A novel index to evaluate CO2-induced wellbore cement degradation in CO2 rich environment

Abstract

A new index named CT-derived carbonation index is proposed to evaluate CO2-induced cement degradation in CO2-rich corrosive environment. This index is obtained by conducting CT scanning of cement samples before and after exposure to CO2, calculating grayscale difference and applying an algorithm to correlate grayscale with CaCO3 content. To demonstrate how to calculate the carbonation index, a CO2-cement interaction experiment along with X-ray computed micro-tomography characterization of a cement sample before and after exposure to CO2 was carried out to investigate the degree of cement carbonation. CO2 was dissolved in 1 wt% NaCl solution under a CO2 partial pressure of 17 MPa and a temperature of 63 °C. The carbonate shell in the cement sample was segmented from the grayscale difference images and the average penetration depth of the carbonate shell was obtained. The carbonation index was then calculated as the ratio of the average penetration depth of the carbonate shell to the radius of the sample, divided by the reaction time. In this study, the carbonation index of the cement sample exposed to CO2 for 14 days was 0.01 day−1. Based on the results in this study and other studies, a carbonation index of 0.05 day−1 or above corresponds to heavy degradation of cement by CO2.