The effects of lateral property variations on fault-zone reactivation by fluid pressurization: Application to CO2 pressurization effects within major and undetected fault zones

• Major fault zone is characterized by a thick and continuous damage zone.
• In Major fault, fluid flows easily and pressurization is too low to trigger a seism.
• Minor fault zone is characterized by a discontinuous damage zone.
• In Minor fault, fluid accumulation can trigger seismic events.

In this study, we performed in situ multidisciplinary analyses of two different fault zones in carbonate formations. One is a seismically active fault zone several kilometers long (the Roccasseira Fault Zone); the other is a small fault zone a few hundred meters long (the GAS Fault Zone). The smaller, “immature” fault zone displays a discontinuous damage zone, because tectonic deformations have been accommodated differently according to the initial properties of the host rock. The larger, “mature” fault zone displays a continuous damage zone caused by the presence of secondary fault cores embedded in a heavily fractured area inside the damage zone. These markedly different fault-zone architectures were reflected in two hydraulic and geomechanical fault models, both generated from a coupled fluid-flow and geomechanical simulator, to examine the impact of hydromechanical property distribution on fault stability when the faults are reactivated by CO2 injection. In the smaller fault zone, marked differences in hydromechanical properties (Young's modulus and permeability) favor fluid accumulation, inducing high pressurization in parts of the damage zone, potentially resulting in small seismic events. On the other hand in the mature fault zone, fluid flows more easily and thus fluid-induced earthquakes may not readily occur, because the fault-zone pressurization is much lower, insufficient for triggering a seismic event.