Underground » Ventilation, Gas Drainage and Monitoring
This project is an extension of work conducted in Project C6024 (Wold and Choi, 1999) and C9023 (Choi and Wold, 2003). Some of the results from Project C11030 (Wold et al., 2006) were also utilised in this project. The aims of the research project were to:
· Improve the understanding of coal properties pertinent to gas drainage and outbursts;
· Improve the understanding of outburst mechanism; and
· Provide a rational basis for the assessment of existing gas content threshold for safe mining under outburst conditions.
In projects C6024 and C9023, a numerical model for outburst initiation and evolution was developed by linking a geomechanical model with a coalbed methane reservoir simulator. For outburst risk analysis and for outburst control and management, it is important to be able to assess both the likelihood of an outburst event and the consequence in case such an event does happen. These aspects were the main focus of projects C6024 and C9023 respectively. In project C6024, a series of parametric studies was conducted using the "coupled" model to identify which are the key variables in outburst initiation, and which are the less important variables. The model results strongly support the importance of gas pressure and pressure gradient and geological structures in determining threshold values for outburst risks. The influence of other variables such as the orientation of the principal components of the in situ stress, the effects of changes in stress on permeability, rate of mass transport between adsorbed gas and free gas, and heading advance rate were also studied. A degree of understanding of the significance of those variables on outburst initiation was obtained. In contrast to the general experience that areas of high CO2 content are more hazardous with respect to outburst compared to areas with high CH4 content, the model predicted, a slight reduction in outburst initiation potential with an increase in CO2 proportion in the gas composition for the same initial reservoir and desorption pressures.
One of the outcomes of C9023 was the development of a model which can be used to predict the likely volume of coal that can be expelled, and the volume of gas that can be emitted, taking into account the influence of gas composition. A better understanding of the cause of the observed differences between CH4 and CO2 outbursts was also obtained during the project.
The results of projects C6024 and C9023 suggest that the mechanisms for outburst initiation have to be distinguished from those for outburst evolution. The effects of gas composition on outburst propensity and intensity under different conditions still need further investigation.
The effects of coal seam heterogeneity on outburst propensity have been studied during Project C11030. The results further reinforced the importance of gas pressure and pressure gradient and coal strength on outburst initiation.
This project provides a better understanding of outburst mechanisms through laboratory model outburst tests conducted using cored coal samples, especially the effects of gas composition on outburst, and further validates the numerical outburst model that has been developed to date.
The important results from this project are:-
1. The laboratory triaxial tests did not show any observable difference in stiffness when the samples were saturated with helium, nitrogen or methane, but a slight increase in stiffness was observed in samples saturated with carbon dioxide. This could be caused by the higher degree of matrix swelling during CO2 adsorption leading to the closure of cleats, pre-existing cracks and/or macro-pores.
2. Laboratory uniaxial compression tests showed that carbon dioxide may cause a reduction in peak strength which could be due to mechanical damage caused by differential matrix swelling during CO2 adsorption.
3. Reconstituted coal samples have been developed which are highly homogeneous with properties reproducible in the laboratory.
4. There was no observable difference in outburst initiation and severity on tests conducted using reconstituted coal samples saturated with either CO2 or nitrogen, the latter has even lower adsorption affinity to coal compared to methane.
5. Outburst initiation is dependent on pore pressure, pressure gradient, porosity and geometry of the connected pore space, and the strength of the coal samples.
6. The numerical outburst model has been further tested and validated through its ability to predict the results of the laboratory model outburst tests.
7. Better understanding of outburst mechanisms has been obtained which has been applied to explain some of the field and laboratory observations reported in the literature.
8. The results of this project have demonstrated the importance of pressure gradient force and hydrodynamic (or drag) force of the free gas, coal strength and structures on outburst initiation. The importance of these factors was also supported by field observations and laboratory model outburst tests conducted by the authors and others. Most importantly, it seems all outbursts can be explained by careful consideration of these factors, and it is possible, based on the outburst mechanisms, that all outbursts are associated with some forms of structures whether they are pre-existing or mining induced.
9. The current outburst management protocol based on gas content and composition is a simple approach that should work well in preventing major outbursts. However, such an approach may be overly conservative for coal which is reasonably strong and with very low porosity and permeability, and reasonably free of major outburst prone structures. Also, the measured gas content may not correctly reflect the actual pressure distribution in the seam as mining progresses, especially if flow occurs mainly along some high permeability fractures or flow paths. For seams with very low porosity and permeability, a more rigorous approach based on numerical modeling which takes into account stress, coal strength, pressure, pressure gradient and any likely structures should be conducted. Depending on the conditions for a particular mine, it is possible that mining may be able to proceed safely at higher gas contents than the current threshold values with the monitoring of some important field parameters such as reservoir pressure and gas flow rate in the drainage holes, and the conditions of the coal around the face. It is important to ensure that there is not enough free gas to initiate an outburst when outburst prone structures are encountered either through gas drainage or some other suitable control and preventive measures depending on the particular conditions.