Mitigating CO₂ reaction with hydrated oil well cement under geologic carbon sequestration using nanoclay particles

The reaction of the cement with CO₂ saturated brine leads to cement carbonation which results in numerous chemo-mechanical changes in the cement matrix. In this study, the mechanism of nanoclay (NC) to enhance the carbonation resistance of class G oil well cement (OWC) is addressed. Two cement slurries with 0% and 1% of the modified montmorillonite NC were prepared. The samples were characterized with the powder X-ray diffraction (PXRD) technique which confirmed the decrease in the portlandite (CH) content and the increase in the most thermodynamically stable calcium silicate hydrates (CSH) for the sample with 1% NC. The samples then immersed into 0.5 M NaCl solution at 95 °C and 10 MPa for 30 days and the changes in the compressive and tensile strengths, permeability, and pore structure were evaluated with the carbonation time. The results revealed that incorporating NC into OWC paste increased the concentration of the CSH, decrease the reduction in the compressive and tensile strengths to 8.56% and 9.82%, respectively, compared with the compressive and tensile strengths retrogressions of 45.67% and 27.39% for the base cement. The final permeability of the NC-based sample is 51.67% less than the base sample permeability. The decrease in the cement permeability and CH content and the increase in the CSH are the main mechanisms which enhanced the NC-based cement resistance to carbonation.