页岩干酪根吸附规律的分子模拟研究

Molecular simulation of adsorption law for shale kerogen

页岩气是以CH_(4)为主赋存在有机质页岩的非常规天然气,其中的吸附气是页岩气后期产量的主要来源,因此,研究页岩的吸附机理对页岩气开发有重要作用。利用Ⅱ型(混合型干酪根,来源于浮游生物)干酪根分子建立Ⅱ型干酪根模型,采用蒙特卡洛方法和分子模拟方法研究CH_(4)在Ⅱ型干酪根中的微观吸附行为和机理,通过实验数据验证了该模型,并进一步探究了孔径、温度和压力对吸附行为的影响。研究取得的认识如下:(1)孔径越大,CH_(4)超额吸附量越大;温度越高,CH_(4)超额吸附量越低;压力越大,CH_(4)绝对吸附量先快速上升后逐渐平缓,CH_(4)超额吸附量先上升后下降;(2)随着孔径的增加,CH_(4)的吸附热越来越少,并从吸附热看,CH_(4)在干酪根的吸附行为是物理吸附;(3)孔径小于1 nm时,CH_(4)在干酪根中为吸附相;孔径大于1 nm时,CH_(4)在干酪根中为吸附相与游离相共存。

Shale gas is unconventional natural gas,mainly CH_(4),occurring in organic shale.The adsorbed gas is the main source for later production of shale gas.Therefore,studying the adsorption mechanism of shale plays an important role in shale gas development.By using typeⅡkerogen molecules,a typeⅡkerogen model is established.Then,Monte Carlo method and molecular simulation method are used to study the micro adsorption behavior and mechanism of CH_(4) in typeⅡkerogen.Experimental data are used to verify this model,and the effects of pore size,temperature and pressure on the adsorption behavior are investigated.The findings are as follows:(1)The higher the pore size,the greater the excess adsorption capacity of CH_(4).The higher the temperature,the lower the excess adsorption capacity of CH_(4).With the increase of pressure,the absolute adsorption amount of CH_(4) increases rapidly at first and then gently,and the excess adsorption amount of CH_(4) increases first and then decreases.(2)The adsorption heat of CH_(4) decreases with the increase of pore size.The adsorption of CH_(4) in kerogen is physical adsorption.(3)When the pore size is smaller than 1 nm,CH_(4) is the adsorption phase in kerogen;when the pore size is larger than one nanometer(1 nm),CH_(4) is the coexistence of adsorption phase and free phase in kerogen.