页岩不同类型干酪根内甲烷吸附行为的分子模拟

目前对于页岩中不同类型有机质干酪根对甲烷的吸附影响规律尚不清楚。为此,根据页岩干酪根的元素含量、分子结构构建了腐泥型、混合型和腐殖型的干酪根分子模型,利用巨正则蒙特卡罗方法和分子动力学模拟方法研究甲烷在干酪根中的吸附行为,并进行了实验验证。进而探讨了温度、地温梯度、干酪根分子组成、比表面积对甲烷—干酪根吸附系统的影响,以及甲烷在页岩干酪根内的微观吸附机理。结果表明:(1)腐殖型干酪根对甲烷的吸附量最大,混合型次之,腐泥型最小;(2)干酪根的化学结构对甲烷的吸附有着重要的影响,富含芳香族的干酪根对甲烷具有更强的亲和力,碳、硫原子对甲烷在干酪根中吸附的影响较大;(3)甲烷在干酪根中吸附属于物理吸附,温度越高甲烷吸附量越小,平均等量吸附热均小于42 k J/mol;(4)随着地层深度的增加,甲烷吸附量先增加后减少,在地层深度介于2 000~2 500 m甲烷绝对吸附量达到峰值,地温梯度越小相同埋深下甲烷吸附量越多;(5)甲烷吸附量与干酪根的比表面积呈线性正相关关系。

Molecular simulation of methane adsorption behavior in different shale kerogen types

The impact of different kerogen types on the methane adsorption behavior is still not clear at present. In view of this, a model was built according to the molecular structures of three kirogen types: TypeⅠ—Sapropel, TypeⅡ—Sapropel–Humic, and Type Ⅲ—Humic), in which the adsorption behaviors of methane were investigated by using the Grand Canonical Monte Carlo (GCMC) and Molecular Dynamic (MD) methods, and this model and results were validated. On this basis, the effects were also discussed of temperature, geothermal gradient, and the molecular composition and the specific surface area of kerogen on the adsorption of a methane–kerogen adsorption system, and the microscopic adsorption mechanism of methane in kerogen was revealed. The following results were obtained. (1) The adsorbed methane amount in Type Ⅰ kerogen was the largest, followed by Type Ⅱ, and Type Ⅲ the smallest. (2) The chemical structure of kerogen significantly influences its adsorption amount of methane: the kerogen with more aromatic hydrocarbons and cyclanes have a stronger affinity for methane, and the carbon and sulfur atoms play an important role in the adsoprtion of kerogen to methane. (3) As temperature increases, the average isosteric adsorption heat of methane decreases, and it is always less than 42 kJ/mol, indicating that this adsorption is a physical adsorption. (4) As the depth of stratum increases, the adsorption amount first increases and then decreases, reaching the maximum at 2000-2500 meters. At the same depth, the smaller the geothermal gradient, the more the methane adsorption amount. (5) The adsorption amount of methane has a linear relationship with the specific surface area of kerogen.