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Thermodynamic Modelling on Wellbore Cement Integrity During Underground Hydrogen Storage in Depleted Gas Reservoirs


 
 
 Underground hydrogen storage (UHS) has been raising more interest to safely and cost-effectively store hydrogen at large-scale to help the transition from fossil fuel to sustainable energy and to achieve net-zero emission target. During hydrogen subsurface storage particularly in depleted gas reservoirs, the wellbore plays an important role in injection and reproduction to meet seasonal energy demand. However, it is still unclear how wellbore cement would react with stored hydrogen in the presence of formation brine, which may effect long-term cement integrity. We thus performed thermodynamic modelling on cement reactions with hydrogen and water at reservoirs conditions.
 
 
 
 The dissolution of individual components of cement including C3S, C2S, C3A, C4AF and gypsum of Class G/H, and potential precipitation of twenty secondary minerals were simulated at an infinite time scale at reservoir temperature and pressure (representing the worst case scenario of cement degradation from geochemical perspective; in real case, the degree of cement degradation would be much less than the results from thermodynamic modelling as it is a time-dependent process). The extent of cement mineral reactions with hydrogen was compared with that of methane and carbon dioxide to assess the wellbore cement integrity during UHS compared to UGS and CCS.
 
 
 
 The cement hydration process would lead to the transformation of the major cement compositions C3S and C2S to C1.5SH (CSH) and portlandite. Adding hydrogen would only slightly change the percentage of C1.5SH and portlandite and generate a small fraction of new mineral mackinawite. As a comparison, adding methane would generate a considerable amount of calcite. When CO2 is involved, all CSH compounds would transform to calcite through the cement carbonation process. Overall, the compositional mineral phases of cement after cement hydration is more closed to the case involving H2 compared to CH4 and CO2, implying a relatively low risk of wellbore cement degradation during UHS.
 
 
 
 Our work underlines the importance of incorporating geochemical modelling in hydrogen geo-storage evaluation when using existing old wells and new drilled wells.
