Reactive Transport Modeling of Interactions between Acid Gas (CO2 + H2S) and Pozzolan-Amended Wellbore Cement under Geologic Carbon Sequestration Conditions

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• 作者：Liwei Zhang ; David A. Dzombak ; David V. Nakles ; Jean-Patrick Leopold Brunet ; Li Li
• 刊名：Energy & Fuels
• 出版年：2013
• 出版时间：November 21, 2013
• 年：2013
• 卷：27
• 期：11
• 页码：6921-6937
• 全文大小：920K
• 年卷期：v.27,no.11(November 21, 2013)
• ISSN：1520-5029

文摘

The implementation of acid gas cosequestration requires investigation of the potential for acid gas leakage along existing wellbores at sequestration sites. In this study, the interaction between pozzolan-amended wellbore cement (35 vol % pozzolan/65 vol % cement, hereafter referred to as 35:65 sample) and acid gas (e.g., a mixture of CO₂ and H₂S) was simulated using the reactive transport code CrunchFlow. The model was applied to describe, interpret, and extrapolate scanning electron microscopy-backscattered electron (SEM-BSE) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) results on pozzolan-amended cement samples exposed to a 1 wt % NaCl solution saturated with an acid gas mixture of 21 mol % H₂S and 79 mol % CO₂ under the temperature of 50 掳C and pressure of 150 bar. Simulation outputs included calcite volume percentage, total Ca and S weight percentages in the solid phase, porosity, and effective permeability from the surface to the interior of pozzolan-amended wellbore cement. The model reproduced the observed calcite zone formed in the brine-cement interface region of the sample after 2.5 days of exposure. The model also predicted that the calcite layer became dense (calcite vol % in the layer reached 55%) after 90 days of exposure, consistent with the experimental observation. C鈥揝鈥揌 was the primary Ca²⁺ source to form the calcite layer, followed by C3S and Ca(OH)₂. The main observed products of reaction between the 35:65 sample and H₂S were pyrite and ettringite. Pyrite was primarily formed within 0.5 mm from the brine-cement interface; ettringite mainly formed within 1 mm from the interface. The model simulated these reactions that only the interface region (up to 2 mm distance from the surface) of the 35:65 sample became porous after 30 years of exposure. However, this narrow porous region could still serve as a migration pathway for acid gas, which was indicated by the increase in effective parallel permeability values determined from the simulation results. Those results show consistency with results of neat cement samples exposed under similar conditions. An increase in H₂S content (in the range of 0 mol % to 40 mol %) results in more dissolution of Ca-bearing minerals in cement and more precipitation of calcite. Overall, this study indicates that an increase of porosity and permeability of pozzolan-amended wellbore cement at the cement interface with brine saturated with CO₂ and H₂S can cause significant changes in effective permeability of the cement.