Floor Seam Gas Emission Characterisation and Optimal Drainage Strategies for Longwall Mining

Gas emission from floor coal seams during longwall mining can cause significant safety and productivity issues. It can result in substantial gas make in the ventilation return, exceeding the ventilation system's capacity to dilute the gas to a safe level. Sudden releases of gas into the working face, can also shut down longwall production systems.

Effective management of floor gas requires a clear understanding of floor-seam gas-emission processes and patterns. The lack of understanding in this area means that floor-gas emission predictions often display large discrepancies from actual experience and measurements. Mining induced strata behaviours determine the desorption and migration processes of gases into and from the floor seams. An insight into floor-strata behaviour and its dynamics is key to the effective prediction of gas emission and design of optimal gas drainage systems. To date, the majority of studies on strata behaviour have focused on overburden strata. However, floor strata behave differently from the roof strata, as floor strata do not experience the caving processes that occur in the roof strata.

With the current trend of longwalls towards high production and greater depth, and the requirement for low emission mines, it is necessary to study gas emission and optimal drainage from both roof and floor coal seams.

This project aimed to characterise floor-seam gas emissions and identify strategies for optimal gas drainage in longwall mining. The scope of work included:

• Field-gas data collation and analysis to obtain floor-gas emission characteristics experienced in Australian coal mines, and to understand existing gas-drainage practices and performance;
• Field-experiment data analysis to understand floor-strata porepressure responses to mining, and identify the extent and degree of floor-seam gas emissions;
• Geomechanical and two-phase coupled numerical modelling to characterise floor-strata stress changes, deformation, permeability changes and gas-emission patterns;
• A literature reviews to help establish a conceptual model characterising floor-strata fracturing and permeability enhancement;
• Computational fluid dynamic (CFD) simulations to investigate floor-gas flow dynamics into the goaf and workings, and mechanisms of gas drainage;
• Integrated analysis to develop strategies and optimal approaches for floor-gas drainage.