CO2 IN AUSTRALIAN COAL SEAMS, STORAGE AND GAS DIFFUSIVITYPROPERTIES AT LOW GAS PRESSURE

摘要

Many Australian coals contain significant amounts of carbon dioxide. This has beenrnthe cause of numerous gas outbursts in coal mining in Australia in the last half arncentury. In order to control the gas outburst, thresholds limits on the CO2 content ofrncoal have been set by coal mining industry. For instance in the coalfields of NewrnSouth Wales in Australia, a limit of 5 m3 per tonne of coal has been set as statutoryrnlimit to ensure safe mining operations. This CO2 content limit was based empiricallyrnon numerous outburst experiences with headings driven into geological discontinuitiesrnand so far this limit have proved to be adequate.rnStudies have been undertaken to qualify and quantify the nature of outbursts and thernrole of gas type in the onset and the amount of energy released during such geomechanicalrnevents. The capacity of coal to store carbon dioxide together with itsrnability to release this gas are two factors among others that influence the occurrencesrnof gas outbursts. In order to accumulate enough energy to break mechanically the coalrnand rocks extensive volumes of free gas are required. Therefore for an outburst tornoccur coal must have stored a large amount of gas within the coal in an adsorbed staternwhich can be released from the in a short period of time. The adsorption capacity andrndiffusivity of coal for CO2 have therefore important roles in determining whether suchrnoutbursts occur.rnOver the last decade these two factors, namely coal CO2 storage capacity and therndiffusivity of gas through coal, were investigated by CSIRO. Numerousrnmeasurements of coal adsorption isotherms on crushed coals and measurements ofrnCO2 diffusivity through solid coal discs were undertaken. To measure the adsorptionrncapacity of coal a gravimetric system was developed and used for Australian coalrnseams with in-situ thermodynamic conditions of up to 6 Mpa pressure andrntemperatures below 50 ℃. A system for measurement of diffusivity was also built tornmeasure the flux of gas through solid coal discs under constant concentration gradientrnat ambient temperature. Coefficients of diffusivity were determined for the same coalsrnfor which CO2 adsorption isotherms were measured.rnThis paper presents some of these measurements and discusses the results. The resultsrncan be used in other areas were CO2 storage and flow are of interest such as inrnenhanced coalbed methane studies and CO2 sequestration.