**4. Caprock integrity**

The effective and shear stress in the formation continues to change during the injection of CO2. The tensile or shear damage will occur when the effective or shear stress reaches a certain critical point, forming fractures and providing leakage channels for CO2.

### **4.1 Failure type**

Two types of failures can occur during the continuous injection process, that is, tensile and shear failure. The effective stress continues to decrease to 0 as the pore pressure increases during the continuous injection of CO2. Then it grows in the opposite direction and changes from compressive stress to tensile stress. Tensile failure will occur when the tensile stress exceeds the tensile strength of the rock. The principle is the same as that of hydraulic fracturing. Therefore, it is necessary to calculate the fracture pressure of the formation before injection, and then inject CO2 at a pressure lower than the fracture pressure. Typical fracture pressure calculation methods can be found in hydraulic fracturing-related studies. The shear failure will

**Figure 6.** *Mohr circle variation considering stress path coefficients.*

occur when the shear stress at a point reaches its shear strength. The Mohr-Coulomb criterion is used to determine whether a shear failure has occurred in the rock.

The maturity of hydraulic fracturing technology has made the determination of tensile failure easier. And the existence of stress path coefficients leads to a more complicated determination of shear damage. More importantly, the shear effect of injecting CO2 varies in different fault regimes [35–39].

### **4.2 Normal fault regime**

In a normal fault regime formation, as shown in **Figure 7a**, the maximum principal stress is the vertical (overburden) stress and the minimum principal stress is the minimum horizontal principal stress. In this stress regime, the initial Mohr circle of the formation is shown in **Figure 7b** and the variation of the Mohr circle with the injection of CO2 is shown in **Figure 7c**. The variation of the maximum effective stress is greater as compared to that of the minimum effective stress when the pore pressure increases. The phenomenon that can be observed is that the radius of the Mohr circle will decrease and move slightly to the left, which implies the reduction of the shear stress. Therefore, at the early stage of CO2 injection, the Mohr circle will move away from the damage envelope and the stability of the caprock and fault will increase to some extent. The shear stress decreases continuously with the injection of CO2 until the maximum and minimum effective stresses are equal, and then the Mohr circle reverses [40, 41].
