1. INTRODUCTION
Brazilian tests are commonly used in petroleum rock
mechanics for estimation of tensile strength [1, 2]. Such
tests do however also capture basic information of how
fractures initiate and grow on the mm to cm scale, and
this may have relevance for hydraulic fracture initiation
and growth on the scales of a well within a reservoir. Of
particular interest is the role of stress anisotropy versus
textural rock anisotropy in gas shales, in order to
understand, predict and optimize the fracturing process.
In the field, natural fractures increase the complexity of
the problem, whereas here, we concentrate on laboratory
experiments with intact samples of an outcrop (Mancos)
shale.
Fracture patterns observed through the presence of a
central diametric crack provide no “real time”
information regarding the crack initiation process. For
anisotropic samples when the bedding is inclined to the
load axis, the locus of fracture initiation between the
load points is uncertain [3]. There is both a basic and an
1 Current address: ExxonMobil, Stavanger, Norway
applied research interest in learning where the first crack
initiates and how it propagates [4]. For many years,
acoustic emission (AE) monitoring techniques have been
used in order to detect the processes that accompany
fracture initiation in isotropic materials [5]. In addition,
conventional video cameras have been used in some
studies to observe fracturing effects [6], however
depending on the rock type, conventional video (25-100
frames per second) is not sufficient to monitor in detail
the fracture initiation point and fracture growth [5]. In
this study the fracture initiation point for Mancos Shale
was captured through the use of a high-speed camera
(filmed at 5,000 frames per second) supported by a
mounted acoustic emission set up.
In the following, an overview of current theoretical
understanding of crack initiation in Brazilian tests is
given in Section 2. Mancos Shale is described in Section
3, focusing on its anisotropic and also heterogeneous
character, along with the experimental set-up and the
applied test procedures. In Section 4, test results are
presented for samples with different orientations. The
Brazilian tensile strength estimates are shown as a
function of the angle between load direction and the
ARMA 14-7399
Failure Mechanics of Anisotropic Shale
during Brazilian Tests
Simpson, N.D.J.1
DTU, Lyngby, Denmark and NTNU, Trondheim, Norway
Stroisz, A. and Bauer, A.
SINTEF Petroleum Research, Trondheim, Norway
Vervoort, A.
KU Leuven, Belgium and SINTEF Petroleum Research, Trondheim, Norway
Holt, R.M.
NTNU, Trondheim, Norway and SINTEF Petroleum Research, Trondheim, Norway
Copyright 2014 ARMA, American Rock Mechanics Association
This paper was prepared for presentation at the 48th US Rock Mechanics / Geomechanics Symposium held in Minneapolis, MN, USA, 1-4 June
2014.
This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.
ABSTRACT: Experiments have been performed with Mancos Shale under Brazilian tensile test conditions, addressing the effect
of the angle between layer or bedding planes and the loading direction. A high-speed camera with digital image correlation
software is used in combination with acoustic emission recording to monitor the fracture initiation and growth processes during
loading. Although a clear anisotropy is observed in the variation of the P-wave velocity with the inclination angle, a significant
effect on the Brazilian tensile strength is not observed. The mode of failure depends however on sample orientation. For all
specimens, a main diametrical central fracture is induced first. It originates in the middle of the specimen and grows as a straight
line or as a zig-zag line, depending on the orientation of the sample with respect to load direction. The zig-zag fracture is then a
combination of a fracture along the weak direction and in other directions. Its evolution is an order of magnitude slower than that of
a brittle straight diametrical central fracture.
normal to the bedding plane, while recordings from the
high-speed camera are used to demonstrate differences
in fracture propagation for different orientations. A brief
discussion of the results and their possible applicability
is given in Section 4. The main conclusions are drawn in
Section 5.
2. THEORETICAL BACKGROUND AND
INTERPRETATION
Based on the loading type, there are three basic crack
propagation modes in a fracture process, namely: Mode I
(tension, opening), Mode II (shear, sliding) and Mode III
(shear, tearing). Accordingly, a crack can propagate
according to any of these modes or a combination of
them [7]. In fact the combination of mode I and mode II
(i.e. mixed mode I-II) is more common in rocks [7].
The Brazilian tensile test is one of the most widely used
tests to determine the tensile strength of rock material.
Although the test is easy to perform, requires little
sample preparation, and is relatively fast, the stress
distribution is much more complex than often assumed.
Hence, the detailed interpretation of the induced fracture
pattern is not always straightforward.
The basic theoretical stress distribution is developed for
a line load applied on a circular sample at diametrically
opposite sides, whereby the material is considered to be
continuous, homogeneous and isotropic, and behaves in
the linear elastic domain [1]. This means that as soon as
one of these conditions is not fulfilled, the basic
theoretical stress distribution changes. The reason why
the Brazilian test is used to determine the tensile strength
is that the theoretical solution indicates a constant tensile
stress on the plane between the two loading lines (and
normal to it). The formula for indirect tensile strength,
σt, perpendicular to the loaded diameter is given in Eq.
(1).
σt = 2F
Dt (1)
where F is the applied force at which the specimen fails;
D and t are the diameter and thickness of the disc
specimen, respectively. However, along the plane
between both loading lines, the stress state is far from
uniaxial. The minor principal stress is tensile, but the
major principal stress is compressive. The latter is about
three times the absolute value of the tensile stress in the
central part of the loaded diameter, but much more close
to the loading lines, i.e. in the analytical solution [8] the
end points of the loaded diameter are singular points in
the stress field. This analytical solution is also based on
the plane stress assumption; hence, the intermediate
principal stress is assumed to be zero. Away from the
loaded plane, tensile stresses are also present, but their
magnitudes decrease. At about a distance of half the
radius from the loaded plane at the centre of the
specimen, the tensile stress is about half its maximum
value.
The explained stress distribution should lead to a
diametrical splitting of the sample creating a single
extension type of fracture between both loading lines.
However, various other types of fracturing may occur.
Due to the stress concentration close to the loading lines,
failure in shear may occur, resulting in a V-type of
fracture close to the loading lines. Also, a set of parallel
fractures may be induced rather than a single fracture.
As indicated above, the conditions do not necessarily
conform to the assumptions in the basic theoretical or
analytical solution. Some researchers have concentrated
on the description of the load applied on the sample.
This is in reality not a perfect line with a zero thickness.
Lavrov and Vervoort [9] have quantified in a theoretical
way what the effect would be, if not only normal stresses
are applied on the circular sample, but also tangential
stresses. Markides et al. [10] have investigated the effect
of a non-uniform radial loading arc. However, the largest
difference between the theoretical solutions and the
reality is linked to the material. Rock is seldom
homogeneous, isotropic and purely continuous. Micro-
cracks are often present and influence locally the stress
distribution [11, 12]. Fracturing starts where the strength
is exceeded locally by the stress state. This is well
illustrated when one maps the acoustic emission hits.
Development of failure in quasi-brittle materials is
linked to the occurrence of micro-cracks, which release
energy in the form of elastic waves (i.e. acoustic
emission). At the start of the loading, the amount of hits
is low and diffuse over a large part of the specimen [12,
13, 14]. When the material’s strength is approached, the
acoustic emission activity increases and is mainly
situated in critically stressed regions. As damage
increases and peak stress is reached, a coalescence or
localization of damage occurs. During unloading further
damage may occur.
3. EXPERIMENTAL METHOD
3.1. Sample description The experiments presented here are conducted on
Mancos outcrop shale purchased from TerraTek Inc.,
Salt Lake City. The block of Mancos material was sealed
when received, and was subsequently stored in inert oil
after removal of the seal to avoid desiccation effects.
The main constituents are quartz (40 to 45%), clay
(around 20-25%), carbonates (about 20%) and some
organic material (slightly in excess of 1 weight %). The
porosity is about 6 - 8 %. The bulk density of the
Mancos shale investigated was 2.57 g/cm³ as measured
on cores used for rock mechanical testing.
Mancos shale is considered an analogue to gas shales.
The definition of gas shale that best describes the
reservoir is “organic-rich, and fine grained” [15].
Mancos shale fails however to classify as shale in a
geological context, according to its low content of clay
minerals. It still exhibits many features and
characteristics of shale behavior, such as anisotropy. For
instance, as shown by Fjær and Nes [16], the unconfined
strength is minimum between 45° and 60° inclination,
and the static E-modulus is 50 % higher when loading
parallel rather than normal to bedding.
Fig. 1 shows examples of bedding and of a structural
heterogeneity that may exist within a sample of Mancos
Shale.
Fig. 1 Cross section of two Mancos Shale samples. Sample A
shows how the layering is assumed to exist for the majority of
samples (samples are cored parallel to bedding; see circle).
Sample B shows how a minority of samples may exhibit a
more complex structure due to the natural variation in the
rock.
3.2. Sample Preparation The specimens were cored parallel to bedding, all from
the same block of Mancos shale. The cores were then cut
into test specimens of approximately 24 mm thickness
and 48 mm diameter, giving a thickness-to-diameter
ratio (t/D) of 0.5.
Prior to testing the sample end faces were hand polished,
labelled and axes marked at predefined increments (θ =
0°, 15°, 30°, 45°, 60°, 75° and 90°) in relation to the
bedding planes (Fig. 2). The inclination angle θ in the
Brazilian tests is defined as the angle between the
applied load and the bedding plane.
In total 35 samples were tested. The samples were
divided into 5 sets each with the 7 different angles θ.
Sample surfaces were modified in order to optimize
imaging. Three sets were tested without an oil coat; one
set coated with oil and the remaining set lightly sprayed
with white and black mat paint prior to Brazilian testing.
Spraying was done to optimize the quality of the video
recordings.
Fig. 2 Disc-shaped sample and configuration of layers in
Brazilian tests. θ varies between 0° and 90°. Set of inclined
parallel lines symbolize average layer direction (bedding) with
the red arrow representing the direction of the principal tensile
stress, based on a theoretical solution. This configuration has θ
= 30° and the specimen is cored parallel to bedding.
3.3. Test Procedure The experimental system (Fig. 3) was designed to
investigate fracture development during the (Brazilian)
indirect tensile strength test on Mancos Shale. The
loading was applied using a MTS (10 kN) load frame.
To soften the steel rock contacts, the discs were wrapped
by a layer of paper masking tape before being placed
inside the curved steel jaws and preloaded to 400 N.
Fig. 3 Experimental set-up: (1) MTS Frame, (2) Acoustic
Emission sensors (3) High-speed camera (4) Tungsten lamp
(5) LED lamp (6) Camcorder camera.
Four acoustic emission sensors were then mounted on
the rear of the sample in order to detect and record the
elastic waves generated during formation and
propagation of fractures. Acoustic Emission (AE) was
measured using an AMSY-5 System by Vallen GmbH.
Each signal was amplified by a preamplifier with an
amplification of 34 dB. Four wide-band sensors were
used of type B1025 (Digital Wave Corp., USA), which
have a radius of 4.5 mm and a frequency range from 50
kHz to 2 MHz. The sensors were held in place using a
customized holder, and grease was chosen as the
coupling medium. The minimum threshold to measure
signals was set to 23.8 dB. Once mounted, the sensors
were calibrated in order to improve their behavior in
terms of signal strength and relative response to each
other. The test was then continued and loaded until
failure with a loading rate of 0.003 mm/s. Failure
occurred within 30 minutes of the sample initially being
removed from the inert oil.
Fracture development was captured with the use of a
high-speed camera (Phantom v12). The camera's
memory can store approximately 4 seconds of footage
(at 5,000 fps) after it is manually triggered. The trigger
was initiated after the primary fracture was observed
with the assistance from the MTS recording (sudden
drop in loading) and acoustic emission (increased
number of acoustic events). Digital images were
recorded at a rate of 5,000 frames per second. Image
resolution was set to 704 × 704 pixels with an exposure
time of 190 µs. Up to two 1,000 Watt tungsten lamps
and a Magicshine (MJ-880E) 2,200 lumen LED bike
lamp were utilized to supply sufficient lighting when
required. As the time period of the high-speed camera
was limited to 4 seconds an additional Camcorder
camera was used to capture images at 25 frames per
second for the full entirety of the test. Visible fracture
growth did however largely restrict itself to the 4
seconds period captured by the high-speed camera, so
further analysis of the Camcorder recordings have not
been pursued.
The Digital Image Correlation (DIC) method was used
to assess the local displacement fields at the sample
surface with “7D” software [17]. For each test, the initial
image is split in square elements that create a virtual grid
upon the sample surface. The resolution of this grid
(extensiometric base) is set to 10 × 10 pixels. The
correlation process consists in looking for the most
probable deformed pattern in the neighborhood of each
node of this grid in terms of color level. The
displacement fields of each element are then assessed by
the means of a bilinear interpolation [17].
Analog output channels from the MTS frame and high-
speed camera were linked into the acoustic emission
system to synchronize the time of the collected data.
In addition, the ultrasonic compressional P-wave
velocities were determined. The pulse at frequency 1
MHz were excited with the electrical pulse supplied by a
function generator (Agilent 33220A, Agilent
Technologies) and amplified with 50 dB power amplifier
(ENI 2100L RF). The signal from the receiver was
recorded with an oscilloscope (TDS3012B, Tektronix)
and stored for further data processing. Seven angles, at
15° increments, with respect to bedding were tested
radially with one axial measurement taken centrally
through the disc. The wave velocities were determined
by picking the arrival time at the first zero-crossing. The
velocities were corrected for system time delay by
testing a reference material.
4. TEST RESULTS AND OBSERVATIONS
4.1. Acoustic Velocity
The P-wave velocity perpendicular to the bedding is
about 3,800 m/s. It increases with the inclination angle
to about 4,125 m/s parallel to the bedding. The velocity
change represents the acoustic anisotropy within the
sample. Velocities measured in the axial direction were
in agreement with measurements taken at 90° in the
radial direction.
The spacing between the different parts of the box plot
help indicate the degree of spread and skewness in the
data. It appears that the variability is relatively large
parallel to the bedding plane (a difference of about 300
m/s between the smallest and largest value recorded).
Perpendicular to the bedding this difference is about half
(150 m/s). Note that all samples were cored from the
same block.
Fig. 4 Box and whisker plot of P-wave velocity variation with
bedding orientation. The red arrows in the imbedded picture
represent the direction of velocity from the sensors
(represented in green).
4.2. Brazilian tensile strength Fig. 5 presents the variation of the unconfined strength
(Fig. 5a) and the Brazilian tensile strength (Fig. 5b) as a
function of the inclination angle for all samples. At first
sight no significant variation of the tensile strength as a
function of the inclination angle is observed when
looking at all data. The red dashed curve represents the
arithmetic average of all measured samples without
coating for each angle, but the scatter in the data exceeds
the variation of the curve. The largest value recorded is
4.3 MPa (0°), while the smallest is 2.3 MPa (15°). The
variation between minimum and maximum value per
inclination angle is situated between 0.8 MPa (15°) and
nearly 2 MPa (0°). Thus, the variation among all
samples is larger than a possible effect of the inclination
angle.
The difference with the variation of the P-wave velocity
as a function of the inclination angle (Fig. 4) is striking
and significant. A possible explanation for this
observation is that any flaw or micro-crack within a
sample may act as an initiation point for tensile failure,
and hence contribute to a large scatter in experimental
data. Velocities would be less affected, unless a
significant crack density has been established, i.e.
probably only very close to failure.
Looking at all data of Fig. 5, one can have the
impression that the uncoated samples result in smaller
strength values than the oil coated and sprayed samples.
It may be argued that any coating may suppress the
effect of initial flaws that connect to the sample surfaces.
However, for an inclination angle of 75°, the uncoated
specimens have higher strength than the coated ones.
These observations require further research in order to
be properly explained.
4.3. High-speed observation of fracture initiation
and propagation The fracturing process during the Brazilian test has been
imaged with the use of the high-speed camera at 5,000
fps. Fig. 6 and Fig. 7 display images of Mancos Shale
for θ = 30° and 75°, respectively. The selected images
represent the fracture evolution, including initiation and
propagation, at crucial stages. The images include: the
original image, a divided image, the x-displacement and
the y-displacement. Image division (the first image
divided by the corresponding pixel values of the second
image) was performed using commercially available
software (Adobe Photoshop). This makes it possible to
improve the visualization of the fracture using the
“naked eye”. Further information may be obtained using
digital x- and y-displacements with DIC software [17].
The color scale given in these figures provides the
direction and magnitude of displacement, where the
maximum positive x- or y-displacement (right or up) is
marked with red and the maximum negative x- or y-
displacement (left or down) is marked by blue.
Fig. 6 and Fig. 7 also show that propagation times may
vary as a fracture propagates through an anisotropic
sample inclined at a certain angle to bedding. Typically,
the time interval for angles of θ ≥ 60° is very rapid, i.e.
being less than a few frames, while the propagation
process for angles θ < 60° is taking an order of
magnitude more (tens of frames) to complete. Although
we can see that fracture development differs for various
angles, the exact time of propagation is difficult to
quantify using visual indicators.
(a) UCS
(b) BTS
Fig. 5 Unconfined compressive strength UCS (a) and Brazilian strength BTS (b) at various inclination angles, with an average
marked by a dashed line. Note: UCS values are reproduced after Fjær and Nes [16].
Fig. 6 High-speed video images of a uniaxial Brazilian Test on Mancos Shale at θ = 30°. Images displayed include: original image
(far left), divided image (center left), x-displacement map (center right) and y-displacement map (far right). The color convention
for the displacement is: positive values are right or up, while the negative are left or down.
Fig. 7 High-speed video images of a uniaxial Brazilian Test on Mancos Shale at θ = 75°. Images displayed include: original image
(far left), divided image (center left), x-displacement map (center right) and y-displacement map (far right). The color convention
for the displacement is: positive values are right or up, while the negative are left or down.
The fracture sequence as shown in Fig. 6 and Fig. 7 is
schematically illustrated in Fig. 8, and proceeds as
follows:
(i) The main diametrical central fracture (Fig. 8b)
originates in the middle of the specimen (for all
samples) and grows towards the loading jaws
(a) Central fracture paths are generally fairly
straight for 60° ≤ θ ≤ 90° and θ = 0° (Fig.
9a)
(b) Central fracture paths may be zig-zagged for
15° ≤ θ ≤ 45° (Fig. 9b)
(ii) Non-central fractures may then originate from
the edges of the sample (Fig. 8c) and propagate
along the bedding towards the centre
In some cases evidence of existing cracks were visible
on the surface of the sample prior to failure. However,
these cracks had no impact on the diametrical fracture
initiating in the middle of the sample.
(a) (b) (c)
Fig. 8 Fracture sequence for Mancos Shale: (a) before loading,
(b) central fracture, (c) non-central fracture (bedding
activation)
(a) 60° ≤ θ ≤ 90° & θ = 0° (b) 15° ≤ θ ≤ 45°
Fig. 9 Central fracture types for certain inclination angles.
After failure different types of fracture patterns are
observed (Fig. 10). For this fracture pattern analysis only
the central fracture is considered as it was identified as
the first fracture to propagate (Fig. 8). The fractures that
were roughly parallel to the bedding planes are denoted
as “layer activation” and the remaining fractures located
in the central part of the sample are further called “other
direction fractures”. The percentage of layer activation
versus other directions to the total fracture length has
been measured according to inclination angle (Fig. 11).
4.4. AE recording and location of events Acoustic emission (AE) enables us to determine number,
magnitude and, to some extent, localization of acoustic
events. In this study AE is utilized to confirm the
moment of tensile failure.
Fig. 12 shows the progression of acoustic events leading
to failure of the sample. Prior to failure the average
number and intensity of events is relatively low, which
may signify system noise or micro-fractures rather than
primary fracture generation. The highest number and
intensity of events is seen in the close vicinity of the
ultimate tensile stress, when the central fracture
initiation and propagation takes place. The time between
early acoustic activity and indicated failure is less than a
second, for a loading rate of 0.003 mm/s. The amount of
energy released during deformation depends on the
amplitude and the duration of the acoustic waves.
A location map was generated using the acoustic
emission software, which assumes an isotropic medium.
Given the anisotropic nature of Mancos Shale it is clear
that the isotropic model does not produce reliable
results. Debecker and Vervoort [21] present an algorithm
for improving event localization (in 2D and 3D) for
transversely isotropic media using a least squares
method.
5. DISCUSSION
We observed that samples taken from the same core,
prepared in the same manner and tested at the same
inclination angle reveal scatter both in P-wave velocity
and tensile strength (Fig. 4 and Fig. 5). These differences
may be ascribed to the inherent heterogeneity between
samples as shown in Fig. 1. The scatter in P-wave
velocities, however, is much less than that of the tensile
strength, which indicates that rock strength is affected
stronger by heterogeneity (possibly small cracks acting
as nucleus for fracturing) than rock stiffness. The angle
dependence of P-wave velocity is clear and significant,
with the P-wave velocity parallel to bedding being by
almost 9% higher than the P-wave velocity
perpendicular to bedding (Fig. 4). Because of the large
scatter in the tensile strength data, there is no clear trend
observed for the orientation dependence of the tensile
strength (Fig. 5b). By averaging the tensile strength data
obtained from the uncoated samples for each orientation
(data from coated samples is not included in the
averaging since the samples have been prepared in
different ways), a trend may be seen (dashed line in Fig.
5b). However, as this trend is not seen for the oil-coated
or sprayed samples, this is not conclusive. It is
interesting to note, though, that the trend in orientation
dependence of the Brazilian tensile strength (in
particular the appearance of a peak between 45° and 90°)
depicted by the dashed line in Fig. 5(b), as well as the
observed orientation dependence of the UCS values (Fig.
5a) are in qualitative agreement with experimental and
modeling results for Asan gneiss and Boryeong shale
reported by Park and Min [18] and further studies on
sandstone [19] and schist [20].
(a) θ = 0° (b) θ = 15°
(c) θ = 30° (d) θ = 45°
(e) θ = 60° (f) θ = 75°
(g) θ = 90°
Fig. 10 Disc-shaped specimens after failure in Brazilian tensile strength tests for Mancos Shale at various inclination angles (θ).
Images displayed include: original image (left), divided image (center) and drawn image of the central fractures only (right). The
parallel grey lines on the drawn image represent the layer direction. Layer activation is represented in green and fractures in other
directions in magenta.
Fig. 11 Variation in central fracture length percentage corresponding to layer activation and fractures in other directions for
Mancos Shale.
10mm
10mm 10mm
10mm 10mm
10mm 10mm
(a) θ = 30°
(b) θ = 75°
Fig. 12 Brazilian tensile stress measurements until failure for (a) θ = 30° and (b) θ = 75°, and related acoustic emission activity
given in terms of AE amplitude (red dots) and AE energy (green dots).
The surface conditions appear to have an effect on the
tensile strength of Mancos shale, since the sprayed
samples can, at least on average, sustain higher stresses
than the uncoated samples (Fig. 5b). There are, however,
several possible explanations for a different strength.
Desiccation of uncoated samples could result in surface
cracks that could act as nuclei for tensile fractures. A
thin coating (paint) could prevent desiccation and crack
formation. Furthermore, it could strengthen existing
surface cracks (e.g. created during cutting of the sample)
by reducing the stress concentration at crack tips during
loading (in the same way as coatings of glass fibers
prevent the fibers from breaking). A more systematic
study would be required to investigate the impact
desiccation and surface defects on the Brazilian tensile
strength, and the potential need for surface coating in
Brazilian tests.
The high-speed video footage provided a useful insight
for the fracture propagation both in Mode I (tension),
namely 60° ≤ θ ≤ 90° & θ = 0°, and mixed Mode I and II
(i.e. tension and shear), namely 15° ≤ θ ≤ 45°. The
fracture initiation, a point of interest for this study, refers
to the local tensile failure process i.e., where fracture
initiation, fracture propagation and crack coalescence
take place almost instantaneously. The results from the
high-speed images, together with x- and y-
displacements, suggest that the diametrical crack
initiates in the middle region for all specimens.
Typical fracture sequences are illustrated in Fig. 6 and
Fig. 7. Analysis of the propagation sequences revealed
that secondary fractures occur along the bedding plane
after the formation of the main diametrical central
fracture. It is believed that secondary fractures are
artefacts created due to the shape of the Brazilian
cylindrical discs and therefore not likely to occur in the
formation in situ. Tensile failure (Mode I) is fast, the
fracture initiates and propagates through the whole
sample within one frame taken with the high-speed
camera, i.e. within 0.2 ms (see Fig. 7), whereas mixed
mode failure (Mode I-II) seems to be about an order of
magnitude slower (see Fig. 6). During mixed mode, at θ
= 30°, a central crack first initiates along the bedding
layers, possibly due to the shearing. With a time delay of
a few ms, probably needed for stress build-up, the crack
propagates further, across the layers towards the loading
points.
The kinetics of the fracturing process still needs to be
explored further both by experiments and modeling. A
possible explanation for slower fracture propagation in
the θ = 30° case is that the initial crack formation along
bedding involved shear displacement that resulted in a
tensile stress reduction, requiring additional vertical
loading (vertical displacement) for fracture propagation
in the vertical direction. In recent years, there has been
an increased interest in the behavior of transversely
isotropic rock material under stress, and recent studies
have shown that there is a large difference in the
behavior and failure of such rocks [3, 22, 23]. The ratio
of the strength parallel to the planes of isotropy versus
normal to these planes varied between 2 % for a slate
[23] and nearly no anisotropy for a sandstone [3].
However, for a layered sandstone [24] and a shale [22] a
ratio of about 70 % was observed. The anisotropy ratio
observed for the Mancos shale seems to be rather small.
This can be explained by the observation that the strata
investigated contain larger grains than typical shale and
that the layering is far from perfect (see paragraph 3.1).
The small or even absence of anisotropy between
loading parallel and perpendicular to the layering is
probably also the cause for a systematic splitting of the
specimens. For the slate, mentioned above, with a
anisotropy ratio of 2 %, the fractures in the weak
direction were dominant over the entire interval of all
inclination angles [23]. For large inclination angles,
about 70 % of all induced fractures were, on average, in
the weak direction or the schistosity direction, and the
failures were not linked to the diametrical splitting of the
specimens. For the interval between 20° and 65°, the
failure that occurred was a shear failure along one of the
weak planes, and the final fracture pattern did not
connect the two loading lines. The failure was by
splitting only for small inclination angles. Of course, this
is an extreme case of anisotropy, but also for less
anisotropic rock, differences have been noted between
the fracturing process, as a function of the inclination
angle.
For the rocks investigated by Cho et al., 2012, Dan et al.,
2013 and Vervoort et al., 2012 [3, 22, 24], there is a
significant difference in the cross-over from dominant
fractures in directions other than the weak planes to
dominant fractures along the weak planes. For rocks
with a large anisotropy, the position of this cross-over
point is already situated at about 75°, while for rocks
which are nearly isotropic, this position is at about 15°,
which is also the case for the Mancos under
investigation here.
Despite the fact that for all experiments there is a clear
fracture between both loading lines and which is on
average vertical, the detailed observation of the induced
fractures shows clearly the influence of the local
structure of the material on the fracture. From an
inclination angle of 60° and less, there is a systematic
increase of the portion which clearly follows the layer
direction. For an inclination angle of 60°, this is about 7
%, while for lower inclination angles (e.g. 15°) this is
about 30 %. For 0°, the fracture does not follow a single
weak layer or bedding plane and is also not perfectly
straight, but slightly curved. The reason for it is probably
a combination between the small anisotropy and the
local variation in the orientation of the bedding planes.
This results for 0° in still about one third of the total
fracture length not being in the direction of the bedding
planes.
6. CONCLUSIONS
Brazilian tensile tests have been conducted on discs
cored from the same block of Mancos shale, by loading
in different directions with respect to the bedding plane.
Images captured with a high-speed camera were
analyzed in detail to identify how fractures were initiated
and how they would develop. Simultaneous recording of
acoustic emissions show that the AE activity coincides
with the main fracture evolution process, and was also
used as a trigger for the high-speed camera.
The video recordings show that fracture initiation occurs
through a main diametrical central fracture, originating
in the middle of the specimen (for all samples), and
growing towards the loading jaws. This central fracture
has either a generally straight path for 60° ≤ θ ≤ 90° and
θ = 0°, or a zig-zagged path for 15° ≤ θ ≤ 45°.
Secondary, non-central fractures may originate from the
edges of the sample and propagate along the bedding
towards the centre.
Although one can see that fracture development differs
for various angles, the exact rate of fracture propagation
is difficult to quantify, even with use of the high-speed
camera. Qualitatively, the main diametrical central
fractures develop rapidly, typically within a few frames,
which mean less than 1 ms. The mixed mode zig-zagged
fractures are seen to develop at a rate which is about an
order of magnitude (tens of frames) slower.
Prior to failure the average number and intensity of
acoustic emission events is relatively low, which may
signify system noise or micro-cracks rather than primary
fracture generation. The highest number and intensity of
events is seen in the close vicinity of the ultimate tensile
stress, when the central fracture initiation and
propagation takes place. The time between early acoustic
activity and indicated failure is less than a second, for a
loading rate of 0.003 mm/s.
Although the variation of the P-wave velocity as a
function of the orientation is clear and significant, this is
not confirmed for the Brazilian tensile strength. The
overall variation in the strength is too large to
distinguish a clear and systematic trend.
The main conclusion of the research conducted is that
the applied combination of high-speed camera images
and acoustic emission observations is a good way to
learn more about fracture initiation and growth leading
to final failure and this even for an unstable fracture
growth, as is normally assumed for Brazilian tensile
failure. The knowledge gained can have practical
implications, especially in the field of hydraulic
fracturing of gas shales. The pure tensile fractures appear
to propagate significantly faster than the zig-zags. This
coincides with more brittle behaviour, whereas the zig-
zag clearly would be a more permeable feature not
requiring proppants to stay open if it is generated in a
shale reservoir. Then, "brittleness" is detrimental to high
productivity, which would be an important contribution
to the discussion around brittleness as an index
parameter for "fracability".
When conducting this research, some new questions
were generated requiring additional attention. As
different techniques were used as part of the sample
preparation, a possible difference is observed between
the uncoated samples and the oil coated and sprayed
samples. Further experiments are needed to see if the
uncoated samples have indeed a smaller strength.
However, this could be an interesting observation and
could tell us something on the effect of heterogeneities
and existing flaws on the behaviour of shale. The
acoustic emission data could also be further analyzed to
conduct a better localization, i.e. taking the transversely
isotropic media into account. Some interesting
observations are made on the different fracture types
(along weak directions versus other directions). To better
understand this and to be able to make a comparison
with other observations of transversely isotropic rocks,
numerical simulations allowing a direct simulation of the
fracture patterns are needed.
ACKNOWLEDGEMENTS
This work was partly financed through the strategic
institute program "Gas Shale for Exploration and
Exploitation" at SINTEF Petroleum Research. Nathaniel
Simpson wishes to acknowledge the technical assistance
provided by the staff at SINTEF Petroleum Research and
SINTEF Materials and Chemistry and also the academic
institutions, DTU and NTNU, for giving him the
opportunity to participate in this research for his MSc
Thesis [25].
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