Abstract: Changing the wettability of a reservoir and decreasing the oil-water interfacial tension is an effective method for solving the high pressure water injection problem in low and ultra-low permeability oil fields. However, during in-lab displacement experiments, the surfactant concentration not only affects the interfacial tension between oil and water, but also the wettability of the rock surface. As such, the effect of the surfactant wetting ability on depressurization cannot be accurately evaluated. Lattice Boltzmann models can control the interfacial tension independently through the interaction parameters between fluid and solid, and simulate any change in contact angle. Based on a lattice Boltzmann two-dimensional plane model, considering surface roughness and creating a micro-ladder structure, this study investigated the effects of changing wettability on depressurization rate under the dimensionless parameters of capillary number, Bond number, and viscosity ratio. The results show that with decreasing capillary number, the Bond number and viscosity ratio increase, and with decreasing contact angle, the depressurization rate and depressurization effect improve. There is an optimum range for the contact angle. Continued decrease in the contact angle outside this range produces no obvious depressurization. The changing wettability has a more pronounced effect on the depressurization rate for a porous medium than for a single channel.

Key words: surfactant, lattice Boltzmann, wettability, interfacial tension, contact angle