1. INTRODUCTION

Fault zones are classically described like thick zones

(metres to kilometres depending mainly on the slip

dimension) with a complex internal structure made

up of: (1) a single or multiple core zones, where

most displacements are accommodated, and (2) a

fractured damage zone surrounded by (3) the host

rock [1]. Recent studies on carbonate reservoirs,

have shown that the heterogeneity of a multilayer

sedimentary system could strongly control the fault

zone architecture [2,3].The lithology variations

(shale and limestone) have an important impact on

the fault zone characteristics [4,5]. Shale can be

either a brittle material or a plastic one(s), its

rheology is independent of temperature, depth (or

burial) or strain [6,7]. One of the main consequences

is the capacity of the clays to retard the propagation

of fractures, leading to a strong decrease of the fault

zone thickness in clay formations [3]. In limestone

formations, the damage zone thickness will depend

on the initial properties of the sedimentary layers. In

the porous layers, there is an important

accommodation of the deformation by micro-

mechanisms resulting in a progressive decrease in

the porosity toward the fault core [8]. Inversely, in

the low-porosity layers, deformations are

accommodated toward the fault core by: an increase

in the fracture porosity, in the micro-cracks porosity,

and by displacements along pre-existing fractures

[4]. The fault zone appears as relatively stiff and low

permeable zones intercalated with low stiffness and

high fracture permeability zones that extend one to

tens of meters from the fault following the initial

ARMA 13-246

Relation between fault zone architecture, earthquake

magnitude and leakage associated with CO2 injection in

a multilayered sedimentary system

Pierre Jeanne, Antonio Pio Rinaldi, and Jonny Rutqvist. Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA,USA

Frédéric Cappa.

Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA,USA

GeoAzur, University of Nice Sophia-Antipolis, Cote d’Azur Observatory, Nice, France

Yves Guglielmi.

CEREGE (UMR7330), Aix-Marseille University, CNRS-IRD, France

Copyright 2013 ARMA, American Rock Mechanics Association

This paper was prepared for presentation at the 47th US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, USA, 23-26

June 2013.

This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.

ABSTRACT: In this study, we have examined the influence of the fault zone characteristics on pressure diffusion and fault

reactivation by CO2 injection. Especially, we studied the effect of lithological and rock physical properties on the fault zone

response inside a multilayer sedimentary system. Through numerical analysis, we compared four models where the complexity of

the fault zone internal architecture is considered. Results show how the presence of hydromechanical heterogeneity influences the

pressure diffusion, as well as the effective normal and shear stress evolutions. The more complex the fault zone architecture is and

the more heterogeneities that are present, the faster the pressurization within the damage zone occurs. But, these hydromechanical

heterogeneities (i) strengthen the fault zone resulting in earthquake of smaller magnitude, and (ii) impede fluid migration along the

fault. We also show that the effects of the hydromechanical heterogeneities within the reservoir are negligible relative to those

between the caprock and the reservoir.

properties contrasts and geometry of the sedimentary

layers [4]. Hydraulic injection into boreholes reveals

horizontal and vertical contrasting permeability

values (up to two orders of magnitude) within the

damage zone [9].

Over the last decades, numerous studies based on

hydromechanical models have shown that the

presence of fluids within the fault zone may lead to

the appearance of overpressure that could trigger

earthquakes [10]. This phenomena has been noticed

in projects related to: CO2 sequestration [11], oil

industry [12], or naturally by the upwelling of deep

fluids [13]. Cappa [13] demonstrated that the

damage zone properties significantly affect the

locations and the evolution of fluid overpressures.

Hydromechanical models represent fault cores with

a very low permeability allowing its pressurization

during the fluid injection. Damage zones are

considered either as negligible for small fault zone

[11], or as continuous and thick layers adjacent to

the fault core for mature fault zone. In this last case

the damage zone is represented either with uniform

materials adjacent to the fault surface [14,15], with

material properties varying symmetrically on both

sides of the fault [13,16] or with dissimilar materials

across the fault [17,18]. But, the impacts of the

variations of damage zone thickness related to the

lithology and to the rock physical properties

variations of the host rock (especially in multilayer

reservoir) are never taken into consideration. In this

paper we address the two following questions: (1) is

there a link between fault zone architecture and its

capacity to be reactivated by CO2 injection, and (2)

what are the impacts on the induced seismicity and

on the CO2 leakage? We first present the

methodology used to estimate the magnitude of

seismic events occurring during the fault rupture.

Then, we analyze the hydromechanical behavior of a

fault zone represented either by: (1) a continuous

damage zone, or by a discontinuous damage zone

caused by (2) lithology variations, and plus by (3)

the initial properties of the sedimentary layers in a

limestone reservoir.

2. ESTIMATION OF EARTHQUAKE

MAGNITUDE

The mechanical behavior of the fault core is

represented by a combination of solid elements and

ubiquitous-joints oriented as weak planes in a Mohr-

Coulomb model [10]. The Mohr-Coulomb criterion

for failure can be written as [19]:

nsc ' (1)

where τ is the critical shear stress necessary for slip

occurrence, c is the cohesion, and μs is the static

friction (μs=tanφ, where φ is the friction angle). In

order to allow us to model a sudden slip, we used a

Mohr-Coulomb model with strain softening

frictional strength properties, consistent with a

seismological slip-weakening fault model. In our

model the frictional coefficient varies from a static

value of 0.6 to a value of 0.2 when the shear strain

within the fault zone is greater than a certain critical

value (10-3

), and a rupture occurs.

Following the approach used by Cappa and Rutqvist

[10,20,21] and Mazzoldi et al. [11], the magnitude of

a seismic event is estimated using seismological

theories. The energy releases during an earthquake is

directly correlated with rupture area along the fault

plane and displacement [22,23]. Hanks and

Kanamori [24] proposed a relationship to link these

two parameters with the seismic moment (M0):

M0 = G × Dave × A (2)

where G is the shear modulus of the host rock, Dave

is the average displacement along the fault surface

and A is the rupture area during a single event. M0

can be directly related to magnitude (M) by the

Equations (3) [25]:

M (log10M0 /1.5) 6.1 (3)

3. NUMERICAL MODEL

The simulations were performed using the coupled

thermo-hydro-mechanical simulator TOUGH-FLAC

[26], which link the flow simulator TOUGH2 [27]

and the geomechanical code FLAC3D

[28]. This

simulator is well described by Rutqvist [26], and it

was previously applied to study fault instability

processes related to fluid overpressure [20,21]. The

hydromechanical coupling used in this study relates

the porosity (hm) to the mean stress ('M), and then

the permeability (hm) depends on the porosity

changes. The formulation was first derived by

Davies and Davies [29] and then modified for

carbon sequestration application by Rutqvist and

Tsang [30]:

hm (0 r)exp(5 108 'M )r

hm 0 exp[22.2(hm /0 1)]

(4)

Fig. 1: The modeled hydro-mechanical environment

where subindex 0 refers to the initial unstressed

value (for both porosity and permeability, 0 and 0),

and r is the residual porosity at high stress. We

applied the changes to the fault zone only (damage

zone and fault core).

Figure 1 shows the geometry and initial conditions

of the models (2 km × 2 km). The model extends

vertically from 500 to 2500 m in depth and

horizontally far enough from the injection zone (2

km) to simulate laterally infinite acting conditions.

The model represents a multi-layered carbonate

system. Two thick shale formations (150 m) with

low permeability (kcap = 10-19

m2) isolate a storage

aquifer 100 m in thickness where CO2 is injected.

This aquifer is composed by an alternation of porous

( = 20%) and low permeable layers (k = 10-14

m2)

with low porosity ( = 5%) and more permeable

layers (k = 10-13

m2). Each layer is 5 m thick. The

reservoir above and below the two shale formations

have permeability of 10-14

and 10-17

m2 respectively.

The fault zone is 1 km long, dipping 80° and it is

500 m away from the injection well. Considering a

1000 m-long well the CO2 would be injected at a

constant rate of 20 kg/sec, which corresponds to the

industrial rate, such as the case of In Salah (Algeria)

[31]. The injection occurs during five years.

The fault core is represented by constant

hydromechanical properties (E= 5GPa, K= 10-17

m2),

and is embedded in a damage zone. The damage

zone is 45 m thick, and composes of two parts: a 5 m

thin zone embedding the core (DZ2), surrounding by

a 40m thick area (DZ1). In this way, we can

consider the horizontal hydromechanical

heterogeneity reflecting increases of deformations

toward the fault core. Then, we have performed

three simulations to consider vertical

hydromechanical heterogeneities.

From Model 1 to 4 the damage zone complexity was

added incrementally. Model 1 represents a thin

damage zone (only DZ2) without vertical

heterogeneity. Model 2 considers the impact of

variations in the initial host rock properties on the

damage zone properties, where the permeabilities

increase of two orders of magnitude relative to the

host rock and the Young modulus decrease up to 5

GPa. Model 3 take into account the different

rheology behavior between shale and limestone. The

damage zone is only 5 m thick in the caprock, while

the permeability increases one order of magnitude

and the Young modulus decreases to 9 GPa (Fig.

2c). In Model 4, we also considered different

rheology linked to the variation in rock physical

properties within the limestone (Fig. 2d). In the

initial porous and un-fractured layers, the damage

zone is 5 m thick, while the permeability increase

one order of magnitude

Table 1: Rock characteristics common to the four models in the simulations.

and the Young modulus decrease to 12 GPa. In the

low-porosity and fractured layers the properties are

similar to those used in Model 2 and 3. The

hydromechanical properties common to all the

simulation are given in Table 1.

We set the initial conditions as a linear pore pressure

and temperature gradient (9.81 MPa/km and 25

˚C/km, respectively) with an atmospheric pressure of

0.1 MPa and a temperature of 10 ˚C at the ground

surface. This results in a pore pressure of 5 MPa and

temperature of 22.5 ˚C at the top of our model (at

500 m depth).

Boundaries were open for fluid flow (i.e. at constant

pressure and temperature), except for the left

boundary, where no flow occurred. The simulations

were conducted in an isothermal mode, implying

that the temperature gradient is maintained during

the simulation. Null displacement conditions were

set normal to the left and bottom boundaries,

whereas constant stress was imposed normal to the

right and top boundaries (Fig. 1).

4. MODELLING RESULTS

Common to all the simulations, the results show

sharp stress drops and subsequent fault slips after

several months of CO2 injection, whereas the CO2 is

mainly located near the injection point. Ruptures

nucleate at the bottom of the reservoir and spread

mainly along the fault portion inside the reservoir

and inside the lower caprock. Numerous studies

have described this behavior [11,20]. The pore

pressure increase, inside the reservoir, reduces the

effective stress normal to the fault surface, hence

reducing the shear strength of the fault. The shear

stress along the fault is reduced resulting in a

minimum value at the top of the reservoir. These

shear stress changes are caused by poroelastic

effects occurring within the aquifer, as the pressure

increase (i.e., reservoir swelling). The sum of the

two effects results in a rupture that nucleates at the

bottom of the reservoir.

The four different models show variations at the

rupture in: the timing (129 to 200 days), the seismic

events magnitude (from 2.15 to 3.12), the pressure

change (Fig. 3a), the rupture lengths (from 185 to

596 m) (Fig. 3b), the maximum displacement along

the fault (2.9 to 8.9 cm) and the shear stress

evolution (Fig. 3d). Parameters explaining the time

delay between these models are the evolutions of the

effective normal stress and the shear stress. In Model

1, fluids can flow easiest through the thin damage

zone within the reservoir and the caprock. The good

fluid diffusion prevents an important reservoir

swelling, and the shear stress evolves slowly.

Rupture occurs after 200 days, it extends over 596 m

and results in seismic event of magnitude 3.12. This

large rupture zone is due to the pressure change

within the fault core. The absence of heterogeneity

allows larger pore pressure diffusion (Fig. 3, red

line). In Model 2 and 3 the vertical hydraulic

heterogeneities impedes diffusion (Fig. 3a). The

fluid pressures within the damage zone in the

reservoir evolve faster, leading to a fast decrease of

the effective normal stress and a higher reservoir

Figure 2: Schemes representing the four models of fault zone architecture and their hydro-mechanical properties. Damage

zones are represented by (a) thin and continuous with homogenous properties (Model 1), (b) thick and continuous with

properties varying along with the initial host rock properties (Model 2), or as a discontinuous damage zone caused by (c)

the variation of lithology (Model 3), and by (d) the variation of initial host rock properties inside the reservoir aquifer

(Model 4).

swelling. More important, the larger the permeability

contrast between the caprock and the reservoir are,

the earlier ruptures occurs (Fig.3c). In Model 2 slip

occurs after 155 days, with a seismic magnitude of

2.44 and a rupture extending over 260 m (Fig. 3,

green line). In Model 3, slip occurs after 129 days,

with a magnitude of 2.15 and a rupture extending

over 185 m (Fig. 3, black line). Results for Model 3

and 4 (Fig. 3, blue line) are similar. This means the

effects of the hydromechanical heterogeneities

within the reservoir are negligible relative to those

between the caprock and the reservoir.

Hydraulic heterogeneities influence also the fluid

migration through the fault zone. The most

important leakage occurs in Model 1 (Fig. 4a and b).

4.38% of the total CO2 injected has leaked through

the upper caprock after 5 years, and 5.33% after 20

years. We can notice that most of the leakage

appears during the injection. In Model 2, 3 and 4 no

leakage appears (Fig.4 c, d, e and f). The presences

of low permeable layers within the damage zone

prevent a good fluid migration along the fault, and

help the CO2 to be confined within the injection

reservoir. Less leakage occurs for a more complex

fault zone architecture.

Figure 3: Results of (a) pressure change, (b) slip offset of the fault walls, (c) slip and (d) shear stress evolution at point P

(located at the bottom of the reservoir within the fault) in simulations with TOUGH-FLAC, for the four models.

Figure 4: Evolution of the CO2 plume and leakage at the end of injection (1800 days) and 15 years after (7200 days) for

Model 1 (a & b), Model 2 (c & d), Models 3 and 4 (e & f).

5. CONCLUSION

In this study, we have examined the influence of the

fault zone architecture inside a multilayer

sedimentary system on pressure diffusion within the

fault zone and its activation by CO2 injection. We

have considered the rheological behaviors of the

geological layers affected by the fault. We have

compared deformations and leakages occurring in a

5 m thin fault zone with a homogenous damage zone

and in fault zones nine time bigger (45 m thick) with

heterogeneous properties. It appears that the most

important factors for the hydromechanical behavior

of the fault zone is not its size, but the

hydromechanical contrast within the damage zone

along the fault core. The more complex fault zone

architecture is, the faster slip but with less leakage.

This is caused by the presence of hydromechanical

heterogeneities which (i) favor rapid damage zone

pressurization, and (ii) strengthens the fault zone

(less deformable layers) resulting in small

earthquake, and (iii) prevents a good fluid migration

along the fault. In our simulations, the lack of

difference between Model 3 and 4, indicates that

only the hydromechanical heterogeneities linked to

the lithological variations influence the

hydromechanical behavior of the fault zone.

REFERENCES 1. Mitchell, T.M., Faulkner, D.R., 2009. The nature

and origin of off-fault damage surrounding strike-

slip fault zones with a wide range of

displacements: a field study from the Atacama

fault system, northern Chile. Journal of Structural

Geology 31 (8), 802-816.

2. Jeanne, P., Guglielmi, Y., Lamarche, J., Cappa, F.,

Marié, L., 2012. Architectural characteristics and

petrophysical properties evolution of a slip fault

zone in a fractured porous carbonate reservoir.

Journal of Structural Geology 44, 93-109.

3. Roche. V., Homberg, C., Rocher, M., 2012.

Architecture and growth of normal fault zones in

multilayer systems: A 3D field analysis in the

South-Eastern Basin, France. Journal of Structural

Geology 37 (2012) 19-35.

4. Mandl, G., 1988. Mechanics of Tectonic Faulting:

Model and Basic Concept. Elsevier Sci., New

York.

5. Schöpfer, M.P.J., Childs, C., Walsh, J.J.,

Manzocchi, T., Koyi, H.A., 2007. Geometrical

analysis of the refraction and segmentation of

normal faults in periodically layered sequences.

Journal of Structural Geology 29, 318-335.

6. Mourgues, R., Cobbold, P.R., 2003. Some tectonic

consequences of fluid overpressures and seepage

forces as demonstrated by sandbox modeling.

Tectonophysics 376, 75-97.

7. Cobbold, P.R., Clarke, B.J., Løseth, H., 2009.

Structural consequences of fluid overpressure and

seepage forces in the outer thrust belt of the Niger

Delta. Petroleum Geoscience 15, 3-15.

8. Tondi, E., Antonellini, M., Aydin, A.,

Marchegiani, L., Cello, G., 2006. The roles of

deformation bands and pressure solution seams in

fault development in carbonate grainstones of the

Majella Mountain, Italy. Journal of Structural

Geology 28, 376-391.

9. Jeanne, P., Guglielmi, Y., Cappa, F., 2013.

Dissimilar properties within a carbonate-reservoir’s

small fault zone, and their impact on the

pressurization and leakage associated with CO2

injection. Journal of Structural Geology 47, 25-35.

10. Cappa, F., Rutqvist, J., 2011. Modeling of coupled

deformation and permeability evolution during

fault reactivation induced by deep underground

injection of CO2. International Journal of

Greenhouse Gas Control 5, 336-346.

11. Mazzoldi, A., A. P. Rinaldi, A. Borgia, J. Rutqvist

(2012), Induced seismicity within geological

carbon sequestration projects: Maximum

earthquake magnitude and leakage potential, Int. J.

Green. Gas Contr., 10, 434-442.

12. Wiprut D, Zoback MD. Fault reactivation and fluid

flow along a previously dormant normal fault in

the northern North Sea. Geology 2000;28:595–8.

13. Cappa, F., 2009. Modeling fluid transfer and slip in

a fault zone when integrating heterogeneous

hydromechanical characteristics in its internal

structure. Geophysical Journal International 178,

1357-1362.

14. Andrews, D.J., 2005. Rupture dynamics with

energy loss outside the slip zone. Journal of

Geophysical Research 81, 5679-5687.

15. Rice, J.R., 2006. Heating and weakening of faults

during earthquake slip. Journal of Geophysical

Research 111, B05311.

16. Noda, H., Shimamoto, T., 2005. Thermal

pressurization and slip-weakening distance of a

fault: an example of the Hanaore fault, Southwest

Japan. Bulletin of the Seismological Society of

America 95 (4), 1224-1233.

17. Ben-Zion, Y., Shi, Z., 2005. Dynamic rupture on a

material interface with spontaneous generation of

plastic strain in the bulk. Earth and Planetary

Science Letters 236, 486-496.

18. Shi, Z.Q., Ben-Zion, Y., 2006. Dynamic rupture on

a bimaterial interface governed by slip-weakening

friction. Geophysical Journal International 165,

469e484.

19. Jaeger, J.C., Cook, N., 1979. Fundamentals of

Rock Mechanics. 3rd

ed. New York: Chapman &

Hall, pp. 28–30.

20. Cappa, F., Rutqvist, J., 2011a. Impact of CO2

geological sequestration on the nucleation of

earthquakes. Geophys. Res. Lett. 38(L17313).

21. Cappa, F., Rutqvist, J., 2012. Seismic rupture and

ground accelerations induced by CO2 injection in

the shallow crust. Geophys. J. Int. 190: 1,784–

1,789.

22. Bath, M., 1981. Earthquake magnitude – recent

research and current trends. Earth-Science Reviews

17, 315–398, doi:0012-8252/81/0000-0000.

23. Giampiccolo, E., D’Amico, S., Patanè, D., Gresta,

S., 2007. Attenuation and source parameters of

shallow microearthquakes at Mt. Etna Volcano,

Italy. Bulletin of the Seismological Society of

America 97, http://dx.doi.org/10.1785/

0120050252.

24. Hanks, T.C., Kanamori, H., 1979. A moment

magnitude scale. Journal of Geophysical Research

84, 2348–2350, doi:0148-0227/79/009B-0059.

25. Kanamori, H., Anderson, D.L., 1975. Theoretical

basis of some empirical relations in seismology.

Bull. Seism. Soc. Am. 65(5): 1073–1095.

26. Rutqvist, J., 2011. Status of TOUGH-FLAC

simulator and recent applications related to coupled

fluid flow and crustal deformations. Comput.

Geosci. 37: 739–750.

27. Pruess, K., Oldenburg, C.M., Moridis, G., 2011.

TOUGH2 User’s Guide, Version 2.1. Lawrence

Berkeley Natl. Lab., Berkeley, CA, USA: Paper

LBNL-43134 (revised).

28. ITASCA, 2009. FLAC3d v4.0, Fast Lagrangian

Analysis of Continua in 3 Dimensions, User’s

Guide. Minneapolis, MN, USA: Itasca Consulting

Group.

29. Davies, J.P., Davies, D.K., 2001. Stress-dependent

permeability: characterization and modeling. SPE

Journal 6: 224–235.

30. Rutqvist, J., Tsang, C.F., 2002. A study of caprock

hydromechanical changes associated with CO2-

injection into a brine formation. Environ. Geol. 42:

296–305.

31. Rinaldi, A.P., Rutqvist, J. Modelimg of deep

fracture zone opening and transient ground surface

uplift at KB-502 CO2 injection well, in Salah,

Algeria. International Journal of Greenhouse Gas

Control, Volume 12, January 2013, Pages 155-167.