页岩中超临界甲烷等温吸附模型研究

针对甲烷在页岩储层中呈超临界状态吸附的特点,开展了页岩中超临界甲烷等温吸附模型的研究.引入过剩吸附量,对常规吸附模型(Langmuir,Freundlich,Expand-Langmuir,Langmuir-Freundlich,Toth,D-R和D-A等吸附模型)进行了修正,将常规吸附模型扩展为超临界吸附模型,利用相对误差评价各吸附模型修正前后对页岩中超临界甲烷等温吸附的拟合效果.通过分析模型拟合参数的物理意义,探讨了页岩的吸附特征及吸附机理.各吸附模型的拟合参数所反映的吸附机理存在一定的差异,其中多分子层BET模型(B-BET和T-BET)和Expand-Langmuir模型对部分页岩的拟合参数失去其物理意义,不适合用于页岩中超临界甲烷吸附特征研究,而Langmuir模型和D-A模型拟合的参数能反映页岩的吸附特征.对比页岩中超临界甲烷等温吸附拟合效果,各吸附模型修正后的拟合效果好于修正前,且Freundlich修正模型的拟合效果最差,Toth修正模型和D-R修正模型的拟合效果好于Langmuir修正模型,但总体上拟合效果不好,Langmuir-Freundlich修正模型和D-A修正模型的拟合效果较好.研究结果表明,D-A修正模型的拟合参数能更好地反映页岩中超临界甲烷的吸附特征,是描述页岩中超临界甲烷吸附特征比较理想的模型. 

Isothermal Adsorption Model of Supercritical Methane in Shale

Methane may physically be absorbed on shale in a supercritical state within shale reservoirs. Based on these characteristics, research has been conducted for isothermal absorption models of supercritical methane in shale. Excess adsorption is introduced to correct conventional adsorption models, such as Langmuir, Freundlich, Expended-Langmuir, Langmuir-Freundlich, Toth, B-BET, T-BET, D-R and D-A. Thus, conventional adsorption models are expanded into supercritical adsorption models. In addition, relative error is used to assess fitting results for isothermal adsorption of supercritical methane on shale before and after correction for those adsorption models. By analyzing the physical significance of fitting parameters in these models, it is possible to investigate features and mechanisms of adsorption in shale. There are certain differences in absorption mechanisms reflected by fitting the parameters of the absorption models.In particular, multi-molecular layers BET models (B-BET and T-BET) and Expand-Langmuir model have no physical significance for some shale samples. Accordingly, these models can no longer be used to determine features of supercritical methane adsorption. Furthermore, fitting parameters generated through Langmuir model and D-A model can accurately reflect supercritical methane adsorption characteristics. Comparison of the fitting results shows that the corrected adsorption model fits better than the originalone. The corrected Freundlich model fits badly, while the corrected Toth and D-R models display better performances than the corrected Langmuir model. But the overall fitting performances are not satisfactory. The corrected Langmuir-Freundlich model and D-A model have better performance in terms of fit. Research results show that fitting parameters determined by using the corrected D-A model are suitable for fitting the supercritical isothermal adsorption of methane in shale. Accordingly, the corrected D-A model can be seen as a desirable model for representation of supercritical methane adsorption characteristics in shale.