Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 490

Box 10.4.1 ( Continued )

+

(b)

We see that for an angle θ = 0 the shear stress is zero and the normal stress
σ n = σ 3 . For an angle θ = 90º we obtain σ s = 0 and σ n = σ 1 . The maximum shear stress
is obtained for an angle of 45º.

Fluid pressure
increasing

σ 3 σ 1

σ n (Effective normal stress)

Figure 10.4.3 Shear stress vs. effective normal stress
Shear stress vs. effective normal stress for rocks undergoing changing pressurization
due to injection.

Many factors complicate the above explanation of induced seismic-
ity. One of these is the existence of faults that are very near to failure
naturally, i.e., a natural earthquake may be only a few years away. The
natural increase in tectonic stress is augmented by an injection process
which effectively triggers the earthquake. Such an earthquake may be
quite large and not related directly to the amount of fl uid pressure