Mechanism of Low Chemical Agent Adsorption by High Pressure for Hydraulic Fracturing-Assisted Oil Displacement Technology: A Study of Molecular Dynamics Combined with Laboratory Experiments

This study investigates the influence of physical parameters such as porosity, permeability, pore-throat radius, and specific surface area/volume on the adsorption capacity of surfactants in the pore surface of reservoirs. In the meantime, the hydraulic fracturing-assisted oil displacement (HFAD) technique can effectively improve the permeability and porosity of pores in the reservoir, which may affect the adsorption capacity of surfactants in low-permeability reservoirs. This may help to reduce the adsorption loss of surfactants in low-permeability reservoirs. Based on physical simulation methods, dynamic adsorption experiments were conducted to clarify the dynamic saturation adsorption capacity effect of high-pressure and low-pressure displacement agents by the HFAD technique. In addition, the molecular dynamics simulation method was used to study the effect of high-pressure conditions of HFAD on the adsorption capacity of surfactants on weakly lipophilic silica walls. Under the condition of high injection pressure by the HFAD technique, the fluid flow velocity and the initial kinetic energy of molecules increase, while the absolute value of the electrostatic potential energy in the system decreases. In addition, the van der Waals potential energy increases. In other words, the smaller the gravitational attraction experienced by the surfactant molecules during adsorption, the greater the repulsive force. Under the dual action of electrostatic force and van der Waals forces, the absolute values of the adsorption energy and the free energy decrease. The adsorption capacity of the surfactant molecules is weakened. Moreover, the decrease in adsorption capacity has little effect on the improvement of wettability, indicating that the adsorption of the surfactant reduced by HFAD technology is mostly ineffective adsorption.