10th Asian Regional Conference of IAEG (2015)

Potential risks of fault reactivation associated with the production of shale gas byhydraulic fracturing
Haijun Zhao, Fengshan Ma, Jie Guo
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of
Sciences, 100029 Beijing, China
E-mail: zhaohaijun@mail.iggcas.ac.cn.
Abstract
As an unconventional natural gas, shale gas has increasingly attracted attention
around the world. Horizontal drilling and hydraulic fracturing are the primary technologies,
which make the extraction of tightly bound natural gas from shale formations economicallyfeasible. However, their potential environmental effects remain controversial, especiallythose related to water pollution. Among various possibilities, the conductive faults shouldbe treated as the focus of attention because they serve as major pathways for gas orcontaminant discharge to shallow groundwater. During the hydraulic fracturing, a criticallystressed fault can be reactivated because of the increased pore water pressure and thedecreased effective stress on the fault planes. In general, fault reactivations are highly
related with seismic events during and post hydraulic fracturing operation, and thiscorrelation has been record in many different places around the word. Although there islack of sound scientific filed observations and peer-reviews articles on the effects of fault
reactivation on shallow groundwater quality, the probability of causal relationship is high.
In this study, we reviewed the seismicity induced by shale gas development and other fluid
injection engineering. Mechanism of fault reactivation associated with the production of
shale gas by hydraulic fracturing was discussed and the formation of the transport pathwaysof contaminants was depicted to help manage the environmental risks in the production.
Keywords: shale gas, hydraulic fracturing, fault reactivation, water pollution
1.

Introduction

surface after hydraulic fracturing only 9% to 34% of
the injected fluid (Alleman 2011). Although many
investigations have been done to study the hydraulicconnectivity between the deep shale gas formationand the shallow drinking water aquifers, such as
Osborn et al. (2011), Warner et al. (2012), Myer(2012), and Vidic et al. (2013), but still lack astrongly convinced evidence.
Mechanically, injection-induced seismicity andhydraulic fracturing induced seismicity share thesame mechanics, which all involves the principle ofeffective stress and slip or failure of rock
discontinuities on different scales. Elevated fluidpressure can change the stress state of thesurrounding rock masses, especially the rock
discontinuities on different scales.
Failures of the rock discontinuities are usuallyaccompanied with microseismic activities. The larger
the structure planes slip, the greater the magnitudes ofthe seismicity come into being. Seismic monitoring
provides valuable data for assessing the earthquake
potential of the injection or fracturing operations (DasThe presence and application of the horizontal
drilling and hydraulic fracturing technologies makethe extraction of unconventional sources of gas
economically feasible. Recently, there has been
considerable attention focused on earthquakes
associated with the hydraulic fracturing of shale gas
formations (Rutqvist et al. 2013; Hummel andShapiro 2013). Public attention to internal relations
between earthquakes and fluid injection begin fromearly seismic events in the wastewater injection,
geothermal energy development, enhanced oil
recovery, and carbon capture and storage.
Fluid injection and hydraulic fracturing induced
fractures and fault reactivation can provide permeable
pathways for fluids at variety of scales. Publicconcerns about ground water contamination fromhydraulic fracturing are prevalent because thehydraulic fracturing fluids are toxic mixtures, which
consist of water, proppants, and chemicals. The
amount of injected fluid that returned to the ground1

10th Asian Regional Conference of IAEG (2015)

and Zoback 2011; Frohlich 2012). In this study, theearthquakes induced by traditional fluid injection andhydraulic fracturing of the shale gas werere-evaluated and mechanisms of fault reactivation
associated with the injection and production of shale
gas by hydraulic fracturing was discussed. The
factors that influence the fault reactivation in
hydraulic fracturing were evaluated to help manage
the environmental risks in the production.
2.

Arsenal approximately 6 miles northeast of thedowntown Denver. Although the Denver had beenpreviously considered to be in an area of low
seismicity, more than 1,500 earthquakes wererecorded between 1962 and 1967 because of the deep
injection operations. As illustrated in Fig. 1, there wasa significant relation between the volume of waste
injection and the earthquake frequency. The farthest
earthquake migrated about 10 km from the injection
well that tracked a critical pore pressure front of 3.2
MPa. The Rocky Mountain Arsenal earthquake
illustrates that the diffusion of pore pressure within an
origin fault zone can initiate earthquakes many
kilometers from the injection wells (Ellsworth 2013).
That is, the earthquakes can delay several months oreven many years after injection.
At Paradox Valley, the fluid-injection project has
been under way since 1996 in order to reduce salinity
in the Colorado River. Between 1996 and 2014, morethan 5,700 earthquakes were recorded around 12 kmof the injection point where only 3 tectonic
earthquakes occurred within 15 km of the injection
well during 1985 and 1996. The maximum
earthquake was Mw 4.3 in May 2000. As illustrated
in Fig. 2, a significant relation existed between thevolume of fluid injection and the earthquake
frequency. The paradox Valley experience illustrates
that long-term, high-volume injection can lead to thecontinued expansion of the activated area andtriggering of earthquakes many kilometers from theinjection point more than 15 years after the initialearthquake occurrence (Ellsworth 2013).

Lessons from injection-induced
earthquakes

Although it is known that long-term injection
operation in the deep subsurface can induce
earthquakes in some circumstance, it is still difficult
to discriminate the man-made and natural tectonic
earthquakes because of incomplete information on theinitial stress state, geologic structure, hydrogeology,
injection history, and the pressure changes
surrounding the injection wells. Among the numerous
cases of earthquakes that were likely induced byinjection operations, several confirmed andwell-documented cases are available to learn the
pre-injection stress state, injection history, as well as
the earthquake relations.

Fig. 1 Histograms showing relation between volume
of waste injection and earthquake frequency in the
Rock Mountain Arsenal well. Some of theearthquakes were sensible and about 13 of them withmagnitudes larger than Mw 4.0 occurred during theabove period. The maximum magnitude occurredwith magnitude of Mw 4.8 in August 1967 (adaptedfrom Evans (1966); Healy et al. (1968); McClain(1970); Hsieh and Bredehoeft (1981); National
Research Council (2013)).

Fig. 2 Histograms showing relation between volume
of waste injection and earthquake frequency in the
Paradox Valley (adapted from Block (2011); National
Research Council (2013)).

Among these injection-induced earthquakes, themost notable one was the US army’s injection of fluid
into a 12,000 ft deep well at the Rocky Mountain2

10th Asian Regional Conference of IAEG (2015)

Both the mic-seismic events and felt earthquakes
are a performance of release of elastic strain energy
stress release of rock discontinuities on different
scales. Under normal circumstance, the structuralplanes or fault surface are remain locked because offrictional resistance on the structural planes caused by
the in situ stresses. The fault slips when the shear
stress is large enough to overcome the friction forces,
resulting in a seismic event. The fluid injection or thehydraulic fracturing can increase the pore pressure in
the rock discontinuities, which act to counteract thenormal stress on the structural planes and result in adecrease in the frictional force. Once initiated,
frictional resistance drops and seismic waves radiateaway.
The mechanisms are illustrated in the schematic
maps of Figs. 3 and 4. In the macroscopic view, therock masses slip along an entire fault plane andrelease the elastic strain energy. In the microscopic
view, rock bridge failure among multi-cracks caused
local slip damage of the fault, rather than one large
reactivation along the entire fault plane.

Similarly, other long-term injection operations,such as geothermal injection in Geysers (USA) andBasel (Switzerland) (Majer et al. 2000), oil and gas
recovery in Rongchang (China) (Li et al. 2008),
Zigong (China) (Li et al. 2013), and potential sourcesin areas where the geologic conditions are favorable
for the occurrence of earthquakes.
3. Earthquakes
inducedbyhydraulic fracturing
Hydraulic fracturing is an essential technologythat involves the injection of fluid under pressure tocreate network of open fractures in reservoirs
especially in the ultralow permeable shale gas
reservoirs, thereby increasing the permeability ofrock formations. In general, the horizontal drill wells
were drilled several kilometers within the reservoirs
and tens of thousands of cubic meters of fracturing
fluid were pumped into the shale reservoirs withextremely high pressure to create tensile fractures.
The aforementioned injection-induced earthquakes
are a good analog for the potential for earthquakes tobe triggered by hydraulic fracturing.
Recently, several cases have been reported inwhich earthquakes large-than-usual seismic eventswere associated directly with the hydraulic fracturingof the shale gas reservoirs. One of the most notablecases that has been documented and confirmed inwhich hydraulic fracturing induced felt earthquake
was occurred in Blackpool, England, in 2011 (DePater and Baisch 2011). Site investigation recorded
two prominent seismic events of magnitude Mw 2.3
and Mw 1.5, which were induced by injection of alarge volume of injection fluid into a fault zone thathas not been previously found in the early geologicalprospecting. In the Eola Field of Garvin County,
Oklahoma, a series of felt seismic activity that ranged
in magnitude from 1.0 to 2.8 were recorded within 5
km of from the well due to hydraulic fracturing
operation. In addition, an unusual series of seismic
events were recorded in the Hom River Basin of
British Columbia during hydraulic fracturing inadjacent to a pre-existing fault (Ellsworth 2013). In
this instance, there were 21 seismic events with Mw
3.0 and the larger were recorded and the maximum
was Mw 3.6. However, the quality of the evenlocations was not adequate to full establish a directcausal link to hydrofrac treatment. Beyond that, there
are some other examples of hydrofrac induced
seismic events (Mw<2). However, all these seismic
events are small and which can still be classified as
mic-seismicity. In other words, the present process ofhydraulic fracturing for shale gas recovery does notpose a high risk for production and public safety.
4.

Fig. 3 Schematic map of the mechanics of fault
reactivation induced by fluid injection.

Fig. 4 Schematic map of the stressed state of a fault
plane. The normal and shear stresses acting across thefault depend on the vertical and horizontal stress andthe fault inclination.
Mechanically, fluid injection/hydraulic fracturing
is a dynamic mechanical process. Fluids are pumped
into the well under high pressure to open the existing
fractures or initiate new fractures. The direct changeof pore water pressure resulting from fluid injection
can generate significant changes in effective stresses.

Mechanics of induced fault reactivation3

10th Asian Regional Conference of IAEG (2015)

The change of effective stress is related to the porewater pressure change and the total stress changes canbe expressed as follows (Soltanzadeh and Hawkes2009):

S h are the vertical and horizontal geo-stress,

 ij   ij  Pf  ij

(1)

 ij is the total stress changes, ij is theeffective stress,  is Biot’s coefficient,

Pf is the

Where

Where  is the dip angle of the fault plane, S v andrespectively. In an injection scenario, the likelihoodof fault reactivation can be evaluated through the
Coulomb Failure Criteria:  C   ( n  Pf )pore pressure changes, and  ij is the Kronecker delta.

Where C is the cohesive strength of the fault plane;and  is the coefficient of friction in the fault plane.

For the convenient of poroelastic analysis, we

The coefficient of friction is in the range 0.6-0.85(Byerlee, 1978). Using the chart method in Fig. 7, allpossible orientations of faults lie within the shadearea. The horizontal distance between any orientationand the failure envelope is used to evaluate thepropensity of the plane to failure.

define the stress arching ratio  ij which is the ratio,of the ij to thePf within the rock mass.

 ij   ij /  Pf

(7)

（2）

As for a fault in which the initial and induced
stress changes in the vertical and horizontal are
principal stresses, the induced horizontal and verticaleffective stress changes within the rock mass can be
expressed as follows: h '   1   h  Pf(3)

 v '   1   v  Pf

(4)

Fig. 5 Three-dimensional Mohr diagram withcomposite Griffith Coulomb failure envelop (revised
from Mildren et al. (2002)).

In the two dimensional case, the shear stresses(  ) and normal stresses (  n ) on a fault plane can be

The change in Coulomb Failure Stress can beevaluated as (King et al. 1994):

solved by a subsection method.



1
2
1
2

CFS      n'( z   x ) sin 2   xz cos 2

Where(S v  S h  Pf (2   v   h ) sin 2   xz cos 2

 and

(8)

 n are shear stress and effective

normal stress in a fault plane. Thus, the fault
reactivation factor (  ) can be defined and used to22evaluate the characterization of reactivation
 n   x cos    z sin   2 xz cos 222 (S h   h ) cos   (S v   v ) sin   2 xz cos 2 (Soltanzadeh and Hawkes 2009). In addition, the slip
tendency of the fault elements can be calculated(6)
through a ratio of shear to the effective normal stress(5)4

10th Asian Regional Conference of IAEG (2015)

For a fault located in the influence area of thehydraulic fracturing, the critical pore pressure ranges
between the upper and lower threshold of the pore

( Ts ).

  CFS /  Pf
Ts   / ( n  Pf )   faultpressure ( Pc1  Pc  Pc 2 ).

(9)

The injection or hydraulic fracturing fluid cangreatly decrease the effective normal stress andcohesive force on the fault surface, thus decreasing
the shearing strength of the fault. When the porepressure comes to the critical state, the fault occurs
slip failure. Sometimes, the injection fluid canmigrate several thousands feet within a fault. More
importantly, the change of pore pressure tends tocause local slip damage along oblique factures in thefault zone, rather than a larger scale reactivation
event along the entire fault surface. Fault reactivation
could enhance the permeability of the rock mass andprovide flow pathways for gas or fracturing fluid
discharge to shallow ground water.

(10)

5.

Influencing factors of fault
reactivation
Investigations show that many factors influence
the initiation and extent of a fault reactivation when
the fault is stimulated by hydraulic fracturing
operation. Nevertheless, these factors could
be classified into two broad categories.
Geomechanical parametersIn situ crustal stress
Relative position of the fault

Fault mechanicsPermeability of fault zonePore water pressure…
Hydraulic fracturing parametersInjection depthInjection rateInjection duration
Injection volume…

Fig. 6 Sketch of normalized slip tendency in relation
to the Mohr diagram. Principal stressesσ1, σ2, andσ 3, can be expressed in terms of unknown
parameters r1 and r2.
In addition, the slip tendency of a fault plane canbe visually represented by a graphic method. As
illustrate in Fig. 6, the normal stress on the fault plane
can be drawn in the three-dimensional morh’s stresscircle. Connect the origin point and the normal stresspoint to form a straight line. The slope of the line
represents the degree of risk for the fault slip.
Specifically, the larger the slope of the line, the higher
risk of the fault occurs slip damage.
In the critical state of a fault reactivation, theupper limit and lower limit of the pore pressure in thefault plane can be back calculated under the condition
that the friction coefficients of the fault planes are
specified as the minimum and maximum threshold.

 ((1  sin  )l 2  ((1  2 )sin   1)m 2 

r1 
2
Pc1 
 (1  sin  )n )
2 sin  22 2
2
2 2
2 2 1/2
   l m  (1   ) m n  n l  

The interaction and triggering mechanism ofthese factors is a complex issue. But in general, whena critical stress fault located in the optimal positionand direction of the injection region, a minor changeof pore water pressure can destroy the local balanceof friction on the fault plane, resulting slip damage
and seismic events. In ideal circumstances, theincrease in injection rate and net injection volume cangreatly stimulate the reactivation of faults and otherscales of rock discontinuities. As illustrated in Fig 7,
the seismicity shows three forms. First, the number ofseismic events of Mw 1.5 and greater have increased(11) ((1  sin  )l 2  ((1  2 )sin   1)m 2 
r1 2
Pc 2  (1  sin  )n ) 
2 sin 

2 2
2
2
2 2
2 2 1/2  0.6  l m  (1   ) m n  n l  (12)5

10th Asian Regional Conference of IAEG (2015)

The research was supported by the Natural
Science Foundation of China (Grant Nos. 41372325,
41372323, and 41172271), and the Strategic Priority
Research Program of the Chinese Academy of
Sciences (Grant No. XDB10030602). Gratefulappreciation is expressed for these supports.

from almost none in the 1960s to 112 in 1975 andthen to 1,384 in 2006. Second, the annual number ofearthquakes of Mw 3.0 and greater is shown along thebottom of the graph. By 1985, these events occurred
25 annually. Third, seismic events of M4.0 and
greater are shown neat the top. Such events have beenrecorded about one to three of per year since1972.
The maximum magnitude of earthquake was Mw
4.67 in May 2006 (Smith et al. (2000); Majer et al.
(2007)).

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6.

Conclusions

Fault slip is highly related with seismic events
during and post injection/hydraulic fracturing
operation, and this correlation can be identified as anevidence for fault reactivation. Injection/hydraulicfracturing fluid can greatly decrease the effective
normal stress on the fault surface, thus decreasing
the shearing strength of the fault plane. When theshear stress is large enough to overcome the friction
force, the fault slips, resulting in seismic events.
Pressured fluid tends to cause failure ofmulti-cracks and resulting in local slip damage, rather
than one large-scale slip/reactivation along the entire
fault plane. However, even a minor slip failure offault, it could increase the permeability of the fault
zone and may create new flow pathways for upward
fluid migration.
There are many factors influence the performanceof fault reactivation. The geomechanical properties ofthe fault play a decisive role. The injection
parameters are the triggering factors. In general, theincrease in injection rate and net injection volume cangreatly stimulate the reactivation of a critical stressfault or other rock discontinuities on different scales.
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