Evaluation of density influence on resistance to carbonation process in oil well cement slurries

Abstract

The carbon dioxide (CO2), in the presence of water, forms a corrosive medium that reacts with the Portland cement hydrated products in a process denominated carbonation. Carbonation is a topic of great interest for the oil and gas industry due to deleterious and negative effects caused by formations that contain CO2 in their pores to the cements used in cementing operations, as primary and remedial cementing. The corrosive compound present in subsurface can be originated from geological processes, injection techniques for enhanced oil recovery and geological storage. This work evaluated characteristics of CO2 attack in class G Portland cement slurries formulated with different densities: 1890 kg/m3 (15.8 ppg), 1970 kg/m3 (16.5 ppg) and 2030 kg/m3 (17.0 ppg). The slurries were prepared, cured, and subsequently exposed to CO2 during periods of 30, 60 and 90 days in an autoclave with controlled temperature and pressure conditions. After the times inside the autoclave, the samples were analyzed with a pH indicator solution, sedimentation test and scanning electronic microscopy (SEM). The results showed an inversely proportional pattern: the higher the density, the lower the carbonation. The higher content of reactive solids contributed to make the matrix more closed and with a greater availability of hydrated cement products, acting as a barrier to the carbonation advancement.