8 Conclusions

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 that have been developed and employed over decades in oil and gas industry to adequately deal with sour gas (mainly CO2 or H2S) occurrences. In contrast, most existing wells were not designed, drilled, completed and/or abandoned taking into account future CO2 storage purposes. Existing wells comprise operational wells and abandoned wells. Operational wells generally are accessible and can be adapted to fit injection or abandonment of corrosive fluids. Techno-economical considerations determine the feasibility of required workover or abandonment measures. The main issue is with previously abandoned wells that are no longer accessible and therefore cannot be improved when needed, without huge costs.

8.1 Well abandonment techniques and practices

In the oil and gas industry the most common material used for plugging wells is Portland cement. Several different techniques have been developed to emplace the cement in the well, such as the balanced plug method, the dump bailer method and the two-plug method. Of these, the two-plug method provides a maximum of accuracy and a minimum of cement contamination.

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. Inadequate well design, well construction or plugging/abandonment performance may lead to poor isolation. Primary causes of failure are connected to mud contamination as a result of poor mud removal (most common), unstable cement slurries, insufficient slurry volume, and poor job execution.

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.

8.2 Well material degradation

It has been established in numerous laboratory studies (e.g. Barlet-Gouédard et al., 2006; Duguid et al., 2006; Kutchko et al., 2007) that cement degradation rates follow a diffusion law. However, results from multiple experimental studies 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 from Shen and Pye (1989) obtained at quite severe (temperature) conditions, i.e. 204°C and 69 bar. These experimental results correspond to worst case diffusion-based cement degradation involving CO2-brine penetration some 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. However, more recent results show substantially lower rates. In general therefore, present-day abandonment plug specifications seem acceptable, although significant differences in abandonment regulations can be observed worldwide with prescribed plug lengths ranging from 15-100 m.

Consequently, 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 (Scherer et al., 2005). The presence or development of fractures or annular pathways in or along the cement strongly affects the permeability of the cement (Shen and Pye, 1989) and will play an important role in leakage mechanisms, potentially significantly enhancing cement degradation (Bennaceur et al., 2004). Migration pathways are most likely to occur at the interfaces between cement and casing or the interface between cap rock and cement. Therefore, most attention should be paid to the potential migration of CO2 along the different material interfaces. Several mechanisms, both syn- and post-operational, can deteriorate the bond between the cement-steel and/or cement-rock interfaces, such as poor mud removal, cement shrinkage or pressure- or temperature-induced stresses inherent in well operations (Ravi et al., 2002).

This is supported by investigations of downhole cement samples from different field cases maintaining its sealing capacity (Carey et al., 2007; Crow et al., 2008; see Section 3.2.2). Analyses of these samples show that diffusion-controlled degradation of the cement matrix occurred only on a limited scale and does not seem to be a significant hazard to loss of wellbore integrity. In contrast, migration of CO2 along the cement-steel and cement-formation interfaces was reported by Carey et al. (2007) and Crow et al. (2008) during at least 30 years of exposure to CO2. In spite of the obvious, but limited flow of CO2 along the casing-cement interface of the SACROC sample, still no significant corrosion of the casing steel was observed (Carey et al., 2007), possibly as a result of the formation of a partially protecting siderite layer and increased pH values alongside the cement.

8.3 Well abandonment regulations

An overview of available oil well abandonment regulations is presented for a selection of countries and states involved in geological storage of CO2. Obviously oil and gas well abandonment regulations were not enforced simultaneously throughout the world. Moreover, the different regulatory frameworks show diverse levels of stringency. 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.

A general distinction can be observed between European and non-European countries. The main differences lie in the length requirements of the plugs near the deepest casing shoe. 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. When plugging perforated cased sections, the required plug length is in the range of 50 to 100 meters in the considered European countries. The required plug lengths for the studied non-European countries fit in the range of 30 to 60 meters. An exception is formed by the United Kingdom where approximately 30 meter (100 ft) is required in both cases described above, although where possible 150 meter (500 ft) plugs are set. In addition, when mechanical plugs are used, additional cementing is often required. It can be noticed that the required length for additional cementing differs significantly between the countries studied. For instance, in the Netherlands and in China 50 m additional cementing is required, whereas API requires 6 m of cement. Considering the plugs that isolate the permeable zones, the required plug length is again in the range of 50 to 100 meters in most considered countries, both within and outside Europe. Exceptions are the United Kingdom and Alberta (Canada), where a minimum plug length of 30 meters (or 100 ft) is prescribed.

As most abandoned wells that will play a role in future CO2 storage projects were abandoned according to present or historical oil well abandonment regulations, the focus of this study lies in these regulatory requirements. However, at present provisions for CO2 storage are being developed and implemented in many countries. For instance, in Germany, Australia and Norway as well as in the international conventions for marine protection amendments have already been made.

8.4 Risk management methodologies

As most abandoned wells are not easily accessed and, if required, remediated, a comprehensive assessment of risks associated with the wellbores need to be performed when considering geological storage of CO2. Several risk assessment and management methodologies have been developed over the past years. In general a distinction can be made between qualitative and quantitative methods. In principle, qualitative approaches, such as Quintessa’s CO2 FEP Database and TNO’s CASSIF and FEP methodology, 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. Different approaches will be applied for different situations. Operations involving numerous wells benefit the most from grouping of multiple more or less similar wells into classes and using probabilistic risk assessment methodologies. Quantitative methods employed on storage reservoirs comprising few wells can consist of deterministic approaches. However, even these necessarily comprise probabilistic or statistical elements. Several quantitative methodologies have been developed such as the Performance and Risk Management methodology (P&Amp;RTM) and SimeoTM-Stor from Oxand S.A., Semi-Analytical Model and Monte Carlo Simulation presented by Kavetski et al. (2006) and the CO2-PENS method. In general, it should be noted that impact of leakage and therefore the associated risks are highly dependent on surface environment (e.g. population density, level of urbanization). In addition norms and perception related to geological storage of CO2 may vary at different locations or regions worldwide.

8.5 Recommended best practice

Recently a lot of effort has been directed at the evaluation of the suitability of conventional materials for long-term containment of CO2. Furthermore new technology and materials have been developed. 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 was proposed by Carlsen and Abdollahi (2007; In: Randhol et al., 2007), 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 general previously abandoned wells cannot easily be re-abandoned. These wells 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. Finally, many abandoned wells may not be easily accessible without relatively high costs.

Since the wellbore system, especially at the end of its designed life-cycle, proves to be potentially sensitive to adverse effects associated with CO2 storage, the current state of the wells involved needs to be confidently assessed when considering CO2 storage. This especially goes for inaccessible wells, requiring comprehensive risk assessment and monitoring efforts. This involves an evaluation of the abandonment configuration as well as an examination of the current state of the materials involved applying appropriate risk assessment methodologies. Monitoring of well integrity for CO2 storage is part of a broad suite of monitoring techniques that can be employed on a storage site. Most often abandoned wells cannot be inspected or improved, significantly limiting monitoring options to general (near-)surface monitoring of the area around these wells, remote sensing techniques or geophysical methods can be used.

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, if physically possible, to be remediated and sealed again below the surface using appropriate techniques, such as squeeze cementing or expandable tubulars. 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.