Cement Degradation Under Conditions Relevant to Geological Carbon Sequestration

Assessing the potential risks of CO₂ leakage from deep subsurface is one of the major challenges for geological carbon sequestration. Injected CO₂ can dissolve into in situ brine to produce acidity for cement wellbore degradation. This degradation can potentially change the properties of the cement and decrease its structural integrity. Wellbore integrity assessment is essential for predicting the risk of leakage because degraded cement can open pathways for CO₂ leakage. In this work, we develop a reactive transport model based on experimental observations to obtain quantitative and mechanistic understanding of the coupled reactions and diffusion processes involved in cement degradation. This work also aims to quantify the effect of initial cement properties, in particular porosity and the abundance of portlandite, on the rate and extent of cement degradation.

The results show that the leaching of calcium ions during the acid attack of the cement leads to precipitation of calcite near the cement brine interface. This precipitation yields a localized reduction in porosity, which reduces the diffusion process and the penetration of the degradation inside the cement sample. The calcite precipitation is largely affected by the initial porosity and portlandite content. Increasing content of portlandite allows a thicker calcite precipitation thus decreasing the extent of degradation. Increasing initial porosity leads to faster diffusion of the acidic brine and less porosity reduction by the calcite precipitation. As a result, it increases the rate and extent of cement degradation. The ratio of the initial portlandite content and the porosity is a critical indicator of the severity of cement degradation. The results allow us to better understand cement behavior when exposed to CO₂-saturated brine under geological sequestration conditions and to predict the rate and extent of cement degradation with given cement properties.