**1. Introduction**

To date, the application of GCS on a commercial scale is considered to be an effective solution for reducing the greenhouse effect [1]. GCS projects are carried out in highly permeable, porous formations at a certain depth, and CO2 is typically injected in a supercritical state. It is necessary to ensure that the CO2 injected into the subsurface does not or rarely leak within a very long time (at least 1000 years) for large-scale applications and public acceptance [2–6]. The types of CO2 leaks are classified as physical and chemical and the essence of these is the geomechanical issues during CO2 injection and storage, such as excessive stress changes, fault activation, damage to wellbore integrity, and caprock failure due to continuous injection and long-term storage [7–12]. This chapter introduces the geomechanical issues involved in the GCS process from the perspectives of the CO2 trapping mechanism, in-situ stress changes, caprock performance, wellbore integrity, and induced seismicity.

## **2. Trapping mechanisms**

In recent years, an increasing number of studies have focused on the short- and long-term effects of CO2 injection into the subsurface. In most projects, CO2 is

injected in a supercritical state which can be stored in a gaseous, liquid, or supercritical state depending on the formation conditions. At the initial stage of injection, CO2 will move toward the caprock layer due to the temperature-pressure conditions and density difference, and will eventually be blocked by the caprock layer. Then, the CO2 can be captured as residual gas when groundwater intrusion occurs during the movement. In addition, CO2 can be dissolved in groundwater and chemical reactions can occur by contact with rocks, which will contribute to CO2 capture. Therefore, there are four trapping mechanisms for CO2 in the storage process, that is, stratigraphic trapping, residual trapping, solubility trapping, and mineral trapping. These mechanisms are activated at different periods of the sequestration, as shown in **Figure 1**. Stratigraphic trapping is in charge of the initial CO2 storage. Residual trapping and dissolved trapping play an important role in the transport of CO2. Mineral trapping is formed when the CO2 diffuses in the formation and contacts the rock. At this time, CO2 is in the most stable state and the risk of leakage is minimized [14–18].
