Review of integrity of existing wells in relation to CO2 geological storage: What do we know?

Carbon dioxide storage in geological media is a climate change mitigation technology that is based on the ability of certain geological media to retain CO2 in supercritical phase or dissolved in formation water and to prevent its return to the atmosphere for very long periods of time. However, in certain cases there are flow pathways, natural or manmade, conducive to CO2 leakage. Depending on their condition, existing oil and gas wells may provide such leakage pathways due to either mechanical defects developed during well drilling, completion and/or abandonment, or to chemical degradation of well cements and/or casing. In the case of CO2 storage, there is a concern that well cement in existing wells will degrade in the presence of water-saturated CO2 and/or CO2 saturated formation water/brine, thus creating new leakage pathways and compromising the integrity and security of CO2 storage. In this paper we review the status of knowledge in regard to the failure of existing wells, with special attention to the laboratory experiments, field investigations and numerical simulations carried out in the last several years in attempts to elucidate the behavior of well cements in the presence of CO2. Extensive carbonation has been observed in well cements in both laboratory and field studies. However, in CO2-rich environments, severe cement degradation is associated with the dissolution of calcite from the carbonated cement. This is not expected under typical geological storage conditions because CO2-saturated brine is likely in equilibrium with carbonate minerals that are present in virtually all formation rocks.