气源岩吸附试验的机理及吸附特征新认识

吸附常数是煤矿瓦斯防治、煤层气抽采、页岩气开发等工程必备的基础指标。然而，大量吸附试验却得出了两类特征迥异的吸附等温线:Langmuir型和“峰值型”，普遍采用Langmuir模型拟合这些非Langmuir型等温线，不但得出了错误的吸附常数，而且带来了极大的生产安全风险。鉴于此，本文经过吸附试验机理推导、等温吸附试验、等温线校正和误差分析，得出:① 等温吸附试验只能测出由Gibbs吸附量组成的“峰值型”等温线，出现峰值拐点的本质原因是等温吸附原理中减掉了Gibbs舍弃量ρva，并非负吸附、超临界吸附等原因造成的。② Gibbs吸附等温线出现峰值拐点的临界压力是由Gibbs舍弃量ρva决定的，其中，煤体结构决定了吸附相体积va的大小，试验温压条件决定了游离相密度ρ的大小。③ 煤和页岩吸附属于物理吸附，其实际吸附量等温线在达到饱和吸附压力后必然形成Langmuir型等温线。截距法根据Gibbs等温线的下降特征反推吸附相密度所得出的实际吸附量等温线完全符合饱和吸附的物理特征，是合理可靠的;液相法采用液相密度代替吸附相密度，不仅缺乏理论依据而且误差较大;Langmuir模型法运用LM算法拟合吸附相密度，需要人为设定各未知参数的初始值和边界值，存在较大的人为误差。④ 不能将实验室测试出的吸附等温线直接应用于吸附常数计算、非常规气资源量预测等，原因是体积法常因试验压力较低引起未饱和吸附、忽视Gibbs舍弃量等误差，而重量法在高压条件下测得的“峰值型”吸附等温线必须校正后才能使用。

New interpretation of adsorption test mechanism and adsorption law for gas source rock

Adsorption constant derived from adsorption isotherm is a fundamental indicator in coal mine gas control,CBM extraction and shale gas development.However,the current experimental studies have obtained two types of adsorption isotherms with different characteristics:Langmuir type and Central-peaked type.Using Langmuir model to fit non-Langmuir type isotherm not only results in great errors in adsorption constant,but also brings great production safety risk.In this regard,by deducing the mechanism of adsorption test,isothermal adsorption test,correction of adsorption isotherm and error analysis,the following understandings have been obtained:① Isothermal adsorption test can only measure the “peak” isotherm consisting of Gibbs Surface excess,and the essential reason for the peak inflection point is that Gibbs discarded amount ρv a has been reduced in the principle of isothermal adsorption,not negative adsorption,supercritical adsorption and other reasons.② The critical pressure for the peak inflection point of Gibbs adsorption isotherm is determined by Gibbs discarded amount ρv a,in which the volume of adsorption phase va is determined by coal structure,and the density of free phase ρ is determined by test temperature and pressure conditions.③ Coal and shale adsorption belongs to physical adsorption,once the saturated adsorption pressure is reached,their actual adsorption isotherms will inevitably form a Langmuir-type linear platform for saturated adsorption.The intercept method deduces the adsorption phase density and the actual adsorption isotherm by the descending characteristics of Gibbs isotherm,which,and therefore,conform to the physical characteristics of saturated adsorption and are reliable.The liquid phase method regards the liquid phase density as the adsorbed phase density,it not only lacks the theoretical basis,but also has a great error.Langmuir model method needs to set the initial and boundary values of unknown parameters in the process of fitting adsorption phase density with LM algorithm,and thus there is a large artificial error.④ It is unreasonable to calculate the adsorption constant and the unconventional resources by using the unadjusted adsorption isotherm measured in laboratory,because the unsaturated adsorption error and the neglect of Gibbs discarded amount are often caused by the lower test pressure using volumetric method,while the “peak” type adsorption isotherm measured using gravimetric method under high pressure must be corrected before they can be used.