Study on the mechanism of CO2 composite pressure flooding recovery enhancement in deep medium-low permeability heavy oil reservoirs

Abstract

To address the challenges of poor development performance in deep medium-low permeability heavy oil reservoirs, which are attributed to low permeability and high viscosity of crude oil, a CO₂ composite pressure flooding technology has been proposed in the field. This study investigates the interaction mechanisms among solubilizer, CO₂, and heavy oil through the performance of CO₂ solubilization and synergistic viscosity reduction. High-temperature and high-pressure micro-displacement experiments were conducted to examine the oil displacement mechanisms of CO₂ composite pressure flooding from the perspectives of seepage characteristics and residual oil distribution. The results indicate that the addition of solubilizing and enhancing agents can increase CO₂ solubility by more than 25% under pressures ranging from 5 MPa to 30 MPa, with a synergistic viscosity reduction rate exceeding 95%. Microscopic experiments demonstrate that the dissolving-carrying-extraction effect of CO₂ composite pressure flooding is significant, as it reduces the viscosity of crude oil through dissolving diffusion, leading to alterations in the morphology, volume, and flow state of oil droplets, thereby facilitating rapid extraction. The residual oil primarily exists in the form of disconnected phase pore blind ends and oil film-like crude oil. The CO₂ composite pressure flooding technology enhances oil recovery through the mechanisms of solubilization and viscosity reduction, energy assistance, and mass transfer efficiency, enabling the effective utilization of deep medium-low permeability oil reservoirs and providing technical support for the effective development of deep heavy oil.