Underground » Ventilation, Gas Drainage and Monitoring
The coal mining industry in Australia is facing the ever increasing challenge of managing gas emissions. The coal extraction process in underground mines can significantly alter stress and fracture distributions around mine openings, which forms flow channels for gas emissions into longwall and development faces. As a result, gas emissions from adjacent layers can largely contribute to specific gas emissions (SGE). Although gas content measurement for coal seams is a normal practice, the estimation of gas content in coal measures remains a challenging task. When present within certain type of sandstones the free form of gas content would escape quickly during the drilling and coring process, making direct measurements unfeasible. In some underground mines, gas measurement results during longwall production suggest that free gas in interburden can contribute up to about 25% of SGE. In addition, the conventional method to measure gas content in coal seams based on coring and canister desorption is slow, expensive, and labour intensive.
The measurement results of gas content in coal only provide point source data and are not sufficient to cover the large variability throughout the geological strata from the surface to hundred metres deep underground. An accurate and continuous estimation of gas content in both coal seams and coal measures throughout an entire geological formation is critical to predict SGE, design gas drainage and ventilation strategies, and maintain compliance with gas concentration in the underground environment.
Geophysical logs, on the other hand, are routinely obtained that provide continuous underground information at fine resolution and have the potential to estimate gas content across various intervals along the entire length of wellbores. Although geophysical logs have been widely used in the oil and gas industry to evaluate hydrocarbon resources, their potential application for estimating gas content in the Australian coal mining sector has not been extensively explored. The quality of geophysical logs obtained from the mining industry is believed to be less reliable/accurate than the petroleum counterparty, in terms of using it as a scanning tool for fine resolution and detecting unit thickness.
The objective of this project was to develop reliable methods for estimating gas content in coal measures and adjacent coal seams to the mining level using geophysical logs for underground coal mines. This project aimed to establish the framework of using geophysical logs to estimate gas content in coal/coal measures. Although the accuracy of the prediction models is one of the key performance indicators, the focus at the initial stage was to demonstrate the feasibility of proposed methodologies, rather than striving for the highest prediction accuracy. Several key components were developed and ensembled to form a framework for gas content estimation, which may require further calibrations for field implementation.
This report includes a comprehensive literature review, laboratory measurements, machine learning modelling, theoretical analysis, and case studies conducted in three different Australian coal mines. The goal was to establish robust techniques that can leverage geophysical logs to provide accurate gas content estimations in coal seams and coal measures, thereby enhancing the understanding and management of gas emissions during underground coal extraction, especially in terms of SGE prediction.
It should be emphasised that the gas content prediction models developed in this project are only applicability to underground mines for the purposes of ventilation management or gas drainage designs. As a conservative prediction, the model tends to adopt the 'worst-case scenario assumptions' that may overestimate the presence of coal/strata gas. This enables mine ventilation or gas drainage engineers to have enough safety factors while designing, however, does not necessarily reflect the amount of gas emissions from the site. The proposed models should NOT be used to estimate gas content or calculate fugitive emissions from open cut mines, due to the conservative nature of the models and lack of cap rocks to retain the gas over a long geological time.