页岩油在干酪根中吸附行为的分子动力学模拟与启示

Molecular dynamics simulation of shale oil adsorption in kerogen and its implications

王民 余昌琦 费俊胜 李进步 张宇辰 言语 吴艳 董尚德 唐育龙

Min WANG Changqi YU Junsheng FEI Jinbu LI Yuchen ZHANG Yu YAN Yan WU Shangde DONG Yulong TANG

有机质及其相关孔隙吸附行为的研究对于揭示页岩油赋存状态与机理有重要意义。不同于以往采用石墨烯模型代替有机质的方法，研究采用真实的干酪根分子模型（Ⅱ-C型），基于GAFF（general Amber force field）力场模拟了有机孔内页岩油多组分体系下的吸附行为。结果表明：①与石墨烯仅能模拟壁面吸附不同，干酪根对页岩油具有吸附和吸收双重作用：壁面上存在页岩油竞争吸附，以极性和重质组分吸附为主，而骨架中则存在页岩油组分吸收现象，小分子迁移距离较远。页岩油在干酪根壁面上的吸附和在骨架中的迁移受控于页岩油与干酪根相互作用能的强弱及分子大小，重质组分表现出“强吸附-弱吸收”、轻质组分呈“弱吸附-强吸收”的特征。②页岩油组分的吸收使得干酪根骨架和孔隙发生变化，表现出新孔隙的形成、原有孔隙的扩大和部分塌陷。干酪根的塑性对吸收页岩油进而膨胀起重要作用，干酪根塑性较强时（干酪根成熟度低），页岩油更容易被吸收从而引发明显的干酪根骨架膨胀，反之，干酪根膨胀较弱。③温度增加会促进干酪根骨架吸收芳香烃分子萘和非极性分子甲酸、乙醇以及噻吩，降低干酪根壁面的吸附作用，同时有利于饱和烃类分子的脱附。压力对页岩油在干酪根中的吸附和吸收影响不明显。研究利用真实的干酪根分子模型，首次创新性地模拟了干酪根吸附和吸收页岩油组分的现象，对于客观揭示页岩油在干酪根中赋存状态及赋存机理具有重要帮助。

Investigating the adsorption behavior in organic matter and its associated pores holds critical significance for revealing the occurrence states and mechanisms of shale oil. Differing from the previous method of replacing the organic matter model with the graphene model， a realistic kerogen molecular model， the Type Ⅱ-C model， is employed to simulate the adsorption behavior of multi-component shale oil within organic pores based on the general Amber force field （GAFF）. The results are as follows. （1） Unlike graphene， which can only be used to simulate surface adsorption， kerogen has the dual functions of both adsorption and absorption. Competitive shale oil adsorption transpires on kerogen walls， dominated by the adsorption of polarity and heavy components， while the kerogen skeleton is characterized by absorption of small molecules moving far away. The shale oil adsorption on the surface and absorption in the skeleton with migration are influenced by the interaction energy between shale oil and kerogen， as well as the molecular size. Specifically， heavy components of shale oil are subjected to strong adsorption but weak absorption， while its light components undergo weak adsorption but strong absorption； （2） The absorption of shale oil components leads to the deformation of the kerogen skeleton and pores， manifested as the formation of new pores and the expansion and partial collapse of original pores. The plasticity of kerogen plays a significant role in its shale oil absorption and further skeleton swelling. Highly plastic kerogen （with low maturity） is more prone to absorb shale oil and swell significantly in skeleton. In contrast， weakly plastic kerogen swells slightly with absorption； （3） An increase in temperature enhances the absorption of aromatic hydrocarbon molecules （like naphthalene） and non-polar molecules （e.g.， formic acid， ethanol， and thiophene） in kerogen skeleton， which reduces the adsorption on kerogen surface， and is conducive to the desorption of saturated hydrocarbon molecules. Additionally， pressure produces insignificant effects on the shale oil adsorption and absorption in kerogen. In this study， the realistic kerogen molecular model is innovatively applied to simulate kerogen’s adsorption and absorption of shale oil components， which is of great help in objectively revealing the shale oil occurrence state and mechanism in kerogen.

10.11743/ogg20230609