**6.2 Seismic moment release**

At present, the principle and calculation formula of subsurface injection-induced seismicity proposed by McGarr [60] are widely used. The theory is based on the following assumptions to calculate the upper bound seismic moment due to fluid injection into geological formations.


If a volume Δ*V* of liquid is injected into a fully saturated formation, the average increase in pore pressure *P* can be calculated as follows.

$$
\Delta P = \frac{3\lambda + 2G}{3} \frac{\Delta V}{V} \tag{8}
$$

where *V* is the volume of the formation weakened by the injection; *λ* and *G* are Lame's elastic parameters, *G* being the modulus of rigidity.

And the upper bound to the cumulative seismic moment is given by:

$$
\sum M\_0 = \frac{2\eta(3\lambda + 2G)}{3} \Delta V \tag{9}
$$

where *η* is the coefficient of friction.

The analytical solutions that can accurately calculate the seismic magnitude are difficult to obtain because of the complexity of the induced seismicity. Therefore, more numerical simulations are used for seismic prediction, including the calculation of activation potential using shear slip criterion, based on continuous medium mechanics approach, through fault hydrodynamic approach, and discrete method modeling approach. A seismic event of magnitude 3–4 with a radius of the damage zone between a few hundred meters and one kilometer of the shallow formation can be perceived based on field experience and extensive numerical simulation studies. It is further demonstrated that high-level seismicity is induced only when the fault has continuous permeability and the pressure is distributed over a sufficiently large fault with simultaneous brittle fracture. In fact, even clearly perceivable seismic events may not open new flow channels over the entire thickness of the caprock, that is, seismically induced flow channels are unlikely to cross a formation with multiple caprocks.

## **7. Conclusions**

GCS is an important way to reduce carbon emissions. There are multiple trapping mechanisms after CO2 injection into the subsurface, including stratigraphic trapping, residual trapping, solubility trapping, and mineral trapping, and each of them plays a role at different times. Geomechanical issues directly determine the success or failure of GCS. The evolution of the in-situ stress and effective stress can be calculated from the pore pressure variation and the stress path coefficient. Further, the integrity of the caprock under different regime conditions can be evaluated based on the variation of in-situ stress. Generally speaking, it is most stable in the normal fault regime, followed by the strike-slip regime, and the most unstable is the thrust fault regime. In addition, acidic fluids or gases can be formed after CO2 injection, corroding the wellbore and cement sheath, creating multiple leak paths, and leading to failure of wellbore integrity. For the general public, the top concern is whether the GCS project will cause earthquakes. Fortunately, it has been identified based on the current studies that the injection of CO2 does trigger microseismic events that can be perceived by humans, but the magnitude of the earthquakes and the energy released would not bring damage to buildings or organisms on the ground.

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