Underground » Ventilation, Gas Drainage and Monitoring
Pre-gas drainage is widely used as an effective method to control gas and coal outburst in underground coal mines. However difficulties of reducing gas content below threshold limit values (TLV) within a given drainage lead time have been encountered in some coal mines in QLD and NSW, especially in carbon dioxide abundant coal seams. By injecting an alien gas such as nitrogen into the coal seam, the gas drainage performance can be improved. In this study, field trials of nitrogen injection enhanced gas drainage were conducted. This study aims to conduct field trials using underground inseam (UIS) directional boreholes to demonstrate the use of nitrogen flushing to accelerate gas drainage rate to enable statutory threshold values to be met in hard-to-drain or low permeable seams.
For the design and implementation of field trials, extensive laboratory studies of the Bulli coal characteristics were conducted using coal samples collected from the top, middle and bottom of the Bulli seam of the planned trial sites at Metropolitan and Appin mines. All samples have a large volume of pores with a diameter up to 6nm while the micro-pores account for most of the pore space and surface area. Permeability changes of the cores are measured under different confining stress mimicking the field geo-stress conditions. The permeability of the coal samples was found in the order of 5~0.01mD under a confining pressure condition from 2MPa to 8MPa. From the permeability tests, we conclude that the coal seam at the proposed trial area is comparatively tight and less permeable under current stress conditions at Metropolitan Colliery. Nitrogen flushing tests are conducted in laboratory using core samples and these tests indicated that a ratio of 3~10 units nitrogen is required to displace 1 unit of CO2.
Numerical reservoir simulations are conducted by the COMSOL Multiphysics software. A binary gas transport model has been developed to simulate the nitrogen flushing process of coal seam gas. The simulations involved an improved permeability and fully-coupled model considering the competitive adsorption of binary gas and its influence on the coal matrix-cleat interaction, and the mechanical response to binary gas state change under in-situ stress condition by constraining core displacement. This model was fully validated by laboratory nitrogen flushing tests. The residual gas content of the core and the gas composition variations of the production gas flow matched well with the simulation results. The simulations demonstrated that coal seam permeability played a vital role in governing the nitrogen flushing process.
This project has demonstrated that nitrogen flushing with UIS boreholes can be used to accelerate gas drainage in hard-to-drain or low flow boreholes. This is a promising technology for stimulating gas recovery from tight coal seams. The field trial has established a work procedure that can be referred to and used in future field work. Future work should consider a full scale field demonstration project using a group of boreholes from a drainage stub and a continued supply of nitrogen source from surface or underground. Outcomes from this project are also useful for other options of seam gas drainage such as surface in-seam (SIS) holes with nitrogen flushing.