煤层吸附气的全解吸过程及组分与碳同位素变化:基于热模拟实验结果

解吸法是测试煤层含气量、评价煤层气地球化学特征的常用方法,但由于存在一定数量的损失气,使得对煤层气的全解吸过程了解甚少,也难以评估损失气拟合计算的可靠性。利用自主研发的煤层气/页岩气生成与解吸实验装置,对一块煤岩样品(Ro=0. 84%)进行了模拟(模拟后样品Ro=1. 80%),精确测定了在设定条件下的损失气、解吸气与残留气的数量、成分与甲烷碳同位素,对比研究了USBM直线法和多项式回归法对损失气拟合计算的制约条件与可靠性,探讨了煤层气解吸过程中成分与甲烷碳同位素分馏的机理。结果表明:损失时间是影响损失气量估算结果可靠性的关键,当损失时间较短(0. 25 h),USBM直线法与多项式法估算的均损失气量较接近真实值;相比之下,多项式法的结果更为可靠。样品气体解吸过程存在成分与甲烷碳同位素分馏,表现为:气体干燥系数(C_1/C_(1-3))总体降低,甲烷碳同位素(δ~(13)C_1)逐渐变重。样品广泛发育纳米孔隙结构,在气体解吸过程中存在的解吸-扩散-运移分馏是导致气体组分和甲烷碳同位素分馏的重要原因。

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 geochemical 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 fractionation 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 (<0.25 h),the average amount of the calculation results of USBM linear fitting and polynomial fitting is close to the actual results,and the calculation result of polynomial fitting is better.The fractionation of chemical and stable carbon isotopic composition of the desorbed gas is found and characterized by the gas drying coefficient declines in total,and the higher methane carbon isotopic composition during whole desorption process.The nano pore structure is extensively developed in the studied samples,and the desorption diffusion migration fractionation is the main reason leading to the fractionation of chemical and stable carbon isotopic composition of the desorbed gas.