Exposure of five cementitious sealant materials to wet supercritical CO2 and CO2-saturated water under simulated downhole conditions

Abstract

Maintaining well integrity is a key challenge to the secure geological storage of CO₂. Here, sealants based on Portland Cement form a key component, providing seals between the steel wellbore and surrounding caprock, as well as plugs for sealing wells that will no longer be used. However, exposure of sealants based on Portland Cement to CO₂-bearing fluids may lead to carbonation, potentially followed by degradation of these materials during prolonged exposure or flow, which may thus negatively impact well integrity. Therefore, new sealant materials need to be developed to help ensure long-term well integrity.

This paper reports exposure of five different sealants to CO₂-saturated water and wet supercritical CO₂ at in-situ conditions (80 °C and 10 MPa). Three of the sealants investigated are based on Portland Cement, while the other two are based on Calcium Aluminate Cement, and a rock-based geopolymer specifically developed for Geological CO₂ Storage (GCS). The five sealants were selected to represent different methods for improving wellbore seal integrity, such as restricting permeability (and porosity), or modifying how the material interacts with CO₂-bearing fluids. Exposures were carried out in a purpose-built batch apparatus, enabling simultaneous exposure of up to 10 samples in total to CO₂-saturated water and wet supercritical CO₂.

After exposure, changes in the sealants’ microstructures and chemical and mineralogical compositions were assessed using scanning electron microscopy with energy-dispersive X-ray spectroscopy, computed tomography scanning, and fluid chemical analysis. The impact of exposure to CO₂-bearing fluids was interpreted in terms of alteration and degradation of the materials, to compare how different sealant design modifications can be employed to enhance wellbore integrity.