Underground » Ventilation, Gas Drainage and Monitoring
Coal seam trapped gas in underground coal mines poses significant health and safety hazards for mine personnel and contributes to environmental degradation through greenhouse gas emissions. Underground inseam (UIS) gas pre-drainage is commonly utilised in Australian underground coal mines as a method to lower the gas content. The efficiency assessment of gas pre-drainage operation is thus critical for coal mining operators.
The objectives of the project were to:
- assess the theoretical and experimental findings on the capability of seismic waves to map coal seam gas content in the target areas and to identify poorly drained areas for evaluating gas pre-drainage practices,
- evaluate the challenges and benefits of implementing time-lapse in-seam seismic (ISS) in gas pre-drainage practices for underground coal mines, and
- assess the ability of ISS to cover large distances in coal seams, such as approximately 500 m.
Earlier project C27027 revealed that wave velocity undergoes variation during the desorption of gas from coal. Specifically, a decrease was observed in wave velocity as gas drainage occurred mainly due to change in overall porosity, indicating a reduction in the gas content within the coal. This finding served as a fundamental basis for this project investigating time-lapse (in-seam) seismic measurements for detecting gassy areas in coal seam on a large scale.
The literature review undertaken as part of this project, revealed that while the seismic and in-seam seismic have been widely utilised for geological investigation in the past, there is a notable scarcity of studies on time-lapse seismic and particularly time-lapse in-seam seismic methods. Specifically, the research landscape lacks comprehensive evidence demonstrating the efficacy of time-lapse inseam seismic measurements in assessing gas pre-drainage. Delving deeper into the literature, it became apparent that in-seam seismic techniques excel in identifying coal seam properties and anomalies compared to surface seismic surveys. This superiority extends to time-lapse inseam measurements, which offer enhanced accuracy and spatial resolution for coal seam characterisation. This advantage is particularly pronounced when the surrounding strata exhibit greater strength than the coal seam, and the coal seam itself demonstrates homogeneity. In such cases, the transmission of seismic waves (channel waves in in-seam seismic) remains relatively unaffected, contributing to the superior performance of in-seam seismic methods.
A clear consensus emerged from the literature that transmission configuration is suitable for mapping the gassy zones in the field measurements although no conclusion is drawn on reflection configuration due to data scarcity. For successful implementation of time-lapse in-seam seismic techniques, careful design considerations are paramount.
The literature review also highlighted a crucial aspect of data interpretation. While traditional in-seam seismic (ISS) data interpretation often involves complex processes to extract and interpret anomalies such as faults, coal seam thickness, fractures, etc., the interpretation of time-lapse ISS data is comparatively more straightforward. In time-lapse ISS interpretation, the focus is primarily on the differences between responses over time rather than connecting them with specific coal seam properties or features. This streamlined approach reduces the complexity associated with interpretation, as the emphasis is placed on identifying changes over time rather than analysing specific geological features.
The literature review also brought to light the utility of resistivity measurements in monitoring changes within the coal seam, particularly through electrical conductivity assessments, which can indicate shifts in water content. However, it was noted that the spatial coverage of high precision resistivity measurements within coal seams is often limited to less than 100 meters, restricting its applicability for large-scale investigations.
An essential consideration in gas content detection is whether to use transmission or reflection modes of wave propagation. While transmission has been employed in a few studies outlined in the literature, practical constraints often limit access to both sides of the coal panel for installing transducers and receivers, making reflection measurements the only viable option. However, existing literature does not clearly establish whether the interface between low- and high-gas content areas can cause measurable wave reflection. If wave reflection cannot detect these interfaces, then transmission emerges as the only option. In transmission, geophones can be placed in drilled pre-drainage boreholes or surface boreholes intersecting the seam. On the other hand, if reflection proves effective, it offers superior spatial resolution and higher coverage percentage, despite being operationally more complex.
While in-seam seismic measurements offer valuable insights into the effectiveness of pre drainage operations, it is crucial to acknowledge the associated limitations. The main limitation of the technique is linked to difficult data interpretation, particularly if the geological structure of the coal seam is complex. The other limitation of the technique is related to the effect of noise and artifacts on collected data, which can reduce the accuracy of the measurements thus needs careful attention. The reliance on differences between surveys over time, rather than interpreting individual surveys independently, reduces the impact of complexities in geological structure and reduces the influence of noise and artifacts. As a result, time-lapse ISS emerges as a promising approach for overcoming these challenges and obtaining more robust assessments of pre-drainage effectiveness.