Evolution of cementing properties of wellbore cement under CO2 geological storage conditions

In the context of CO2 geological storage, the wellbore is a critical location for potential CO2 leakage, with the cement bond surface being the primary pathway. The mechanical bond strength (MBS) of cement is crucial for wellbore seal integrity. To clarify the potential impact of long-term CO2 sequestration environments on MBS and uncover its evolutionary mechanisms, we prepared cement-shale and cement-casing bond samples, establishing both degradation and control groups. A 60-day CO2 high-temperature, high-pressure, static degradation test was conducted, along with a redesign of the MBS testing method. We measured the normal bond strength (NBS) and shear bond strength (SBS) and used various material characterization techniques to study the impact of chemical degradation on the bond surface's microstructure and composition, aiming to reveal the evolutionary mechanisms of MBS. The results indicate that, after 60 days of exposure to a saturated CO2 solution at 60 degrees C and a pressure of 15 MPa, the cement bond surfaces show a greater susceptibility to CO2 degradation compared to the bulk cement. The degradation group showing a 35.0 % decrease in NBS and 54.8 % in SBS for cement-shale, and an 83.9 % decrease in NBS and 84.3 % in SBS for cement-casing. The cement-casing interface deterioration was more severe. Analysis of the bond surface's microstructure and composition identified two main factors contributing to the deterioration: a reduction in the effective bond area due to material leaching and tensile stress from CaCO3 crystallization pressure. This study provides a clear understanding of the cement MBS evolution in long-term carbonic acid environment and explains the deterioration mechanism. It offers valuable insights for wellbore modeling and the design of degradation-resistant cement, serving as a reference for future research.