Experimental Study on CO2 Flooding for Enhanced Oil Recovery in Ultra-Low Permeability Reservoirs of the Songliao Basin

The Daqingzijing Oilfield in the Songliao Basin is characterized by low porosity and ultra-low permeability, exhibiting significant potential for CO ₂ flooding. To further clarify the impact of different CO ₂ injection methods on enhanced oil recovery (EOR), long core flooding experiments were conducted, including water flooding, gas flooding, and gas-water alternative flooding with different slug ratios. The results demonstrate that in such ultra-low permeability reservoirs, gas-water alternative flooding can effectively plug CO ₂ channeling pathways by forming dynamic Jamin effect slugs, overcoming the limitations of using either gas or water injection alone, achieving a synergistic enhancement effect. Multi-cycle injection significantly enhances oil recovery, with increases of 10.83% and 7.82% observed at slug ratios of 1:2 and 2:1, respectively. A slug ratio of 1:1 was identified as the most suitable for reservoir characteristics, yielding an oil recovery increase of 12.28%. Macroscopically, the slug ratio considerably influences mobility control and sweep efficiency; microscopically, the mass transfer between CO ₂ and crude oil predominantly governs flooding efficiency. The composite flooding method of water-alternating-gas can effectively overcome the recovery bottlenecks at different flooding stages, providing technical support for the development of ultra-low permeability reservoirs.