Flow and reaction along the interface between hydrated Portland cement and calcareous rocks during CO2 injection. Laboratory experiments and modeling

Abstract

To better understand the geochemical alteration of the interface between wellbore Portland cement and rock because of potential leakage during CO2 storage operations, we performed percolation experiments using three solid cores each made of two half-cylinders, one of hydrated Portland cement and the other of calcareous rock (limestone, marl or sandstone). These experiments were run under atmospheric (PCO2 = 10−3.4 bar and room temperature) and supercritical (PCO2 = 130 bar and 60 °C) CO2 conditions with an injection pH of 6.4 and 3.2, respectively. The variation in the aqueous chemistry of the outflows was reproduced by 2D reactive transport simulations.The experimental and model results showed that under atmospheric conditions, a slight dissolution of portlandite and C-S-H near the cement-channel interface was responsible for an incipient alteration of cement that was prevented by the precipitation of brucite. By contrast, under supercritical conditions, cement alteration was marked owing to an intense dissolution of cementitious phases (portlandite, ettringite, Si-hydrogarnet and hydrotalcite), causing an increase in porosity.Overall, the results show that potential CO2 leakage during and after CO2 injection will cause an alteration of the hydrated Portland cement, resulting in a loss of its sealing properties. The alteration of low-porosity calcareous rocks, such as the ones used in this study, is only expected to be minor.