Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 488

This seismic energy results in ground accelerations that can damage
structures. When most people hear the word “earthquake,” they associ-
ate it with danger and structural damage. Most of the earthquakes of
which we speak as being caused by injection are too small to be felt. For
this reason, the term induced seismicity is preferred in the geological
carbon sequestration context.
Induced seismicity is a well-known by-product of changing the load-
ing or pore pressure in rock. Induced seismic events have been recorded
as consequences of the fi lling of large surface-water reservoirs [10.40]
and geothermal energy production. Here we focus the discussion on
changes in pore pressure as the impetus for induced seismicity.
In the earth, rocks are in a state of stress characterized by three
mutually orthogonal principal components (see Box 10.4.1 ). The largest
stress is

σ 3
depend on the depth and tectonic forces active in any given block of
rock. In general, the deeper the rock of interest, the larger the vertical
component of stress becomes relative to the horizontal stresses as the
weight of the rock above increases. Negative normal stresses corre-
spond to tensional stresses.
Shown in Figure 10.4.3 is the classic Mohr-Coulomb plot of shear
stress (vertical axis) vs. normal stress (horizontal axis). Recall Figure 9.6.3
which shows the tendency to fracture the rock as a function of pressure
and depth. This tendency to fracture corresponds to the values of shear
and normal stress above which the intact (unfaulted) rock will break, the
so-called intact rock failure envelope (see Figure 10.4.3 ). The other enve-
lope in Figure 10.4.3 describes the region above which a fault has a
tendency to slip, the fault slip envelope . As shown in the fi gure, if the
normal stress across the fault is very large, it will take a larger shear
stress to re-activate the fault.
The two semi-circles in the fi gure represent

σ 1 , followed by σ 2 , and σ 3 . The orientations of σ 1 , σ 2 , and

σ 3 of a formation
in which we inject CO 2 . Because of the injection, the local fl uid pressure
increases. As a consequence, all of the compressive normal stresses
decrease because the fl uid pressure acts against compressive normal
stresses and tends to dilate the rock. This is represented in the fi gure by
the semi-circles moving to the left. Therefore the mechanism of induced
seismicity is that pore pressure arising from fl uid injection reduces the
effective normal stress as shown in Figure 10.4.3 to the point that exist-
ing shear stresses can re-activate the fault. What this means is that

σ 1 and