Abstract:

With the exploration and development of oil and gas resources in China, heavy oil reservoirs account for more than 20% of domestic reservoir development. The efficient exploitation of heavy oil resources is crucial for China’s energy security. CO₂ injection technology has been proven to be an effective method to enhance heavy oil recovery. The mid-deep heavy oil reservoirs in the study area face critical challenges including strong formation water sensitivity and difficulties in large-scale waterflooding development, necessitating a shift from current water injection strategies. To address the unclear mechanism of CO₂ injection, unoptimized huff-n-puff development parameters, and lack of experimental research support in the study area, the mid-deep heavy oil reservoir in the Bei 10 well block of Xinjiang oilfield was selected as the research object. Experiments were conducted to investigate CO₂ diffusion in crude oil, supercritical CO₂ extraction of heavy oil, and the high-temperature and high-pressure long-core huff-n-puff tests, aiming to elucidate the mechanisms of CO₂ injection in heavy oil reservoirs and the efficient CO₂ huff-n-puff development mechanisms. The experimental results indicated that: (1) the diffusion coefficient of CO₂ in heavy oil exhibited a positive correlation with both injection temperature and pressure, with pressure demonstrating a greater influence on the diffusion coefficient than temperature. (2) With increasing extraction cycles, the mass fraction of medium-light components (C₆ to C₁₂) decreased significantly, while the mass fraction of medium-heavy components (C₁₄₊) gradually increased, particularly evident within the first three extraction cycles. (3) For heavy oil reservoirs, the recovery rate under depletion development was only 7.01%, while CO₂ huff-n-puff development proved effective in enhancing recovery, achieving a cumulative recovery rate of 36.94% and an oil replacement ratio of 0.59 t/t (oil production per ton of CO₂ injected). (4) Through huff-n-puff experiments, the optimal soaking time was determined to be 1-2 hours, and the optimal pressure depletion rate was 50 kPa/min. The research findings contribute to clarifying the mechanism of enhanced oil recovery by CO₂ injection in mid-deep heavy oil reservoirs, while guiding the design of CO₂ injection parameters in the Bei 10 well block. Moreover, this study provides critical references for the large-scale application of CO₂ huff-n-puff process in the future and offers valuable insights for CO₂ injection development in similar reservoirs.

Key words: mid-deep heavy oil reservoir, CO₂ huff-n-puff, diffusion, extraction, injection-production parameter optimization