Thermodynamic insights into the degradation of oil well cement by CO2 under geologic sequestration conditions

Geological carbon storage (GCS) is an effective way to reduce CO2 emissions into the atmosphere. The degradation of oil well cement by CO2 poses a risk to the feasibility of GCS. It is critical to understand how the oil well cement degrades. The previous research is mainly experimental. The objective of this paper is to investigate the degradation of the oil well cement by thermodynamic theory in terms of the chemical reaction of individual phase and the phase evolution of the cement-CO2-water system. The results show that hydration products have different degradation resistance. The order of degradation resistance from best to worst is revealed. The thermodynamic equilibrium of the chemical reactions of degradation is affected by temperature and pressure. The higher the temperature and pressure, the less likely a chemical reaction will occur. Temperature tends to affect more than pressure does. The hydration product of the oil well cement is determined by thermodynamic modeling. The hydration products consist of calcium silicate hydrate solid solution (CSHQ), Si-hydrogarnet solid solution (Si-HG), ettringite solid solution (Ettri), portlandite (CH), and OH-hydrotalcite (OH-HT). The hydration products begin to degrade in the following order: CH, CSHQ, Si-HG, Ettri, and OH-HT. The results can be used to improve the degradation resistance of the oil well cement and research new type of degradation resistant cement. The thermodynamic theory is an effective way to investigate the degradation of the oil well cement by CO2 under geologic sequestration conditions.