甲烷在多孔介质煤层中运移时，会发生显著的同位素分馏，目前的模型和机制并不能完全解释瓦斯运移过程中的同位素分馏现象。以不同分子间的竞争吸附作用、扩散能力差异性以及碳同位素分馏解吸—扩散成因等理论为基础，构建了煤层甲烷碳同位素分馏动力学模型。研究结果表明，在解吸过程中，甲烷碳同位素逐渐变重。瞬时同位素值在解吸后期呈指数级变重，累计同位素值随时间的变化与气体的累计脱气量随时间的变化相似。在气体扩散过程中，扩散系数比值(D1/D2)对同位素分馏具有显著的控制作用。煤芯中吸附气的含量占到90%以上，解吸/吸附引起的同位素分馏变化与耦合作用下引起的同位素分馏变化相近，该模型较好的预测了煤芯瓦斯解吸中碳同位素分馏实验。甲烷在孔隙中的解吸、扩散是碳同位素发生分馏的重要原因，在此过程中游离气和吸附气引起的同位素分馏具有阶段性。通过该模型的参数与煤芯的参数匹配后，可以获得煤芯吸附气含量、游离气含量，进而评价煤层瓦斯含量。将该模型应用到煤层钻孔瓦斯抽采过程中，结合现场测试和实验室测试确定模型参数，建立煤层钻孔抽采瓦斯碳同位素值与瓦斯抽采情况的关系。可以判断煤层瓦斯抽采所处的阶段，并计算残余瓦斯含量,为评价煤层瓦斯抽采效果的潜在应用提供理论基础。

Transporting in coal seams with porous media,methane can result in significant isotope fractionation,which cannot be fully explained by the existing models and mechanisms. Therefore,the dynamics model for fractionation of carbon isotopes of methane is developed based on the theories such as competitive adsorption among different molecules,the difference of diffusion capacity and the desorption diffusion causes of isotope fractionation. The results are shown as follows:the carbon isotopes of methane become increasingly heavier during the process of desorption. The instantaneous isotopes become quickly heavier at an exponential speed during the latter period of desorption. The variety of accumulative isotopes is similar to that of accumulative amount of degassing gas. During the process of diffusion,the diffusion coefficient ratio (D1/D2) has a significant effect on isotope fractionation. The content of adsorption gas in coal core accounts for more than 90%,and the variety of isotope fractionation caused by adsorption/ desorption is close to that of isotope fractionation caused by coupling process,which means that the model better predicts the experimental results of carbon isotope fractionation for desorption of methane in coal core. The important cause of isotope fractionation is the process of desorption and diffusion for methane in pore,where the isotope fractionation caused by free gas adsorption gas is of stage wise. According to the match of parameters between model and coal core,the content of adsorption gas and free gas can be obtained to evaluate the content of methane gas in coal seams. When applied to the process of desorption and diffusion for gas in coal seams,its basic parameters can be determined through on site and laboratory measurements. The model is applied to the process of gas drainage,its parameters are determined by combining with field test and laboratory test,and then the relationship between the carbon isotope value of gas extracted from coal seam and gas drainage situation is established. The stage of coal seam gas drainage can be assessed,and the real time residual gas content of coalbed around drainage borehole can be obtained,which can provide a theoretical basis for the potential application of evaluation effect of coal seam gas drainage.

1 模型建立

2 同位素分馏的影响因素

   2.1 纯扩散引起的甲烷碳同位素分馏过程

   2.2 考虑吸附/解吸、扩散的甲烷碳同位素分馏过程

   2.3 模型验证

3 讨论

   3.1 孔隙结构对甲烷碳同位素分馏的影响

   3.2 模型在瓦斯抽采效果评价中的潜在应用

4 结论