## 3 Self-Healing Cement  
Under reservoir conditions, mechanical and chemical effects can create microcracks, which act as primary pathways for leakage and compromise well integrity, as shown in Figure 2. To enhance the ability of cementitious materials to resist cracking, block the penetration of harmful substances, and prolong their service life, self-healing technology for concrete has been extensively studied. Researchers have introduced various additives, such as fibers and elastomers, into cement slurries to improve the performance of the cement sheath and facilitate the self-healing of cracks. Different self-healing approaches have been developed, including the use of microcapsules, microbes, cementitious materials, nanofillers, geopolymers, and shape-memory alloys [29; 30; 31; 32]. Self-healing cement can be categorized into autogenous/natural and autonomous/artificial healing based on the mechanisms involved [5; 6; 7; 8; 9; 30; 31; 32; 33]. Autogenous healing occurs due to unreacted cement particles and the carbonation of hydration products, which form calcium carbonate (CaCO3) deposits within cracks in cement-based materials. CO2-induced self-healing relies on the deposition of calcium carbonate on the surfaces of cement cracks, aiding in their closure. Also, it can occur by blocking cracks by impurities in water or loose cement particles from crack spalling, further hydration of unreacted cement or cementitious materials, and expansion of the hydrated cementitious matrix in crack flanks due to the swelling of calcium silicate hydrate (C-S-H). However, cement recipes with low cement content or water ratio can improve the cement durability. On the other hand, autonomous healing depends on external healing agents that are not inherently part of the cement-based materials. Authors on autonomous healing have primarily focused on using bacteria, microcapsules, organic or inorganic materials, and superabsorbent polymers, which are employed to seal cracks effectively. Autonomous healing techniques have demonstrated superior performance in repairing cracks compared to most autogenous healing methods, which are typically restricted to addressing cracks with widths narrower than 150 \(\upmu\)m [30]. The self-healing capability of cement can be achieved by incorporating bacterial species into the cementitious matrix, as explored by several researchers [6,7,34]. When water penetrates the cracks, it activates dormant microorganisms, which precipitate sufficient microbial calcite. This process effectively seals the cracks and prevents further water intrusion [35,36]. Microencapsulation is a process where tiny particles or liquid droplets are enclosed within an inert shell, protecting the core material from unwanted interactions with the surrounding environment [37,38]. In self-healing applications, microcapsules are embedded into the host matrix during manufacturing. When cracks form, the stress from the crack causes the microcapsule shells to break, releasing the healing agents contained within them. This mechanism allows the material to repair itself effectively. Geopolymer cement is composed of a polymeric Si-O-Al framework, similar to zeolites, but is predominantly amorphous in nature. The term \"geopolymer cement\" typically refers to cement produced by combining aluminosilicate source materials, such as fly ash or ground granulated blast-furnace slag, with an alkaline solution. This reaction forms silicon-oxo-aluminate chains and networks, giving geopolymer cement its unique properties [39]. Pozzolans are widely used in cement formulations to reduce cementation costs and decrease slurry density. Studies have demonstrated that CO\({}_{2}\)-saturated brine and supercritical CO\({}_{2}\) can penetrate pozzolanic cement with a 35:65 pozzolan-to-cement volume fraction to a depth of around 170-180 mm over 30 years. This indicates that the inclusion of pozzolans accelerates the rate of CO\({}_{2}\) penetration compared to neat cement [40].