Subsurface storage of captured CO2 in depleted oil and gas reservoirs, saline aquifers, or unmineable coalbeds is currently seen as one of the most feasible ways of reducingemissions. Key to evaluating this concept and to storage site selection is the assessment of system integrity, with focus on the potential for CO2 leakage. Typically, depleted gas and especially oil are perforated by large numbers of (abandoned) production/exploration wells. Many of these wells also penetrate deep that might be used for geological storage of CO2. Although most of these wells are sealed and plugged with cement at abandonment, the well system may create potential conduits for CO2 leakage. Potential migration of CO2 from subsurface reservoirs along wells is generally recognized as the major hazard associated with long-term storage of CO2 in geological formations or CO2-enhanced recovery (EOR, EGR, ECBM) operations.
With respect to the evaluation of long-term integrity of geological CO2 storage two types of wells can be distinguished, i.e. existing wells and future wells. Future wells can be designed, drilled, completed and abandoned taking into account the preceding CO2 storage operations, using state-of-the-art materials and techniques to adequately deal with sour gas (mainly CO2 or H2S) occurrences. In contrast, most existing wells, comprising operational wells and abandoned wells, were not designed, drilled, completed and/or abandoned taking into account future CO2 storage purposes. Operational wells generally are accessible and can be adapted to fit injection or abandonment of corrosive fluids. The main issue is with previously abandoned wells that are no longer accessible and therefore cannot be improved when needed, without huge costs. This is illustrated by several case studies, clearly demonstrating the implications of previously abandoned wells penetrating prospective CO2 storage reservoirs as well as crucial aspects that are typically associated with these wells.
This report aims to describe current and historical abandonment practices. To this purpose first the basal plugging and abandonment techniques are briefly described. The risk of leakage through abandoned wells primarily depends on the regulations toward drilling and abandonment enforced at the time of plugging, of the diligence expressed by the operator during the plugging, and of the materials used in the plugging operation.
Subsequently, a synopsis of the adverse effects of aqueous CO2 on typical well materials is presented, including chemical degradation of well cement and steel as well as mechanical effects that may be associated with either operational activities or degradation. Results from multiple experimental studies on cement degradation are not univocal and not in full agreement with observed phenomena from field samples. A worst case approach would be the extrapolation of laboratory results obtained at quite severe (temperature) conditions, extrapolating to CO2-brine penetration of 12.4 m into the cement over 10,000 years. Taking into account the general results from laboratory studies, rather than this end member value, up to a few meters of cement may be affected over such time span. In the light of minimum plug length requirements, the mechanical integrity of the cement plug and the quality of its placement might be of more significance than the chemical degradation of properly placed abandonment plugs. The presence or development of fractures or annular pathways in the cement or along interfaces, between cement and casing or cement and caprock, strongly affects theof the cement. These will play an important role in leakage mechanisms, potentially significantly enhancing cement degradation.
Furthermore, an overview has been generated of regulations and guidelines governing well abandonment for a selection of countries and states involved in geological storage of CO2. Although these mainly comprise current regulations an effort was made to identify major changes in abandonment requirements. A general distinction can be observed between European and non-European countries. While in Europe the length of the cement plug is between 50 to 100 meter, in evaluated non-European regulations the length of the plug is between 30 and 60 meter. An exception is formed by thewhere approximately 30 meter (100 ft) is required. When mechanical plugs are used, additional cementing is often required of various lengths. Abandonment practices historically gradually developed to the present high standards. This implies that especially older wells may present problems, and should hence be carefully evaluated when considering their use in CO2 storage. The evaluated regulations primarily comprise prescriptive requirements for plugging and abandonment of oil and gas wells. It should be noted that also complementary regulations on e.g. labour conditions and environmental impact can significantly influence the effective management of well abandonment.
Severalmethodologies are described that can be employed to evaluate the wellbore system. In general a distinction can be made between qualitative and quantitative methods. In principle, qualitative approaches are used in the first stages of risk identification, providing means to comprehensively evaluate the system with respect to potential risks. In general a qualitative assessment will precede quantitative evaluations. Quantitative methodologies can be subdivided in probabilistic and deterministic approaches, where operations involving numerous wells benefit the most from using probabilistic risk assessment methodologies. Quantitative methods employed on storage reservoirs comprising few wells can consist of deterministic approaches.
Finally, recommendations are presented both onof well abandonment and on managing previously abandoned wells regarding long-term geological storage of CO2. As a result of uncertainties, proposed abandonment methodologies for present and future decommissioning of CO2 wells are relatively rigid to ensure safe and efficient storage of CO2. A procedure for permanent abandonment of CO2 wells is described, recommending the use of specialized cement and casing materials. However, newly developed materials, techniques and methodologies can only be applied to future wells and existing, operational wells. In contrast, previously abandoned wells cannot easily be re-abandoned and will form the biggest challenge regarding long-term containment of CO2. The most critical concerns regarding previously abandoned wells arise from the fact that the performed abandonment measures at the time did not take into account the potential application of the reservoir for CO2 storage purposes. As a consequence, the majority of abandoned wells was not completed and plugged using design and materials compliant with storage of corrosive fluids. Furthermore, the time of decommissioning versus historical developments of applying abandonment regulations highly determine the quality and suitability of the abandonment with respect to second-life applications of assets.
When considering CO2 storage the current state of the wells involved needs to be confidently assessed. For inaccessible wells this requires comprehensive risk assessment and monitoring efforts. In worst-case scenarios of leakage through abandoned wells, there is a limited amount of options that could be pursued. Wells could be re-entered to be remediated. Alternatively, several measures can be employed to reduce the reservoir pressure in order to both reduce the pressure gradient that drives migration and reverse potential opening of cracks or annuli. Obviously it would be beneficial if costly remediation operation could be prevented. A starting point for this is a comprehensive assessment of the wells involved. Furthermore, it would be recommendable to assess potential future applications of depleted reservoirs prior to abandonment, so that the abandonment can be tailored to second-life applications.