Desorption process and variation of chemical and carbon isotopic composition of coalbed adsorbed gas based on the results of thermal simulation

The desorption method is commonly used to calculate the coalbed methane gas content and evaluate the geo- chemical characteristics of coalbed methane. However,the whole desorption process and evaluation of the reliability of lost gas fitting calculation are unable to be assessed due to the gas loss. Based on the apparatus for simulating coalbed /shale gas generation and desorption,a coal sample was used for thermal simulation experiments,and the Ro of the coal sample became from 0. 84% to 1. 80% . The gas content,chemical and stable carbon isotopic composition of the lost gas,desorbed gas and residual gas under different conditions were calculated accurately. The restricting conditions and reliability of USBM linear fitting and polynomial fitting to calculate lost gas were compared. The mechanism of the frac- tionation of chemical and stable carbon isotopic composition of the desorbed gas during whole desorption process was discussed. The results show that the lost time is a key factor to affect the reliability of lost gas fitting results. If the lost time is short (