Fault reactivation

Depletion and injection of fluids in the subsurface cause changes in pore pressure and therefore on effective stresses. Fluid injection such as in hydraulic fracturing, water-flooding, and waste-water disposal can reach adjacent faults (if any) and decrease the effective normal stress $\sigma_n = S_n - P_p$ acting on faults. In hydraulic fracturing, the injection of fluids is temporary and limited to the fracture completion size. In water-flooding, pressure build-up is limited by the producer wells. In waste-water injection, the pressure build-up is controlled by the aquifer size, compressibility and compartmentalization.

Figure 5.29: Fault reactivation. Injection of fluids changes the pore pressure and lowers effective normal stress on faults increasing $\tau /\sigma _n$.

The change of stresses produced by increases of pore pressure assuming constant total vertical stress and negligible poroelastic effects is

$$\left\lbrace \begin{array}{l} \Delta \sigma_v = - \Delta P_p \\ \Delta \sigma_{hmin} \leq - \Delta P_p \\ \end{array}\right.$$ (5.15)

The result is a shift of the Mohr circle to the left, closer to the shear failure or “reactivation” line. The magnitude of change of pore pressure needed to reactivate a fault is (at least) equal to the horizontal distance between the point of the Mohr-circle of such fault and the failure line. Hence, a critically oriented fracture needs the lowest $\Delta P_p$ to reactivate.