Mitigating the Effect of Carbon Dioxide Contamination on Oil Cement Slurry

Carbon dioxide contaminates cement slurry and changes the hardened cement physical properties such as porosity, permeability and flow properties. The effects of additives such as diesel and lime in various proportions to the cement slurry to reduce the effect of the carbon dioxide contamination of cement slurry was studied. Also, the possibility of replacing the Portland cement with a white Portland cement was studied. The permeability of the carbonized cement was reduced, on the average for diesel cement and white cement given a minimum reduction of about 29.8% and 4.6% respectively when compared to the minimum porosity of carbonized Portland cement. This is a good indication that the addition of white cement and diesel as additives enhanced a good cement job in terms of sealing off unwanted zone. It was discovered that while diesel additive to the cement slurry has a positive effect on mitigating the effect of the carbon dioxide contamination; the addition of small quantities of lime has adverse effects on the cement if it is contaminated with carbon dioxide leading to the carbonization of the Portland cement. Therefore, whenever there is the possibility of carbonization of cement slurry, diesel cement is recommended to be used instead of the normal Portland cement. For all the additives, carbonization increased the porosity as compared to the uncontaminated cement and this is expected. Increased porosity due to the cement carbonization is a measure of weakened strength of the cement but the observed permeability when the used diesel or white cement is so small that there is possibility that the cement will retain its integrity of isolating unwanted zones. For improved strength, there is a need to add a strength improver to the cement slurry whenever there is a possibility of carbon dioxide contamination.