Experimental Study of CO₂ Huff and Puff Combined with N₂ Foam for Enhanced Oil Recovery by Three-Dimensional Physical Models

The oil increment of a heavy oil reservoir in North China decreases gradually year by year after multiple CO₂ huff and puff operations. In order to improve the developmental effect, CO₂ huff and puff combined with N₂ foam was proposed to enhance the oil recovery. Evaluation experiments on dynamic and static performances of foam systems were conducted to clarify the mechanism of CO₂ huff and puff combined with N₂ foam in enhancing oil recovery and assess the plugging performance of N₂ foam systems. Then, a self-designed three-dimensional heterogeneous physical model was used to carry out laboratory physical simulation experiments on CO₂ huff and puff combined with N₂ foam, with the effect of which on improving oil recovery and related mechanisms studied. Experimental results showed that a stable foam system could be formed by using α-olefin sulfonate (AOS) and polyacrylamide (HPAM) both with a mass fraction of 0.3%, and the plugging ratio could reach 99.57%, which thus effectively plugged high permeable layers. The results of three-dimensional experiments showed that CO₂ huff and puff combined with N₂ foam could improve the oil recovery by 22.74 percentage points, and the water cut could be reduced to as low as 2.07% during huff and puff operations, with its effective action period lasting 2.5–3.0 times that of pure CO₂ huff and puff. The CO₂ huff and puff combined with N₂ foam can effectively enlarge the swept volumes of CO₂ and subsequent water, which provides theoretical support for its future field applications.