页岩中甲烷等温吸附量计算问题及方法改进

Problems of methane isothermal adsorption calculation in shale and method improvement

研究页岩中甲烷气体的等温吸附规律,是页岩气勘探开发研究的基础工作,近年来的相关研究已经取得了一定成果。然而在众多的吸附曲线实验结果中,异常曲线的数量却占据了相当的比例,部分岩心的等温吸附曲线在高压段下拐,甚至出现负吸附值现象。通过多组等温吸附实验结果分析,认为曲线在高压段下拐是正常现象,但也存在不合适的计算及实验方法,其主要问题包括:计算过程中没有区分绝对吸附量和过剩吸附量的差别,实验中没有考虑吸附相体积的影响,不适当的甲烷自由体积测定方法,以及设备系统误差的影响等。利用Langmuir等温吸附模型,建立了三元Langmuir方程,该模型在给定吸附相密度的情况下,选取甲烷自由体积作为未知参数;利用迭代法或Matlab软件计算自由体积、Langmuir体积和Langmuir压力。基于三元Langmuir模型所计算的甲烷自由体积,与氦气所测自由体积具有良好的可比性,采用该方法能更好地修正以前的曲线异常现象。

The studies of isothermal adsorption of methane in shale are the basis for shale gas exploration.Some achievements have been made recently.However,a substantial proportion of the in-lab experiment curves are abnormal,and even drop down,indicating for adsorption amount decrease under high pressure.Some reasons were found based on the studies of isothermal adsorption curves,including improper calculation and simulation methods.For example,the difference between absolute and excess adsorption amounts and the influence of adsorption phase volume might be ignored.What’s more,some improper methods were applied for measuring methane free volume,and equipment error also existed.A ternary Langmuir model was built based on the Langmuir isothermal adsorption equation,which took methane free volume as an unknown parameter while adsorption phase density was fixed.Free volume,Langmuir volume and Langmuir pressure were calculated with iterative method or Matlab software.Methane free volume calculated with the ternary Langmuir model has a good comparability with Helium.The simulation results obtained by this method can better correct the previous curve anomalies.