Research on Enhanced Oil Recovery by CO ₂ –Water/Polymer Composite Alternative Flooding in Heterogeneous Reservoirs

Addressing the challenge of significant physical property variations in heterogeneous reservoirs, where conventional water flooding easily induces channeling, the current research focus lies in utilizing CO₂-water alternating gas (CO₂-WAG) or a CO₂-water/polymer composite alternating flooding to control fluid mobility and reduce gas channeling, thereby enhancing oil displacement efficiency. While existing studies primarily concentrate on single-reservoir types (e.g., purely high permeability or purely low permeability), this research employs core samples from the TX block (predominantly medium-high permeability) and CS block (predominantly medium-low permeability) of a specific oilfield. By adjustment of the size of periodic slugs, parallel long-core flooding experiments were sequentially conducted for CO₂-WAG and CO₂-water/polymer alternating flooding. Nuclear magnetic resonance (NMR) technology was integrated to quantitatively characterize the oil displacement effectiveness of the composite CO₂-water/polymer flooding under different heterogeneous physical properties and microscopic pore characteristics. The experimental results indicate that during CO₂-WAG flooding, the core recovery factor from the TX block (medium-high perm) was higher than that from the CS block (medium-low perm). During CO₂-water/polymer composite alternating flooding, the core recovery factor from the CS block (medium-low perm) exceeded that from the TX block (medium-high perm). CO₂-WAG effectively mitigates gas channeling in medium-high permeability heterogeneous reservoirs, while CO₂-water/polymer alternating flooding effectively reduces gas channeling in medium-low permeability heterogeneous reservoirs and enlarges the microscopic swept volume. The conclusions of this study provide a theoretical basis and a practical reference for the differentiated development of similar reservoirs.