Early Hydration Process of Well Cement Slurries for Cementing CO₂-Contained Gas Reservoirs and Factors Affecting the Performance of the Cement Slurries

In cementing a gas well with CO₂ contained in the reservoir, it is inevitable for the cement slurries to contact with the CO₂. The length of the contact time will possibly affect the early hydration characteristics and the development of the microstructure of the cement slurries, and worse still is the negative effects of this contact on the quality of well cementing. In this study, the change patterns of the early hydration characteristics, the mechanical properties and the permeability of a cement slurry were systematically investigated by controlling the ventilation time of CO₂. Using XRD, TG and SEM etc., the early phase composition and the evolution pattern of the cement slurry were analyzed. The study showed that the fluidity of the cement slurry was improved in the early stage and was then deteriorating with the time of CO₂ ventilation. The setting time of the slurry, on the other hand, was shortened continuously. The hydration heat release curve showed that CO₂ treatment significantly accelerated the cement hydration process, and increased the early cumulative released heat. However, as the time of CO₂ ventilation increased, these effects became weakening. Compressive strength and permeability test results showed that CO₂ treatment significantly enhanced the early strength of the cement slurry, and the best result can be obtained at ventilation time of less than 3 minutes. However, as the ventilation time of CO₂ increased, the strength and permeability of the set cement both gradually decreased. XRD, TG and SEM analyses showed that CO₂ treatment increased the formation of CaCO₃, disrupted the coating structures formed by Ca(OH)₂ and the C—S—H gel, and hence accelerated the hydration process of the cement particles.