Comparative Analysis of Gas Injection and Chemical Enhanced Oil Recovery Techniques for a Moderately Pressure Heavy Oil Reservoir: A Performance Efficiency Study

Abstract Recovering heavy oil from moderately pressured reservoirs in the Niger Delta remains a persistent challenge due to low reservoir pressure, unfavorable mobility ratios, and complex fluid–fluid and fluid–rock interactions. Conventional recovery methods such as gas injection, waterflooding, and chemical flooding are often deployed with varying degrees of success. This study presents a comparative performance evaluation of lean gas injection (nitrogen and carbon dioxide) and a suite of chemical EOR techniques—including polymer, surfactant, surfactant-polymer (SP), alkali-surfactant (AS), and alkali-surfactant-polymer (ASP) flooding—applied to a representative Niger Delta reservoir. The reservoir is characterized by a crude oil viscosity of 8.405 cP, saturation pressure of 1441 psia, formation water salinity of 25,000 ppm, and a gas–oil ratio (GOR) of 222 scf/STB. A fully integrated static and dynamic reservoir model was developed using comprehensive numerical simulation and calibrated with historical field production and pressure data to ensure predictive reliability. Results show that gas injection, particularly with nitrogen, significantly enhanced oil recovery through pressure maintenance and solution gas drive. Due to the reservoir pressure being below the minimum miscibility pressure (MMP = 1875 psia), CO2 injection occurred under immiscible conditions, limiting its capacity for viscosity reduction. Nevertheless, nitrogen injection yielded the highest oil recovery (7.00 MMstb), outperforming CO2 injection (6.87 MMstb) and all chemical EOR methods. Among the chemical methods, polymer flooding demonstrated superior performance (6.10 MMstb recovered), attributed to mobility ratio improvement, reduced fingering, and effective areal sweep. Surfactant-only injection resulted in the lowest oil recovery due to insufficient viscosity control. The findings underscore the technical and operational advantages of combining pressure support (via gas injection) with mobility control (via polymer flooding) for optimal recovery in moderately pressured heavy oil systems.