Mechanisms of Adsorption Reduction and Enhanced Oil Recovery by Silica-Based Active Nanofluid in Hydraulic Fracturing-Assisted Oil Displacement

Abstract Hydraulic fracturing-assisted oil displacement (HFAD) is expected to alleviate the challenges faced by low-permeability and tight oil reservoirs, such as “inability to inject and produce effectively,” low injection efficiency of chemical agents, significant near-wellbore adsorption loss, and poor oil-washing performance in the far-wellbore region. Nanomaterials, with their unique characteristics in EOR, are regarded as a promising technology for addressing low sweep efficiency and low recovery issues in traditional EOR. To further clarify the influence patterns of active nanofluids in HFAD on EOR, this paper constructed a fracture-matrix dual-medium HFAD model. On the basis of the Lambert–Beer law, ultraviolet spectrophotometry and high-temperature and high-pressure core flooding techniques were employed to investigate the adsorption reduction and oil displacement behavior of active nanofluids under various factors. The results showed that the fracture length, core permeability, nanofluid concentration, and injection rate exerted different effects on the adsorption of active nanofluids. With increasing fracture length, core permeability, and nanofluid concentration, the active nanofluid in HFAD more readily reached the dynamic adsorption equilibrium state; however, as the injection rate increased, the time required to reach the dynamic adsorption equilibrium first decreased and then gradually increased. The dynamic saturated adsorption capacity decreased exponentially with fracture length and permeability, increased logarithmically with nanofluid concentration, and decreased logarithmically with the injection rate. In addition, these factors further affected the oil-washing performance and EOR of the active nanofluid in HFAD by altering the effective sweep efficiency, the wettability modification effect on the core surface, or the oil–water interfacial tension. Within a certain range, when the fracture length, permeability, injection rate, and nanofluid concentration were increased, the EOR enhancement by the nanofluid in HFAD changed by 7.77, −0.75, −7.37, and 4.71%, respectively. This study is of great significance for the development and application of active nanofluid HFADs in low-permeability tight oil reservoirs.