Study on the way of destroying hydrated cation bridges by atomic force microscope and molecular dynamics simulation

Abstract For low-permeability/extralow-permeability reservoirs, appreciable amount of residual oil remains after water flooding, which is mainly caused by the formation of hydrated cation bridges between oil and rock surface. The hydrated cation bridges are centered on hydrated cations and connect the oil and rock via hydrogen bonds, which leads to the adhesion of oil film on the rock surface and low-oil washing efficiency of the injected fluid. Therefore, finding the way of destroying the hydrated cation bridges is essential for enhancing oil recovery. In this paper, the force needed to peel off the oil droplets in brine containing Na+, Ca2+, and Al3+ is measured via atomic force microscopy (AFM). Molecular dynamics (MD) is used to study the evolution of the microconformation between the oil and rock during peeling off the oil droplet and to compare the strength of the hydrogen bonds in the hydrated cation bridges (with Na+, Ca2+, and Al3+). The results showed that the hydrogen bonds formed between water molecules in the hydrated Ca2+ cations and polar molecules in oil were the strongest. Therefore, peeling off the oil droplets in CaCl2 solution is the most difficult. Additionally, the number of the hydrogen bonds formed between the water molecules in the hydrated cations and polar molecules in oil decreases during peeling off the oil droplet. It decreases to zero when the oil droplet is completely peeled off. Thus, breaking the hydrogen bonds between the water molecules in the hydrated cations and polar molecules in oil is an effective way to destroy the hydrated cation bridges. This study reveals an effective way of destroying the hydrated cation bridges and provides the theoretical basis and a design idea for the molecular structural design of highly efficient oil film strippers.