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.