A Vertical Equilibrium Model for CO2 Migration in Depleted Gas Fields

Saeid Telvari , Hariharan Ramachandran , Gang Wang, Florian Doster

This study extends the vertical equilibrium (VE) modeling framework to simulate multi-phase flow involving CO₂, methane, and brine in depleted gas reservoirs. Methane presence introduces complexity not captured in traditional VE models. The proposed model integrates a black-oil approximation with VE assumptions, reducing dimensionality and enabling rapid simulation of large-scale CO₂ injection. By representing CO₂ and methane as separate compressible phases, and treating brine as incompressible, the model balances computational speed and physical realism. Pressure-dependent density and viscosity relationships for CO₂ and methane enable accurate property estimation without flash calculations. Three case studies validate the model: (1) a flat, layered reservoir benchmark demonstrating accurate CO₂ plume behavior under buoyancy control; (2) a dual-anticline scenario demonstrating that the VE model reproduces the large-scale plume geometry and inter-anticline mass redistribution observed in compositional simulations; and (3) a field-scale application to the Hugin Formation East, where the VE model replicates key features of CO₂ and methane migration observed in compositional models. The VE model runs orders of magnitude faster than full 3D simulations while capturing large-scale displacement dynamics. Although the formulation neglects explicit mixing, dissolution, and capillary fringe effects, it captures first-order displacement behaviour with speedups up to 100×. This makes it a practical tool for screening, uncertainty quantification, and optimisation in CCS projects targeting depleted gas reservoirs.