煤储层物性对甲烷碳同位素分馏的影响

The Impact of Coal Reservoir Physical Properties on Carbon Isotope Fractionation of Coalbed Methane

煤层气作为一种吸附气,在储层中解吸-扩散运移时甲烷碳同位素发生分馏。笔者通过采集山西晋城和新疆昌吉不同演化程度的煤样进行解吸试验,系统记录整个解吸过程中甲烷碳同位素组成的变化情况,研究了储层物性对甲烷碳同位素分馏的影响。结果表明:对于基质致密的煤样,解吸气体的δ13C1随时间增加逐渐变重,其变化的速率具有先快后慢的阶段性特点。对于基质疏松的煤样,解吸气体的δ13C1随时间增加先轻后重,这是由于取芯操作及煤样解吸过程中的基质收缩变形破坏了煤体原生结构,从而对正常的同位素分馏效应产生了影响。随着成熟度的增高,煤中微孔丰度增加,气体解吸-扩散过程中受的限制增强,同位素分馏效果更显著。压裂裂缝的存在影响了煤层甲烷碳同位素的分馏效果,使得试采过程中井口气样δ13C1的变化规律不明显。

Carbon isotope fractionation of coalbed methane （CBM） as a kind of adsorbed gas occurs during diffusion transport of coalbed methane （CBM） in coal reservoir. In order to investigate the effect of reservoir physical proporties on the isotope fractionation, a canister desorption experiment of coal was carried out with the samples of different maturity, collected from Jincheng of Shanxi and Changii of Xinjiang, and the δ13C1values of all the desorbed gas were recorded carefully. The results show that for the consolidated coal sample, the measured ~3Ca values of desorbed gas increase with time, and change rate is characterized with fast in the beginning and slow at the late stage. For the unconsolidated coal sample, the measured δ13C1 values of desorbed gas decrease firstly and then increase, and this is because the drilling operation and matrix shrinkage which damaged the original structure of coal during desorption influenced the normal isotope fractionation. Micropore abundance increases with increasing coal maturity, which imposes much restriction on the gas flow and results in distinct isotopic fractionation. Meanwhile, due to the effect of fracturing on the isotope fractionation, the δ13C1 values of methane collected in well head change irregularly in trial exploitation.