Investigation on wellbore cement degradation under geologic CO2 storage conditions by micro-CT scanning and 3D image reconstruction

This study presents a CT scanning and image analysis protocol to characterize wellbore cement degradation under geologic CO2 storage (GCS) conditions. The CT scanning and image analysis procedures described in the protocol are as follows: 1) CT scanning of the cement sample before exposure to CO2; 2) exposure of the cement sample to supercritical CO2 or CO2 saturated brine; 3) CT scanning of the cement sample after the exposure experiment; 4) application of 3D rigid registration to align all CT image frames, in order to eliminate pixel location variation between CT image frames due to sample drift; 5) acquisition of grayscale intensity difference images, which are obtained by subtraction of CT images after the CO2 exposure experiment from raw CT images before the CO2 exposure experiment; 6) application of noise filtering technique on grayscale intensity difference images to obtain images with good quality; 7) acquisition of 3D pore structure change of the cement sample after CO2 exposure experiment from grayscale intensity difference images, showing degradation of wellbore cement. To demonstrate the application of the protocol, an experiment of reaction between CO2 and wellbore cement under GCS conditions was conducted and the wellbore cement samples used in the experiment went through aforementioned CT scanning and image analysis procedures. CT image analysis results demonstrate a region with increased porosity in the exterior of the cement samples (Zone 1) and a region with decreased porosity next to Zone I due to CaCO3 precipitation (Zone 2). Next to Zone 2, a region with increased porosity due to Ca(OH)2 and C-S-H dissolution (Zone 3) was observed. In summary, this study proves feasibility to use 3-D CT scanning and CT image analysis techniques to investigate CO2-induced degradation of wellbore cement.