Date of Award

2-23-2026

Thesis Type

PhD

Document Type

Thesis

Divisions

Faculty of Engineering

Department

Department of Chemical Engineering

Institution

Universiti Malaya

Abstract

After polymer gel formation, the residual partially hydrolyzed polyacrylamide (HPAM) and the crosslinker (Cr3+) caused plugging and decreased oil recovery. Microorganisms can degrade HPAM to address the issues, but Cr³⁺ inhibits the process and reduces oil recovery. This study explores the underexamined biodegradation of HPAM and improved oil recovery in the presence of Cr³⁺, offering new perspectives for microbial enhanced oil recovery (MEOR) after polymer gel formation. HPAM and crude oil were innovatively used as the sole nitrogen(N) and carbon(C) sources to isolate the best-growing strain from Daqing Oilfield produced water, identified as Bacillus licheniformis SP01. The oil displacement potential of SP01 was evaluated in the presence of HPAM, where viscosity, surface tension, and interfacial tension were reduced to 75.56%, 20.55%, and 22.42%, respectively; however, these effects were markedly suppressed upon the addition of Cr³⁺. To clarify Cr³⁺ effects on biodegradation mechanisms, experiments under Cr³⁺ stress showed HPAM and crude oil biodegradation decreased by 7.25% and 29.48%, accompanied by declines in OD600 and enzymatic activities, indicating inhibition of growth and metabolism. Multi-analytical characterization further revealed Cr³⁺-induced structural and morphological changes in both substrates, alongside evidence from a novel ¹³C/¹⁵N isotope labeling approach suggesting shifts in degradation pathways. A Box–Behnken Design model predicted maximum biodegradation rates of 37.10% for HPAM and 33.92% for crude oil under 37.4 °C, pH 6.5, and 20 g/L salinity. As microorganisms are key to MEOR, the impacts of Cr³⁺ on SP01 growth were studied. Minimum inhibitory concentration tests revealed that 500 mg/L was the critical concentration at which microbial growth, extracellular polymeric substances (EPS) production, and Cr3+ biosorption peaked. Beyond this threshold, all declined sharply, reinforcing the dose-dependent toxicity of Cr³⁺. SP01 adsorbed Cr³⁺ primarily in EPS, then cell walls, and least in the protoplast, with a peak at 500 mg/L, evidencing a strong interaction between Cr³⁺ and SP01. Notably, microbial growth was also optimal under 500 mg/L, suggesting a possible balance between biosorption and tolerance. Transcriptomic analysis represents a pioneering effort to elucidate the molecular basis of Cr³⁺ resistance in SP01, highlighting stress-induced reprogramming of genes involved in membrane integrity, energy metabolism, and stress response. These findings imply that biodegradation and microbial growth occur through co-metabolic pathways and that an appropriate Cr³⁺ concentration can stimulate SP01’s metabolism, resistance, and growth. Oil displacement experiments across a Cr³⁺ gradient revealed a dose-dependent effect, with 500 mg/L as the critical concentration. At 500 mg/L, SP01 showed optimal growth and EPS production, achieving optimal oil recovery of 58.69%, 9.4% above the microbial control without Cr³⁺ (53.65%). Lower or higher concentrations suppressed microbial activity, reducing EPS secretion and lowering recovery. This study explores the Cr³⁺-resistance mechanisms of SP01 and highlights its practical application in MEOR after polymer gel formation, providing a solution to address reservoir plugging and improve oil recovery.

Initial

khm

Additional Information

Thesis (PhD) - Faculty of Engineering, Universiti Malaya, 2026.

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