## 3 Self-Healing Cement
Observations revealed that pozzolanic cement paste reacts significantly faster with CO\({}_{2}\) than neat cement; however, no measurable degradation in the physical properties of pozzolanic  
Figure 2: Self-healing cement system in a wellbore. The section shows microcracks before and after self-healing, demonstrating the cement’s ability to autonomously repair damage, enhancing durability, zonal isolation, and leakage prevention in subsurface applications. cement was detected [41]. The accelerated penetration in pozzolanic cement is attributed to the increased porosity and higher diffusion rates caused by the pozzolan additives, as well as the reduced portlandite content. This reduction in portlandite minimizes excessive carbonate precipitation, which promotes deeper penetration of CO\({}_{2}\)-bearing fluids into the cement matrix [42]. These findings emphasize the critical influence of pozzolan additives in modifying cement microstructure and enhancing its interaction with CO\({}_{2}\), potentially impacting long-term cement integrity in CO\({}_{2}\)-rich environments. The bacterial species loses its healing capacity owing to the higher pH of the concrete. Moreover, the spores of the bacterial species may become damaged because of the shear forces generated, shrinkage during mixing, and drying of the concrete. Ensuring the long-term integrity of wellbore cement is critical for maintaining zonal isolation, preventing sustained casing pressure (SCP), and mitigating fluid migration in oil and gas wells. Self-healing cement (SHC) technologies have been extensively developed to address these challenges, leveraging various mechanisms such as polymeric additives, elastomers, hydrocarbon-responsive materials, and biomineralization. This review synthesizes advancements in self-healing cement formulations, their underlying mechanisms, and their field applications. Various self-healing cement technologies enhance wellbore integrity in extreme subsurface conditions. Polymer-based cement, incorporating epoxy resins, improves mechanical strength and reduces permeability by 70% [43], effectively sealing 0.5 mm fractures [44]. Elastomer-enhanced cement increases flexibility and crack resistance, proving effective in HPHT wells (>330 \({}^{\circ}\)F, 10,000 psi) [13; 45]. Hydrocarbon-responsive cement, such as NxSHC, swells upon contact with hydrocarbons, sealing fractures in high-pressure gas storage wells [15], while water-responsive agents can seal microcracks within 40 min at 2 MPa [46]. CO\({}_{2}\)-resistant cement, essential for CCS applications, includes reduced-OPC formulations that maintain integrity under CO\({}_{2}\) exposure [18] and polyethylene glycol-based cement, reducing permeability by 85.5% [47]. Fluid-independent self-healing cement, integrating elastomers, seals microannuli without external fluid activation [14]. Biologically inspired cement (MICP) uses urease-producing bacteria to precipitate CaCO\({}_{3}\), improving durability in CO\({}_{2}\)-rich and offshore conditions [48]. Advanced technologies, such as microencapsulation and stress-resistant cement, further enhance self-repair, with polyurethane-based microcapsules sealing HPHT wells [49] and stress-resistant cement preventing annular pressure buildup [16]. These innovations significantly improve long-term well integrity across geothermal, CCS, and deep-well applications.