Core Scale Numerical Simulation of Nitrogen Displacement of Residual Natural Gas in Water-Flooded Zones

The development of natural gas reservoirs faces challenges such as low recovery rates and suboptimal economic performance. In water-drive gas reservoirs, water invasion is particularly prominent, leading to declining well productivity and low recovery efficiency. Conventional development techniques struggle to effectively control water invasion and enhance recovery. Therefore, exploring new enhanced recovery methods, particularly techniques targeting residual gas in water-flooded zones, is crucial for improving gas reservoir recovery and economic viability. This study focuses on the challenge of recovering residual gas in water-flooded zones during the late-stage development of water-drive gas reservoirs. It investigates the mechanisms of switching to nitrogen injection after water flooding and evaluates the effectiveness of nitrogen injection in enhancing gas recovery through core-scale numerical simulation. Experimental results show that at 30 MPa, the remaining gas saturation after water flooding was 28.1%, which decreased to 20.8% upon nitrogen breakthrough and further dropped to 7.99% at the final stage, achieving a recovery rate of 88.9%. Numerical simulations reveal that the high permeability contrast in fracture-matrix systems causes water to rapidly advance along fractures, while capillary forces between fractures and the surrounding matrix promote water imbibition into matrix pores, forming gas entrapment. Nitrogen injection significantly reduces residual gas saturation (by 30–50%), effectively improving gas recovery in water-flooded zones. The findings provide a theoretical foundation for the efficient development of fractured water-drive gas reservoirs.