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Increasing fracture aperture by lowering effective normal stress and by inducing dilatant shearing and thermo-elastic effects is essential for transmissivity increase in enhanced geothermal systems. This study investigates transmissivity evolution for fluid flow through natural fractures in granodiorite at the laboratory scale. Processes that influence transmissivity are changing normal loads, surface deformation, the formation of gouge and fracture offset. Normal loads were varied in cycles between 1 and 68 MPa and cause transmissivity changes of up to three orders of magnitude. Similarly, small offsets of fracture surfaces of the order of millimeters induced changes in transmissivity of up to three orders of magnitude. During normal load cycling, the fractures experienced significant surface deformation, which did not lead to increased matedness for most experiments, especially for offset fractures. The resulting gouge material production may have caused clogging of the main fluid flow channels with progressing loading cycles, resulting in reductions of transmissivity by up to one order of magnitude. During one load cycle, from low to high normal loads, the majority of tests show hysteretic behavior of the transmissivity. This effect is stronger for early load cycles, most likely when surface deformation occurs, and becomes less pronounced in later cycles when asperities with low asperity strength failed. The influence of repeated load cycling on surface deformation is investigated by scanning the specimen surfaces before and after testing. This allows one to study asperity height distribution and surface deformation by evaluating the changes of the standard deviation of the height, distribution of asperities and matedness of the fractures. Surface roughness, as expressed by the standard deviation of the asperity height distribution, increased during testing. Specimen surfaces that were tested in a mated configuration were better mated after testing, than specimens tested in shear offset configuration. The fracture surface deformation of specimen surfaces that were tested in an offset configuration was dominated by the breaking of individual asperities and grains, which did not result in better mated surfaces.
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Permeability Evolution in Natural Fractures Subject to Cyclic
Loading and Gouge Formation
Daniel Vogler
Florian Amann
Peter Bayer
Derek Elsworth
Received: 6 September 2015 / Accepted: 3 June 2016
ÓSpringer-Verlag Wien 2016
Abstract Increasing fracture aperture by lowering effective
normal stress and by inducing dilatant shearing and thermo-
elastic effects is essential for transmissivity increase in
enhanced geothermal systems. This study investigates trans-
missivity evolution for fluid flow through natural fractures in
granodiorite at the laboratory scale. Processes that influence
transmissivity are changing normal loads, surface deforma-
tion, the formation of gouge and fracture offset. Normal loads
were varied in cycles between 1 and 68 MPa and cause
transmissivity changes of up to three orders of magnitude.
Similarly, small offsets of fracture surfaces of the order of
millimeters induced changes in transmissivity of up to three
orders of magnitude. During normal load cycling,the fractures
experienced significant surface deformation, which did not
lead to increased matedness for most experiments, especially
for offset fractures. The resulting gouge material production
may have causedclogging of the main fluid flow channels with
progressing loading cycles, resulting in reductions of trans-
missivity by up to one order of magnitude. During one load
cycle, from low to high normal loads, the majority of tests
show hysteretic behavior of the transmissivity. This effect is
stronger for early load cycles, most likely when surface
deformation occurs, and becomes less pronounced in later
cycles when asperities with low asperity strength failed. The
influence of repeated load cycling on surface deformation is
investigated by scanning the specimen surfaces before and
after testing. This allows one to study asperity height distri-
bution and surface deformation by evaluating the changes of
the standard deviation of the height, distribution of asperities
and matedness of the fractures. Surface roughness, as
expressed by the standard deviation of the asperity height
distribution, increased during testing. Specimen surfaces that
were tested in a mated configuration were better mated after
testing, than specimens tested in shear offset configuration.
The fracture surface deformation of specimen surfaces that
were tested in an offset configuration was dominated by the
breaking of individual asperities and grains, which did not
result in better mated surfaces.
Keywords Fracture mechanics Fracture transmissivity
EGS Fracture surfaces Aperture Gouge
1 Introduction
Anthropogenic intervention and resulting perturbations
(e.g., hydraulic fracturing) in a rock mass at great depth
may result in complex thermal-hydro-mechanical response.
This is of particular relevance when dealing with enhanced
geothermal systems (EGS) and unconventional oil and gas
extraction utilizing hydraulic fracturing. Our focus here is
on EGS, which is considered to be a promising option for
generating electricity from hot but naturally low permeable
deep formations (Tester et al. 2006). Studying the key
THM coupled processes associated with specific reservoir
characteristics in an EGS is of foremost relevance to
establish a reliable heat exchanger capable of achieving the
target production rate while maintaining low background
seismicity. The triggering of elevated seismic activity can
cause the termination of EGS projects due to damage to
&Daniel Vogler
Department of Earth Sciences, Swiss Federal Institute of
Technology Zurich, Sonneggstr. 5, 8092 Zurich, Switzerland
Department of Energy and Mineral Engineering, EMS
Energy Institute, Center for Geomechanics, Geofluids, and
Geohazards, Pennsylvania State University, University Park,
Rock Mech Rock Eng
DOI 10.1007/s00603-016-1022-0
facilities on the surface (e.g., the hot dry rock EGS project
in Basel in 2006). Rock transmissivity and flow rates in
fractures determine the productivity of geothermal reser-
voirs and are controlling factors of whether reservoirs are
economically feasible. Lowering effective normal stresses
by high-pressure injection, dilatant shearing of critically
stressed fractures or thermo-elastic effects (e.g., cooling)
can cause fracture conductivity to increase (Evans 2005).
To relate mechanical and hydraulic effects in fractures,
the mechanical and hydraulic aperture (amand ahyd ) are
generally considered separately (Witherspoon et al. 1980;
Barton et al. 1985; Esaki et al. 1991; Zimmerman et al.
1991; Park et al. 2013). While the mechanical aperture
describes the physical distance between two fracture sur-
faces, the hydraulic aperture describes the aperture
accommodating a particular flux assuming a parallel plate
model. With increasing mechanical aperture the hydraulic
aperture increases, but the relation between the mechanical
aperture and the hydraulic aperture is not one to one (Esaki
et al. 1991,1999; Rutqvist and Stephansson 2003; Xiong
et al. 2011 and McClure and Horne 2014).
The common constitutive equations are based on aper-
ture and linearly relate the flow rate in a fracture to the
pressure gradient. Fluid flow between parallel walls can be
derived from the simplified, incompressible Navier–Stokes
equations (Louis 1969), as,
where Qrepresents the fluid flow rate, ahyd the hydraulic
aperture, wthe fracture width, lthe dynamic viscosity and
pthe fluid pressure.
HM-coupled laboratory investigations reveal that
assuming the same mechanical and hydraulic aperture is
not generally valid (Raven and Gale 1985; Brown 1987;
Cook 1992; Renshaw 1995; Hakami and Larsson 1996;
Oron and Berkowitz 1998; Esaki et al. 1999; Chen et al.
2000 and Lee and Cho 2002).
Laboratory tests on granite, marble and basalt specimens
with tension-induced artificial fractures by Witherspoon
et al. (1980) and on artificial resin fractures by Li et al.
(2008) found the approximation of fluid flow with a par-
allel plate model with an effective hydraulic aperture to be
suitable depending on the fracture roughness characteris-
tics. Findings by Raven and Gale (1985), Cook (1992),
Hakami and Larsson (1996), Oron and Berkowitz (1998),
Esaki et al. (1999), however, showed hydraulic apertures to
be consistently smaller than mechanical apertures. Studies
by Cook (1992) and by Oron and Berkowitz (1998) showed
decreasing mechanical aperture leading to a faster than
cubic decrease in hydraulic conductivity and a nonlinear
increase in contact area. Zimmerman et al. (1991)
demonstrated that the ratio of the hydraulic aperture to the
mean mechanical aperture is related to the ratio of the
mean mechanical aperture and its standard deviation. In
contrast, Renshaw (1995) found the hydraulic aperture to
be constant below a residual hydraulic aperture, while the
mechanical aperture could still decrease further when the
normal load was increased.
Analysis of experimental data by Cook (1992) and Park
et al. (2013) showed mechanical normal joint stiffness and
hydraulic aperture to be largely dependent on the contact
area between fracture surfaces. Joint stiffness, joint closure
and fluid flow were shown to behave highly nonlinearly.
Also, as normal loads on a specimen are cycled, hysteretic
behavior results. The exponent of the aperture is increas-
ingly higher than cubic with progressive fracture closing.
Experimental work by Durham and Bonner (1994), Esaki
et al. (1991), Esaki et al. (1999), Chen et al. (2000), Li et al.
(2008), Park et al. (2013) found the hydraulic conductivity of
natural fractures to be significantly larger with fracture shear
displacements. Esaki et al. (1991,(1999) and Lee and Cho
(2002) also found increases in transmissivity to become
smaller with ongoing shear displacement. This behavior was
controlled by the maximum asperity heights and the distance
between asperities. For repetitive forward and reverse shear-
ing, the development of wear products from fracture surface
deformation (gouge) was found to decrease transmissivity by
an order of magnitude. A commonly used relationship
between shear and normal stress to represent changes in
conductivity was developed by Barton et al. (1985). This
relationship was tested for rock specimens of various sizes and
utilizes surface roughness quantities [i.e., the joint roughness
coefficient (JRC)] to quantify surface roughness.
While the hydraulic aperture inferred from experiments
normally only represents an averaged hydraulic aperture,
fracture flow depends on fracture surface heterogeneity, which
is also strongly fracture size dependent (Renshaw 1995;Yeo
et al. 1998; Oron and Berkowitz 1998; Pyrak-Nolte and Morris
The findings of previous experiments demonstrate that
the cubic law and a linear relation between mechanical and
hydraulic aperture are only partially valid, which serves as
motivation for this study. In this study, several coupled
processes of importance for reservoir modeling are exam-
ined for a single fracture. The overall objective is to define
the evolution of fracture transmissivity in natural fractures
under applied normal and shear displacements as an analog
to the response of fracture networks to changes in applied
effective stresses driven by fluid pressures and thermal
loads. Changes in transmissivity due to changes in con-
fining stress on the specimen are studied for single loading
cycles. This includes an analysis of fracture surface dam-
age and the production of gouge material. The experi-
mental setup allows one to compare changes in mechanical
and hydraulic aperture during specimen loading.
D. Vogler et al.
For increases in confining stress, the effect of the mat-
edness of the two fracture surfaces is examined by per-
forming experiments with well-mated and offset fracture
surfaces. This provides qualitative as well as quantitative
information on the impact of dilation due to shear dis-
placement, production of gouge material, and quantitative
insight into the magnitudes of transmissivity and trans-
missivity changes during cyclic loading.
2 Methods
2.1 Specimen Preparation
Granodiorite specimens were provided by the Grimsel Test
Site, Switzerland. The specimens were obtained from cores
of the CRIEPI fractured rock study (Takana et al. 2014).
Twelve laboratory specimens with a diameter of 2.5 cm
and a length of 6 cm were produced by overcoring pre-
existing fractures in 10.5 cm cores (Table 1; Fig. 1). The
overcoring orientation was chosen so that fractures are
aligned parallel to the specimen’s main axis. The fractures
were classified as tensile (mode I) and shear (mode II)
fractures (Table 1). Classification of the fracture type was
performed by investigating fracture surfaces for slicken-
sides, plumose structures, mineralization and crack propa-
gation through individual grains.
2.2 Experimental Setup
The experimental setup consisted of a pressure-tapped core
holder of the DCH series (Fig. 2, Table 2), as produced by
Core Lab. The core holder setup was described in detail by
Wang et al. (2011).
One Isco pump (model 500D; pump PA) and two Isco
pumps (model 100DM; pumps PBand PC) were utilized to
control the fluid pressure at the specimen inlet pfp (pump
PC), the confining pressure pco (pump PB) and to measure
Table 1 Physical properties of
the specimen under
investigation with original
specimen length, shear
displacement offset, the
resulting effective specimen
length and the distinction
between tensile and shear
fracture mode
Test specimen Specimen length Shear offset Eff. length Frac. mode
(–) (mm) (mm) (mm) (–)
1 61 0 60 I
2 62 0 60 II
3 62 0 60 I
4 60 0 60 I
5 60 2 58 I
6 60 2 58 I
7 60 3 57 I
8 61 3 58 I
9 60 6 54 I
10 62 5 57 II
11 60 1 59 I
12 60 1 59 I
Fig. 1 Example of overcored specimen on top of one fracture surface
Fig. 2 Experimental setup of core holder and specimen. The
pressures were prescribed in pumps PA(axial pressure pax), PB
(confining pressure Pco) and PC(fluid inlet pressure pfp )
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
the axial pressure pax (pump PA). Similar setups for
investigation of rock specimen properties were employed
by Zhu et al. (2007), Wang et al. (2011), Shugang et al.
(2013) and Zhong et al. (2014), but focusing on different
research questions.
The specimen placement with the fracture oriented
parallel to the core holder axis produces fluid flow along
the fracture from the inlet to the outlet. Specimens were
loaded with an initial confining stress of 1 MPa, and
steady-state flow was established. At all times, the fluid
pressure at the outlet was kept at atmospheric pressure patm
and the inlet pressure was recorded. When initial equilib-
rium of fluid pressure for a given flow rate was reached, the
confining stress was increased incrementally by 1–2.5 MPa
up to a maximum of 68 MPa and subsequently decreased
back down to 1 MPa, thereby completing one loading
cycle. After each incremental increase in confining stress,
the confining stress was kept constant until the fluid inlet
pressure necessary to maintain a constant flow rate
Qthrough the specimen reached a steady-state condition.
During the experiment, the flow rate Qwas kept constant
for individual loading cycles. Accuracy of flow rate mea-
surements was 0.5 %of setpoint (i.e., the currently read
pressure in the pump). Between 5 and 10 loading cycles
were performed for each specimen.
2.3 Permeability Behavior of Natural Fractures
Under Various Loading Scenarios
The experimental setup was used to test transmissivity
changes of natural fractures under changing confining
stress conditions. Confining stress is applied radially on
the cylindrical specimen. The fracture surfaces were
investigated in mated and offset configurations with
offsets among fracture surfaces ranging between 1 and
6 mm. An example of open and closed fractures is
shown in Fig. 3. Offset of the fractures occurred in the
axial direction. Differentiating between mated and offset
fracture surfaces enabled us to investigate the effect of
shear displacement on transmissivity and surface
The main measured properties are the inlet fluid pressure
pfp and the confining pressure pco. The effective confining
pressure pco;eff was calculated by
pco;eff ¼pco pfp þpatm
A representative transmissivity Twas derived from the
cubic law (Eq. 1) to allow one to investigate changes in
transmissivity, with the transmissivity being independent
of changes in flow rate in subsequent cycles. If the flow rate
Qis changed between load cycles, the representative
transmissivity Tdoes not change if the changes in pressure
gradient rpare proportional to the changes in flow rate.
where dis the specimen diameter, ahis the hydraulic
aperture, pthe flow pressure and lthe dynamic viscosity of
water. The transmissivity Tis used to make a comparison
between individual experiments because Trelates the flow
rate driven by a unit pressure gradient, rather than a head
gradient in the normal hydrogeological definition. Note that
the transmissivity Tis related to the normal hydrogeolog-
ical definition of transmissivity Thas Th¼Tcwhere cis
the unit weight of the flowing fluid. Reporting the trans-
missivity offers the advantage of reporting all properties
known in the experiment (e.g., Q,DL,Dpand d) in one
variable. This allows straightforward comparison of
experiments performed with different flow rates Qor on
fractures with varying width d. Similar forms of the
transmissivity were also used in Renshaw (1995), Gentier
et al. (2013) and Zimmerman and Bodvarsson (1996).
Besides the hydraulic analysis, surface deformation was
determined and mineralogical characterization of the
fracture surfaces was performed.
2.4 Surface Scans
Prior to testing, replicas of the fracture surfaces were
produced. These were utilized to obtain high-resolution
photogrammetric scans of the surface. Fracture surfaces
were evaluated with surface scans recorded with the ATOS
Table 2 Pumps used in the setup with model, minimum and maxi-
mum pressures pmin and pmax, standard pressure accuracy SPA
pfp pco pax
Model (–) 500D 100DM 100DM
Min. pressure (MPa) 0.67 0.67 0.67
Max. pressure (MPa) 25.9 69 69
Volume (mL) 507 103 103
SPA (% FS) 0.1 0.1 0.1
Fig. 3 Example of overcored specimen aopen and bclosed
D. Vogler et al.
Core 3D scanner from GOM. The ATOS Core sensor
projects fringe patterns on the object surface, which are
recorded by two cameras. The patterns form a phase shift
that is based on a sinusoidal intensity distribution which
enables one to calculate the three-dimensional (3-D) sur-
faces. The photogrammetry scanner is calibrated with two
tests. The diameter and shape of a sphere and the distance
between two spheres that are mounted on a plate are
measured with the photogrammetry scanner to derive cal-
ibration errors and accuracy. All equipments used for cal-
ibration are specifically developed by the company GOM,
which manufactures the scanner. The photogrammetry
scanner was calibrated with length deviation errors
between 0.009 and 0.027 mm and optimized calibration
deviations of 0.014 ±0.001 pixels.
The same process was repeated for the damaged sur-
faces after testing to analyze the surface changes that
occurred during the experiments and compare these to the
changes in transmissivity during the experiment. Surface
damage during testing caused gouge material to accumu-
late in the fracture planes. Analyzing the gouge material
found in the fracture after testing can give insight into
possible relations between the grain size distribution of the
gouge material and transmissivity changes during
mechanical cycling.
A similar analysis is performed by computing the
asperity distributions on the fracture surfaces derived from
the fracture scans before and after the tests. The changes on
the fracture surfaces can also be used to determine probable
relations to transmissivity changes during testing.
2.5 Grain Size Analysis
The grain size distribution of the gouge material produced
during testing was measured by sieving samples larger than
1 mm and by using laser spectrometry (Malvern Instru-
ments Mastersizer 2000) for all gouge material below
1 mm. The gouge material is analyzed for volumetric dis-
tribution of gouge instead of weight. This is done because
laser spectrometry allows a very accurate measurement of
grain size volume down to around 10 micrometers.
2.6 Aperture Changes
The volume change of the confining fluid during cyclic
loading was recorded in each experiment and was used to
calculate the volumetric strain. After removing noise and
accounting for confining pressure changes, these data were
used to quantify relative mechanical aperture changes
during cycling, assuming that the volume change is entirely
associated with fracture deformations.
When rubber jacket and specimen volumes decrease
(Fig. 2), the pump controlling the confining pressure has to
adjust for this increase in volume of the confining pressure
fluid. The volume of the rubber jacket can decrease due to
deformation of the jacket and the specimen enclosed
within. The rubber jacket itself deforms elastically, which
allows one to separate the volumetric changes of the rubber
jacket and the specimen. These changes can be attributed to
the elastic deformation of the specimen itself as well as to
reversible and permanent changes of the fracture. To cal-
ibrate this, an additional test with an intact granodiorite
specimen was performed for loading between 1 and
68 MPa. Therefore, the changes in confining fluid volume
dðVp;conf Þare related to the changes in mechanical aperture
damech by
damech ¼dVp;conf
where dis the specimen diameter and lis the specimen
length. The change in hydraulic aperture derived from fluid
pressure increase can be compared to the mechanical
aperture decrease, and the hydraulic aperture can be cal-
culated with Eq. 3. Changes in hydraulic aperture are
measured by comparing the hydraulic aperture during the
experiment to the initial value at the start of the experiment
with 1 MPa confining pressure.
3 Results
In the performed experiments, we measured the fluid
pressure gradient response to changing confining pressure
on a natural fracture. The flow rate Qwas kept constant for
individual cycles, and the required fluid pressure gradient
rpto maintain Qwas measured. A representative fracture
conductivity can be calculated in form of the transmissivity
(Eq. 4). Hence, transmissivity is used to quantify fracture
conductivity changes during the experiments. This section
reports on the effect of cyclic loading on the fluid flow,
gouge material and the surface damage that can be evalu-
ated after completion of the experiments. To investigate the
effect of shear displacement on cyclic loading, two classes
of tests were performed on specimens with mated surfaces
(tests 1, 2, 3, 4) and on specimens with shear displacement
of 1–6 mm (tests 5, 6, 7, 8, 9, 10, 11, 12). A detailed
discussion follows in the subsequent Sect. 4. The feasible
number of loading cycles was determined for each speci-
men and the experimental device. Due to the large opening
of a natural fracture in comparison with a saw-cut fracture,
each specimen was wrapped in a smaller jacket within the
larger rubber jacket separating the specimen from the
confining fluid. Nonetheless, rupture of the rubber jacket
led to the termination of some tests and a maximum of 10
cycles was performed to limit wear on the material. Other
reasons for termination of experiments were failure of the
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
specimen under high normal loads and fluid inlet pressures
above 30 MPa. It should be noted here that the effective
confining stress in these extreme cases was very variable
along the specimen axis, due to the large pressure gradients
observed between inlet and outlet. The equilibration times
for fluid pressures strongly varied depending on the frac-
ture opening, existing gouge material and applied confining
pressures. These times ranged from minutes up to multiple
hours for individual changes in confining pressure that
were between 1 MPa for small pressures (i.e., 1–10 MPa)
and 2.5 MPa for larger pressures (i.e., 10–68 MPa).
Experimental data that are used for further analysis
include volume changes, time and pressures that are mea-
sured at each pump for confining and fluid flow pumps
(Fig. 2). Transmissivity (Eq. 4) and effective confining
pressure are chosen to represent the experimental data.
Changes in surface properties are characterized by changes
in asperity height and the standard deviation of asperity
height. These are obtained from inspection of the surface
scans. For a concise description of results, we select only
experiments 1, 2, 4, 7 and 10 (Fig. 4a–e). The selected
experiments are considered representative of hysteretic
behavior during normal load increase and transmissivity
changes with ongoing cycling. Tests 2 and 4 (Fig. 4b, c)
display strongly hysteretic fluid inlet pressures, which are
linearly related to the transmissivity. Specimens 2 and 4
were tested in a mated configuration. Specimens 7 and 10
were tested in an offset configuration (3 and 5 mm,
respectively) and show hysteretic fluid pressures responses
for late cycle numbers (e.g., cycles 4 and later for tests 7
and 10) as well.
3.1 Permeability Changes During Load Cycling
Figure 4a–e depicts the transmissivity changes as derived
from Eq. 4for changing effective confining pressure. The
color coding of the data curves ranges from the first cycle
(dark red) to the last cycle (dark blue), and the increasing
confining pressure path of a cycle is marked with a ., while
the decreasing confining pressure path is marked with a /.
The general trend shows fracture transmissivity decreasing
with increasing cyclic loading (Fig. 4a–c). During the first
initial cycles, transmissivity declines rapidly and converges
toward later cycling.
Most experiments (Fig. 4b–e) display hysteretic behav-
ior during cycling as fracture transmissivity is lower during
unloading of the specimen than during loading. The cyclic
loading also allows one to compare transmissivity decrease
due to changes in confining stress and increasing cycling
and accompanying surface damage. During experiment 1
(Fig. 4a), transmissivity decrease due to larger confining
stresses during individual test cycles is on the same order
of magnitude as decreases due to irreversible fracture
closure (e.g., due to surface damage) with repeated load
cycling. Load cycle 4 shows exceptional behavior with
significantly larger initial transmissivity decreases than all
other cycles. This is likely related to clogging of a main
flow channel in the fracture or clogging of the fluid pipes
upstream or downstream of the specimen. The effect of
changing confining stress during individual cycles is more
pronounced for experiments 2 and 4 (Fig. 4b, c) where
transmissivity decreases strongly during initial confining
pressure increase and converges against a constant trans-
missivity value between 10 and 20 MPa effective confining
In experiment 10 (Fig. 4e), the fracture aperture is very
large and the pressure gradient is quite small until
47.5 MPa confining pressure is reached. The fluid pressure
gradient then rises quickly, which means that transmis-
sivity is decreasing quickly.
Depending on the surface of the fractures, different
response patterns of the transmissivity to increased con-
fining pressure can be observed. Specimen 1 (Fig. 4a)
exhibits a small slope that is almost linear on a semilog
plot, which stands in stark contrast to specimens 2 and 4
(Fig. 4b, c). The surface of specimen 1 shows significant
mineralization and slickensides (Sect. 1) with two well-
mated surfaces with a uniform distribution of contact area.
While this leads to almost no hysteretic effects between the
increasing and decreasing confining stress paths, it also
causes a significantly less steep transmissivity decline for
small confining stress (1–10 MPa). Tests with shear offset
show higher transmissivities (Fig. 4d, e). The large range
of transmissivity values for respective confining stresses
can also be observed for all specimens in Fig. 13. While
increased confining stress and surface damage cause
transmissivity values for individual specimen to vary up to
three orders of magnitude, the offset of the specimen only
seems to have an effect up to 1 mm. For offsets between 1
and 6 mm, the total transmissivity range is comparable for
all tests.
3.2 Aperture Changes
Measurements during the experiments only allow one to
deduct changes in mechanical aperture, as absolute values
are not locally known and measured changes are repre-
sented as averages across the whole specimen. Therefore,
changes in the mechanical and hydraulic aperture from the
starting value of each cycle at 1 MPa confining stress are
presented. As both aperture values generally decrease from
their starting value at 1 MPa, positive aperture changes
denote a reduction of the mechanical or hydraulic aperture.
After maximum effective confining stress is reached, the
mechanical aperture changes generally decrease, indicating
fracture opening. Changes in mechanical and hydraulic
D. Vogler et al.
aperture are compared in Fig. 5a–d. As for Fig. 4a–e,
stages in the experiment with equilibrated confining and
injection pressures are displayed with .and /during
increasing and decreasing confining pressure, respectively.
Closely spaced markers, therefore, indicate small changes
during one confining pressure interval change (2.5 MPa)
while markers spaced further apart indicate rapid changes
with confining pressure changes.
The first cycle is not always shown for reasons given
below. The specimen and the experimental equipment (i.e.,
the rubber jacket, see Fig. 2) deform during the first cycle,
which can lead to erratic aperture changes. This behavior
Fig. 4 Tests 1, 2, 4, 7 and 10 are shown in subfigures ae,
respectively. The plots show transmissivity (Eq. 4) versus effective
confining pressure. The first number in each row in the legend denotes
the cycle number, with color coding of the data curves going from the
first cycle (dark red) to the last cycle (dark blue). The second number
in each row in the legend denotes the flow rate during the respective
cycle in (mL/min). The curve from 1 MPa to maximum effective
confining pressure is marked with .while the reverse curve is marked
with /(color figure online)
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
can be especially pronounced for the offset specimens
where the two fracture sides have a small contact area.
Small shear displacements at high confining stresses can
lead to rapid fracture normal closure. For offset fracture
surfaces (eg., tests 7 and 10), oscillating behavior and rapid
changes during initial loading cycles are especially pro-
nounced, due to large initial apertures and fluid flow
pressure oscillations.
During test 1, mechanical aperture changes are more
than one order of magnitude larger than hydraulic aperture
changes (Fig. 5a). For each cycle, the hydraulic aperture
changes increase initially drastically until they start to
converge. Hydraulic aperture changes for test 4 (Fig. 5b)
are also significantly smaller than mechanical aperture
changes. Maximum mechanical aperture changes decrease
with ongoing load cycling while maximum hydraulic
aperture shows no distinct trend. Mechanical aperture
changes (i.e., fracture closure) are largest for test 7 (i.e., a
specimen that contained a fracture that was tested with
3-mm offset, Fig. 5c). For test 10 (5-mm offset),
mechanical aperture initially changes drastically, marked
by wide marker spacing. While mechanical aperture
change increases further as the fracture closes, hydraulic
aperture changes start converging.
For all tests, maximum mechanical aperture changes
(i.e., maximum mechanical aperture closure) remain com-
parable between cycles (around 1 mm except for test 4
shown in Fig. 5b with 0.2 mm). The mechanical aperture
changes of tests 1, 4, 7 and 10 can also be interpreted when
comparing mechanical aperture changes to effective con-
fining pressure (Fig. 6a–d). All tests show larger mechan-
ical aperture changes during initial loading (up to 10 MPa),
which become smaller for higher effective confining
stresses. Other visible trends include small decreases of
mechanical aperture changes with ongoing load cycles and
hysteretic behavior of mechanical aperture changes. For all
tests, the mechanical aperture change is larger than the
hydraulic aperture change after the maximum effective
confining pressure has been reached and confining stress is
decreasing (Fig. 5a–d). Hysteretic effects are more pro-
nounced for mated specimen (Fig. 5a–b), but are still
observable for offset specimen (Fig. 5c–d).
ahyd change [mm]
0 0.01 0.02 0.03 0.04
amech change [mm]
2 - 20
3 - 20
4 - 20
5 - 20
6 - 20
7 - 20
8 - 20
9 - 20
10 - 20
Q [mL/min]
ahyd change [mm] ×10-3
amech change [mm]
2 - 0.1
3 - 0.3
4 - 0.5
5 - 0.1
6 - 0.3
7 - 0.5
Q [mL/min]
ahyd change [mm]
0 0.02 0.04 0.06 0.08
amech change [mm]
2 - 10
3 - 10
4 - 10
5 - 10
Q [mL/min]
ahyd change [mm]
0 0.02 0.04 0.06 0.08 0.1
amech change [mm]
2 - 10
3 - 10
4 - 10
5 - 10
6 - 10
Q [mL/min]
Fig. 5 Mechanical versus hydraulic aperture changes. Changes are
calculated by comparison with the initial values at 1 MPa. Specimens
from tests 1, 4, 7 and 10 are assigned to ad, respectively. The curve
from 1 MPa to maximum effective confining pressure is marked with
.while the reverse curve is marked with /
D. Vogler et al.
Here, it should be noted that for tests depicted in
Figs. 5a–d and 6a–d, only test 4 (Figs. 5b, 6b) experienced
fluid inlet pressures high enough to cause significant dif-
ferences between the confining stress and effective con-
fining stress.
3.3 Analysis of Gouge Material
Due to the experimental setup, fine and coarse grain
material was not collected at the outflow end of the
experiment. The gouge material collected after testing was
analyzed under a magnifying lens, which found the min-
erals quartz, feldspar, biotite and chlorite, all common in
granodiorite. Specimens 2 and 10 showed slickensides.
The collected volume of gouge material (Fig. 7) follows
a log-normal distribution. Since the tests in mated config-
uration did not lead to significant surface damage, the
amount of gouge material was not sufficient to derive a
distribution for specimens without shear offset. Specimen 7
had whole individual grains of rock breakouts with sizes up
to 6 mm that were not monominerals. Larger quartz
crystals were the dominant minerals on the fracture surface
of specimens 1 and 7. Overall, mineral distributions on the
fracture surface and of the gouge material revealed to be
comparable and did not differ largely for individual spec-
imens and between all specimens. No mineralogical
Fig. 6 Mechanical aperture changes versus effective confining
pressure. Changes are calculated by comparison with the initial
values at 1 MPa. Specimen from tests 1, 4, 7 and 10 are assigned to
ad, respectively. The curve from 1 MPa to maximum effective
confining pressure is marked with .while the reverse curve is marked
with /
Size [mm]
10 -3 10 -2 10 -1 10 0
Volume Fraction [-]
Fig. 7 Grain size distribution of gouge material for tests 5, 6, 7, 8, 9,
10, 11 and 12
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
features larger than the average grain sizes exist in the
tested specimens. Changes in transmissivity during normal
load cycling are more likely connected to the surface
geometry and the specimen offset than the mineralogy of
the specimen. This is supported by comparable grain size
distributions across tests (Fig. 7). While mineralogy on
fracture surfaces was comparable across specimens, chan-
ges in transmissivity during cyclic changes in confining
stress showed strong correlation to shear offset during
testing (Fig. 4a–c for mated specimens versus Fig. 4d–e for
offset specimens and Fig. 13).
Larger rock pieces and individual grains that broke off
stayed in place during all tests. Smaller grains and fine
material were found evenly distributed across the fracture
after testing. Therefore, while larger breakouts of material
led to stronger surface alteration, fine material is more
likely to clog flow channels with gouge that leads to
increasing pressure gradients to maintain flow rates.
3.4 Comparison of Surface Scans Before and After
Due to the large maximum confining pressures, significant
surface deformation was expected in the fracture plane of
the specimens. For detailed analysis, photogrammetric
surface scans of the fracture specimens were generated
before and after the experiments.
The surface scans of the individual sides can be matched
to derive the fracture aperture by calculating the distances
between the two surface sides. This is fundamental to study
the effect of surface roughness, connectivity and the cor-
relation length of the fracture surfaces as well as the
respective aperture, which is especially crucial for fracture
Surface roughness can be quantified with the standard
deviation hstd of the asperity height hin a distribution with
an average (mean) asperity height have .
hstd ¼X
ðhave hiÞ2
Another measure to characterize surface roughness is the
correlation length of the asperity height, which gives an
indication of the directional dependence that is to be
expected for shear displacement, which affects fluid flow
through the fracture. The correlation length ncan be found
by studying the convergent behavior of the function H,
which is defined as
i¼1ðzðcxyÞzðcxy þrÞÞ2
where cxy is the spatial coordinate in the x- and y-direction
on the fracture surface, zis the asperity height at cxy and n
is the number of spatial locations that are at a distance of r
from cxy. The correlation function Hwas computed in the x
and y-direction, with the correlation length ndefined as the
point when Hdoes not increase further for r[n. For a
very small correlation length in the offset direction, the
effect of dilation due to the offset is expected to become
constant once the offset distance reaches the correlation
length. This has also been found by Yeo et al. (1998) and
Kim and Inoue (2003). Studies by Iwano and Einstein
(1993) and Hakami et al. (1993) determined the correlation
length in apertures of different rock types and linked
increasing correlation length to smaller areas of contact and
more pronounced flow channels.
The asperity distribution before and after testing shows
drastic changes when comparing the ranges observed for
the cumulative density functions of asperity height distri-
butions on the fracture surfaces (Fig. 11). Figure 11 shows
more comparable distributions of asperity heights across all
fracture surfaces before testing, with asperity height dis-
tributions becoming more heterogeneous after testing when
compared between all tested specimens. Photogrammetric
scans produced profiles of the surfaces, which were ori-
ented according to a best-fit plane in the xycoordinates.
Therefore, the mean asperity height is close to 0 (Figs. 8,
9). However, for visualization and comparison, an asperity
height of 0 mm is assigned to the lowest point of the sur-
face for Fig. 11. While asperity distribution for individual
specimens is similar to both fracture surfaces before test-
ing, the two sides show different distributions after testing.
This can be attributed to surface damage at individual
contact points occurring on the specimen side with the
lower asperity strength.
Surface damage also caused the measured correlation
lengths to change (Table 3). While the correlation function
tended to converge slightly slower for the mated tests (tests
1–4), convergence was changed more significantly for offset
tests, with the correlation length increasing (e.g., tests 7 and
8), or even decreasing for one of the surfaces (e.g., surface B
in test 9). An example of the correlation length function in x-
and y-direction for specimen 6 is shown in Fig. 12.
Transmissivity values for the maximum effective con-
fining pressure (varies between experiments) and for 1 MPa
after each load cycle are shown in Fig. 13a, b, respectively.
The marker size changes from early cycles (small circle) to
later load cycles (large circle), which enables one to observe
quantitative changes of the transmissivity with ongoing load
cycles and shear displacement. The impact of effective
confining pressure is more pronounced for mated fractures.
This becomes apparent when comparing Fig. 13a, b, where
specimen with shear offset show transmissivity changes
between maximum effective confining stress and at 1 MPa
confining stress of one order of magnitude or smaller.
Specimens in mated configuration, however, can show
D. Vogler et al.
transmissivity differences of up to two orders of magnitude
(e.g., specimens 3 and 4 in Fig. 13a, b). The transmissivity
behavior at maximum effective confining stresses does not
appear to be influenced by additional shear offset past 1 mm
in Fig. 13a, while there is a trend of increasing minimum
transmissivity values for 1 MPa effective confining pressure
after cycling (Fig. 13b). The general trend of transmissivity
decrease for ongoing load cycling is evident in all specimens
at maximum effective confining stress and at 1 MPa con-
fining stress after each cycle, with changes in magnitude
varying from half an order of magnitude (test 1, 0 mm shear
displacement) to three orders of magnitude (test 6, 2 mm
shear displacement).
4 Discussion
4.1 Fracture transmissivity
Two fundamental behaviors were observed during this
study. First, in the case of mated fractures the
transmissivity decreases rapidly during individual load
cycles for effective confining stresses below 10 MPa
(Fig. 4a–c). The minimum transmissivity observed during
each cycle only shows small changes significantly below
one order of magnitude between cycles. However, the
minimum transmissivity generally decreases with an
increasing number of load cycles (Fig. 4a–c). Secondly, in
the cases of offset fractures, the transmissivity does not
always decrease significantly during individual cycles, with
maximum differences between subsequent cycles of one
order of magnitude or lower (Fig. 4d, e). Exceptions are
cycles 2 and 6 in experiment 10 (Fig. 4e). Changes of
minimum transmissivity between cycles were more pro-
nounced than for individual cycles and mated specimen,
with observed changes of multiple orders of magnitude
(Fig. 4d, e).
The fact that fault gouge was produced (Fig. 7) during
cyclic loading suggests that the transmissivity decrease in
both cases is associated with the transport of fault gouge
material downstream after surface damage occurred.
Surface A Surface B
Test 1
Test 2
Test 4
Test 7
Test 10
Fig. 8 Surface scans before testing. Specimens from tests 1, 2, 4, 7
and 10 (top to bottom,ae) with surfaces A (left) and B right before
testing. A reference figure for the surface scan dimensions and
asperity height colorbar is shown in f(color figure online)
Surface A Surface B
Test 1
Test 2
Tes t 4
Test 7
Tes t 1 0
Fig. 9 Surface scans after testing. Specimens from tests 1, 2, 4, 7, 10
(top to bottom) with surfaces A (left) and B (right) before testing. A
reference figure for the surface scan dimensions and asperity height
colorbar is shown in f(color figure online)
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
Gouge material transport likely occurs during decreasing
confining pressures when the fracture aperture increases
again and gouge material that was previously lodged in
place can be transported downstream. This observed gouge
material production could potentially cause the observed
hysteretic behavior by subsequently clogging flow paths,
thus lowering fracture transmissivity (Fig. 4b–e).
The results for the mated fractures in Fig. 4show that
most of the transmissivity changes between load cycles
occured during the initial cycling (Fig. 4a–c). Mated spec-
imen have more contact area, which can be seen in their
aperture fields (Fig. 10a–c). This likely leads to less surface
damage as indicated by the small gouge production mea-
sured for mated specimen (Fig. 7). The smaller changes
observed in transmissivity behavior between load cycles for
mated specimen could therefore be linked to less pro-
nounced surface damage. During individual cycles, high
effective confining stresses greater than 20 MPa (Fig. 4b,
c) do not lead to further transmissivity decrease for mated
specimens. As mechanical aperture changes were recorded
for effective confining stresses above 20 MPa, this indicates
that additional changes in mechanical aperture (Fig. 6a, b)
do not affect fluid flow. One possible explanation for this is
a crucial change in flow regime from distributed flow across
the whole specimen width to channelized flow. This
hypothesis is supported by the converging hydraulic aper-
ture changes while mechanical apertures are still decreasing
(Fig. 5a, c, d). The slow decrease in transmissivity observed
in specimen 1 (Fig. 4a) is in contrast to specimens 2 and 4
(Fig. 4b, c). The aperture fields (Fig. 10a–c) obtained from
specimens before and after testing give indications that this
could also be related to surface damage. When comparing
aperture fields before and after testing for specimen 1, there
is no significant aperture decrease across the fracture
(Fig. 10a). Specimen 2 is a shear fracture and has smaller
aperture values across the fracture than other specimens,
both before and after testing (Fig. 10b). Significant surface
damage in form of a reduced aperture across the fracture is
apparent for specimen 4, however (Fig. 10c). Observed
magnitudes in transmissivity changes during cyclic loading
could therefore potentially be strongly linked to the specific
fracture and corresponding aperture field configura-
tions (Fig. 13a, b).
Transmissivity changes during load cycles are not as
pronounced for shear offset specimens. However, shear
offset specimens experience large transmissivity changes
between cycles. For shear offset fractures, the aperture
fields with small regions of low aperture suggest larger
contact stresses between the fracture surfaces (Fig. 10d, e).
While small transmissivity changes during each cycle
could stem from relatively open aperture fields (Fig. 10d,
e), the more pronounced changes between cycles could
Table 3 Correlation length in direction of the specimen axis (X) and
normal to axial direction (Y) before and after testing for fracture
surfaces A/B of each specimen, respectively
Test Pre Pre Post Post
Specimen XYXY
(–) (mm) (mm) (mm) (mm)
124;19 þ;þ27;28 þ;þ
221;20 14;14 25;24 12;12
328;26 13;þ27;28 12;12
416;18 08;10 17;21 10;11
520;21 10;10 20;21 09;11
625;23 15;þ26;25 13;þ
707;07 13;08 15;20 þ;þ
808;08 09;þ09;24 09;þ
925;26 þ;þ27;19 þ;þ
10 þ;þ14;12 15;þþ;þ
11 22;20 þ;þ25;20 þ;þ
12 16;18 08;10 17;21 10;11
Correlation lengths that did not converge over half of the total length
in x- and y-direction are marked with a þ
Pre-Testing Post-Testing
Test 1
Test 2
Tes t 4
Tes t 7
Test 10
Fig. 10 Aperture fields as derived from surface scans (Figs. 8,9).
Specimens from tests 1, 2, 4, 7 and 10 (top to bottom) with apertures before
(left)andafter(right) testing. Reference figure for the aperture field
dimensions and aperture size colorbar is shown in f(color figure online)
D. Vogler et al.
result from larger contact point stresses that could result in
asperity failure and wear product removal.
The transmissivity for mated and offset fractures is
always larger after the first cycle than after subsequent
cycles (Fig. 13b), and the general trend is a decrease in
transmissivity from the first cycle (smallest circle) to the
last cycle (largest cicle). The finding of no significant
increase in transmissivity after initial shear displacements
(i.e., 1 mm, see Fig. 13) is congruent with previous find-
ings by Kim and Inoue (2003) and Esaki et al. (1999).
In most cases, the transmissivity decrease after the first
cycle is considerably larger then decreases for later cycles,
which agrees with the previous literature on fracture clo-
sure and hydraulic aperture changes during repeated load-
ing (Witherspoon et al. 1980; Gentier et al. 2013). This
illustrates the importance of reservoir history on expected
reservoir performance. Repeated fluid injection under high
pressures leads to fracture opening, which can potentially
cause a redistribution of gouge material along the main
flow paths. Lowering of injection pressure may cause
asperities and gouge material to be further comminuted,
which could close flow pathways.
4.2 Aperture Changes
Changes in mechanical and hydraulic aperture from the
initial values at 1 MPa confining stress are shown in
Fig. 5a–d, while changes in mechanical aperture versus
effective confining stress are shown in Fig. 6a–d. Note that
here one marker symbol (.and /for increasing and
decreasing confining pressure, respectively) represents an
increase in confining stress of 1 MPa (up to 10 MPa con-
fining stress) or 2.5 MPa (between 10 and 68 MPa con-
fining stress), respectively. Generally, changes in amech are
more drastic than those for ahyd for all tests. Similar to the
changes in transmissivity (Figs. 4a–e, 6a–d), aperture
changes display hysteretic behavior. Generally, changes in
mechanical and hydraulic aperture are larger during
decreasing effective confining stress (Figs. 5a–d, 6a–d).
Mechanical and hydraulic aperture changes in tests 1, 7
and 10 can be categorized similarly (Fig. 5a, c, d). For low
confining stresses, mechanical aperture changes increase
strongly (Figs. 6a–d, 5a–d) until aperture changes (equiv-
alent to aperture closure) increase monotonically (Fig. 6a,
c, d) or converge against a constant value (Fig. 6b).
During initial mechanical aperture changes, the
hydraulic aperture remains relatively unaffected, but
changes strongly once initial mechanical aperture change
has occurred (Fig. 5a–d). This change in behavior occurs
between mechanical aperture changes between 0.15 and
0.25 mm (test 1), 0.5 and 0.6 mm (test 7) and 0.1 and
0.2 mm (test 10) amech . The sudden decrease of ahyd could
be caused by the transition from uniform flow through most
of the fracture to channelized flow. Hydraulic aperture
changes become smaller after this increase in ahyd change
despite mechanical aperture changes increasing further
with effective confining stress (Fig. 6a, c, d). This behavior
can be explained by fluid flow confined to individual flow
channels, which are not closed despite further increase in
mechanical aperture changes. Once the maximum confin-
ing stress is reached and confining pressure is lowered
again, the mechanical aperture amech change reverts slower
than during initial loading (Fig. 6a–d). However, when
comparing changes in amech to ahyd (Fig. 5a, c, d), the
mechanical aperture change recovers much faster than the
hydraulic aperture change after peak loading. With fluid
flow potentially concentrated within single channels,
increasing amech (i.e., decreasing amech changes) would not
lead to a redistribution of fluid flow since the contact area
of the fracture surface is where the initial comminution of
asperities and the resulting formation of gouge material
would be located. Once amech opens sufficiently for the
failed asperties to be flushed from the system, ahyd
decreases significantly between a change in the mechanical
aperture of 0.2 and 0.4 mm. Mechanical aperture changes
do not decrease more rapidly during that regime (Fig. 6a–
d), since the gouge material is only flushed from the system
if compressive stresses on the gouge material are low and
therefore do not significantly contribute to the continued
propping of fractures. The rapid decrease in flow channels
for confining stress increases up to 10 MPa is consistent
with prior observations by Gentier et al. (2013).
4.3 Analysis of Gouge Material
Grain size distribution of the comminution products
(Fig. 7) organizes in two different families with a more
pronounced log-normal distribution for tests 5, 11 and 12
than for tests 6, 7, 8, 9 and 10. Tests 5, 11 and 12 have
small shear offsets (2, 1 and 1 mm, respectively), making
the breakout of monominerals much more likely than in the
other specimens, where large shear offsets lead to isolated
contact points that can cause more rupturing of asperities
during each loading cycle. With ongoing surface damage,
the number of contact points may be increased until the
local contact stress is then insufficient to overcome the
asperity strength. This hypothesis is consistent with the
analysis of mechanical and hydraulic aperture changes in
Sect. 4.2.
4.4 Surface Scans
Figures 8and 9compare the surfaces before and after
testing and show strong alterations in the surfaces, but not
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
decreasing asperity height during testing for all speci-
mens (Fig. 14). Due to the small length scale of the frac-
tures under investigation, the breaking of individual grains
and asperities can lead to fractures that fill with gouge
material, but do not develop better correlation between the
two sides than before testing. This phenomenon is influ-
enced by the displacement of the two fracture surfaces
against each other. Displacement offsets of a few mil-
limeters may lead to more point loads than in mated
specimens (Fig. 10a–e), which can cause higher stresses in
grains and asperities that can lead to failure, as mentioned
above. The relationship between the decrease in contact
area upon displacement can be related to the correlation
length in the x-direction (Table 3), which influences the
dilatancy of the fracture surfaces upon shear displacement.
The large correlation lengths indicated in Table 3could
suggest that mechanical apertures should continually
increase even for large shear displacements. However,
most specimens show a strong increase of the correlation
function H in the first few millimeters of specimen offset
(e.g., Fig. 12), with convergence only occurring for large
correlation lengths beyond the specimen scale. This indi-
cates that the correlation decreases strongly during the first
few millimeters of shear displacement and has no large
effect for displacements larger than 5 or 10 mm as the
correlation length has not converged yet. While the cor-
relation length can be used as an indicator after which shear
displacement of an offset specimen will not see an aperture
increase anymore, it cannot be used to estimate the amount
of contact area for a given displacement, which is what
ultimately effects contact stresses between the two speci-
men sides. The contact area plays a very significant role
since this determines the likelihood of surface damage in
Asperity Height [mm]
CDF [-]
Asperity Height [mm]
CDF [-]
Normalized Asperity Height [-]
0 0.5 1
CDF [-]
Normalized Asperity Height [-]
0 0.5 1
CDF [-]
Fig. 11 CDF of asperity height (a) and asperity height normalized to
the maximum asperity height (b) for specimens 1–12 for fracture
sides A and B before (left) and after (right) testing
r [mm]
0 5 10 15 20 25 30
H(z) [mm2]
r [mm]
0 5 10 15
H(z) [mm2]
Fig. 12 Example of the correlation length function in x- (left) and
y-direction (right) for specimen surface side A of test 6
Fig. 13 Transmissivity values for all normal loads versus specimen
offset for all specimens. Shown is one marker for each load cycle at:
amaximum effective confining stress and b1 MPa confining stress at
the end of each load cycle. Cyclic loading progresses from the first
cycle (smallest circle) to the last cycle (largest circle)
D. Vogler et al.
the fracture for a given normal load, which can counteract
changes in mechanical aperture caused by an increase in
offset of a millimeter or two more. Therefore, we deduce
that the correlation length is only a suitable metric to
determine transmissivity in fractures for the first few mil-
limeters of shear offset, with the amount of contact area
being more significant for defining the important role of
fracture damage under high normal loads.
Figure 11 shows a wider distribution of asperity heights
after testing when comparing the values for the CDF at 0.5
and 1.0. While all but two fracture surfaces in Fig. 11a
(left) have a maximum asperity height of 2 mm for the
midpoint of the CDF before testing, more than half of all
specimens surpasses this value for fracture surfaces after
testing. Asperity height distributions across specimens
show (Fig. 11) that maximum asperity height differences
become more pronounced after testing, showing larger
maximum values for asperity height and a weaker resem-
blance of the CDF between individual specimens. The
effect of increased local normal stresses observed in
Fig. 11a and b (e.g., Fig. 9c–e with strong surface changes)
shows that repetitive loading does not necessarily lead to
smoothed out asperity distributions and surfaces. While a
few specimen surfaces became more homogeneous, it was
observed on tests on offset specimens that cracks formed as
a consequence of normal loading and the associated sur-
faces were fairly rough. Such cracks are shown in Fig. 9c
(on surface B) where a crack parallel to the y-direction is
visible at an xvalue of around 20 mm and in Fig. 9e (on
surface A) where a crack parallel to the y-direction is
visible at an xvalue of around 10 mm. Mineral inclusions
of quartz and felspar in granodiorite have a high strength,
which makes crack propagation more likely to occur at the
boundaries with lower strength in between grains. Grain
boundaries and grain size will then dominate the effect of
surface deformation after increases in confining stress.
Iwano and Einstein found evidence that asperity distribu-
tion in tensile fractures strongly depends on the overall
grain size (Iwano and Einstein, 1993). Mineral grain sizes
of intact core material were estimated to be between 2 and
7 mm, which can explain the large asperity heights after
testing (Fig. 11b).
These findings indicate the influence of surface damage
on fracture transmissivity, especially for fractures that
experienced shear offset. Gouge production induced by
surface damage can strongly counteract the transmissivity
increase produced by shear offset (Figs.12,13,14).
5 Conclusions
Granodiorite specimens with natural tensile and shear
fractures were subjected to cyclic loading between 1 and
68 MPa confining pressure. Constant fluid flow rates
through the specimen were established, which made it
possible to measure the fluid pressure response to con-
fining pressure. A total of 12 tests were performed, with
four specimens tested in a mated configuration and eight
specimens tested with shear displacement between 1 and
6 mm. Fracture surfaces were scanned before and after
testing to give insight into surface deformation during
testing. The gouge material produced by asperity damage
was collected to study the impact of a gouge layer on
While all specimens showed a decrease in transmissivity
with increased confining pressure, transmissivity decrease
in mated and offset specimens shows fundamentally dif-
ferent behavior. Mated specimens only show strong
decrease during individual load cycles for small confining
pressures, suggesting fluid flow confined to channels for
high confining pressures. Offset specimens show signifi-
cant transmissivity decrease between load cycles and sur-
face damage which leads to gouge production.
Permeability generally decreased with ongoing loading
cycles, indicating nonelastic deformation of the fracture
surfaces. Specimens 2, 4, 7 and 10 showed hysteretic
effects during individual loading cycles, with lower per-
meabilities during unloading of the specimen for respective
effective confining pressures. During testing, the normal
loads were large enough to keep most gouge material in
place, which led to increased fluid pressures to sustain
constant flow rates. It is hypothesized that breaking
asperities, which are only flushed out of the system during
decreasing normal loads, contribute strongly to the hys-
teretic transmissivity behavior observed during tests.
sample [-]
asperity height STD [mm]
Pre (A)
Pre (B)
Post (A)
Post (B)
Fig. 14 Comparison of the standard deviation of asperity height on
the surfaces A and B before (red marker) and after testing (blue
marker) (color figure online)
Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
Mechanical and hydraulic aperture changes are compared,
showing more pronounced changes in the mechanical aper-
ture for all tests. Mechanical apertures change most drasti-
cally for low confining stresses. The most pronounced
changes in hydraulic apertures occur after initial mechanical
aperture closure can be observed. This could be explained by
mechanical aperture closure causing fluid flow displacement
to individual flow channels. This behavior changes again for
high confining pressures, when mechanical aperture changes
(i.e., closes) further while the hydraulic aperture does not
change significantly. These nonlinear changes in the rela-
tionship of mechanical and hydraulic aperture changes can be
attributed to increased surface damage and fracture closure
for high confining pressures, while fluid flow is already
confined to channel flow and is not strongly affected by
compression of the fracture.
This work illustrates the importance of fatigue behavior,
which is especially crucial for EGS with reservoir opera-
tion times of multiple decades. During operation of a
reservoir, interruption of injection wells can lead to cyclic
lowering and raising of the effective normal stress that
fractures are subjected to. On the laboratory scale, this
study shows differences of up to three orders of magnitude
of the transmissivity, which suggests that the history of the
fracture surfaces in a reservoir plays a significant role in the
evolution of fracture transmissivity after initial stimulation.
Acknowledgments The authors want to thank two anonymous
reviewers for their constructive suggestions which helped to improve
this work. The authors further want to thank the National Cooperative
for the Disposal of Radioactive Waste (Nagra), Switzerland, and the
CRIEPI fractured rock study Takana et al. (2014) for providing us
with the specimen material for our study. The authors further want to
thank the chair of geosensors and engineering geodesy at ETH Zurich
for their support with the photogrammetry scanner. This work was
partially supported by the GEOTHERM II project, which is funded by
the Competence Center Environment and Sustainability of the ETH
Domain. This project benefitted from partial funding from DOE DE-
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Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
... Extracting geothermal energy from underground is of major interest in the transition from energy recovered from conventional resources such as coal or oil towards renewable energies (Vogler et al. 2016). Geothermal energy is expected to have a great potential to meet future energy demands. ...
... These are believed to be connected to pre-existing natural fractures in the reservoir (Li et al. 2015). The efficiency and sustainability of an EGS are critically dependent on sufficient water flow through the fracture network and on the conductive properties of the separate fractures (Vogler et al. 2016). This prerequisite, in addition to a relatively high geothermal gradient, is necessary to successfully run an EGS over several years (Milsch et al. 2008;Voltolini and Ajo-Franklin 2020). ...
... Injecting fluid into a fluid-bearing fracture with a different composition may lead to dissolution-precipitation reactions due to local changes in the chemical equilibrium, which typically results in a decrease in fracture transmissivity (Gutierrez et al. 2000;Milsch et al. 2008;Cheng and Milsch 2020b;Cheng et al. 2021). The transmissivity may 105 be also reduced by the clogging of flow channels due to the migration of fine particles, e.g., clay (Carey et al. 2015;Zhang et al. 2015;Vogler et al. 2016;Walsh et al. 2016) or by the production of a fine-grained gouge layer resulting from shear displacement (Rutter and Mecklenburgh 2018). ...
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Deep geological repositories represent a promising solution for the final disposal of nuclear waste. Due to its low permeability, high sorption capacity and self-sealing potential, Opalinus Clay (OPA) is considered a suitable host rock formation for the long-term storage of nuclear waste in Switzerland and Germany. However, the clay formation is characterized by compositional and structural variabilities including the occurrence of carbonate- and quartz-rich layers, pronounced bedding planes as well as tectonic elements such as pre-existing fault zones and fractures, suggesting heterogeneous rock mass properties. Characterizing the heterogeneity of host rock properties is therefore essential for safety predictions of future repositories. This includes a detailed understanding of the mechanical and hydraulic properties, deformation behavior and the underlying deformation processes for an improved assessment of the sealing integrity and long-term safety of a deep repository in OPA. Against this background, this thesis presents the results of deformation experiments performed on intact and artificially fractured specimens of the quartz-rich, sandy and clay-rich, shaly facies of OPA. The experiments focus on the influence of mineralogical composition on the deformation behavior as well as the reactivation and sealing properties of pre-existing faults and fractures at different boundary conditions (e.g., pressure, temperature, strain rate). The anisotropic mechanical properties of the sandy facies of OPA are presented in the first section, which were determined from triaxial deformation experiments using dried and resaturated samples loaded at 0°, 45° and 90° to the bedding plane orientation. A Paterson-type deformation apparatus was used that allowed to investigate how the deformation behavior is influenced by the variation of confining pressure (50 – 100 MPa), temperature (25 – 200 °C), and strain rate (1 × 10-3 – 5 × 10-6 s-1). Constant strain rate experiments revealed brittle to semi-brittle deformation behavior of the sandy facies at the applied conditions. Deformation behavior showed a strong dependence on confining pressure, degree of water saturation as well as bedding orientation, whereas the variation of temperature and strain rate had no significant effect on deformation. Furthermore, the sandy facies displays higher strength and stiffness compared to the clay-rich shaly facies deformed at similar conditions by Nüesch (1991). From the obtained results it can be concluded that cataclastic mechanisms dominate the short-term deformation behavior of dried samples from both facies up to elevated pressure (<200 MPa) and temperature (<200 °C) conditions. The second part presents triaxial deformation tests that were performed to investigate how structural discontinuities affect the deformation behavior of OPA and how the reactivation of preexisting faults is influenced by mineral composition and confining pressure. To this end, dried cylindrical samples of the sandy and shaly facies of OPA were used, which contained a saw-cut fracture oriented at 30° to the long axis. After hydrostatic pre-compaction at 50 MPa, constant strain rate deformation tests were performed at confining pressures of 5, 20 or 35 MPa. With increasing confinement, a gradual transition from brittle, highly localized fault slip including a stress drop at fault reactivation to semi-brittle deformation behavior, characterized by increasing delocalization and non-linear strain hardening without dynamic fault reactivation, can be observed. Brittle localization was limited by the confining pressure at which the fault strength exceeded the matrix yield strength, above which strain partitioning between localized fault slip and distributed matrix deformation occurred. The sandy facies displayed a slightly higher friction coefficient (≈0.48) compared to the shaly facies (≈0.4). In addition, slide-hold-slide tests were conducted, revealing negative or negligible frictional strengthening, which suggests stable creep and long-term weakness of faults in both facies of OPA. The conducted experiments demonstrate that dilatant brittle fault reactivation in OPA may be favored at high overconsolidation ratios and shallow depths, increasing the risk of seismic hazard and the creation of fluid pathways. The final section illustrates how the sealing capacity of fractures in OPA is affected by mineral composition. Triaxial flow-through experiments using Argon-gas were performed with dried samples from the sandy and shaly facies of OPA containing a roughened, artificial fracture. Slate, graywacke, quartzite, natural fault gouge, and granite samples were also tested to highlight the influence of normal stress, mineralogy and diagenesis on the sustainability of fracture transmissivity. With increasing normal stress, a non-linear decrease of fracture transmissivity can be observed that resulted in a permanent reduction of transmissivity after stress release. The transmissivity of rocks with a high portion of strong minerals (e.g., quartz) and high unconfined compressive strength was less sensitive to stress changes. In accordance with this, the sandy facies of OPA displayed a higher initial transmissivity that was less sensitive to stress changes compared to the shaly facies. However, transmissivity of rigid slate was less sensitive to stress changes than the sandy facies of OPA, although the slate is characterized by a higher phyllosilicate content. This demonstrates that in addition to mineral composition, other factors such as the degree of metamorphism, cementation and consolidation have to be considered when evaluating the sealing capacity of phyllosilicate-rich rocks. The results of this thesis highlighted the role of confining pressure on the failure behavior of intact and artificially fractured OPA. Although the quartz-rich sandy facies may be considered as being more favorable for underground constructions due to its higher shear strength and stiffness than the shaly facies, the results indicate that when fractures develop in the sandy facies, they are more conductive and remain more permeable compared to fractures in the clay-dominated shaly facies at a given stress. The results may provide the basis for constitutive models to predict the integrity and evolution of a future repository. Clearly, the influence of composition and consolidation, e.g., by geological burial and uplift, on the mechanical sealing behavior of OPA highlights the need for a detailed site-specific material characterization for a future repository.
... 9,25-30 Moreover, the fracture slip leads to gouge particles formation which could decrease the permeability by blocking the flow path in the fracture. 31 Many numerical studies have investigated the fault (or fracture) activation as a result of change in poroelastic stress, static stress transfer, migration of seismicity. 1,3 However, there are still a lot of uncertainties regarding the magnitude of stress perturbation required to induce the seismic events, 32,33 and the distance of influence of the stress perturbation. ...
... Gouge particles formed due to abrasion and crushing of fracture asperities during triaxial shear experiments and flow through experiments have been reported in previous studies. 9,11,31 Gouge particles were only observed in the case of rough fractures, which is indication of the asperity's degradation in the rough fractures. Larger amount of gouge particles was observed in case of the tensional fault compared to that in the compressional fault (Fig. 12). ...
Anthropogenic activities such as underground fluid injection/extraction, well stimulation, and dam impoundment will perturb the state-of-stress in the subsurface, which could activate critically stressed pre-existing fractures and faults. Fault and fracture activation can induce seismicity, change flow behavior, and damage asperities on the fracture surface to alter reservoir permeability. In this study, triaxial shear tests were conducted on saw cut and rough fracture Barre granite specimens to better characterize how a decrease in normal stress and increase in shear stress will influence crystalline rock fracture hydromechanical properties. In a triaxial set up, confining pressure (CP) and differential stress (DS) were used to induce normal and shear stress on a 45° oriented fracture. Fracture slip and the corresponding stress relaxation was observed when the confining stress decreased. Fracture permeability increased when the fracture slipped. Moreover, new micro-cracks observed by x-ray images indicated the creation of new flow paths for fluid movement. The experimental outcomes of this study indicate that decrease in normal stress due to stress perturbations along the faults/fractures in the extensional stress regime can induce the slip along the fracture. This could have a significant influence on slope stability in quarries, stimulation and injection strategy in deep geo-resources, and potentially minimize the risk of induced seismicity. The observed stress dependency of hydraulic aperture in rough fracture during pre- and post-slip stages could provide key inputs for field-scale simulation of geo-resources.
... This stress-path dependency has been associated with inelastic deformation mechanisms due to which a portion of the pores and cracks do not fully re-open during unloading thus causing a reduction in permeability between the first and subsequent loading cycles. These inelastic processes have been attributed to: (a) regions of oblique contact between the fracture surfaces were shear stresses develop locally, causing frictional sliding at the micro scale (Scholz & Hickman, 1983), (b) plastic (Yoshioka, 1994) and/or brittle deformation of asperities coming into contact, leading to a permanent increase in contact area, (c) clogging of the fluid flow channels by fine-grained material produced from crushed asperities (Vogler et al., 2016) and (d) time-dependant fracture closure . The question of which of these corresponds to the dominant process responsible for fracture permeability hysteresis remains enigmatic. ...
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Fluid flow through the brittle crust is primarily controlled by the capability of fracture networks to provide pathways for fluid transport. The dominant permeability orientation within fractured rock masses has been consistently correlated with the development of fracture intersections; an observation also made at the meso‐regional scale. Despite the importance attributed to fracture intersections in promoting fluid flow, the magnitude of their enhancement of fractured rock permeability has not yet been quantified. Here, we characterize the hydro‐mechanical properties of intersections in samples of Seljadalur Basalt by generating two orthogonal, tensile fractures produced by two separate loadings using a Brazilian test apparatus, and measuring their permeability as a function of hydrostatic pressure. We observe that intersecting fractures are significantly more permeable and less compliant than two independent macro‐fractures. We formulate a model for fracture intersection permeability as a function of pressure by adding the contributions of two independent fractures plus a tube‐like cavity with an effective elastic compressibility determined by its geometry. Permeability measurements during cyclic loading allowed determination of the effective stress coefficient (α in pe = pc − αpp) for fracture and intersection permeability. We observe a trend of lower αintersection values with respect to αfracture, which suggests that the channels controlling fluid flow have a higher aspect ratio (are more tubular) for the intersections relative to independent fractures. Our results suggest that fracture intersections play a critical role in maintaining permeability at depth, which has significant implications for the quantification and upscaling of fracture permeability toward reservoir‐scale simulations.
... Most laboratory shear-flow tests on fractures were based on the force-driven fracture slip (Esaki et al. 1999;Gutierrez et al. 2000;Li et al. 2008;Olsson and Barton 2001;Wenning et al. 2019;Zhang et al. 2017) or the fracture slip generated by manually displacing the fractured specimen (Crawford et al. 2017;Ishibashi et al. 2015;Vogler et al. 2016). The ...
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The increased fluid pressure in faults/fractures can trigger the decrease of shear strength and induce the faults/fractures to become instable. A series of novel fluid-induced fracture slip experiments are conducted in laboratory on the prefabricated flat fracture under various normal pressures. Our experimental results show that the effective normal stress and shear strength of fractures decrease as the fluid pressure increases. However, for the fracture with high normal stress or smooth surface, the relationship between fluid pressure and effective stress is not suitable for the conventional effective stress principle. With the increase of fluid pressure, the fractures in the critical stress state experience three slip intervals, namely, quasi-static slip (slip velocity < 1 μm/s), slow slip (1 μm/s < slip velocity < 4 μm/s), and rapid slip (slip velocity > 4 μm/s). The evolution of fracture permeability in the slip process is determined by the slip state of fracture. When the fractures are in “quasi-static” slip or slow slip interval, even if the fluid pressure increases, the increase in fracture permeability is limited, or even slightly decreases. Once rapid slip initiates, the fracture permeability exhibit larger permeability enhancement. Our experimental results imply that the areas with low in-situ stress should be paid more attention to, in deep underground engineering that under high water pressure.
... (4.1)) validity, and leads to the disparity observed between b m and b h . The (CL) b h is consistently smaller than the (micro-CT) b m (Figure 4.5i), aligning with studies suggesting these apertures cannot be equated (Olsson and Barton, 2001;Vogler et al., 2016). b h reduction (42 %) is smaller than that of b m (50.8 %) from 5 to 13.8 bar σ' due to larger apertures being forced to close, while, the void configuration between stressed asperities enables preferential flow channels to exist (Kang et al., 2016;Nemoto et al., 2009), yielding comparably smaller b h changes. ...
... These test specimens are primarily square specimens, and the maximum normal stress and shear displacement can reach 60 MPa and 20 mm, respectively, but due to a lack of proper sealing around the shear box, the maximum seepage pressure that can be applied in the test can reach 0.6−1 MPa. In addition, the damage of the fracture surface (Boulon et al. 1993;Liu et al. 2016), accumulation of filling material (Koyama et al. 2009;Vogler et al. 2016), and evolution of the geometric characteristics of the fracture surface (Xiong et al. 2011;Gui et al. 2017;Mofakham et al. 2018;Li et al. 2019) are the roughness mechanisms affecting the shear-seepage coupling characteristics (Rong et al. 2018). ...
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To reveal the shear-seepage coupling characteristics of fractured specimens under cyclic loading and unloading, the specific test device and test method were designed in this study. The cyclic loading and unloading shear-seepage coupling test on the fractured rock mass under different confining pressures and seepage pressures was carried out by processing “double L-shaped” specimens, and the change laws of the shear characteristics and seepage characteristics of fractured specimens with different roughness were experimentally investigated. The results indicated that the peak shear stress, residual shear stress, and shear stiffness of rough fractures all increase with increasing confining pressure, while the change in normal dilatation displacement is the opposite. Under a constant normal stress, the permeability of rough fracture decreases, increase, and then stabilizes with increasing shear displacement. The peak shear stress of the smooth fracture is 3.7 times lower than that of the rough fracture with the same shear displacement, and the smooth sandstone specimens are all in a shear shrinkage state, with the normal shrinkage displacement of less than 1.0 mm. In addition, during unloading, permeability increases to some extent but cannot recover to the original value. The confining pressure causes permanent damage to the permeability of fractured rock mass. The permeability of sandstone specimens changes primarily in the early loading stage and late unloading stage. Based on the test results, the relationship between permeability and confining pressure follows a negative exponential function under cyclic loading and unloading conditions.
Fluid injection into rock masses is involved during various subsurface engineering applications. However, elevated fluid pressure, induced by injection, can trigger shear slip(s) of pre-existing natural fractures, resulting in changes of the rock mass permeability and thus injectivity. However, the mechanism of slip-induced permeability variation, particularly when subjected to multiple slips, is still not fully understood. In this study, we performed laboratory experiments to investigate the fracture permeability evolution induced by shear slip in both saw-cut and natural fractures with rough surfaces. Our experiments show that compared to saw-cut fractures, natural fractures show much small effective stress when the slips induced by triggering fluid pressures, likely due to the much rougher surface of the natural fractures. For natural fractures, we observed that a critical shear displacement value in the relationship between permeability and accumulative shear displacement: the permeability of natural fractures initially increases, followed by a permeability decrease after the accumulative shear displacement reaches a critical shear displacement value. For the saw-cut fractures, there is no consistent change in the measured permeability versus the accumulative shear displacement, but the first slip event often induces the largest shear displacement and associated permeability changes. The produced gouge material suggests that rock surface damage occurs during multiple slips, although, unfortunately, our experiments did not allow quantitatively continuous monitoring of fracture surface property changes. Thus, we attribute the slip-induced permeability evolution to the interplay between permeability reductions, due to damages of fracture asperities, and permeability enhancements, caused by shear dilation, depending on the scale of the shear displacement.
Sustainable and profitable energy production from Enhanced Geothermal System (EGS) requires a comprehensive understanding of the coupled effects of elastic and plastic deformation on the hydraulic evolution of geothermal fractures. Four flow-through tests were conducted on granite samples with a rough single fracture at 25–180 °C. Each test was performed on three cycles of loading-unloading processes within a confining pressure of 5–30 MPa. Experimental results indicated that the hydraulic properties are negatively correlated with confining pressure in a logarithmic manner. The coupled effects of elastic and plastic deformation induced by stress loading are the main factor affecting fracture hydraulic properties. Plastic deformation is associated with the mineral grains crush occurring in fracture contacting asperities, which is a permanent and irreversible process. Compared to the results at 25 °C, a larger reduction in permeability is observed at 180 °C, as revealed by a maximum reduction in hydraulic aperture up to >40% at this temperature. This means that larger plastic deformations in the fractures are associated with higher temperatures. Therefore, the addition of proppant is very important for sustainable geothermal development for fractured geothermal reservoirs under high-stress conditions. The ion concentration detection of the effluent solution confirms the existence of free-face dissolution in high-temperature scenarios. Generally, free-face dissolution is beneficial to low-permeability geothermal reservoirs due to its positive effects on fracture hydraulic properties. As the number of loading-unloading cycles increases, the hysteresis effect induced by plastic deformation becomes less and less. Compared to the first loading-unloading stage, the maximum decline of permeability decreases from 73% to 6% and 1% during the second and third stages, respectively. However, pressure dissolution under long-term stress loading should be further investigated, because it may disrupt the self-propping balance of geothermal fracture asperities.
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We determine the evolution of frictional strength, strain weakening behavior and permeability in fractures subject to dissolution and precipitation. We establish these relations through slide-hold-slide experiments, with hold times from 10 to 3000 s, on split limestone core, under hydraulically open and closed conditions. Fracture friction and permeability are measured continuously throughout the experiments. The limestone displays velocity-strengthening behavior (stable slip) under incremented velocity steps of 1-6 μm/s. Frictional healing is observed to be time- and stress-dependent, showing higher gains in strength at both longer hold times and under lower effective stresses. Activation of healing is greater in wet samples than in dry samples. Flow-through experiments for flow rates in the range of 1-10 ml/min are conducted to further investigate the role of flow and mineral redistribution in contributing to healing. These experiments show strength gains are lower at higher flow rates where advective mineral dissolution and redistribution is enhanced and cementation concomitantly limited. Concurrently measured permeability decreases throughout the slide-hold-slide sequences indicating that mean fracture aperture reduces during sliding. We combine models representing pressure solution and stress corrosion as models for the growth in fracture contact area and represent the observed time-dependent behavior of strength gain and permeability evolution. The simulated results represent the observed strength gain at long hold times (~1000 s), but underestimate strengthening at short hold times. We conclude that the evolution of strength and permeability are significantly controlled by mechanisms of fluid-rock interactions and that the strengths and nature of feedbacks on these linkages are critical in understanding the mechanical and hydraulic behavior of faults.
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A new type apparatus for shear-flow coupling test, named coupled shear-flow-visualization apparatus, was developed to investigate the hydromechanical behavior and the fracturing process of intact soft sedimentary rock. With this apparatus, it is possible to simultaneously carry out direct shear test and constant head flow test, and also to observe specimen surface during the experiment. Under controlled shear load, normal load, flow rate and shear displacement can be obtained as raw data. Moreover, fracture area and fracture apertures can be estimated using image-processing techniques. The shear and normal stress capacities of load cells are 4.08 MPa and 3.33 MPa, respectively. The measurable maximum permeability of apparatus is about 5.80×10−3 cm/s under a hydraulic gradient of 40 cm/cm. According to the observation of visible fractures on specimen surface, the fractures were not completely propagated at the peak of shear stress. The developed testing method with image processing techniques enabled us to analyze the relationships between fracture flow rate, hydraulic aperture and two fracture apertures defined in this study.
The paper deals with the phenomenon of flow in jointed media and the stresses transmitted by seeping water on fissured rock. Systems of plane parallel joints are investigated. Such systems exist in the majority of practical cases. It was shown that flow is governed mainly by the geometry of the joint systems and that the permeability of rock material compared to that of the joints is negligible small. The factors influencing the geometry of the joint systems are described. With the help of theoretical hydraulics and model tests, flow equations are developed for seepage through openor filled joints. The form and the surface roughness of the joints are taken into consideration. Based on this, graphical and analytical methods for the determination of the potential distribution in jointed media are developed. For a given potential distribution and joint system, the stresses transmitted by the seeping water on the rock are determined for the three dimensional case. These stresses are composed of shear forces, seepage pressures and buoyancy.
Flow and stress transmitted by seeping water on fissured rock; systems of plane parallel joints are investigated; flow equations are desired for seepage through open or filled joints; form and surface roughness of joints are taken into consideration.
The classical concept of hydraulic fracturing is that a single, planar, opening mode fracture propagates through the formation. In recent years, there has been a growing consensus that natural fractures play an important role during stimulation in many settings. There is not universal agreement on the mechanisms by which natural fractures affect stimulation, and these mechanisms may vary depending on formation properties. One potentially important mechanism is shear stimulation, in which increased fluid pressure induces slip and permeability enhancement on pre-existing fractures. We propose a tendency-for-shear-stimulation (TSS) test as a direct, relatively unambiguous method for determining the degree to which shear stimulation contributes to stimulation in a formation. In a TSS test, fluid injection is performed while maintaining the bottomhole fluid pressure slightly less than the minimum principal stress. Under these conditions, shear stimulation is the only possible mechanism for permeability enhancement (except, perhaps, thermally induced tensile fracturing). A TSS test is different from a conventional procedure because injection is performed at a specified pressure (rather than a specified rate). With injection at a specified rate, fluid pressure may exceed the minimum principal stress, and it may cause tensile fractures to propagate through the formation. If this occurs, it will be ambiguous whether stimulation was because of shear stimulation or tensile fracturing. Maintaining pressure less than the minimum principal stress ensures that the effect of shear stimulation can be isolated. Low-rate injectivity tests could be performed before and after the TSS test to estimate formation permeability. An increase in formation permeability would indicate that shear stimulation has occurred. The flow-rate transient during injection may also be interpreted to identify shear stimulation. Numerical simulations of shear stimulation were performed with a discrete-fracture-network (DFN) simulator that couples fluid flow with the stresses induced by fracture deformation. These simulations were used to qualitatively investigate how shear stimulation and fracture connectivity affect the results of a TSS test. Two specific field projects are discussed as examples of a TSS test, the Enhanced Geothermal Systems (EGS) projects at Desert Peak, Nevada, and Soultz-sous- Forêts, France.
This paper summarizes more than a decade's research at BRGM on the hydromechanical behavior of natural fractures in granite under normal and shear stress. The paper's emphasis is on the importance of understanding the role of fracture geometry in fluid flow and, in particular, the evolution of fracture flow paths with changes in stress. Experimental results were obtained by modifying classical hydromechanical tests to allow detailed analysis of fracture geometry under zero load and of the spatial organization of flow. Fracture-wall geometry is analyzed using profilometry; a casting methodology is used to determine the geometry of the fracture's void space. Tracer tests show that the decrease in transmissivity that occurs with increasing normal stress is associated with increasingly distinct channeling. This channeling is strongly linked to correlation lengths identified from geostatistical analysis of surface profiles and data obtained from the casts of fracture void space. Modeling results show that deformation of fracture surfaces with increasing normal stress causes substantial, nonuniform changes in void-space geometry that can change the flow regime. To better understand the mechanical behavior of fractures under shear stress, image analysis techniques are used to identify geometrical parameters that affect the micromechanical behavior and the evolution of damage zones during shearing. Laboratory experiments indicate that a fracture's mechanical response to shear stress can be broken down into at least five phases, which are shown to be associated with changes in flow. In general, application of shear stress induces an opening of the fracture, sometimes preceded by a closure phase, that causes a very large increase in global transmissivity that is associated with a reorientation of flow subperpendicular to the shear direction. Reorientation culminates just after peak shear stress is reached. During the subsequent softening and residual phases, flow tends to return to a more isotropic pattern.
We report laboratory experiments to investigate the role of gas desorption, stress level and loading rate on the mechanical behavior of methane infiltrated coal. Two suites of experiments are carried out. The first suite of experiments is conducted on coal (Lower Kittanning seam, West Virginia) at a confining stress of 2 MPa and methane pore pressures in the fracture of 1 MPa to examine the role of gas desorption. These include three undrained (hydraulically closed) experiments with different pore pressure distributions in the coal, namely, overpressured, normally pressured and underpressured, and one specimen under drained condition. Based on the experimental results, we find quantitative evidence that gas desorption weakens coal through two mechanisms: (1) reducing effective stress controlled by the ratio of gas desorption rate over the drainage rate, and (2) crushing coal due to the internal gas energy release controlled by gas composition, pressure and content. The second suite of experiments is conducted on coal (Upper B seam, Colorado) at confining stresses of 2 and 4 MPa, with pore pressures of 1 and 3 MPa, under underpressured and drained condition with three different loading rates to study the role of stress level and loading rate. We find that the Biot coefficient of coal specimens is <1. Reducing effective confining stress decreases the elastic modulus and strength of coal. This study has important implications for the stability of underground coal seams.