Three-Phase VE Simulation of CO2–Methane–Brine Flow in Reservoirs

This study presents a three-phase vertical equilibrium (VE) model for simulating CO₂ injection into depleted gas reservoirs containing methane and brine. Using a black-oil formulation, CO₂ and methane are treated as compressible, immiscible fluids. Vertically integrated mass balance and flux equations are derived to account for density variations and implemented in the open-source MATLAB Reservoir Simulation Toolbox (MRST). Benchmarking against a high-resolution compositional simulation of CO₂ injection into an anticlinal trap shows that the VE model accurately captures plume geometry, phase ordering, and final trapping volumes, while reducing computational cost by over two orders of magnitude. Deviations are limited to the CO₂/methane mixing zone, which the immiscible formulation cannot resolve. The model therefore provides a practical tool for rapid screening, well-placement optimization, and uncertainty analysis where full compositional simulations are computationally prohibitive.