Synergistic mechanism of enhanced oil recovery and CO₂ sequestration by CO₂-N₂ mixed gas gravity drainage in fractured tight oil reservoirs

To address the challenges of weak gravity segregation, high gas channeling risk, and unclear carbon sequestration mechanisms during gas injection in fractured tight oil reservoirs, a method utilizing CO₂-N₂ mixed gas top injection gravity drainage is proposed. This study systematically investigates the synergistic mechanism of enhanced oil recovery and CO₂ sequestration. By coupling a modified Peng-Robinson state equation with CoolProp thermodynamic module, a phase-behavior model for CO₂-N₂ mixed gas was established. Incorporating with a discrete fracture network(DFN), a compositional numerical simulation model for fractured tight oil reservoirs was constructed. By using this model, the impact of mixed gas composition, injection-production parameters, and fracture natures on oil displacement and CO₂ sequestration was studied, and key parameters were optimized. Results show that optimal gravity segregation occurs at a CO₂ molar fraction of 70%, yielding a density difference of 330.2 kg/m³ against crude oil. In the reservoirs with fracture density of 0.05 fractures·m⁻¹, top gas injection engaged at a rate of 2×10⁴ m³/d with a injection-production well spacing of 250 m improves the ultimate recovery by 27.3% compared to depletion development. Moreover, 8.7×10⁴ t CO₂ sequestration is achieved. Carbon sequestration is primarily achieved through dissolution trapping and residual pore trapping, with sequestration efficiency showing a trend of first increasing and then decreasing with fracture connectivity. Long-term stable CO₂ sequestration is achievable under reservoir conditions of 20 MPa and 57 °C. This research determines the synergistic mechanism and prospect of CO₂-N₂ mixed gas gravity drainage for oil displacement and carbon sequestration in fractured tight oil reservoirs and provides technical support for the synergistic practice of field development and carbon sequestration in similar reservoirs.