**4.3 Strike-slip fault regime**

In the strike-slip fault regime as shown in **Figure 8a**, both the maximum and minimum principal stresses are in the horizontal direction. In this regime, the initial

**Figure 7.**

*(a) Normal fault regime; (b) initial Mohr circle; (c) schematic of Mohr circle variation.*

**Figure 8.**

*(a) Strike-slip fault regime; (b) initial Mohr circle; (c) schematic of Mohr circle variation.*

*Geomechanics of Geological Carbon Sequestration DOI: http://dx.doi.org/10.5772/intechopen.105412*

Mohr circle of the formation is shown in **Figure 8b**. The maximum and minimum effective stresses have the same variation and the diameter of the Mohr circle is constant when CO2 is injected into the strike-slip regime formation because the horizontal stress path coefficients are the same. However, the Mohr circle rapidly moves to the left and closer to the damage envelope due to the reduction of the effective stress, causing the formation to become unstable, as shown in **Figure 8c** [42].

#### **4.4 Thrust fault regime**

In a thrust fault regime, as shown in **Figure 9a**, the maximum principal stress is the horizontal stress, while the minimum principal stress is the vertical stress. In this regime, the initial Mohr circle of the formation is shown in **Figure 9b**. The maximum effective stress changes less than the minimum effective stress when CO2 is injected. Therefore, the radius of the Mohr circle increases and rapidly moves to the left during the injection process, and the shear stress increases (**Figure 9c**). The distance between the Mohr circle and the damage envelope will decrease rapidly, and then the caprock stability will decrease. Therefore, according to the trend of the Mohr circle, the reservoir and caprock will be more stable in the normal fault regime than in the thrust regime [40, 43].

In summary, the changes in Mohr circles are very different when CO2 is injected into formations with different stress regimes. Generally, the formation is most stable in the normal fault regime, followed by the strike-slip regime, and the least stable is the thrust regime.

## **5. Well integrity**

Well integrity is generally defined as the ability of a well to produce or inject a fluid while preventing harmful fluid leaks to reduce the risk of uncontrolled leakage of fluids formation throughout the life of the well. Well integrity is key to the success of geologic carbon sequestration and CO2 enhanced recovery operations. Modern wells are designed with multiple barriers to create a controlled injection or production pathway and to isolate the fluid in the formation along with its depth. Wells will be impacted by physical, chemical, and mechanical stresses, which can reduce the effectiveness of the barrier and ultimately lead to loss of integrity. In CO2 injection areas, these effects may be amplified because of high temperature and pressure variations in the injection well, higher injection pressure, and reactions between the CO2-brine

**Figure 9.** *(a) Thrust fault regime; (b) initial Mohr circle; (c) schematic of Mohr circle variation.*

mixture with the well material. A well with compromised integrity may not be able to prevent upward transport of injected CO2 and other formation fluids (e.g. brine, hydrocarbons). The leaked liquid will become greenhouse gas if it is released into the atmosphere and will contaminate drinkable groundwater resources if it leaks into subsurface water formations [44, 45].
