Molecular Dynamics Study on Mechanism of Preformed Particle Gel Transporting Through Nanopores: Deformation and Dehydration

Understanding the translocation mechanism of preformed particle gel (PPG) through the nanoporous medium is crucial for gel treatment during enhanced oil recovery. On the basis of nonequilibrium molecular dynamics simulation, the translocation process of PPG in silica nanopores consisting of two different diameters was investigated. During the simulation, an external pulling force was applied to PPG representing the injection pressure. The simulation results suggest that a synergetic deformation and dehydration of PPG occurs during the translocation from the wide side into the narrow side. The energy barrier of the translocation process mainly result from the conformational energy change of PPG (mainly from the angle bend and dihedral torsion) and the dissociation energy barrier between PPG’s hydrophilic groups and water. Furthermore, the nanopore size has a crucial impact on the translocation mechanism of PPG, not only the degree of the deformation and dehydration near the entrance, but also the translocation mechanism after they entered the nanopore. For a nanopore with a large diameter, PPG can reabsorb water to induce a complete hydration layer around it after entry. However, for the nanopore with a small size, the compression from the pore restricts PPG’s rehydration ability. Without the screen and lubrication of the hydration layer, the pulling force needed to drive PPG increased rapidly, which means a larger injection pressure in the macroscopic view. The findings are helpful for understanding the translocation process of PPG in porous media on molecular level and, also, will facilitate technology developments for enhancement of recovery efficiency of petroleum.探明预成型颗粒凝胶（preformed particle gel, PPG）在纳米多孔介质中的运移机制，对提高采收率作业中的凝胶处理工艺至关重要。本研究基于非平衡分子动力学模拟，针对两种不同孔径的二氧化硅纳米孔内PPG的运移过程展开了系统研究。模拟过程中，通过施加外拉力以模拟实际作业中的注入压力。仿真结果表明，PPG从宽孔段向窄孔段运移时，会发生协同变形与脱水现象。运移过程的能垒主要来源于两部分：一是PPG自身的构象能变化（主要来自键角弯曲与二面角扭转），二是PPG亲水基团与水分子间的解离能垒。此外，纳米孔径对PPG的运移机制具有关键性影响，不仅体现在入口处的变形与脱水程度，还体现在PPG进入纳米孔后的运移行为模式。对于大孔径纳米孔，PPG在进入孔道后可重新吸附水分，在其表面形成完整的水化层；而对于小孔径纳米孔，孔壁的压缩作用会限制PPG的复水能力。由于缺乏水化层的屏蔽与润滑作用，驱动PPG所需的外拉力会急剧上升，宏观上即表现为需要更高的注入压力。本研究结果有助于从分子层面理解PPG在多孔介质中的运移过程，同时也将助力石油采收率提升技术的进一步发展。