超临界二氧化碳萃取深层稠油组分的分子动力学模拟

为了从微观层面探究超临界CO_2对稠油组分的萃取机制,采用分子动力学模拟方法分析了稠油组分在砂岩表面的密度分布和吸附特征,研究了超临界CO_2对岩石表面稠油组分的萃取特点和扩散规律。研究结果表明,稠油四组分在稠油聚集体中呈现不均衡分布状态,沥青质自缔合能力强,胶质紧密包裹在沥青质周围,构成复杂的空间网状结构;芳香烃和饱和烃分布在沥青质、胶质周围,显示出稠油分子结构的层次性。沥青质与岩石表面相互作用能较大,超临界CO_2难以在沥青质中运移,扩散系数低,萃取难度大,萃取率接近于0;而芳香烃、饱和烃与岩石表面的相互作用能较小,超临界CO_2容易在芳香烃、饱和烃中溶解、运移,扩散系数大,容易被超临界CO_2萃取,萃取率可分别达到53%和28%。运用分子动力学方法揭示的微观动力学机制对于宏观认识超临界CO_2萃取稠油轻质组分具有重要意义。

Molecular Dynamics Simulation for Supercritical Carbon Dioxide Extraction of Heavy Oil Components in Deep Reservoir

In order to study the molecular extraction mechanism, desorption characteristics and variation law of heavy oil from microscopic perspective, the molecular number density and radial distribution function of heavy oil molecular aggregates were calculated by molecular dynamics simulation method. The density distribution and adsorption characteristics of heavy oil components on sandstone surface were analyzed. The influence of supercritical CO2 on extraction characteristics and diffusion law of heavy oil components on rock surface was studied. The results show that the four components of heavy oil are unevenly distributed in the heavy oil aggregates. Asphaltenes have strong self-association ability and high degree of self-aggregation. Colloids are tightly wrapped around asphaltenes, forming a complex space network structure. Light components like aromatic hydrocarbons and saturated hydrocarbons are distributed around asphaltenes and colloids as dispersion media, showing the hierarchy of molecular structure of heavy oil. The results also demonstrate that the interaction energy between asphaltene molecules and rock surface is large and the adsorption is strong, which makes it difficult for asphaltene to migrate. Therefore, reducing the interaction energy is the key to reduce asphaltene precipitation. Owing to small interaction energy between light components and rock surface and simple molecular structure, it is easy to be extracted by supercritical CO2. The extraction rates for aromatic hydrocarbon and saturated hydrocarbon can reach 53% and 28% respectively, followed by colloids of 9%. The extraction rate of asphaltene is close to 0. Supercritical CO2 is easy to dissolve and migrate in aromatic hydrocarbons and saturated hydrocarbons with small molecules and big diffusion coefficient. However, colloids and asphaltene components are difficult to diffuse and migrate in CO2 due to the adsorption on rock surface and entanglement between molecules. The diffusion coefficient is the smallest. The micro dynamic mechanism revealed by molecular dynamics method is of great significance for macro understanding of supercritical CO2 extraction of heavy oil light components and enhancing heavy oil recovery.