Numerical Simulation of CO2 Huff-and-Puff Mechanism and CO2/N2 Synergistic Huff-and-Puff in the Edge-Bottom Water Reservoirs

With the steady advancement of China’s “Dual-Carbon” goals, CO2 huff-and-puff technology has become one of the mainstream methods for enhancing oil recovery (EOR) in oilfields. However, differences in sweep radius of CO2, CO2-oil interaction mechanisms, injection parameters, and huff-and-puff modes between conventional heavy-oil and light-oil reservoirs still require further investigation. The NP oilfield consists of an upper heavy-oil zone and a lower light-oil zone, with the reservoir inclined at a certain angle. Taking this oilfield as the study area, a positively rhythmic reservoir geological model was established. A compositional numerical simulation approach was employed to analyze the sweep radius of CO2 and the mechanisms of oil-saturation reduction of various huff-and-puff cycles under edge-water reservoir conditions. To address the issue of insufficient energy supply in well groups within closed fault blocks, the effects of synergistic CO2–N2 huff-and-puff on the development performance of conventional heavy-oil and light-oil reservoirs were further investigated. The results show that the sweep radius of CO2 huff-and-puff in heavy-oil reservoirs is significantly larger than that in light-oil reservoirs. Under a 10 m oil-column height, the maximum sweep radius reaches 100 m for heavy oil reservoirs, compared with 65 m for light oil reservoirs. Moreover, the average oil-saturation reduction per cycle in heavy-oil reservoirs is 0.58% higher than that in light-oil reservoirs. For well groups in closed fault blocks with weak energy supply, synergistic CO2–N2 huff-and-puff can effectively enhance oil recovery, benefiting from the gravity override effect of N2. Based on numerical simulation, the optimal CO2: N2 injection ratios for synergistic huff-and-puff were determined to be 2.1:1 for heavy oil and 1.67:1 for light oil. These findings provide important theoretical support for optimizing CO2 flooding development strategies in reservoirs of similar types.