Abstract:

CO₂ flooding is currently one of the most important enhanced oil recovery technologies. In North America, it has entered a stage of stable industrial deployment, while in China, it has entered a stage of rapid development in industrial application. A comprehensive comparison reveals that CO₂ flooding is mainly applied to medium-to-shallow, low-temperature, low-permeability light oil reservoirs, predominantly using miscible flooding in the United States. In contrast, CO₂ flooding reservoirs in China are characterized by deeper burial, higher temperatures, lower permeability, and higher crude oil viscosity, resulting in poor injectivity and significant challenges in implementing miscible flooding, thereby limiting the effectiveness of CO₂ flooding. Focusing on CO₂ flooding in deep, low-permeability reservoirs in East China, this study systematically analyzed the displacement characteristics of CO₂ in such reservoirs. The main challenges included high miscibility pressure, low permeability, relatively poor injectivity, difficulty in effectively replenishing energy through water or gas injection, and significant challenges in implementing miscible flooding. However, miscible flooding can still be achieved through early high-pressure injection to supplement reservoir energy. Additionally, large-slug CO₂ injection requires over-pressured gas injection to maintain the miscible process. Gas-alternating-water injection could effectively achieve gas channeling control, expand sweep efficiency, and improve development performance of gas injection. A field pilot test of CO₂ flooding in the deep, low-permeability reservoir of Caoshe oilfield was conducted. The results showed that the primary gas injection employed an “early-stage injection, large slug, and full-process tracking” approach to conduct high-pressure miscible flooding tests, achieving a 12.4% increase in oil recovery and a CO₂ storage rate of over 85%. Currently, secondary gas injection is being implemented using a strategy of “layered development, low-speed high-interval injection, and variable-frequency alternating injection”. Based on the principles of "controlling override, preventing channeling, and curbing water flooding", a secondary gas injection adjustment plan is formulated. Field tests continue to show promising results, with an additional 5.1% increase in oil recovery and a storage rate maintained at 75%, demonstrating strong application potential.

Key words: deep low-permeability reservoirs, CO₂ flooding, injectivity, pressure-maintained miscibility, water-alternating-gas, sweep efficiency