Utilization of CO₂ resistant cement in CCUS wells

Abstract

With the growing number of Carbon Capture, Utilisation & Storage（CCUS）wells being planned and drilled across the globe, the importance of well integrity is becoming increasingly critical from both environmental and economical perspectives.

Objective of this paper is to provide comprehensive and fair discussions on the need for CO₂ resistant cement utilization in CCUS wells to mitigate potential CO₂ leakage.

Many studies demonstrated the weakness of Portland cement against CO₂ corrosion and the effectiveness of CO₂ resistant cement through lab testing.

On the other hand, many other studies insist（partly or indicatively, not directly）that Portland cement is good enough under CO₂-rich environment primarily due to its self-sealing effect across micro-annulus. However, some papers report that larger defects across micro-annuli may not be adequately sealed by Portland cement’s self-sealing effect. Also, self-sealing behavior was found to be absent at elevated temperatures.

Many papers suggest that CO₂ leak is most likely to occur through the micro-annulus. Hence, it is essential to minimize the micro-annulus in order to avoid the CO₂ leak to surface. To minimize the micro-annulus, expandable cement additive should be added to the cement system.

It is unlikely for Portland cement sheaths to get degraded by CO₂ all the way from the reservoir to surface since the progress of CO₂ corrosion in Portland cement is shown to be small in several different papers. But it is a possible scenario that cement is degraded to the extent where the leak path is connected from the reservoir to the existing micro-annulus and CO₂ migrates to the surface through the micro-annulus. Therefore, to minimize the CO₂ migration risk, CO₂ resistant cement should be utilized across the critical intervals（e.g. from reservoir till a certain height into caprock）.