**5.1 Leak path**

The function of cement sheath in oil and gas wells is to seal formation fluids, support and suspend casing, protect the wellbore, and provide a certain alkaline environment to avoid casing corrosion and ensure wellbore integrity. However, after carbon dioxide is injected into the subsurface, it will corrode the cement sheath and casing under suitable humidity and pressure conditions cause problems, such as a decrease in the strength of the cement sheath and an increase in its permeability. Furthermore, the corrosion effect, cement hardening, and alternate injection processes will impact casing and cement sheath strength and stress distribution in the near-well region, exacerbating wellbore integrity failure. **Figure 10** shows the CO2 leak path due to wellbore integrity failure [45, 47].


**Figure 10.** *Schematic diagram of leak pathways related to wellbore integrity (reproduced from [46]).* (5) Leakage along formations damaged during drilling (#6)

(6) Casing-cement and cement-formation interface debonding resulting in microfractures (#7, #12)

(7) Poor cement consolidation quality with permeability (#8)

(8) Fractures or gas channels in cement (#10, #11)

#### **5.2 Chemical well integrity failure**

Silicate cement is usually used as a sealing material when completing a well. The reaction occurs when the cement comes into contact with CO2, resulting in the degradation of the cement. The continuous reaction of cement with CO2 increases the porosity of the cement matrix, which also allows chloride ions to pass through the matrix causing corrosion of the casing [48, 49].

However, the consequences of the reaction between cement and CO2 are not always harmful. The slight carbonation of CO2 can reduce the permeability and porosity of the cement without causing failure of well integrity if the cement consolidation quality is excellent. However, a high degree of carbonation will certainly lead to failure or fracturing of the cement structure, failing well integrity. The corrosive effect of CO2 will be more pronounced if the cement quality is poor with existing defects such as tiny pores or fractures [50, 51].

There are numerous mechanisms of chemical reactions impacting well integrity, and it is necessary to clarify the CO2 and formation properties, and then the various chemical changes during the interaction of cement materials, wellbore materials, and carbonic acid can be further investigated.

#### **5.3 Mechanical well integrity failure**

Geomechanics is a key factor impacting well integrity with effects throughout the life cycle of a GCS project. During the drilling, completion, and application of the well the stresses in the wellbore, the cement sheath, and the near-well area keep changing. The reasons include pore pressure changes due to injection, leakage, and diffusion, thermal stress changes due to temperature differences, and in-situ stress changes due to tectonic shifts or seismic activity. Among them, the stress change in the cement sheath is the most significant. After the cement is poured, it will go through two stages of hardening (liquid cement transforms into solid cement) and shrinkage (solid cement volume shrinks and continues to harden). The change in cement rheology is more obvious during the hardening process. However, in the post-hardening phase, the shrinkage of the cement not only has a large effect on the stresses (because it is a solid shrinkage), but also the shrinkage may lead to plastic deformation or cause debonding at the interface between the cement and the formation or casing. In addition, the casing position may offset from the center of the wellbore, resulting in uneven stresses on the casing and cement sheath, increasing the risk of casing deformation and interface debonding. Thermal stresses are particularly important when CO2 is injected into the subsurface, especially in the case of cyclic loading, causing coupling between thermal stresses and other stress fields, promoting fracture growth within the cement or debonding at the cement interface [46, 52, 53].

In conclusion, well integrity issues may be encountered during the entire GCS life cycle due to the influence of multiple factors such as coupled stress-thermal-chemical. Therefore, it is necessary to take into account the combined effect of these factors in GCS engineering for well integrity problems, which are generally studied using numerical simulation.
