Original Theoretical Framework for Reactive Transport Modelling of Diagenetic Alteration During CO2-Enhanced Water Flooding in Carbonate Reservoirs of the Benue Trough

Carbon dioxide-enhanced water flooding in carbonate reservoirs induces geochemical reactions between the acidic CO2-water mixture and carbonate minerals that alter reservoir porosity, permeability, and wettability in ways that conventional reservoir simulation frameworks do not capture, presenting a fundamental modelling gap for CO2 EOR in carbonate systems. This study develops an original theoretical framework for reactive transport modelling of diagenetic alteration during CO2-enhanced water flooding applicable to carbonate reservoirs of the Benue Trough. The theoretical development couples the PHREEQC geochemical speciation code with a dual-porosity porous media transport model through a novel operator-splitting scheme that preserves mass conservation and thermodynamic equilibrium at the fluid-mineral interface during advection-reaction time stepping. An original pore-structure evolution model is derived from micro-CT imaging of pre- and post-reaction Benue Trough limestone core samples, expressing porosity-permeability coupling through a modified Kozeny-Carman relationship parameterised for calcite dissolution-dominated alteration. Reactive transport simulations for a representative Benue Trough carbonate section under CO2 water flooding conditions predict a 4.2 to 7.8 absolute unit increase in porosity in the near-wellbore matrix after 180 days of continuous injection, accompanied by preferential wormhole development in high-permeability streaks. Wettability alteration toward water-wetness induced by CO2 acidification is incorporated using a dynamic contact angle function linked to mineral surface charge calculations. Keywords: reactive transport, CO2 flooding, carbonate reservoir, Benue Trough, diagenetic alteration.