Influence of roughness on the water flushing process of NAPL in self-affine fractures based on the lattice Boltzmann method

Water flooding is a remediation technique to remove nonaqueous-phase liquids (NAPLs) from underground water. Roughness plays an important role in the two-phase flow of water and NAPL in fractures. To study the effect of roughness on the two-phase flow of water and NAPL when water displaces NAPL, a self-affine fracture was created and decomposed by the wavelet technique to obtain a smooth fracture without roughness for comparison. The Shan-Chen multicomponent multiphase lattice Boltzmann method (SC LBM) was adopted to simulate the two-phase flow when water flushes NAPL in smooth and rough fractures under different driving forces and fracture surface wettability values. The results show that under identical conditions, the final residual NAPL saturation in the rough fracture is 0.28 higher than that in the smooth fracture. When the driving pressure increases from 1 to 15 Pa, the final residual NAPL saturation in the rough fracture increases to be 0.22 more than that in the smooth fracture. When the fracture surface is water wet, due to the roughness of the fracture surface, 18.04% of NAPL is trapped in the rough fracture. When the fracture surface is NAPL wet, the flushable NAPL in the rough fracture is 31.39% less than that in the smooth fracture. The roughness makes the water flushing process of NAPL more sensitive to the driving force and wettability, which counteracts the NAPL flushing. This study may enhance our understanding of the role of roughness in the displacement of NAPL by water in fractures.