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].

REFERENCES

1. Jaeger, J.C., and N.G.W. Cook. 1979. Fundamentals of

Rock Mechanics. London, Chapman and Hall, 593 pp.

2. Fjær, E., R.M. Holt, P. Horsrud, A.M. Raaen and R.

Risnes. 2008. Petroleum Related Rock Mechanics.

Elsevier. 491 pp.

3. Dan, D.Q., H. Konietzky and M. Herbst. 2013.

Brazilian tensile strength tests on some anisotropic

rocks. Int. J. of Rock Mech. Min. Sci. 58: 1-7.

4. Li, D. and L.N.Y. Wong. 2012. The Brazilian Disc Test

for Rock Mechanics Applications: Review and New

Insights. Rock Mech. Rock Eng. 46: 269-287.

5. Jasarevic, H., A. Chudnovsky, J.W. Dudley and G.K.

Wong. 2009. Observation and Modeling of Brittle

Fracture Initiation in a Micro-Heterogeneous Material.

Int. J. Fract, 158: 73-80.

6. Debecker, B. and A. Vervoort. 2009. Experimental

observation of fracture patterns in layered slate. Int. J.

Fract. 159: 51-62.

7. Ke, C.-C., C.-S. Chen, C.-Y. Ku, and C.-H. Chen.

2009. Modeling crack propagation path of anisotropic

rocks using boundary element method. J. Numer. Anal.

Meth. Geomech. 33: 1227-1253.

8. Andreev, G.E., 1995. Brittle failure of rock materials,

test results and constitutive models. Balkema,

Rotterdam. 446 pp.

9. Lavrov, A. and A. Vervoort. 2002. Theoretical

treatment of tangential stress effects on the Brazilian

test stress distribution. Int. J. Rock Mech. Min. Sci. 39:

275-283.

10. Markides, Ch.F., D.N Pazis and S.K. Kourkoulis. 2012.

The Brazilian disc under non uniform distribution of

radial pressure and friction. Int. J. Rock Mech. Min. Sci.

50: 47–55.

11. Van de Steen, B. and A. Vervoort. 2001. Non-local

stress approach to fracture initiation in laboratory

experiments with a tensile stress gradient. Mechanics of

Materials. 33: 729-740.

12. Van de Steen, B., A. Vervoort and J.A.L. Napier. 2005.

Observed and simulated fracture pattern in

diametrically loaded discs. Int. J. Fract. 131: 35-52.

13. Labuz, J.F., S. Cattaneo and L.-H. Chen. 2001.

Acoustic emission at failure in quasi-brittle materials.

Constr. Bldg. Mats. 15: 225-233.

14. Lavrov, A., A. Vervoort, M. Wevers and J.A.L. Napier.

2002. Experimental and numerical study of the Kaiser

effect in cyclic Brazilian tests with disk rotation. Int. J.

Rock Mech. Min. Sci. 39: 287-302.

15. Rokosh, C.D., J.G. Pawlowicz, H. Berhane, S.D.A.

Anderson and A.P. Beaton. 2009. What is Shale Gas?

An Introduction to Shale-Gas Geology in Alberta.

Technical report, Energy Resources Conservation

Board, Edmonton, Canada.

16. Fjær, E. and O.-M. Nes. 2013. Strength anisotropy of

Mancos shale. In Proceedings of the 47th US Rock

Mechanics / Geomechanics Symposium, San Francisco,

23-26 June 2013, 7 pp.

17. Vacher, P., S. Dumoulin, F. Morestin and S. Mguil-

Touchal. 1999. Bidimensional strain measurement

using digital images. Proceedings of the Institution of

Mechanical Engineers, Part C: Journal of Mechanical

Engineering Science. 213: 811-817.

18. Park, B. and K.-B. Min. 2013. Discrete Element

Modeling of Transversely Isotropic Rock. In

Proceedings of the 47th US Rock Mechanics /

Geomechanics Symposium, San Francisco, 23-26 June

2013, 10 pp.

19. Barla, G., 1974. Rock anisotropy: Theory and

laboratory testing, Vol. 13. Springer-Verlag.

20. Chen, C.-S., E. Pan and B. Amadei. 1998.

Determination of deformability and tensile strength of

anisotropic rock using Brazilian tests. Int. J. Rock

Mech. Min. Sci. 35: 43-61.

21. Debecker, B. and A. Vervoort. 2011. Localization by

Acoustic Emission in transversely isotropic slate.

Advances in Acoustics and Vibration. 10 pp.

22. Cho, J.-W., H. Kim, S. Jeon, and K.-B. Min. 2012

Deformation and strength anisotropy of Asan gneiss,

Boryeong shale and Yeoncheon schist. Int. J. Rock

Mech. Min. Sc. 50: 158-169.

23. Debecker, B. and A. Vervoort. Two-dimensional

discrete element simulations of the fracture behavior of

slate. Int. J. Rock Mech. Min. Sci. 61: 161-170.

24. Vervoort, A., B. Debecker, and A. Tavallali. 2012.

Fracturing of transversely isotropic rock: an

experimental study. In Proceedings of SHIRMS2012,

Southern Hemisphere International Rock Mechanics

Symposium, 14-17 May 2012, Sun City (South Africa),

17-32.

25. Simpson, N.D.J. 2013. An analysis of tensile strength,

fracture initiation and propagation in anisotropic rock

(gas shale) using Brazilian tests equipped with high-

speed video and acoustic emission. MSc Thesis at DTU

& NTNU, 118 pp.