Predicting long-term geochemical alteration of wellbore cement in a generic geological CO2 confinement site: Tackling a difficult reactive transport modeling challenge.

The safety of the future CO2 geological storage is largely dependent on the integrity of existing surrounding wells. Well integrity is of major concern in confinement sites where the number of abandoned wells is particularly high, such as it often occurs in depleted gas and/or oil fields. The degradation of the cement filling of these wells is a key issue to insure the confinement of the CO2. Laboratory experiments are unable to produce data for long periods of interaction; therefore, numerical modeling stands as a powerful means to predict the long-term evolution of the cement plugs, and to assess well integrity and leakage risk for the confining system. We thus present the results of a set of numerical simulations that predict the evolution of fluid chemistry and mineral alteration in the cement of an idealized abandoned wellbore at the top of the Dogger aquifer in Paris Basin, France, where CO2 geological disposal is currently under consideration. A continuum-based reactive transport formulation has been adopted which accounts for multi-component reactivity under water saturated and diffusion-controlled mass transfer conditions. Simplified two-dimensional models have been applied to simulate the complex geochemical interactions occurring at the interfaces between cement, aquifer and caprock domains. The simulations predict a two-stage evolution of the cement porous matrix, after interaction with acid fluids from reservoir: (i) a first, "clogging" stage, characterized by a decrease in porosity due to calcite precipitation, and (ii) a second stage of porosity reopening, related to the disappearance of primary cement phases, and the re-dissolution of secondary minerals, such as zeolites. Overall, the interaction with acid fluids causes a severe mineralogical alteration of the cement and the development of a carbonated, low-porosity layer near the reservoir interface. As the caprock imposes a high partial pressure of CO2, some mineralogical alteration of the cement is promoted also at the interface with the caprock. This pattern of reaction results in a large increase in porosity that might lead to the formation of vertical ascent route for reservoir fluids.