Abstract:

CO₂ sweep efficiency and gas channeling are key factors restricting the effectiveness of CO₂ flooding in low-permeability tight reservoirs. To address the technical issues of low sweep efficiency and gas susceptibility to channeling in CO₂ flooding, a viscoelastic fluid system (CMS) was proposed to improve CO₂ flooding in low-permeability tight reservoirs. A systematic study was conducted on the viscoelasticity, interfacial activity, injectivity, plugging performance, and oil displacement performance of CMS under oil reservoir conditions. The synergistic flooding performance of CO₂-CMS was explored, and field trials were carried out. Experimental results demonstrated that at reservoir temperatures ranging from 30-80 ℃, CMS with a mass fraction of 0.5% showed certain viscoelasticity, with elasticity as the dominant characteristic, and both viscosity and elasticity decreased as temperature increased. At an oil reservoir temperature of 45 ℃, the viscosity of CMS was 3.27 mPa·s, and it exhibited strong elasticity. Furthermore, CMS could effectively reduce the oil-water interfacial tension to 2.68×10^-2 mN/m. After core samples were immersed in the CO₂-CMS system, the water-phase contact angle decreased to 8.75°, indicating enhanced hydrophilicity. The CO₂-CMS system demonstrated good injectability in low-permeability tight cores, with smaller slug sizes yielding better injection performance. In long-core displacement experiments, using a 0.3 PV slug of CMS followed by CO₂ flooding increased the recovery rate by up to 27.79%. In double-tube parallel core displacement experiments, alternating CMS and CO₂ proved most effective, as it successfully sealed the high-permeability cores and mobilized low-permeability cores, resulting in a 26.28% increase in recovery. Field trial data from well groups indicated that after applying CMS, the overall liquid and oil production increased, and the CO₂ volume fraction in gas-channeling wells significantly decreased. This research provides new technical insights and practical solutions for improving the efficiency of CO₂ flooding in low-permeability tight reservoirs.

Key words: CO₂ flooding, viscoelastic fluid, low interfacial tension, low-permeability tight reservoir, synergy, plugging