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1. INTRODUCTION
Although hydraulic fracturing has been widely used for
decades (Roberts, 1866, Bugbee, 1942, Clark, 1949), and
the technology to implement and interpret the induced
fractures has been continuously evolving (Yost, 1988,
NRC, 2001, Fri, 2006, NETL, 2011, Trembath et al.,
2012, Saldungaray & Palish, 2012), many aspects are still
not understood. Specifically, this includes the fracture
geometry and its interaction with natural features such as
existing fractures, bedding planes, and various
heterogeneities. The objective of this research is to gain a
fundamental understanding of the hydraulic fracturing
processes in shales through controlled laboratory
experiments, in which the mechanisms underlying
fracture initiation, propagation, and interaction with
geologic features in the rock are visually captured and
analyzed. Once these fundamental processes are properly
understood, methods that allow one to induce desired
fracture geometries in reservoirs can be developed.
Extensive work has been done at MIT to study fracture
initiation, -propagation, and coalescence (Reyes, 1991,
Bobet, 1997, Wong, 2008, Miller, 2008, Morgan, 2015,
Gonçalves da Silva, 2016, AlDajani, 2017, AlDajani,
2018, Gonçalves da Silva, 2018). These studies were done
on prismatic specimens with two pre-existing artificial
fractures (flaws) without and with the influence of
hydraulic pressure (Figure 1). Specimens were subjected
to uniaxial or biaxial compressive loading, and fracture
initiation and propagation mechanisms (tensile & shear)
were captured using a high-speed camera and a high-
resolution camera while simultaneously measuring the
stress-strain behavior. These experiments were
conducted on different materials: gypsum (artificial
material), different marbles (metamorphic rock), granite
(igneous rock), and different shales (sedimentary rock).
Figure 1 – Testing progression to study the fracture initiation,
propagation, and coalescence in rocks under various loading
conditions. a) Uniaxial or biaxial loading to failure. b)
Constant uniaxial load and pressurizing flaw to failure. c)
Constant biaxial load and pressurizing flaw to failure.
Modified from Gonçalves da Silva (2016).
ARMA 19–1552
Isotropic versus Anisotropic Stress Field Effects on
Hydraulic Fracture Mechanisms in Opalinus Shale
AlDajani, O.A.1, Germaine, J.T.2, & Einstein, H.H.1 1Massachusetts Institute of Technology, Cambridge, Massachusetts, USA 2Tufts University, Medford, Massachusetts
Copyright 2019 ARMA, American Rock Mechanics Association
This paper was prepared for presentation at the 53rd US Rock Mechanics/Geomechanics Symposium held in New York, NY, USA, 23–26 June 2019. 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: Although hydraulic fracturing has been widely used for decades, and the technology to implement and interpret the induced fractures has been continuously evolving, many aspects are still not understood. Specifically, this includes hydraulic
fracture initiation and propagation mechanisms and the effect of stress state and rock fabric. The objective of this study was to
determine the differences between hydraulic fracturing under isotropic and anisotropic stress conditions.
The rock used in this study is Opalinus Shale prepared into prismatic specimens with a pre-existing artificial fracture (flaw) in the
middle. Different external biaxial stresses are applied to simulate in-situ stress conditions followed by hydraulic pressurization of
the flaw until failure. Internal flaw pressure is measured throughout the pressurization and fracturing process. High-speed and high-
resolution cameras are used for visual analysis.
Two experiments are presented, discussed in detail and compared: 1- a specimen with a bedding plane orientation of 30° relative to
horizontal is subjected to a vertical stress of 3 MPa and a lateral stress of 1 MPa (anisotropic stress). 2- a specimen with the same
bedding plane orientation of 30° is subjected to biaxial isotropic stresses of 2 MPa (isotropic stress). The results show that the
combination of rock fabric and stress state affect the initiation and propagation of hydraulic fractures in shale. This adds to
fundamental knowledge on how fractures behave and may provide insight into strategic hydraulic fracture treatments for field
applications.
Hydraulic fracture geometries and mechanisms are
affected by stress state (Perkins & Kern, 1961, Simonson
et al., 1978, Cleary, 1980, Warpinski et al., 1982), rock
fabric (Fisher & Warpinski, 2012, Suarez-Rivera et al.,
2013), and other factors (Daneshy, 1978, Teufel & Clark,
1981, Biot et al., 1983, Blaire et al., 1989). In any given
petroleum reservoir, the stress state can vary spatially,
even along a single wellbore, due to the complex geologic
structures such as salt domes and/or folds (Zoback, 2010).
Rock fabric, i.e. bedding plane orientation, natural
fractures, and localized heterogeneities, affect the local
stress field and may influence the propagation of the
hydraulic fractures, and thus play an important role in
dictating the complexity of the induced fractures. In this
paper, we specifically investigate the effect of the biaxial
(quasi true triaxial) stress regime on the produced
hydraulic fractures in shale. The two stress states
investigated are isotropic stress (𝜎1 = 𝜎3) and anisotropic stress (𝜎1 ≠ 𝜎3).
1.1. Specimen Mineralogy
The rock used in this study is Opalinus Shale from Mont
Terri, Switzerland, which often has distinct alternating
layers as shown in Figure 2.
Figure 2 - Image of Opalinus Shale showing two distinct
alternating layers, a dark clay-rich layer & a light quartz- and
carbonate- rich layer.
The mineralogy was measured using X-ray diffraction
and is presented in Table 1.
Table 1 – Bulk mineralogy analysis results of Opalinus Shale
core sample from X-ray diffraction.
Mineral %
Quartz 33.0
K-Feldspar 3.4
Plagioclase 1.8
Calcite 5.2
Dolomite 0.7
Siderite 1.8
Anatase 0.4
Pyrite 0.9
Muscovite 2.6
Chlorite (Tri) 2.3
I+I/S-ML 32.1
Kaolinite 15.8
1.2. Mechanical Properties
The mechanical properties were measured through
unconfined compression tests and are presented in Table
2. Although these results were for other Opalinus Shale
cores, they fall within range of the extensive mechanical
properties tested and presented by Bock (2001).
Table 2 – Mechanical properties of intact (no flaw) Opalinus
Shale prismatic specimens subjected to unconfined
compression tests (AlDajani, 2017). UCS, MPa E, MPa ʋ
load ⊥ to bedding 17.26 1327 0.33
load ∥ to bedding 5.76 1947 0.26
1.3. Specimen Preparation
Prismatic specimens are prepared by dry cutting cored
borings with various bedding plane orientations and a pre-
cutting a flaw in the middle. The intricate preparation
techniques used are described in AlDajani (2017) and are
meant to preserve the shale’s chemical and mechanical
integrity from in-situ conditions until testing.
In the previously mentioned studies, most experiments
were conducted on specimens with double flaws to
determine their interaction. In this study, only a single
vertical flaw is cut to investigate the effect the stress state
has on hydraulic fractures emanating from a single source,
and observe the interaction of the produced hydraulic
fractures with the rock fabric. The specimen dimensions
and loading configuration are shown in Figure 3. Note
that the specimens used in this study have a bedding plane
orientation of 30° from horizontal.
Figure 3 – Schematic of a prismatic specimen subjected to a
constant biaxial load and a pressurized prefabricated flaw to
induce hydraulic fractures to study fracture mechanisms.
2. EXPERIMENTAL SETUP
The novel and unique hydraulic fracture experimental
setup introduced by Morgan et al. (2017) and described in
detail by AlDajani (2018) is shown in Figure 4.
2.1. Testing Setup & Procedure
The main challenge and advantage of this setup was the
ability to induce hydraulic fractures in an externally
loaded rock specimen and be able to capture the fracture
mechanisms in detail. This involved the flaw
pressurization device (Figure 5).
Figure 5 – Three-dimensional rendering of updated flaw
pressurization device components (isometric view) showing
transparent polycarbonate window and larger flaw seal with
front injection needle inserted into the flaw.
This device, its components, and operation were
described in detail by AlDajani (2018). When placed into
the load frame, it allows one to apply external stresses
onto the specimen and pressurize the flaw to produce
hydraulic fractures while simultaneously capturing visual
and acoustic observations of the detailed fracture
mechanisms.
After applying the external biaxial stresses, the device
allows one to apply stress on the specimen face by
clamping the front viewing window to the rear steel plate,
i.e. creating a true triaxial stress state around the flaw.
This stress (𝜎2) was determined to be approximately 2 MPa by controlling the clamping screws’ torque and
measuring the force on a load cell in place of the
specimen. The same torque, and hence intermediate
stress, was uniformly applied for all experiments.
Computer control is used to apply loads at specified rates
in a synchronized fashion. Specifically:
• In the isotropic test, 2 MPa are simultaneously applied in the axial and lateral directions at a rate of
3.5 MPa/min, following an isotropic compressional
stress path, and then to hold 2 MPa biaxially.
• In the anisotropic test, 1 MPa was applied simultaneously in the axial and lateral directions at a
rate of 3.5 MPa/min, initially following an isotropic
compressional stress path. After the lateral stress was
held at 1 MPa, the axial stress was increased at the
same rate until 3 MPa is achieved.
Figure 4 – Schematic of the experimental setup used in this study. The load frame subjects a constant biaxial stress on to the specimen
and then the pressure volume actuator (PVA) injects fluid into the pre-cut flaw. The fractures are observed using a high-speed
camera, a high-resolution camera, and an acoustic emission system, simultaneously. The fluid pressures were measured in the PVA
as well as internally in the flaw. Modified from Morgan et al. (2017).
The stress paths for these two tests are shown in p-q space,
where 𝑝 =1
2(𝜎1 + 𝜎3) and 𝑞 =
1
2(𝜎1 − 𝜎3).
Figure 6 – Stress paths for the two tests in p-q space where 𝜎1 is the axial load and 𝜎3 is the lateral load.
After loads are applied, the flaw is initially saturated by
pumping fluid from the pressure-volume-actuator (PVA)
through the tubing into the flaw (see Figure 4), and out
through the flaw pressure measurement needle (see
Figure 5). Once saturated, a pressure transducer is
attached to this needle to close the system. The flaw is
then pressurized at a constant injection rate of 1.33
mL/min for both tests. Pressure and volume
measurements are taken at the PVA, and internal flaw
pressure is measured with the pressure transducer probing
the flaw. The fluid injected is hydraulic oil to prevent the
hydration of clays, and has a dynamic viscosity of
approximately 4 cP.
Imagery acquisition was done with a high-speed (HS)
camera at 1,000 frames per second (fps) on a 1 megapixel
(MP) sensor and a high-resolution (HR) camera at 0.5 fps
on a 20 MP sensor. The HS camera is manually triggered
to capture the failure of the specimen, i.e. the end of the
test. The HR camera captures time lapses of the test from
beginning to end. The acoustic acquisition system
samples data at 5 MHz from 8 acoustic sensors, which are
spring-loaded in specialized platens surrounding the
specimen. Acoustic observations are not discussed in this
paper.
To show practical relevance, the concept of this
experiment is similar to bringing the rock to in-situ stress
conditions, the saturation phase is analogous to drilling
mud in the wellbore, and the pumping phase follows what
is done in field operations to induce hydraulic fractures.
In our tests, the following data are acquired to be
analyzed:
a. HS video b. HR images c. Internal flaw pressure d. PVA pressure and volume e. Acoustic emissions (not discussed in this paper)
The imagery is then analyzed, and what is visually
captured is analyzed and drawn into sketches for a clearer
graphical representation of what happened.
3. RESULTS AND DISCUSSION
Recall that two stress states investigated: anisotropic and
isotropic (Figure 7).
Figure 7 – Schematics of tested load configurations. a)
Anisotropic stress state acting on a specimen with 30° bedding
planes. b) Isotropic stress state acting on a specimen with 30°
bedding planes.
3.1. Anisotropic Stress State (𝜎1 = 3 𝑀𝑃𝑎, 𝜎2 ≈2 𝑀𝑃𝑎, 𝜎3 = 1 𝑀𝑃𝑎)
The data collected from the anisotropic hydraulic
fracture experiment are shown in Figure 8. The
internal flaw pressure is the red curve, and the
injected volume is the blue curve. The black
triangles indicate where sketches were taken for
image analysis. A sketch is taken from the HS or
HR images when a significant event occurs such as
fracture initiation or a specific interaction of the
hydraulic fracture with features in the rock.
Figure 8 – Pressure and volume data acquired for the
anisotropic stress hydraulic fracture test. The internal flaw
pressure is the red curve, and the injected volume is the blue
curve. The black triangles indicate where sketches were taken
for image analysis.
The fracture progression is shown in Figure 9,
where each sketch number corresponds to the image
of the specimen at the denoted
pressures/times/volumes in Figure 8 (designated by
the labeled black triangles).
Figure 9 – Sketches of fracture progression throughout
pressurization of the flaw in the anisotropically loaded
specimen. Sketch numbers refer to numbered black triangles on
pressure curve in Figure 8. Fracture initiation is denoted with
red letters. The flaw seal boundary is indicated by the rounded
square. (T) indicates opening in tension, and subscript bp
indicates propagation along a bedding plane.
The final sketch (Sketch 10) is enlarged and shown
in Figure 10.
Figure 10 – Final sketch (Sketch 10 in Figure 8) of the
anisotropic stress hydraulic fracture test.
As shown in Figure 8, fluid is injected at a constant
flow rate, and the internal flaw pressure response is
measured. The maximum pressure was 4.32 MPa,
but the first fracture A(T)bp initiated at the top flaw
tip at 3.49 MPa (Sketch 1in Figure 9) and started
propagating along the bedding plane. A second
fracture B(T)bp initiated at a bedding plane
intersecting the middle of the flaw at 3.92 MPa
(Sketch 2) while A(T)bp continued to propagate. As
propagation of A(T)bp and B(T)bp continued, C(T)bp
branched from A(T)bp (Sketch 3). At this point,
A(T)bp arrested and propagation continued along the
intersecting bedding plane C(T)bp (Sketch 4). The
non-linear pressure response between sketches 3
and 4 reflects the dilation as a result of fracture
propagation while the drastic pressure drop shortly
afterwards is due to the fracture reaching the seal
boundary (Sketch 5). Despite that, the fractures
continued propagating until they reached the
specimen boundaries (Sketch 10 in Figure 10). The
pressure record shows that the pressure to initiate
fractures is greater than that needed to propagate
fractures, as was established by Irwin (1956) and
Feng & Gray (2017). An image taken at the end of
the test is shown in Figure 11 which corresponds to
the final sketch in Figure 10.
Figure 11 – Image of hydraulically fractured specimen
subjected to anisotropic stress at the end of the test.
3.2. Isotropic Stress State (𝜎1 = 𝜎2 ≈ 𝜎3 =2 𝑀𝑃𝑎)
The data collected from the isotropic stress hydraulic
fracture experiment are shown in Figure 12. The plotted
curve colors and symbols are the same as in Figure 8
(section 3.1.).
Figure 12 – Pressure and volume data acquired for the isotropic
stress hydraulic fracture test. The internal flaw pressure is the
red curve, and the injected volume is the blue curve. The black
triangles indicate where sketches were taken for image analysis.
As shown in Figure 12, fluid is injected at a constant rate,
and the internal flaw pressure response is measured. The
maximum pressure in this test was 7.70 MPa. It is worth
noting that no fractures were detected in the image
analysis prior to the drastic pressure drop, which is a sign
that a hydraulic fracture(s) has propagated past the seal
boundary. The likeliest explanation was that the
fracturing started on the rear face of the specimen before
becoming visible on the front (imaged) face. The front
and rear faces of the specimen were photographed after
the test and they show good correspondence (Figure 13).
The earlier fracturing at the rear is supported by the wider
wet region around the fractures on the rear face, indicating
longer exposure to the injected fluid.
Figure 13 - Images of the front and rear face (mirrored for
comparison) of the hydraulically fractured specimen taken after
the test. The fractures show good correspondence and are likely
to have started on the rear face before appearing on the front.
Regardless of this fact, the visual observations on the
imaged face of the rock throughout the test were analyzed
as this test still shows the entire fracture behavior of this
specimen under isotropic loading.
The fracture progression is shown in Figure 14, where each sketch number corresponds to the image of the
specimen at the denoted pressures/times/volumes in
Figure 12 (designated by the labeled black triangles).
Figure 14 – Sketches of fracture progression throughout
pressurization of the flaw in the isotropically loaded specimen.
Sketch numbers refer to numbered black triangles on pressure
curve in Figure 12. Fracture initiation is denoted with red
letters. The flaw seal boundary is indicated by the rounded
square. (T) indicates opening in tension, and subscript bp
indicates propagation along a bedding plane.
The final sketch (Sketch 6) is enlarged and shown in
Figure 15.
Figure 15 – Final sketch (Sketch 6 in Figure 12) of the isotropic
stress hydraulic fracture test.
The first crack to initiate is A(T)bp (Sketch 1 in Figure 14),
which is a bedding plane that opened in tension due to the
hydraulic pressure. By Sketch 2, A(T)bp had propagated
to the seal boundary. By Sketch 3, tensile fracture B(T)
initiated near the middle of the flaw and started
propagating across bedding layers, while A(T)bp had
propagated well past the seal boundary. Afterwards, the
two fractures continued propagating to the boundaries of
the specimen, with A(T)bp simply along the same bedding
plane, and B(T) across bedding layers. An image taken at
the end of the test is shown in Figure 16 which
corresponds to the final sketch in Figure 15.
Figure 16 – Image of hydraulically fractured specimen
subjected to isotropic stress at the end of the test.
3.3. Discussion First and foremost, there is one consistent behavior
among the two tested loading conditions. As the flaw is
pressurized, the first thing to occur is that a bedding plane
near the flaw tip, where the highest tensile stresses exist,
opens in tension. Propagation continues along this
bedding plane. This is strong evidence that the rock fabric
influences hydraulic fracture initiation and propagation.
Though tensile strength of the bedding planes was not
directly measured, it is evident that they are planes of
weakness in shales. In other words, the hydraulic
fracturing process is controlled by stress concentration
and fabric.
This is quite interesting, especially in the anisotropically
loaded test. Given the stress state applied, one would
expect, theoretically, the hydraulic fractures to propagate
in the direction of 𝜎1, i.e. vertically in this testing configuration. This was shown experimentally by
AlDajani (2018) where specimens with horizontal
bedding and a single vertical flaw were subjected to
uniaxial stress, which can be regarded as a special case of
anisotropic biaxial stress. These experiments all produced
hydraulic fractures propagating in the uniaxial direction.
Stress concentration is more extreme in the uniaxial case,
and the horizontal bedding planes did not provide
initiation locations. This confirms the effect of a
combination of fabric and stress concentration effect.
Other observations indicate that there is a combination of
the effects of stress concentration and rock fabric:
• The second hydraulic fracture occurs at and propagates along a bedding plane in the anisotropic
test, but propagates through the matrix in the isotropic
test. Furthermore, the second fracture in the isotropic
stress propagated horizontally with no preferential
direction.
• The maximum pressure reflects how much pressure is required to propagate a fracture to the seal boundary,
and it is significantly higher in the isotropic test.
One initial explanation is that in an anisotropic stress
field, the stress concentrations around the flaw are more
extreme than in an isotropic stress field. However, further
work is necessary to determine how stress concentration
and rock fabric interact.
4. SUMMARY & CONCLUSIONS
The objective of this paper was to study the differences
between hydraulic fractures produced in shale specimens
subjected to isotropic and anisotropic stress conditions.
The hydraulic pressure testing setup at MIT allowed real-
time analysis of fracture initiation and propagation by
utilizing a transparent flaw pressurization device and
imagery equipment.
The anisotropic stress test applied and held the following
external stresses: σ1 = 3 MPa, σ2 ≈ 2 MPa, σ3 = 1 MPa, and the flaw was pressurized at a constant flow rate of
1.33 mL/min. This resulted in hydraulic pressure opening
up bedding planes and propagating along them.
The isotropic stress test applied and held the following
external stresses: σ1 = σ2 ≈ σ3 = 2 MPa, and the flaw was pressurized at the same constant flow rate of 1.33
mL/min. This resulted in one hydraulic fracture
propagating along a bedding plane and the other across
bedding layers.
For both stress conditions, the first hydraulic fracture to
initiate was at the flaw tips where the tensile stress is
highest and at the intersection with a bedding plane. The
fracture continued to propagate along the same bedding
plane. Thus, the rock fabric has a strong effect in dictating
fracture initiation and propagation. However, the
characteristics of the secondary hydraulic fracture and the
pressures were different between the two tests.
While the combined effect of stress state and fabric may
explain the differences in hydraulic fracture initiation and
propagation, more work is needed to fully understand
their interaction.
The results from such experiments can be very insightful
to interpret fracture complexity in past field operations or
in the planning stage for future treatments, where the
stress state can vary spatially, even along the same
wellbore. They can also be used as a validation tool for
theoretical and numerical models.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of this
research by TOTAL in the context of the Multi-scale
Shale Gas Collaboratory project. We not only received
financial support, but were also helped through many
constructive discussions with our technical contacts. We
also would like to acknowledge the Underground
Research Laboratory in Mont Terri, Switzerland which
provided the shale used for this study. The authors
also acknowledge the help in designing and fabricating
from Stephen W. Rudolph in the Department of Civil
and Environmental Engineering at MIT. The authors also
thank their colleagues Dr. Stephen P. Morgan and Bing
Q. Li for their help running the experiments.
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