气体扩散、渗流过程中同位素分馏机理及定量表征研究

The isotopic composition and its fractionation of natural gas, which are the most important carrier of the vital geochemical information, have been widely used to evaluate the geological information for exploration, and show great potential to accurately predict the gas content and the ratio between the adsorbed and the free gas under geological condition. The isotopic composition of natural gas is the results of comprehensive action of hydrocarbon generation, dissolution/precipitation, adsorption/desorption, diffusion, transfusion and so on. Therefore, the quantitative evaluation of the isotope fractionation in each process is the foundation for further analyzing the geological information from the isotopic composition of natural gas. The diffusion and the transfusion are the important processes causing the isotope fractionation of natural gas, but they were not separated effectively and investigated in detail. In this study, the diffusion and the transfusion would be separated effectively by changing the media and the gas in the experiment, and illuminate the law and its key factor of the isotope fractionation in the diffusion and transfusion; then based on the theoretical simulation on the molecule level, the diffusion coefficient, velocity of molecule movement, slip length in gas molecules with different isotopic composition will be compared to reveal the micro-mechanism of natural gas isotope fractionation in the processes of diffusion and transfusion; according to the law and the micro-mechanism achieved above, the coupling model could be built to quantitatively evaluate the gas isotope fractionation in the diffusion and transfusion.

天然气同位素组成及分馏效应是其蕴含关键地球化学信息的最主要载体，在对关键地质信息评价中得到了广泛应用，并在页岩气/煤层气的含气性及吸附/游离气比例评价中展现出了巨大潜力。天然气同位素组成是生烃、溶解/脱溶、吸附/解吸、扩散和渗流等多个热力学和动力学过程综合作用的结果，因此，定量刻画每个过程中同位素分馏效应是进一步准确解析天然气同位素所蕴含地质信息的重要基础。其中，扩散和渗流过程是导致天然气同位素发生分馏效应的主要过程之一，但尚未被有效分离并深入研究。本项研究将通过改变扩散和渗流实验中的介质和气体，实现对扩散和渗流过程的有效分离，以此阐明扩散和渗流过程中同位素宏观分馏规律及其主控因素；基于分子级别的理论模拟，通过对不同同位素气体的扩散系数、分子运动速度、滑移长度等参数的对比，揭示扩散和渗流过程中气体同位素分馏微观机理；在此基础上，建立耦合模型对扩散和渗流过程中气体同位素分馏效应进行定量表征。

天然气同位素组成及分馏效应是其蕴含关键地球化学信息的最主要载体，在对关键地质信息评价中得到了广泛应用，并在页岩气/煤层气的含气性及吸附/游离气比例评价中展现出了巨大潜力。天然气同位素组成是生烃、溶解/脱溶、吸附/解吸、扩散和渗流等多个热力学和动力学过程综合作用的结果，因此，定量刻画每个过程中同位素分馏效应是进一步准确解析天然气同位素所蕴含地质信息的重要基础。其中，扩散和渗流过程是导致天然气同位素发生分馏效应的主要过程之一，但尚未被有效分离并深入研究。.本次研究分别通过对井口产气过程中甲烷碳同位素分馏效应长时间持续观测、对井场解析实验过程中甲烷碳同位素分馏效应的检测，以及对实验室解析实验过程中甲烷碳、氢同位素分馏效应的检测，阐明了页岩气解析过程中碳同位素呈变轻-变重-变轻的趋势变化；通过分子模拟方法，揭示了页岩气解析过程同位素发生分馏效应的本质原因为12CH4和13CH4的脱附速率差；建立二元气体渗流扩散模型对页岩气解析过程中甲烷同位素分馏效应进行定量表征，并进一步探索同位素分馏模型在页岩气含气量与吸附-游离气比例评价中的应用。.本次研究的成果已经在涪陵页岩气田和川东南常压页岩气田的甲烷碳同位素分馏机理探索中得到了成功的应用，为进一步利用同位素分馏效应评价地质条件下的吸附/游离气比例奠定了坚实基础，在未来的页岩气勘探中有望发挥重要作用。