Microscale Laboratory Studies for Determining …...is seen in the horizontal force profile. Comparing the horizontal force to the AE time series, the AE peaks are found to decline

URTeC: 2903021 Microscale Laboratory Studies for Determining Fracture Directionality in Tight Sandstone and Shale during Hydraulic Fracturing Ante, M. A. Manjunath, G. L., Aminzadeh, F., Jha, B. Petroleum Engineering, University of Southern California, USA Copyright 2018, Unconventional Resources Technology Conference (URTeC) DOI 10.15530/urtec-2018-2903021

This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Houston, Texas, USA, 23-25 July 2018.

The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper by anyone other than the author without the written consent of URTeC is prohibited.

Abstract Hydraulic fracturing is a widely-used solution to improve continuity and connectivity across thin layers and to bypass near-wellbore damage. However, predicting and controlling the initiation and cessation of a hydraulic fracture remains a challenge due to compositional and poromechanical heterogeneity, which causes stress concentration, and due to inelasticity at the grain scale. The presence of organic matter also affects the rock-mechanical properties and directionality in fracture initiation and propagation processes. Overall, understanding the fracturing behavior of a rock at the microscale plays a critical role in predicting the performance of hydraulic fracturing during and after the “frac job”. In this paper, we propose a method to investigate the fracture initiation and propagation behaviors in tight sandstones and shales by estimating fracture toughness and directionality using micro-scale mechanical scratch tests. Scratch tests provide a means to account for grain-scale heterogeneity and inelasticity during measurement of fracture properties in different directions. Three uniformly-spaced, consecutive scratch tests are performed in a sample for a representative coverage of the sample surface in terms of the number of grains. The characterization of scratch track using Scanning Electron Microscopy provides a means of identifying the initiation and propagation of failures at critical points during the loading process. The fracture toughness and fracture directionality values of tight sandstone and shale samples are compared to understand the effect of lithology on fracture initiation and propagation processes, respectively. We discuss the influence of the packing density of matrix, porosity and pore size distribution on the fracture processes. Introduction A multiscale understanding of the fracturing behavior of a material is important since the creation and propagation of fractures is inherently a multiscale process (Lawn, 1993). Fracture toughness is a material property that describes the resistance of a material to dynamic crack propagation. It is also termed the critical stress intensity factor, KIC, in linear elastic materials. Fracture toughness is used in design and simulation of hydraulic fracturing jobs. For brittle elastic materials, it can be determined through loading experiments at maximum load. Due to the different factors that contribute to nature of geomaterials such as chemical composition, burial depth, and even scale of analysis, the fracture toughness value is difficult to constrain and must be characterized experimentally for each material. Laboratory studies at nanometer, micrometer and centimeter scales allow for an understanding of how the different constituents of a rock affect its fracturing behavior (Akono and Kabir, 2016). At the nanoscale, the effect of clay particles and their interaction with kerogen can be studied using atomic force microscopy, field emission electron microscopy, and nanoindentation. At the microscale, porous clay structure consisting of minerals, particles, organic matter and pore spaces can be evaluated using microscopy, micro-indentation and micro-scratching. Finally, at the macroscopic scale, the rock can be considered as a layered composite and the length, directionality and connectivity of fractures can be quantified with respect to rock’s macroscopic features such as bedding planes and porosity.

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A typical micro-scratch test consists of measuring the horizontal contact force between the scratching stylus and the material while a prescribed vertical force on the stylus drives it along a line on the material. The vertical force usually increases linearly with time to allow the stylus to penetrate deeper into the material as it moves along the line. Acoustic emissions (AE) are also measured during a typical scratch test to identify the occurrence of damage events during the scratch and characterize their mechanical energy. Similar to nano-indentation, micro-scratch testing can be performed on small volumes of a material, it is reproducible, and it can measure material strength (Richard et al., 2012) and toughness (Akono et al., 2012) with values agreeing with those measured from conventional loading tests. It is argued that micro-scratch tests provide laboratory means of obtaining the degree of adhesion and cohesion on the microscale and, therefore, the material toughness value. However, it is also argued that the scratch process is dominated by the strength of the material instead of its toughness (Lin and Zhou, 2015; Le and Detournay, 2016; Zhou, 2017). Use of micro-scratch is popular in thin films and coatings to determine the functional behavior and the degree of adhesion of a film or coating to the substrate material (Lu et al, 2011). Similar applications in ceramics, metals, polymers and biological materials such as teeth and bone are well-documented (Kanematsu, 2003; ASTM C1624–05; ASTM G171–03; Qing et al, 2015; Kataruka et al, 2017). Applied load can range from milliNewton to hundreds of Newton with the lower limit suitable for thin protective coatings, optical materials and the higher limit more suitable for hard coatings and substrate materials. Akono and Kabir (2016) apply a combination of imaging (optical and electron microscopy), fracture mechanics and microscale mechanical testing to study different types of shale. They report microscopic fracture toughness values that are twice of the macroscopic fracture toughness. The explanation is given by examining the fracture surface, which reveals toughening mechanisms such as crack bridging, particle pull out and crack front roughening. Cracks in the short transverse orientation are seen when both the fracture plane and the fracturing direction are parallel to the bedding plane. The horizontal force is increasing with the vertical force, but an irregular saw-tooth waveform pattern is seen in the horizontal force profile. Comparing the horizontal force to the AE time series, the AE peaks are found to decline rapidly with the horizontal force indicating that local micro-cracking events are occurring in the material. Microscopy indicates that the fracture width is increasing from the tip as the vertical load increases. Here, we examine fracture behaviors of tight sandstone and shale through their response to micro-scratch testing along two directions: parallel and perpendicular to the bedding plane. The different directions allow for an analysis of material anisotropy and honor the fact that fracture mechanics is orientation-dependent. Understanding the role of anisotropy on fracture initiation and propagation is even more important in layered materials such as sedimentary rocks (Barpi et al, 2012). Anisotropy due to preferential deposition and layering of sediments creates variation in the pore structure and the stress field along different orientations. This usually results in higher toughness values along a bedding plane than across it (Chandler et al., 2013). Deep shale reservoirs are under overburden load that may be higher compared to the horizontal tectonic stresses. This results into higher vertical stresses, which can promote vertical-to-quasi-vertical fractures across horizontal bedding planes. Below we provide details of our sample, pre-scratch sample preparation, the scratch procedure and post-scratch scanning electron microscopy (SEM). Parameters obtained from the scratch test are used to determine the directional fracture toughness of the rock samples. The acoustic emission log during the scratch testing can be related to the fracture energy and we examine this as well. We perform post-scratch SEM to study and understand the crack propagation behavior in terms of the effects of individual constituents and material anisotropy on the crack geometry. Theory, Materials and Methods Materials The sandstone and shale samples used for our analyses was provided by California Resources Corporation, a petroleum company operating in California. Sandstone and shale samples were obtained from wells drilled into the Monterey formation. The Monterey formation covers a large area of 1,750 square mile along the coasts of San Francisco and Los Angeles. Some parts of the formation are over a mile (>5000 ft) thick with most areas about half a mile thick. It is known to have a complex lithology that contains calcite, dolomite and silicate minerals and varies laterally in stratigraphy. The Miocene strata is highly siliceous but also contains significant amount of interbedded clastic shale, mudstone and sandstone. Due to its unique geology, lithologic complexity and a rapidly changing thickness, it is difficult to make general characterizations of the entire Monterey formation (Bramlette, 1946). Its proximity to the San Andreas Fault places it in a region of active tectonics with evident faulting, folding and natural fractures.

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The sandstone and shale samples were obtained from the depths of 9783.25 ft and 12468.45 ft, respectively. Nano indentation tests have previously been carried out on the same sample depths with specimen cut parallel to bedding plane in order to obtain hardness and elastic moduli (Ante et al., 2018).

Figure 1: Monterey Formation Locations.

Sample Preparation Prior to testing, the samples were cut 0.5 mm thick parallel and perpendicular to bedding plane using a Buehler precision saw at low speed. The low cutting speed was important considering the thin samples and the need to minimize granular structure disturbance at the testing surface. This was especially important for the sandstone sample, which has grain sizes larger than that of shales. Larger grains can easily be dislodged and result in pitting. The bulk of the smoothing job is at this cutting stage so that grinding and polishing can be kept to a minimum. Next, wet mechanical polishing using a Buehler grinder-polisher was performed to polish the two parallel surfaces of each sample. This was carried out using the following order of grinding paper grit sizes: 180, 240, 320, 400, 600, 1200, 2400 and 4000. Each parallel surface was polished for approximately 2 minutes. After this, dry polishing using micro-cloths and polycrystalline diamond suspension was used to polish each of the parallel surfaces starting with 3 µm and then 1 µm, 0.25 µm, and finally 0.05 µm to achieve a smoothness level that is necessary for the scratch tests. Fracture Toughness Theory For linear elastic isotropic materials under plane strain conditions, crack propagation occurs when the energy release rate, G, is equal to the fracture energy (Griffith, 1921):

𝐺 =1− 𝜈&

𝐸𝐹)&

2𝑝𝐴(1)

2𝑝𝐴 = 4(tan𝜃/cos𝜃)𝑑7 (2)

where, A = contact area due to load in horizontal direction E = Young’s modulus FT = horizontal force p = fracture surface perimeter

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ν = Poisson’s ratio 2pA = Shape function of the probe (conical approximation for a conical probe) d = penetration depth 𝜃= half-apex angle The scratch toughness, KS, is related to the fracture energy release rate as

𝐾9 = 𝐹)

:2𝑝𝐴 (3)

Under the assumptions of fracture-driven loading of a linear elastic isotropic model, the scratch toughness, KS, is the fracture toughness, KC (Akono and Kabir, 2016) Micro-scratch Testing A micro-scratch testing process involves drawing of an indenter probe across the surface of the material to be tested with an increasing vertical load that promotes mechanical failure. The load is applied by the moving indenter to the rock sample and the accompanying depth of penetration is measured in addition to the acoustic emission (AE). The speed of scratching is kept constant. The depth of penetration provides the reversible (elastic) and irreversible (plastic) portions of deformation in the material. The AE sensor records the damage events caused by stress to the material. Therefore, the AE data represents the irreversible, hence inelastic, deformation that has occurred in the rock in the form of plastic or brittle fractures. As scratch progresses, the degree of elastic and plastic deformation increases. The scratch track and the damage within and outside the scratch track can be viewed with an optical microscope or a scanning electron microscope (SEM). We used a sphero-conical diamond-tip stylus that was drawn at a constant speed across each sample. Figure 2 illustrates the orientation of sample with respect to the bedding plane and the normal and frictional (horizontal) forces acting on sample during testing. Table 1 lists the sample nomenclature and orientation, and Table 2 lists the input parameters used for scratch testing.

Figure 2. (a) and (b): samples are cut parallel and perpendicular to the bedding plane. (c) A schematic of the sphero-conical diamond tip stylus. (Courtesy of Nanovea, 2018). (d) Vertical (normal) and horizontal (frictional) forces acting on a sample during testing. Microscopic observation using a micrograph provide a panoramic view of scratch area and a means of detecting surface damage. Fluctuations in the frictional force vs. the scratch length data can be used with the micrograph to study failure events. For composite materials such as ours, a change in the coefficient of friction (ratio of frictional force and normal force) may be attributed to a change in constituents and/or grain/pore that comes in contact with the

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probe along the scratch path. Significant/sudden depth changes in the penetration depth vs. the scratch length data may indicate large failure/fracture events. Along with the pre- and post-scanning microscopy images of the scratch, the depth data may be used to study plastic and elastic deformation. Finally, the acoustic emission data in some materials indicate critical loads of failure initiation. Table 1: Sample Material and Orientation

Sample Number Material Direction of Cutting

1 Sandstone Parallel to bedding

2 Sandstone Perpendicular to bedding

3 Shale Parallel to bedding 4 Shale Perpendicular to bedding

Table 2: Input Parameters for Scratch Testing for All Samples

Parameter Value

Initial Load, N 0.02

Final Load, N 30 Loading Rate, N/min 30

Scratch Length, mm 3

Scratching Speed, mm/min 3

Indenter Apex Angle, o 120 Indenter Tip Radius, µm 200

Indenter Material Tip Diamond

Results and Discussion To account for the material and elastic heterogeneity of our sandstone and shale samples, we perform multiple scratch tests at different locations on each sample. Micrograph and summary plots of each scratch test are shown in Figures 4-15. Below we analyze our results by each measurement variable. Maximum Depth of Penetration: For both sandstone and shale, the maximum penetration is higher on surfaces cut parallel to bedding plane (Table 3). The reason is that the cohesive force between grains is weaker along a bedding plane than perpendicular to the bedding plane. At the microscopic scale where our examination is focused, other factors such as interaction between mineral composites may also be considered causative. The effect of lithology is such that the sandstone samples register a significantly higher maximum penetration depth compared to shale regardless of the sample orientation. This is due to a larger average grain size, pore size and degree of cementation in sandstone compared to shale. Frictional Force: On an average, the frictional force increases with the scratch length. A high-frequency oscillatory or saw-tooth trend imposed on a linear background trend can be used to decompose the frictional force vs. scratch length profile in shales (Figures 13-18). The sandstone samples, however, show large oscillations in frictional force and do not have a predictable trend. A slope change in the frictional force vs. scratch length profile usually indicates the occurrence of a failure event. For example, in Sample 1 Scratch 1 sandstone test, the frictional force increases linearly with increase in the normal force until the normal force exceeds 15N at which point the frictional force suddenly drops which could indicate sudden movement of a grain or cracks in the cementation. At the normal force of 18N, the normal force versus the true depth relationship changes such that the depth is no longer increasing with the normal force. The frictional force also flattens out. This could indicate the presence of stiffer contact configuration at this point. Scratch/Fracture Toughness: Both shale and sandstone show higher values of fracture toughness for samples cut perpendicular to bedding plane. The toughness values for the shale and sandstone lithologies in the transverse

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orientation are quite similar. It is also important to point out that given our scale of investigation, local changes in constituent minerals, pore/grain structure affect the fracture toughness value. We see in the shale samples that for the orientations parallel to bedding, the scratch toughness converges to an average value as the scratch length increases (Figure 4a, b, c). Similar behavior has been observed in homogenous materials and other shale systems (Akono & Kabir, 2016) under brittle fracture-driven deformation. High oscillations seen in frictional force are also present in the toughness profile of shale Similar conclusion cannot be drawn for sandstone (Figure 3). A reason may be that, unlike shales with an almost uniform grain size, scratch on a sandstone surface transitions between fine and coarse grains, which exhibit different mechanical properties. Perpendicular to bedding, the fracture toughness profile for shale (Figure 4c, d, e) is like that of sandstone. This could be due to scratch path encountering more than one layer and thus fine-coarse grain transition. Acoustic Emission: Acoustic emission, AE, can be combined with SEM to identify zones of fracture/microcrack events. For the sandstone samples we studied, AEs are associated with a decline in the frictional force for both orientations studied (Figure 7-12). Table 3: Comparison of Results from Micro-scratch Test

Sample Number

Maximum Depth of Penetration

(mm)

Fracture Toughness (MPa.m1/2)

Scratch 1 Scratch 2 Scratch 3 1 0.56 1.64 ± 0.17 1.89 ± 0.32 2.09 ± 0.40

2 0.35 2.44 ± 0.23 6.87 ± 0.59 3.96 ± 0.33 3 0.26 9.41 ± 1.73 9.06 ± 0.90 8.09 ± 0.57 4 0.21 8.22 ± 2.80 10.70 ± 1.25 11.63 ± 0.56

Figure 3: The scratch toughness, Ks, for sandstone samples parallel (a, b and c) and perpendicular (c, d, e) to the bedding plane. The values show convergence with respect to the scratch length.

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Figure 4: The scratch toughness, Ks, for shale samples parallel (a, b and c) and perpendicular (c, d, e) to the bedding plane. Post-Scratch SEM Scanning electron microscopy allows us to observe fracturing mechanisms induced by scratching and identify phenomena such as branching, crack/fracture bridging, particle pull-out, and crack deflection. It can also be used measure the quality (roughness/smoothness) of the fracture surface. We use a Field Emission Gun Scanning Electron Microscope (Model: JEOL 7001) at different magnification levels to study the sample. In Figure 6, we observe that due to progressive loading, fracture width is increasing as the stylus is drawn across the surface. Particle pull-out locations are marked as (1). Other noticeable phenomena include surface flattening and crack bridging marked as (3) and crack deflection marked as (2) where the stylus encounters a pre-existing fractured surface and briefly takes the path of less resistance. In Fig 3(a), we notice that the scratch stylus does not leave behind a clean groove in Sample 3. Rather, we have a pile of deformed material atop the scratched surface.

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Figure 6: Post-Scratch SEM of samples showing different fracture mechanisms. Fracture width increases as stylus is drawn across the surface. Particle pull-out (1), crack deflection (2), and crack bridging (3), which are fracture initiation and propagation mechanisms, are marked. Conclusions We have studied the directional deformation response of shale and sandstone samples under microscratch loading to understand the effect of lithology and orientation on fracturing mechanisms. Our results identify the role of grain size and bedding plane direction on fracture width, directionality, and toughness at the microscopic scale. Higher scratch toughness values perpendicular to the bedding plane are observed in both sandstone and shale lithologies. Relative variation in the toughness value among multiple scratch tests per sample quantifies the effect of heterogeneity and anisotropy of a given lithology on the fracturing process. We propose that this work will improve microscopic understanding of mechanical failure processes in sandstone and shale, which can be used to design hydraulic fracturing jobs and improve oil recovery. Acknowledgments We want to thank the management of California Resources Corporation (CRC) for supporting this project and providing the core samples. We want to thank Robert Gales, Max Willis, and Rad Sobczyk of CRC for helping us in selecting the coring locations. We thank Core Laboratories, Larry Kunkel and Christopher Lalonde for assistance in cutting the core plugs.

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Appendix

Figure 7: Scratch Test and Micrograph for Sample 1 (Scratch 1)


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Microscale Laboratory Studies for Determining …...is seen in the horizontal force profile. Comparing the horizontal force to the AE time series, the AE peaks are found to decline