分形界面吸附行为初探

A preliminary study of coalbed methane adsorption at fractal interfaces

煤层气吸附解吸机理研究是揭示煤层气成藏机理及高效开发煤层气的基础。已有研究表明,煤储层孔隙结构非常复杂且具有分形特征,煤层气在孔-固界面的吸附行为明显受到煤孔隙结构特征的影响。对比分析几种常用气-固界面上的吸附模型,总结这些模型的特点和适用条件,指出当前吸附模型在分形界面吸附行为的描述和应用上均未摆脱吸附选择性的平稳假设,尚未考虑吸附厚度的尺度不变特征。而分形拓扑理论可以有效标定分形对象的尺度不变属性,为分形界面的等效表征提供了理论支撑。因此,结合上述模型对气-固界面吸附行为的描述,借助分形拓扑理论提出煤储层孔-固界面上分形吸附行为的相关假设及其控制机理,构建基于吸附拓扑的单层吸附模型。在此基础上,通过设置不同的吸附拓扑参数组合获取其等温吸附曲线,分析得出,随着吸附压力的升高,吸附覆盖率表现出指数、线性和对数3种不同的增长趋势,而吸附热表现为对数减小趋势。结果显示,不同的吸附拓扑参数组合会得到不同类型的等温吸附曲线,在一定程度上弥补了Langmuir方程只能描述单一类型等温吸附线的不足。为了验证吸附模型的适用性,结合沁水盆地武乡区块富有机质泥页岩的液氮吸附实验数据,对比了实际等温吸附曲线与模拟吸附曲线的差异。结果表明,通过调整各吸附拓扑参数之间的组合关系,可以使等温吸附模拟曲线与实际吸附曲线的趋势始终保持一致。最后,探讨了吸附解吸机理的研究方向,类比电子层提出了吸附层的概念,指出发展分形动力学描述模型是解释煤层气吸附解吸规律的关键。

The adsorption and desorption mechanisms of coalbed methane(CBM)are foundational for revealing CBM accumulation mechanisms and achieving efficient CBM exploitation.Studies have indicated that coal reservoirs exhibit complex pore structures with fractal features,significantly influencing CBM adsorption at pore-solid interfaces.Based on a comparative analysis of several common adsorption models for gas-solid interfaces and a summary of their charac-teristics and applicable conditions,this study suggests that all these adsorption models still rely on the stationary assump-tion of adsorption selectivity for the description and application of CBM adsorption behavior at fractal interfaces while overlooking the scale invariance of the adsorption thickness.The fractal topography theory can effectively calibrate the scale invariance of fractal objects,providing theoretical support for the equivalent characterization of fractal interfaces.Hence,in combination with the description of CBM adsorption behavior at gas-solid interfaces in the above models,to-gether with the fractal topography theory,this study proposed related hypotheses and control mechanisms of fractal ad-sorption behavior at pore-solid interfaces in coal reservoirs.Furthermore,it constructed an adsorption topography-based monolayer adsorption model,obtaining adsorption isotherms under different combinations of adsorption topological parameters.The analysis reveals that as the adsorption pressure increased,the adsorption coverage exhibited three dis-tinct growth trends:exponential,linear,and logarithmic,while the adsorption heat displayed a trend of logarithmic de-crease.The results of this study show that different combinations of adsorption topological parameters can yield differ-ent adsorption isotherms,making up for the deficiency that the Langmuir equation can merely describe a single type of adsorption isotherms.To verify the applicability of the model constructed,this study compared the actual adsorption iso-therms with the simulated ones using the liquid nitrogen adsorption data of organic-rich argillaceous shale samples from the Wuxiang block of the Qinshui Basin.The finding indicates that adjusting the combinations of adsorption topological parameters can align the trends of simulated adsorption isotherms with those of actual ones.Finally,this study explored the research direction of the adsorption and desorption mechanisms and proposed the adsorption layer concept through analogy with the electron layer,emphasizing that developing fractal dynamics description models is essential for illumin-ating CBM adsorption and desorption regularities.