Supercritical CO2 (ScCO2) invades oilwell cement under geological CO2 sequestration conditions. With the penetration of ScCO2, cement structure prone to damage when the coupled effects of capillarity and carbonation were found. Microstructural evolution of oilwell cement samples was investigated by the CT scanning and the quantitative image-based analysis and show that ScCO2 with the high humid condition would penetrate much deeper than the dry ScCO2 because of the capillarity effects. Due to the deep saline condition in the sequestration formation, the penetration of ScCO2 was retarded by the salt deposition, comparing with the ultrapure water (UP water) conditions. For further assessment of this coupled mechanism, the permeability property and contact angle changes were proposed to analyse the interface region between ScCO2, saline/UP water and oilwell cement.

This presentation demonstrates a CT scanning and image analysis workflow to characterize wellbore cement degradation under corrosive geologic CO2 storage (GCS) conditions. The workflow includes 1) acquisition of raw CT images of the cement sample (before and after exposure to CO2); 2) application of rigid registration to align raw CT images; 3) acquisition of grayscale intensity difference images; 4) application of noise filtering technique to obtain images with good quality; 5) acquisition of 3D pore structure change of the cement sample after CO2 exposure from grayscale intensity difference images, showing degradation of wellbore cement. To demonstrate an application of the workflow, an experiment of reaction between CO2 and wellbore cement under corrosive 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 sample (Zone 1) and a region with decreased porosity next to Zone 1 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 3D CT scanning and CT image analysis techniques to investigate CO2-induced degradation of wellbore cement.