The effects of lateral property variations on fault-zone reactivation by fluid pressurization: Application to CO 2 pressurization effects within major and undetected fault zones Pierre Jeanne a, * , Yves Guglielmi c , Frédéric Cappa b , Antonio P. Rinaldi a , Jonny Rutqvist a a Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA, USA b Geoazur, University of Nice Sophia-Antipolis, Côte d’Azur Observatory, 06550 Sophia Antipolis, France c CEREGE, Aix-Marseille University, 13331 Marseille, France article info Article history: Received 31 May 2013 Received in revised form 18 December 2013 Accepted 22 January 2014 Available online 8 February 2014 Keywords: Fault zone architecture Damage zone continuity Coupled hydromechanical processes Fault reactivation abstract 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 frac- tured 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 geo- mechanical simulator, to examine the impact of hydromechanical property distribution on fault stability when the faults are reactivated by CO 2 injection. In the smaller fault zone, marked differences in hy- dromechanical 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. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In the context of reservoir operations, the influence of fault zones on fluid flow must be known, because such zones can act either as barriers to or conduits for fluid circulation (Aydin, 2000; Jourde et al., 2002). Moreover, coupled hydromechanical pro- cesses are very important for fault-zone hydraulic and mechanical behavior. Under some conditions, fluids can facilitate slip on faults at stress levels lower than those required under dry conditions. If fluid pressures in the fault core and damage zone increase, the effective normal stress across the fault decreases. This effect will in turn reduce fault strength, according to the Coulomb failure crite- rion (Wibberley and Shimamoto, 2005; Rice, 2006). If faults are reactivated during an underground injection operation, such as that associated with geologic sequestration of CO 2 , this could potentially compromise the hydraulic integrity of the caprock that seals the reservoir, wells might be sheared and permanently damaged, and in extreme cases, earthquakes could be triggered (Sminchak and Gupta, 2003). Studies have shown that the hy- draulic and mechanical properties of faults are strongly related to their architecture (Cappa et al., 2007; Guglielmi et al., 2008; Jeanne et al., 2012a, 2013a). Faults are classically described as (1) a single or multiple core zones generally filled with gouge, where most of the fault throw is accommodated, (2) a fractured damage zone sur- rounded by (3) a host rock (Mitchell and Faulkner, 2009). Typically, the fault-core permeability is two-to-three orders of magnitude lower than that of the host rock, and the fault core Young’s modulus ranges from 1 to 10 GPa (Cappa and Rutqvist, 2011b). In the damage zone, the hydromechanical properties evolve progressively from the host rock to the fault core with the degree of fracturing. The Young’s modulus decreases and the permeability increasing up to two orders of magnitude relative to the host rock close to the fault core (Gudmundsson, 2004; Faulkner et al., 2006). * Corresponding author. Tel.: þ1 510 486 6261. E-mail addresses: pjeanne@lbl.gov, pierrejeanne06@yahoo.fr (P. Jeanne). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg http://dx.doi.org/10.1016/j.jsg.2014.01.017 0191-8141/Ó 2014 Elsevier Ltd. All rights reserved. Journal of Structural Geology 62 (2014) 97e108