STUDY ON CHARACTERISTICS AND MECHANISM OF IMMISCIBLE DISPLACEMENT OF SUPERCRITICAL CO₂ IN HETEROGENEOUS POROUS MEDIA

The study aimed to gain a deeper understanding of the immiscible displacement process of supercritical CO₂ in heterogeneous reservoirs, investigating its flow characteristics and impact on fluid dynamics in porous media. This research provides a scientific basis for CO₂-enhanced oil and gas recovery as well as geological carbon storage technology. A microscopic two-phase flow model, based on the coupling of the Navier-Stokes equation and the Cahn-Hilliard equation, was employed. Combined with Voronoi polygon construction technology, the study established a heterogeneous porous medium model that realistically represents pore structure characteristics. Through simulation, the fluid dynamic behavior and interface characteristics of the CO₂ displacement process under different conditions were detailed, and the factors affecting the evolution of the fluid interface morphology and displacement efficiency were analyzed. The results indicated that after the injection of supercritical CO₂,multiple preferential flow channels and complex immiscible interfaces were formed. The characteristics of the pore structure determine the magnitude of capillary forces, which affect the stability of the CO₂ displacement front. Larger pores are prone to fingering, whereas narrower pores, with higher capillary resistance, can cause CO₂ flow paths to divert or branch, leading to an uneven distribution of pore pressure. When the contact angle is acute and small, the suppression of CO₂ fingering becomes more apparent. The properties of CO₂ also directly affect the displacement effect, with adjustments in its viscosity and density helping to improve displacement efficiency. The injection rate significantly influenced CO₂ displacement, with high-rate injection favoring the expansion of preferential permeable channels and improving displacement efficiency. Although an extended displacement process significantly increased CO₂ saturation, it also involved high energy consumption, with displacement efficiency gradually decreasing in the later stages after CO₂ breakthrough. Therefore, regulating the physical properties of supercritical CO₂ and optimizing the injection rate can effectively improve CO₂ distribution and displacement pathways in the formation. These strategies are crucial for enhancing CO₂ displacement efficiency and optimizing reservoir development.