gas has to be high enough for this to occur. This is however not always the most cost effective way, depending on the permeability of the coal. It should be noted that during gas drainage, water is expelled and the pore space is replaced by gas. Some outburst may only occur after enough water has been expelled and there is enough gas in the pore space to drive an outburst. Water infusion can be used to reduce the pore space occupied by gas (or the amount of free gas) (Díaz Aguado and González Nicieza, 2007). Kuzmin (1995) found that outburst propensity can be reduced by reducing the permeability. Depending on the particular conditions of the mine and whether gas drainage or restriction of in-seam gas migration is more desirable, permeability enhancement or reduction can be a better option.
5. Considering the importance of strength as an important factor for outburst initiation, it is of no surprise to see that strengthening of the rock by filling up the cracks with hardening material has also been found to be an effective (Kuzmin, 1995) method for outburst prevention. Another closely related approach is through the minimisation of mechanical damage (or strength reduction) to the coal using destressing roadways and destressing slots (Wood and Hanes, 1982).
6. Gas drainage is effective in reducing the pressure gradient and the amount of free gas. Gas drainage should therefore be able to prevent or reduce the severity of an outburst. This should be the first-line preventive measure if there is no problem in draining the seam. This is also for mine ventilation considerations if both the gas content and permeability are high.
7. The effects of gas composition on outburst initiation or evolution should be able to be explained by its influence on some of the factors mentioned in (1) and (2) above. Seams rich in CO2 can be more outburst prone under certain conditions where the rate of desorption is high enough to have a significant influence on the gas pressure and the amount of free gas, such as in regions where sheared or mylonitic coal is present. For more intact coal, whether CO2 is more outburst prone or the gas content threshold value for CO2 should be in general lower than CH4 needs more research and back-analyses of field data.
8. Depending on how the rate of gas desorption may contribute to the spatial and temporal variation in pressure distribution as mining progresses, desorption rate may contribute to outburst initiation through a higher drag force for coal in the form of very small particles. However, mylonitic coal can be more outburst-prone simply because of its low strength and higher porosity than normal coal. The effect and mechanism of gas desorption rate on outburst propensity and severity need further investigation.
9. Gas content provides an indication of how "gassy" the coal seam is before mining. From the sorption isotherms, we can infer desorption pressure from gas content, and if the coal seam is also in a state of sorption equilibrium, we can also assume that desorption pressure is the same as reservoir pressure and estimate reservoir pressure from the gas content. This is however only true if the free gas is at sorption equilibrium with the adsorbed gas. Such a state may not be attained if the seam is being actively drained, and if both seam permeability and rate of face advance are relatively high compared to the rate of desorption. Gas content may also not correctly reflect the reservoir pressure if flow occurs mainly along some localised high permeability flow paths and the size of the matrix blocks between fractures are reasonably large. Actual reservoir pressure can be higher or lower than indicated by gas content. Also, if mineralisation occurs in the micro- and meso-pores of the coal matrix, diffusion of the gas can be very slow and gas desorption/adsorption may continue for some time before new sorption equilibrium is achieved. Even though gas content may provide some quick indication of how "gassy" the seam may be, gassiness also needs to be considered in relation to porosity, potential sources of free gas and permeability, and the total potential volume of gas that will be available to drive an outburst. There can be insufficient gas even if the gas content is high either because of very low porosity or the gas content does not actually reflect the true reservoir pressure due to the low rate of desorption. In a mechanistic sense, one should be monitoring the pore pressure distribution in the seam behind the face, and the rate of gas that is producing from the drainage holes. This may also provide a useful indication of the possibility of an outburst prone structure at some distance ahead of the face, and whether drainage to below certain critical pressure is required. Some of this information can be obtained from the drainage holes and flank holes. However, it is important to ensure that the drainage holes are not blocked by drill cuttings and caved in coal or by water standing in low parts of the borehole.
10. Past experience appear to suggest that some of the hard to drain areas can be mined through at gas content above threshold values without any outburst incident until some weak structures are intercepted which is in agreement with the outburst model predictions obtained from previous ACARP projects. However, the assessment requires numerical modelling studies based on the field and reservoir conditions, and mining operations of the different mines. It is important to understand the reasons for areas which are hard to drain. Is the ground boggy because of high stress or highly deformed coal associated with some major geological structures or tectonic history? Is the low permeability caused by infill of the cleats by minerals and the coal has not experienced any degradation in mechanical properties? This can have strong implication on how these areas can be mined through safely and cost effectively.
11. Coal can fail/weaken under stress when the face approaches zones of weaker coal which may be associated with geological structures. The duration taken for the transition from a meta-stable to unstable state depends on stress, properties of the coal, amount of gas and gas pressure. During the transition, bumps may occur and the face may become "hardened". These can be signs of an imminent outburst and immediate safety measures should be taken.
12. It is possible that most outbursts, if not all, for mining in the Bulli Seam were associated with some types of structures.
13. Because of the usually highly heterogeneous nature of coal and the difficulty to detect some small local heterogeneity such as some pockets of very weak materials, some very small scale (or localised) "outburst" can be difficult to avoid, such as the formation of some small conical cavities. Focus should however be on the detection of major structures.
14. If it is practical and achievable, it may be more important to monitor reservoir pressure in addition to gas content.
15. The relative importance of the various factors and parameters will depend on the conditions of individual mines. It is necessary to treat the coal-rock-stress-structure-gas interaction as a system. One way to do this is to use a numerical model that can model the individual processes and their interaction, which is where the numerical outburst model that has been developed to date would be useful. The model may be used to help a particular mine to identify the critical factors for outburst control and management purpose especially when mining under new conditions such as at much greater depth where no previous experience can be used as reference.