Capture and subsurface co-sequestration of acid gas (H2S and CO2) is one sequestration approach that can reduce the emissions of both CO2 and H2S from combustion of fossil fuel. Before the implementation of acid gas co-sequestration, the potential of acid gas leakage along existing and abandoned wellbores at target sequestration regions needs to be evaluated. Reactive transport modeling is a powerful tool to mimic the interactions between wellbore cement and CO2 + H2S, so as to evaluate the potential of acid gas leakage along existing and abandoned wellbores. In this study, the interactions between pozzolan-amended wellbore cement and acid gas were simulated using the reactive transport modeling program CrunchFlow. Key simulation outputs include, for example, calcite weight percentage, pH, porosity, pyrite weight percentage and ettringite weight percentage from the exterior to the interior of the pozzolan-amended wellbore cement. To validate model simulation outputs, SEM-BSE and SEM-EDS analyses on real pozzolan-amended cement samples exposed to CO2 and H 2S were conducted. Simulation results are consistent with SEM-BSE and SEM-EDS analyses results from real samples. Both model simulation and experimental results show that under the experimental exposure conditions (T = 50°C, P = 151 bar, 21 mole % H2S in 79 mole % CO2 and samples immersed in 1 wt% NaCl solution), the alteration of pozzolan-amended wellbore cement porosity is not significant after 28 days of exposure.