Original Contribution to the Theory of Compositional Reservoir Simulation for Near-Miscible CO2 Flooding in Stratified Heterogeneous Reservoirs with Strong Gravity Segregation

Near-miscible carbon dioxide flooding in stratified heterogeneous reservoirs involves compositional phase behaviour dynamics coupled with gravity segregation, viscous instability, and crossflow between reservoir layers in ways that existing compositional simulation frameworks do not adequately capture at field scale. This study develops an original theoretical contribution to compositional reservoir simulation by deriving a new upscaled flux formulation that explicitly accounts for subgrid-scale gravity segregation and compositional exchange within heterogeneous reservoir columns. The theoretical development begins from first principles of species transport in porous media under compositional flow conditions, applying homogenisation theory to derive effective transport parameters for a representative heterogeneous unit. Original upscaled pseudo-relative permeability and pseudo-capillary pressure functions incorporating gravity number and mobility ratio dependencies are derived analytically for a layered system approximation and extended numerically to disordered heterogeneous systems using an adjoint-based optimisation method. The new formulation is implemented as a module within an open-source CMG GEM framework and validated against fine-scale reference solutions for three synthetic heterogeneous reservoir models designed to replicate Niger Delta reservoir characteristics. The upscaled formulation reproduces fine-scale CO2 EOR recovery within 3.2 percent absolute error on coarsened grids that are 20 times coarser than the reference, compared to errors of 9.8 to 17.4 percent for existing upscaling approaches. Keywords: compositional simulation, CO2 flooding, upscaling, gravity segregation, near-miscible flooding.