Water/scCO2 Injection for EOR in Fractured-Vuggy Reservoirs Using Full-Diameter Cores

Abstract

Ordovician fracture-vuggy carbonate reservoirs, characterized by complex fracture networks and karst cavities, face critical challenges including short production plateau periods and rapid water breakthrough during water flooding. Conventional physical models fail to simultaneously replicate high-temperature (120 °C), high-pressure (65 MPa), and high-salinity (190,000 mg/L) reservoir conditions, with internal fluid distribution remaining unobservable. To address these limitations, two full-diameter core models emulating representative fracture-vuggy configurations were designed. Simulated crude oil and formation water were formulated based on PVT (pressure–volume–temperature) analysis and formation water composition, achieving unprecedented petrophysical replication accuracy. Six EOR schemes (water/gas injection with variable rates and modes) were tested under rigorously controlled thermo-hydro-chemical conditions on unfilled and partially filled vuggy systems. Significant recovery enhancement was demonstrated through adaptive development mode adjustments. CO2 injection for enhanced oil recovery also enables carbon storage, as CO2 occupies cavern spaces, thereby improving the ecological environment, reducing atmospheric greenhouse gas levels, and contributing to a carbon cycle. Crucially, in situ quantification of residual oil distribution was achieved via NMR (nuclear magnetic resonance) technology, and fluid redistribution mechanisms triggered by scheme modifications were revealed. This integrated approach provides a validated experimental framework for optimizing EOR strategies in complex fracture-vuggy reservoirs.