Behavior of oil well cement slurry systems aged in CO2-saturated water under high pressure and temperature

Abstract

This paper aims at studying the effect of the exposure of hardened cement slurries, intended for oil wells, to carbon dioxide (CO2) in the state of water saturation. Four different systems of cement slurry with water/solid (w/s) ratio of 0.33 ± 0.01 and density of 1.98 ± 0.06 g cm−3 were design. The first cementitious system was a reference cement slurry (PR) commonly used for the cementation of wells. The other systems were obtained modifying the reference slurry with the addition of styrene-butadiene rubber (PRSBR), silica fume (PRSF) or crystalline silica (PRCS). The slurries were optimized in order to fulfill the rheological, physical and mechanical requirements to the cementation of oil fields. Cement slurries samples were aged in CO2-saturated water at 65 °C and 21 MPa pressure, for 30, 60 and 90 days. The degradation of the slurries was studied by thermogravimetric analysis. The carbonation rate with time and the carbonation profile along the diameter of the cylindrical samples were evaluated. Gas permeability and total porosity tests were carried out before and after aging also were determined to complement the analysis. From the results obtained, it was observed that the PR and PRSF mixtures presented the smallest amounts of calcium carbonate, especially in the internal layers of the sample. This happens due to the low permeability and total porosity of these mixtures that presented a high content of hydration products, including C–S–H. The PRSBR and PRCS mixtures presented high porosity and permeability and consequently higher carbonation than the PR mixture for all ages of CO2 exposition evaluated.