Immobilized Microorganisms for Targeted Release in COD Reduction of Oilfield Wastewater and Heavy Oil Viscosity Reduction

Under the dual-carbon policy, the extraction of heavy oil faces significant challenges. Microbial enhanced oil recovery (MEOR), with its sustainability and environmental friendliness, continues to hold a pivotal position. This study proposes an immobilized carrier material (SA/L hydrogel microspheres) capable of targeted microbial release. By leveraging the hydrogel properties of sodium alginate (SA) and the oil solubility of lecithin, the composite material encapsulates microorganisms and is fabricated into microspheres. These microspheres are applicable for reducing the chemical oxygen demand (COD) in oilfield wastewater and decreasing the viscosity of heavy oil, thereby enhancing heavy oil recovery. Positive experimental results indicate that the inoculation of Bacillus subtilis is most efficient at 5%–7%, with an optimal pH range of 6.5–7.0, salinity not exceeding 5000 ppm, and a temperature around 40°C. COD reduction experiments reveal that with SA/L hydrogel microspheres, a mere 5% inoculation rate can achieve a reduction exceeding 60%, and a 7% inoculation rate can surpass 80%. Viscosity reduction experiments for heavy oil demonstrate that with SA/L hydrogel microspheres, a 5% inoculation rate can achieve a viscosity reduction rate of up to 81.6%, with the greatest reduction in saturated hydrocarbons followed by aromatic hydrocarbons. Finally, experiments confirm a recovery enhancement of 23.0%, indicating that SA/L hydrogel microspheres as immobilized carriers are significant.