Application of Engineered Cementitious Composites for Enhanced Wellbore Integrity During Geologic Carbon Sequestration

Abstract

Geologic carbon storage (GCS), which entails the capture of carbon dioxide (CO2) from large CO2 emitters and storage in deep geologic formations, is being proposed as a bridging technology to combat global climate change caused by the emission of anthropogenic CO2. However, leakage of CO2 from storage reservoirs, such as through damaged wellbore cement sheaths, remains a major challenge for GCS. Laboratory and field studies carried out over the past decade have provided significant insight regarding the alteration mechanisms in traditional wellbore cement exposed to CO2 during GCS and indicate that deterioration of the physical and mechanical properties of the wellbore cement could occur in the long-term when exposed to CO2. However, very little work has been done to investigate substitute materials that can prevent such cement sheath deterioration during GCS. In this dissertation, a novel ultra-ductile fiber-reinforced cementitious composite, engineered cementitious composites (ECC), is proposed as a substitute material for wellbore cement used during GCS to ensure superior mechanical performance of cement sheaths. Conventional ECC was exposed to CO2 under typical GCS reservoir temperature and pressure conditions and X-ray computed tomography and scanning electron microscopy analyses were carried out on the reacted and unreacted materials to understand the unique changes in ECC’s microstructure. Results showed that the interaction of ECC with CO2 would alter its microstructure uniquely. Particularly, the fiber/matrix interfacial transition zone (ITZ) will be densified due to carbonation reactions. Further studies carried out to investigate the implication of the ITZ densification on a version of ECC rheologically re-engineered for wellbore cementing applications showed that the densification of the ITZ reduced the ductility of ECC. However, the altered ECC continued to exhibit superior ductile performance in comparison to conventional wellbore cement. Additionally, ECC exhibited fluid transport characteristics comparable to conventional wellbore cement, which suggests that it will be an effective diffusive barrier to limit the movement of fluids through the cement sheath. To investigate cyclic crack healing in damaged ECC, pre-cracked specimens were exposed to CO2 under typical GCS environmental conditions and permeability evolution was determined at various time points. Crack healing occurred in several specimens under the different conditions tested. Specimens exposed to CO2-acidified water, which simulates the environmental conditions near the downhole region of the wellbore, exhibited the most significant crack healing, accompanied by a substantial reduction in permeability. Similarly, specimens exposed to CO2-acidified water recorded the least decline in tensile strain capacity and a significant increase in tensile strength post carbonation, indicating that the application of ECC as a primary cementing material is most viable in the downhole region of GCS. This study establishes ECC as a promising candidate for consideration as a substitute wellbore cementing material during GCS to ensure secure storage of CO2.