DESORPTION PROPERTIES OF SOME COAL RESERVOIRS AND METHANE RECOVERY RECOVERY RATE IN CHINA

Based on desorption experiments under normal pressure and the data obtained from coalbed methane(CBM) well tests,it is suggested that the desorption properties of coal reservoirs in China are very different from each other and the desorption rates are controlled primarily by buried depths and coal ranks The optimum desorption depth ranges from 400 to 600m However,it increases with coal rank if the vitrinite reflectance(R _o,max) is less than 3%,and decreases with coal rank if R o,max larger than 3% In China there is no apparent correlation between adsorption time and coal ranks,however,in some places of China the adsorption time is related to gas content If the gas content is low,the adsorption time must be long,especially when the gas content is less than 8m³/t The existing data show that the adsorption time of coal reservoirs in China is generally less than 9 d.The coalbed gas recovery rate is not only controlled by gas content,adsorption and desorption properties of coal,and by in place pressure system,but also by drilling techniques and mining technology According to experiences in the U S and measurement in China,the average theoretical recovery rate of CBM in China is estimated as 27%.