Study of coal sorption characteristics and gas drainage in hard-to-drain seams

The subject of coal sorption characteristics and investigations into the reasons for coal seam gas drainability of the Bulli seam in Sydney Basin were undertaken by focusing on Metropolitan Colliery, where certain parts of the seam have been found to be especially hard-to- drain. Specifically, one part of this study was to examine gas generation and flow mechanism in coal, to understand coal isotherm testing and calculation methods and the environmental influences on the coal sorption characteristics such as temperature and moisture content and coal particle size variations. Another part of this study was to investigate the possible reasons resulting in hard-to-drain coal at Metropolitan Colliery, including coal cleat system variation, sorption capacity, coal microstructure, coal permeability, gas content and composition. Laboratory tests were conducted to investigate the impact of nitrogen injection to promote gas desorption from hard-to-drain coals. Worldwide, different types of apparatus and methods are used to generate coal sorption isotherms, but in general, there are of two types; volumetric and gravimetric methods. In this study, both methods were introduced in detail and comparatively examined. The unique gravimetric apparatus in the University of Wollongong, called the indirect gravimetric method apparatus, including its setup, operation procedure, calculation theory, and calculation methods were comprehensively modified and introduced. Different factors influence coal sorption characteristics. Concluded from both dry and moist coals\u27 test results, the adsorbed volume of CO2 and CH4 is decreased with increasing temperature. The adsorption capacity of coal (Langmuir volume) also decreases with coal moisture. Ash content of coal samples and the density of coal determined by helium were found to increase with increasing coal particle size. The experiments of CO2 and CH4 adsorption of different particle size coal sample were conducted. The tested coal isotherms were compared and Langmuir volumes were examined and concluded. Coal sorbed volume decreased with the increasing coal particle size, both on a dry basis and dry ash free basis. Coal surface free energy theory was found to fit the experimental test result, and it could be used to explain theoretically the sorption behaviour of coal at higher temperature and moist conditions. In the investigation of the main reasons resulting in hard-to-drain coal at Metropolitan Colliery, coal cleat systems were identified both in the lab and field. Geological background was examined and geological variations could be the explanations of coal permeability change and CO2 concentration variation, hence inducing gas drainage problem. Scanning Electron Microscopy (SEM) was used to analyse the coal microstructures. It was observed that the microstructures of the hard-to-drain coal samples appeared to be tighter and less porous when compared with the easy-to-drain samples. Two different types of the permeability tests were conducted and showed that the Metropolitan coal permeability decreased with the increasing gas pressure and stress. The permeability converges to a steady level below 1 mD under high triaxial stress conditions. Such a relative low permeability could be another one of the possible factors causing the problem of hard-to-drain in the Bulli seam. As the hard-to-drain area is typically CO2 rich, the CO2 isotherm should affect the gas drainage more than the CH4 isotherm. Evidently, the experiment results shows coal adsorption capacity for CO2 is much higher than CH4, indicating that coal seams with higher CO2 concentration and high gas adsorption capacity (7% ash content) can result in low gas saturation, contributing to poor gas drainage problems as these in the Bulli Seam. A critical examination of the whole gas database (519 samples) and typical hard-to-drain database (97 samples) at Metropolitan were conducted. The relationships of Q1, Q2 and Q3 gas content and their ratio in response to total gas content QT were statistically analysed. Comparative analysis of gas content and composition between the whole gas database and the hard-to-drain area was also carried out. Apparently, a direct warning index for the hard-todrain area in the field can include relatively lower gas content (6-10 m3/t), high gas composition of CO2 (CO2\u3e80 %, CH4\u3c20 \u3e% or CH4/(CH4+CO2) Laboratory tests were conducted to systemically analysis the N2 injection enhancement process. It was observed that the coal seam gas (CO2 and CH4) can be flushed out by N2 injection. The experiments results in terms of gas concentration, gas volume and gas content were examined in different stages. Furthermore results also show both N2 gas flushing and gas desorption processes have influences on the coal swelling and shrinkage behaviour. Results from these tests provide invaluable knowledge for field trials of this innovative technology that could potentially lead to enhanced gas recovery from hard-to-drain or low permeability seams