Bacillus subtilis-mediated synthesis of ZnO and Ag-ZnO nanoparticles for eco-friendly displacement agents: Enhancing oil recovery in middle/low permeability reservoirs with nanofluid

Microorganisms, known for their widespread distribution and nonpolluting nature, are extensively used as raw materials. In this study, ZnO nanoparticles (ZnO NPs) and Ag-ZnO nanoparticles (Ag-ZnO NPs) were synthesized using Bacillus subtilis, and their efficacy as nano-displacement agents for enhancing oil recovery in medium and low permeability reservoirs was investigated. The morphology, structure, and properties of these nanomaterials were analyzed using various characterization methods. The results indicated that ZnO NPs are irregularly blocky with an average particle size of 44.7 nm, while Ag-ZnO NPs are approximately spherical with an average particle size of 35.5 nm, both exhibiting high purity and stability. These two nanoparticles can effectively reduce oil–water interfacial tension and improve rock wettability, with Ag-ZnO NPs showing superior performance. In core displacement tests at 60 °C, the recovery rates for intermediate permeability cores (30–40 mD) were 12.43% for ZnO nanofluids and 14.10% for Ag-ZnO nanofluids. For low-permeability cores (<10 mD), the recovery rates were 8.63% and 10.26%, respectively. Microscopic oil displacement experiments revealed that the mechanism of oil displacement by nano-displacement agents includes altering rock surface wettability, penetrating narrow pores, emulsifying crude oil, and exerting viscoelastic effects. In summary, these two nanomaterials significantly improve oil recovery in reservoirs, offering an important reference for their application in the oilfield and pointing toward a new direction for developing green and efficient alternatives to chemical flooding agents.