Cleats are natural fractures in coal that serve as permeability avenues for darcy flow of gas and water to the well bore during production. Theoretically, the development of cleat and coal-seam permeability is related to the rank, type and grade of the coal concerned. The permeability of a coal seam, moreover, may change during gas production, due to either matrix shrinkage, cleat closure or both. Matrix shrinkage and cleat closure are also affected by numerous geological factors, including coal rank, desorption character and geological setting. A method integrating geochemical and petrographic analysis, reservoir engineering diagnosis, geophysical data and production characteristics has been developed, and used to determine the initial permeability of coal seam on a metre by metre scale. This overcomes the constraint of conventional well test by refining the test intervals. The effect of coal rank, grade and type on the initial permeability of coal seams was also investigated, with the special reference to the coals of the Galilee Basin. The permeability was estimated using analytical equations based on the permeability data obtained from well tests and from cleat descriptions within the seam section. This aspect of the study showed that the coal type, rank and grade strongly influence the initial permeability of individual coal seams. Increase in ash content has negative effect on cleat development and permeability. On contrast, increasing coal rank and proportion of bright coal lead to reduction in cleat spacing and increase in permeability. Twenty three core samples collected from the Qinshui Basin in China were evaluated in the laboratory to investigate the effects of coal grade, rank and type on the change in permeability during pressure depletion. The experimental factors included the coal's geochemical properties, the permeability against changing pressure, and strain with pore pressure depletion. This part of the study fund that permeability changes with pore pressure depletion in relation to coal rank, grade and type. The strain values determined by the experiments with pressure depletion were used to identify the mechanical principles associated with changes in permeability during pressure depletion in relation to the rank, grade and type of the coal concerned. A reservoir simulation study was used to investigate the effects of desorption pressure, geological setting and coal rank on the variation in permeability under in-situ conditions during coalbed methane production, based on a study in the Hedong area, Ordos Basin, China. The simulations allowed history matching of gas and water production from 12 wells with the actual well conditions specified as the model pressure. Good agreement was achieved between the model yields and the actual production data, suggesting that the changing permeability interpreted from the simulation is a realistic representation of the in-situ reservoir properties. The reservoir simulation study found that the decreases in permeability with production exceeded the increase in permeability caused by matrix shrinkage for nearly all wells in the Hedong area. The magnitude of the decrease in permeability increases as the gap between initial pressure and desorption pressure increases. The decrease in permeability is slower in the zone closest to the fault. The reservoir simulation has demonstrated that coal rank influences significantly the change in permeability during coalbed methane production.
| Identifer | oai:union.ndltd.org:ADTP/257242 |
| Date | January 2007 |
| Creators | Wang, Xingjin, School of Biological, Earth & Environmental Science, UNSW |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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