(132d) Reactive Transport Modeling and Scanning Electron Microscope Analyses to Evaluate Interactions Between Pozzolan-Amended Wellbore Cement and Acid Gas (CO2 + H2S) Under Typical Acid Gas Sequestration Conditions

Capture and subsurface co-sequestration of acid gas (H₂S and CO₂) is one sequestration approach that can reduce the emission 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 potential sequestration sites needs to be evaluated. Reactive transport modeling is a powerful tool to simulate 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 with the reactive transport modeling program CrunchFlow.  Key outputs from simulation include calcite weight percentage, pH, porosity, and pyrite and ettringite weight percentages from the exterior to the interior of the pozzolan-amended wellbore cement. SEM-BSE and SEM-EDS analyses on pozzolan-amended cement samples exposed to CO­₂ and H₂S were conducted, and results were used to calibrate the CrunchFlow simulation model. The model simulation results are consistent with SEM-BSE and SEM-EDS analyses results from the experimental samples. Both model simulation and experimental results show that at the experimental exposure conditions (T = 50 ^oC, P = 151 bar, 21 mole % H₂S : 79 mole % CO₂, samples immersed in 1 wt% NaCl solution), the alteration of pozzolan-amended wellbore cement is not significant. However, model simulation reveals that an increase of CO₂ partial pressure or H₂S partial pressure by 50% can both lead to significant alteration of the properties of wellbore cement and perhaps increase the risk of acid gas leakage.  Results of the wellbore cement exposure experiments and the related reactive transport modeling are being employed to guide development of a reactive transport model for acid gas migration along an entire wellbore.