Underground » Ventilation, Gas Drainage and Monitoring
The relationship between gas production from underground-to-inseam (UIS) drainage boreholes and various coal seam properties and operational factors were examined. Gas production from 279 UIS gas drainage boreholes was collated and assessed relative to a variety of coal geological properties and operational factors. The reasons for poor coal seam gas drainage performance from particular zones were investigated and actions to improve gas drainage performance have been recommended. Investigation were focused on gas drainage performance from the Bulli seam of the Sydney Basin, focusing on West Cliff Colliery, where gas production was highly variable and many zones found to be difficult to drain.
The degree of saturation (DoS) was found to have a significant impact on coal seam gas drainage, with decreased gas production from highly undersaturated zones with low permeability.
From analysis of operational factors, drainage time was found to have a significant impact on gas production and appeared to be closely related to DoS indicating that coal with lower DoS required increased drainage time. Borehole length and orientation were found to have some impact on gas production with maximum gas production achieved from boreholes between 500 and 1,000 m long oriented between 5 and 60O to the face cleat and between 0 and 40O to the maximum horizontal stress. Boreholes drilled up-dip, with an apparent dip between 0.0 and +3.0O achieved increased gas production and the relationship was strongest in highly undersaturated coal. In saturated coal the initial gas flow rate tends to be high and the increased gas flow velocity supports the borehole to self-clear water and fines. With increasing age, gas flow velocity reduces which appears to affect the ability of the borehole to self-clear, particularly in boreholes oriented down-dip. Undulations such as troughs existing along the length of the boreholes also allow water and fines to accumulate which impedes gas drainage. No evidence was found to support a relationship between applied suction pressure and gas production. However where high suction pressure is applied to boreholes increased leakage may occur. A new method for enhancing coal seam gas production using cyclic injection of inert gas is proposed.
The nature of coal seam gas emission from both fast and slow desorption gas testing methods was investigated using results from 4 185 gas tests collected from eight Australian underground coal mines, four located in Queensland and four in New South Wales.