Establishing and maintaining the integrity of wells used for sequestration of CO2

Abstract

There is a tendency for the CCS industry to believe that everything is already known about CO2 injection wells, based upon the experience of miscible fluid injection for the purposes of tertiary oil recovery, combined with the general experience of gas re-injection and gas production internationally. In fact, analysis of the technical issues identifies that CO2 injection wells for sequestration may be more challenging in a number of ways, both in the fluids and pressures they must handle and the long term duration for which full well integrity will be required. Well integrity means the achievement of fluid containment and pressure containment within the well throughout its whole life cycle. The technical challenges of CO2 injection wells are highly dependent on the individual well design parameters, principally the formation being injected into (saline aquifer versus depleted gas formation) and the quality (impurity levels) found in the CO2 source gas. These factors will impact the potential corrosion and other material degradation challenges which may threaten the well components including injection tubing, injection casing, cement and packer materials. The long term integrity of these components is critical to the injection phase of the life and the suspension and abandonment phases (depending upon the strategy selected for well abandonment). A key part of the acceptance of CO2 sequestration as a safe and reliable greenhouse gas control mechanism will be the proof that the well is truly leak-free. This integrity management element requires a comprehensive monitoring and pro-active warning system which highlights developing integrity issues before they become acute. Key well parameters requiring daily monitoring and intermittent testing will be identified in line with industry international standards and regulations. Acceptance criteria for well test results will be defined in the context of a CO2 injection well. The oil and gas industry is only now commencing with comprehensive well integrity management systems and the CCS industry can make good use of the experience which is being established and software products such as the Intetech Well Integrity Toolkit (iWIT) which are proven to cover the comprehensive challenge that carbon dioxide injection wells represent.