甲烷流动过程中碳同位素分馏实验

Experimental study on carbon isotopic fractionation of methane flow

页岩气在页岩多孔介质中的流动受扩散、渗流以及吸附—解吸等作用的影响,其主要成分甲烷的同位素分子(^(13)CH_(4)、^(12)CH_(4))在吸附性能、扩散性能上均存在明显的差异,因此甲烷气体在流动过程中会发生碳同位素分馏现象。为明确甲烷流动过程中碳同位素分馏机理,开发了一维流动甲烷同位素分馏实验装置及在线同位素监测方法,通过空柱与伊利石填充柱的甲烷流动碳同位素分馏对比实验研究,表明流动过程中造成甲烷浓度梯度的扩散作用是引发甲烷碳同位素分馏的重要因素,建立的对流—扩散耦合方程可以很好地拟合和解释空柱实验同位素数据。填充柱实验中发现初期甲烷碳同位素较原始值明显负向偏移,然后CH;碳同位素组成快速变重,相对于甲烷碳同位素的原始值偏高可达5‰,而后再逐渐变轻至甲烷碳同位素组成的原始比值,同位素变化曲线呈现出明显的拐点,这是扩散与吸附—解吸共同作用的结果,揭示了流动过程中甲烷作用于固体分子筛吸附位上显示出明显的逆同位素效应。

The transport of shale gas through porous media can be affected by diffusion,advection,and adsorption-desorption,and the isotopic molecules of methane(^(13)CH_(4)、^(12)CH_(4))are different in adsorption affinity and diffusion coefficients,so carbon isotope fractionation occurs with these processes. In order to clarify the mechanism of carbon isotope fractionation in methane transport,one-dimensional flow-through experimental device and on-line isotope monitoring method are developed. By means of the carbon isotope fractionation comparison experiment of methane passing through illite-packed column and the empty column,the diffusion effect caused by concentration gradient in the process of methane flow being the important factor of isotopic fractionation are demonstrated. An analytical solution of coupled advection-diffusion equation is developed to fit well the carbon isotope data and interpret the results from each experiment of empty column. In the experiments of illite packed column,it is found that the carbon isotopes of methane in the initial stage are significantly negative shift com-pared with the original value,and then the carbon isotope compositions of methane rapidly become heavier,and the maximum relative value relative to the original value can reach 5‰. After then the carbon isotopes of methane becomes lighter gradually to the original values. It is found that the curve of isotope composition change shows an obvious inflection point,which are affected by the combination result of diffusion and adsorption-desorption. It also reveals that methane acts on the adsorption sites of solid molecular sieves in the process of flow,showing obvious reverse isotope effect. This is the result of the interaction of diffusion and adsorption-desorption,and it reveals that methane has an obvious inverse isotope effect over the adsorption sites of molecular sieves in the transport.