In-seam drilling for the reduction of gas in underground coal mine operations, and for the production of coal seam methane reserves, is increasing in Australia. The flow behaviour of water and gases into a borehole within coal is not well understood and is an extremely complex process. The stress distribution around the borehole, fluid properties
and the unique nature of coal still produce unusual fluid drainage responses during gas drainage and production. A poor understanding of the gas drainage mechanisms and drill design can result in costly ramifications for mines and methane producers. As coal seams are exploited, both within an underground coal mine drainage and commercial gas production sense, operations will tend to deeper depths where the seam fluid pressures and stresses are higher. The increasing technical challenges in extracting the gases at greater depths will rely on a good understanding of the drilling technology and fluid drainage mechanisms around a borehole within a coal seam.
conducted in this thesis involved both laboratory and field coal based fluid drainage research separated into the following sections:
· Laboratory determination of coal shrinkage
behaviour at different gas pressures
· Laboratory permeability testing to observe the effects resulting from changing effective stresses and fluid flow
· Field water injection and interference testing to determine coal seam reservoir permeabilities
· The development of a new gas drainage drilling system utilising
high pressure water jet technology. In addition, high pressure water jets were utilised for the purpose of slotting and stimulating gas drainage boreholes for enhancing fluid drainage.
The inherent variability of laboratory coal permeability and shrinkage was found to support previous research and be related to coal characteristics. The high rank and brighter coals exhibited higher shrinkage rates. There was no relation found in this study to relate the permeability of HQ coal samples with either the bright banding within the coal or rank. Permeability is generally directly related to the effective stress to which the coal is subject. Coal samples tested in this study,
with minimal cleating or brightness observed, exhibited permeability effects that were insensitive to effective stress changes.
The results from water injection testing at German Creek have shown that increasing the injection fluid pressure results in a reduced borehole skin (additional pressure drop at borehole wall) effect. This is due to the extension of a reduced effective stress zone that progresses outwards from the borehole wall as the fluid injection rate (and therefore pressure) is increased. This result makes it difficult to contain any additional pressure drop effects near the wellbore into permeability determination using a single numercial skin
The cleated coal may be considered to represent, in oilfield terms, the matrix permeability whilst the major fractures represent a fracture permeability. The water injection tests conducted clearly identified the matrix and fracture permeability values for the Aquilla and German Creek coal seams, particularly in the analysis of Homer build up plots. As these permeabilities obtained are time dependent, extending the test interval (i.e. duration of testing) would help to confirm the real reservoir fluid flow behaviour. Interference tests were also conducted. Interference tests involve pressure monitoring remote from what is usually an injection well. Their advantages are two
fold. Firstly because they are remote they do not suffer from near wellbore effects associated with skin or local effective stress changes around the wellbore. Secondly if more than three observation points are used they permit the measurement of directional permeability. Finally in a single-phase situation they permit the estimation of reservoir storage parameters.
New underground gas drainage drilling technology was developed which utilises high pressure water to assist conventional rotary drilling. The benefit of this technology is the reduction in feed and torque forces on the drill that results in straighter gas drainage holes across the longwall panel. The high pressure water
pump (developed for the water jet rotary drilling technique) was also utilised to demonstrate slotting of underground gas drainage holes for stimulation of gas flow rates. Three out of five holes stimulated clearly showed improvement in gas flow rates within a poor draining region of the Tahmoor coal mine.
The coal permeability stress and shrinkage properties were implemented, together with varying coal seam conditions, into a one dimensional linear elastic steady state model developed to estimate the fluid flow effects around a borehole. Comparisons of varying shrinkage and permeability stress relations on gas drainage performance are
Impacts of the results from the research in determining coal reservoir properties and drainage performance have also been provided. The main findings from the modeled results were:
1. Coals, which have permeability relationships insensitive to stress, would benefit from having larger gas drainage drill holes in the coal seam. The modified stress distribution around the borehole would not adversely affect the seam permeability and more connectivity to the seam in-situ fractures would result. This is partly supported by the slotting experimentation conducted at Tahmoor mine where three of the five holes slotted exhibited increased gas drainage flow rates.
2. At low alpha values (coal sensitive to effective stress changes), coal shrinkage clearly produced higher gas flow rates by modifying the effective tangential stress zone close to the borehole resulting in a higher permeability zone. High alpha values obviously make the coal drainage characteristics insensitive to varying shrinkage rate.
3. Fluid pressure also influences the stress distribution and water flow into a borehole. In high alpha conditions, the fluid pressure gradient across the seam is seen to significantly influence the stress and permeability profile around the borehole. Increased fluid pressure close to the borehole wall reduces the effective tangential stress and results in increased permeability. This supports the finding of reduced skin effect with increasing injection pressure during water injection testing.