Biosurfactants are surface-active substances produced by microbial metabolism, such as from bacteria and yeasts. Common types include rhamnolipids, sophorolipids, and lipopeptides. Their core mechanism for enhancing crude oil recovery lies in their ability to significantly reduce the interfacial tension between oil and water. The interfacial tension between crude oil and formation water is typically high, causing crude oil to behave like mercury droplets, making it difficult to deform and flow through narrow rock pores. Biosurfactant molecules adsorb at the oil-water interface, with their hydrophilic groups extending into the water phase and hydrophobic groups inserting into the oil phase. This greatly weakens the attractive forces between molecules at the interface, reducing the interfacial tension from its original 10⁻¹ mN/m scale reduced to 10⁻³ mN/m or even lower. The reduction in interfacial tension makes it easier for residual oil droplets, originally trapped in rock pores by capillary forces, to deform, break apart, and flow toward production wells along with the displacing fluid (water).

Additionally, biosurfactants can enhance oil recovery efficiency by altering the wettability of rock surfaces. Polar components in crude oil can shift the surface of reservoir rocks from hydrophilic to oleophilic, causing oil to adhere more firmly. Biosurfactants adsorb onto the rock surfaces, reversing them from oleophilic back to hydrophilic. This makes oil films easier to detach, roll up, and be carried away by water flow.

Compared to chemically synthesized surfactants, biosurfactants offer superior biodegradability and environmental compatibility. They can be produced in situ by microorganisms or directly injected into reservoirs, even under extreme conditions, allowing for a broader range of applications. Field application cases have demonstrated that biosurfactants can effectively enhance oil displacement in challenging development environments, such as heavy oil fields and low-permeability reservoirs, providing a viable approach to improving crude oil recovery rates—a "green" solution.