7 Recommended best practice

7.1 Existing wells and future wells

Two general types of wells can be distinguished regarding long-term geological storage of CO2, i.e. existing wells and future wells (Watson and Bachu, 2007).

Future wells comprise wells directly related to the CO2 storage operations, such as CO2 injection or monitoring wells, and wells penetrating or transecting a CO2 storage reservoir aimed at other structures or deeper reservoirs e.g. for production of hydrocarbons or geothermal energy. These wells can be designed, drilled, completed and abandoned taking into account the preceding CO2 storage operations, using state-of-the-art materials and techniques. The well section running through the storage reservoir and its caprock should be equipped with corrosion resistant materials using appropriate techniques. Similar practices have been employed over decades in oil and gas industry to adequately deal with sour gas (mainly CO2 or H2S) occurrences. Therefore, suitable skills and technology are available to effectively seal and isolate the CO2 storage reservoir.

In contrast to wells drilled after CO2 storage activities, existing wells did not benefit from the prescience on the presence of corrosive fluids in the storage reservoir during their design, drilling, completion and/or abandonment phases. Therefore, their configurations may not agree with such purpose. Existing wells can be subdivided in operational wells and abandoned wells.

If necessary, operational wells can usually be adapted to fit injection or abandonment of corrosive fluids. Although costly, such operations can be performed using suitable workover materials and techniques that are currently available in the petroleum industry. In principle techno-economical considerations determine the feasibility of these measures, and therefore, of the storage project. An important advantage over abandoned wells is that operational wells in principle are accessible and need to be plugged during abandonment. Although the design, drilling and completion phases were developed not taking into account CO2 storage, the crucial abandonment phase can be tailored to specific requirements associated with long-term CO2 containment. If existing wells were converted for CO2 injection, these should be tested to ensure their integrity under pressure (Randhol et al., 2007).

The main issue is with previously abandoned wells. These wells are no longer accessible and therefore cannot be improved when needed, without huge costs. As described in the preceding chapters, 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. Oil and gas well abandonment regulations were not enforced simultaneously throughout the world. Moreover, the different regulatory frameworks show diverse levels of stringency (see Chapter 5).

7.2 State-of-the-art well abandonment for corrosive fluids

Recently a lot of effort has been directed at the evaluation of the suitability of conventional materials for long-term containment of CO2. As a result of uncertainties in reported material degradation rates, 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. Similar to oil and gas wells, sealing elements should consist of multiple pressure barriers and prevent cross-flow. Specific attention in the case of CO2 storage is directed at materials that are chemically inert to wet CO2 and provide sufficient bond strength. The described methodology can be applied to future wells and existing, operational wells.

The proposed procedure (Figure 7.1) involves pulling out of hole of tubing and packer, followed by placement of a cement plug at the bottom of the well. A specially designed fluid (e.g. polymer, resin or other advanced materials) could be injected into the reservoir to cause intentional clogging of the near-well area in the reservoir to displace the CO2 and to delay or reduce contact between CO2 and well materials. In order to avoid potential leakage along cement-casing micro-annuli, then the casing is milled out at the level of the caprock. Subsequently cement can be injected in the perforations and placed along the open hole interval. A cement squeeze job is proposed both at the bottom and top of the caprock, filling and closing any micro-cracks that could have developed during previous operations (Randhol et al., 2007). Emplacement of a cement plug along a milled-out section is also called a ‘pancake plug’. The well then should be filled with a non-corrosive completion fluid. At more shallow depth, if present at the level of a secondary sealing formation, an extra cement barrier is recommended, again placed after removing and milling out of the casing strings. Finally a surface plug is put in place.

Figure 7.1 CO2 storage well before (left) and after abandonment (right) according to the methodology described by Carlsen and Abdollahi (2007; In: Randhol et al., 2007).

7.2.1 Advanced materials

Benge (2008) presents an overview of improvements of the isolating capacity of wellbore sealants regarding geological storage of CO2. Efforts directed at enhancement of the Portland cement-based sealing system have focused on reduction of the cement’s permeability after curing and decreasing the concentration of materials that react with dissolved or wet CO2. These materials can be applied in drilling, completion, workover and abandonment operations.

7.2.1.1 Reduced cement permeability and reactivity

Reducing the cement matrix permeability is a relatively easy measure to decrease the cement’s reactivity with CO2. This can simply be done by reducing the water to cement ratio. However, this also causes an increase of cement density and therefore enhanced hydrostatic pressures in the well. Addition of specialty materials (such as specifically sized particles that fill the cement pore space) provides an alternative method to reduce permeability. This technique also enables modification of the slurry density over a wide range of values to appropriate levels for specific cases. Furthermore, addition of specialty materials at least dilutes the relative amount of reactive species, but can also be tailored such that they protect the reactive species of Portland cement. BarletGouedard et al. (2009) tested Schlumberger’s CO2 resistant cement – EverCRETE – with expansion property, aimed at mitigating the risk of microannulus formation during CO2 injection. The evaluation shows linear expansion results that can be adequately constrained and optimized by the concentration of expansion agent in the cement at different temperatures. The expansion property has no effects on its mechanical performance after exposure to CO2.

7.2.1.2 Non-Portland cements

As Portland based cements will react with wet CO2, the application of non-Portland cements (e.g. calcium (sulfo)aluminate-based cements, geopolymeric or alkali aluminosilicate cements, magnesium oxide cements, hydrocarbon-based cements and ceramic-based cements) could be considered for CO2 storage operations (Benge, 2008). Unfortunately, these materials are incompatible with Portland cements and cross contamination has to be eliminated. Furthermore, the effective density range for non-Portland cements is narrower than for Portland-based cements. Limited availability and higher costs are additional disadvantages of these cements (Benge, 2008).

7.2.1.3 Self healing cements and swelling packers

In addition, specialty materials have been developed, such as ‘self-healing’ cements and in-situ swelling packers (Benge, 2008). Self healing cements contain specific additives designed to react with fluids to clog cracks or debonding annuli and eliminate potential flow. However, swelling technologies sofar focused on hydrocarbon swellable materials, rather than additives interacting with CO2. In addition, developments in slurry design concentrated on slurry design to prevent failure after placement, such as including flexible materials or reducing Young’s modulus of the cured cement. Furthermore, swellable packers have been developed to isolate flow in the event of any failure of the cement sheath, rather than to act as initial wellbore seal. Swelling packers are placed on the outside of the casing and are designed to swell when coming into contact with various materials, e.g. hydrocarbons, water or both (Benge, 2008).

7.3 Managing previously abandoned wells

As previously abandoned wells generally are not accessible anymore, these cannot easily be re-abandoned. The majority of abandoned wells was not completed and plugged using materials compliant with storage of corrosive fluids. In the preceding chapters conventional techniques and materials as well as its potential reactivity with aqueous CO2 were extensively described.

7.3.1 Lessons learnt from field cases

An excellent example of typical issues that could arise when considering second life applications of fields is provided by the De Lier case in the Netherlands (Section 3.1). The most critical concerns 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. Furthermore, the case clearly illustrates the important role of historical developments of abandonment regulations. In general wells that have been decommissioned before significant amendments in regulations were enforced, may hold higher risks with respect to CO2 storage operations. For the De Lier case the fact that the field consists of a stack of reservoirs lead to increased complexity. While CO2 storage was proposed for the shallowest reservoir, several wells transect this reservoir aiming at deeper strata. At the time of abandonment of these wells the most shallow reservoir was already depleted and no plugs were required at its cap rock level as long as no perforations were present. Finally, some of the wells in need of measures were no longer accessible as a result of urban expansion.

Based on the outcome of the well evaluation, the operator decided that geological storage of CO2 in the De Lier field was not economically feasible at the time. However, based on the lessons learnt the operator improved its company best practice on well abandonment: although not required by regulations, sites that are earmarked for potential future CO2 storage are abandoned in a manner compatible to such purpose.

7.3.2 Risk assessment

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. Special attention should be paid to previously abandoned wells. This involves, first of all, an evaluation of the abandonment configuration. Re-evaluation of wells that were successfully abandoned upon finishing production in the De Lier case evidently showed that securing long-term isolation of stored CO2 required substantial additional measures (see Section 3.1), predominantly resulting from the fact that the target storage reservoir was transected by various wells that were aimed and abandoned for reservoirs at greater depth. Second, the current state of the materials involved should be examined, extrapolating from data gathered prior to abandonment. If detailed information on, for instance, the quality of the cement sheath is lacking, no decisive conclusions can be drawn on the safety of the well.

In Chapter 6 different methodologies are described that aim to evaluate risks of CO2 storage projects associated with the well system. The presented approaches were subdivided in qualitative and quantitative risk assessment methods, the latter being classified in deterministic and probabilistic methods. In general qualitative evaluations precede a subsequent quantitative assessment. Qualitative methods are very well suited for making an inventory of all potentially adverse effects on the well system resulting from CO2 storage operations. While qualitative risk evaluations could provide assistance to arrive at a comprehensive risk assessment, these in themselves lack the possibility to calculate probabilities and impacts connected with the hazards defined. Quantitative methodologies enable predictions of the evolution and performance of the well system.

In general 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. Furthermore, 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.

7.3.3 Monitoring and remediation

Monitoring of well integrity for CO2 storage is part of the entire 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. Monitoring of such wells will generally be constrained to general (near-) surface monitoring of the area around these wells. Potential migration of CO2 through or along (parts of) abandoned wells could be indirectly detected by e.g. monitoring soil gas concentrations and fluxes, air concentrations and fluxes indicating surface seepage, and regular groundwater chemistry measurements (Benson and Myer, 2002). For this purpose (near-)surface measurements, remote sensing techniques or geophysical methods can be used. However, detection of diffuse leaks may be difficult as its signal is within the range of natural CO2 fluxes (Benson and Myer, 2002). The addition of tracers to the injected CO2 would facilitate easier detection and discrimination of leakage of the stored gas over natural CO2. Monitoring strategies are discussed in more detail by e.g. Benson et al. (2002).

In worst-case scenarios of leakage through abandoned wells, they should be re-entered, if physically possible, to be remediated and sealed again below the surface using appropriate materials. Techniques commonly employed in oil and gas industry involve squeeze cementing or the use of expandable tubulars, either on the inside or outside of the casing. If remediation will not eliminate leakage, the reservoir pressure might have to be released to both reduce the pressure gradient that drives migration and reverse potential opening of cracks or annuli. This could be realized by several measures, such as reducing CO2 injection pressure, abortion of injection or reproducing injected CO2 to the surface. Alternative options to decrease the reservoir pressure would be peripheral extraction of formation fluids or increasing reservoir capacity by hydrofracturing. Evidently, the latter measure requires great attention to not (further) damage wells or caprock. 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.