Research and Application of Enhanced Oil Recovery by Huff and Puff Using N₂–CO₂ Composite Gas in Complex Fault-Block Reservoirs: A Case Study of the Shallow Reservoir in Nanpu 1–3 Block

CO₂ huff-and-puff techniques have demonstrated favorable oil recovery improvements in the shallow reservoirs of the Nanpu 1–3 Block. However, in small, complex fault-block units, formation energy depletes rapidly, rendering single-component gas injection insufficient for sustainable long-term development. Nitrogen (N₂), with its superior pressurization capability, offers synergistic potential when combined with CO₂ as a composite injection gas. In this study, a numerical simulation model was developed to evaluate the N₂–CO₂ composite gas huff-and-puff process in fault-block reservoirs. The model assesses the sensitivity of key geological parameters—crude oil viscosity, oil saturation, formation dip, and oil-bearing area—and defines screening criteria for effective implementation. Results indicate that composite gas huff-and-puff is most effective when crude oil viscosity exceeds 10 mPa·s, oil saturation is above 40%, formation dip exceeds 5°, or oil-bearing area is larger than 0.04 km2. Field application in Well 302 of the Nanpu 1–3 Block confirmed the model predictions, achieving an additional 845 tons of oil and a 7% increase in recovery factor compared to control wells. These findings offer practical guidance for optimizing recovery in geologically complex fault-block reservoirs.