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Dynamic microscale crack propagation in shale
Engineering Fracture Mechanics 228 (2020) 106906
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Dynamic microscale crack propagation in shale Junliang Zhaoa, Dongxiao Zhangb, a b
⁎
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BIC-ESAT, ERE and SKLTCS, College of Engineering, Peking University, Beijing 100871, PR China School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, PR China
ARTICLE INFO
ABSTRACT
Keywords: Mechanical heterogeneity Crack propagation Micro fabrication In situ fracture experiments Fracture mechanisms
Hydraulic fracturing and horizontal well are the two key techniques for the development of shale oil/gas. Mechanical heterogeneity and anisotropy of shale strongly influence the effects of fracturing. While numerous studies have focused on the fracture behavior of shale, the microscale fracture mechanisms of shale remain poorly understood. In this study, three representative test areas, which were dominated by a stiff mineral, a blob of organic matter and clay layers, respectively, were selected in a terrestrial shale sample from the Yanchang Formation. Micro cantilever beams were manufactured using the micro fabrication method in each test area, and in situ fracture experiments were performed to investigate crack propagation under a scanning electron microscope. The crack propagation process was characterized at micrometer scale in shale. The crack paths and the load-displacement curves revealed various fracture mechanisms. Crack deflection and crack branching owing to mechanical contrast, toughening in organic matter, and crack bridging of clay layers were observed and discussed. This study provided a novel insight into micro crack behavior in shale and presents a new framework for the investigation of the fracture characteristics of shale. The microscale shale characterization and crack mechanisms can serve as a basis or building blocks for mesoscale modeling of fracturing in shale, and the load-displacement data and fracture behaviors can provide a valuable dataset for validating modeling approaches.
1. Introduction Shale oil/gas reservoirs are characterized by their extremely low permeability [1,2]. To increase the permeability, horizontal well and hydraulic fracturing have been widely utilized in the development of unconventional shale oil/gas. During fracturing, waterinjection induced cracks propagate into the rock formation and create a path for the migration of hydrocarbons. The mechanical property of shale constitutes an important factor influencing the fracturing effect and the design of the fracturing plan [3–5]. As a kind of natural composite material, shale is a clay-based mixture of various inorganic and organic constituents [6,7]. In this multiphase system, clay minerals usually play the role of the matrix, and non-clay minerals and organic matter are the inclusions. Like other materials possessing hierarchical structures, such as bone [8,9] and wood [10], the macroscopic mechanical behavior of shale depends on the microscopic properties of different constituents [11,12]. Taking advantage of micromechanical test methods, numerous works have investigated the elastic properties of the main constituents in shale [13–16]. Quasi-static nanoindentation, modulus mapping by nanoindenter, and quantitative nanomechanical mapping by atomic force microscope (AFM) are commonly used micromechanical techniques. These previous works found strong mechanical heterogeneity in shale. The reported Young’s moduli of inclusion minerals, such as quartz, feldspar and carbonate, are ⁎
Corresponding author. E-mail address: [email protected] (D. Zhang).
https://doi.org/10.1016/j.engfracmech.2020.106906 Received 29 September 2019; Received in revised form 22 January 2020; Accepted 23 January 2020 Available online 03 February 2020 0013-7944/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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generally above 50 GPa, while the Young’s modulus of organic matter is approximately 10 GPa [14,15,17]. In addition, compared with the inclusion minerals and organic matter, the mechanical properties of clay minerals exhibit significant anisotropy [18–20]. Besides elastic behavior, mechanical variation within the micro constituents may also affect fracture behavior during hydraulic fracturing. Field-scale and mineral-scale simulations, as well as core-scale experiments, suggested that besides the macroscopic stress field, the existence of different minerals with microscale mechanical heterogeneity and the microstructure in shale may also influence the development of the hydraulic fractures, resulting in more tortuous fracture surfaces and more complex fracture patterns [21–23]. Therefore, the study of the microscale fracture behavior of shale is important. Conventional investigations of fracture propagation in shale focused on macroscopic mechanical tests, such as uniaxial/triaxial compression [24–26] and laboratory fracturing simulation experiments [22,27–29]. In these studies, the researchers cannot predict or determine what kind of constituents will interact with the crack before the test. The summary of the crack phenomenon and the analysis of the fracture mechanism are usually based on the image of the broken sample after the test. However, since the distributions of the constituents are random, the crack propagation paths can also be random, which means that it is difficult to make the selected constituents fail, and the test results may be unable to provide the phenomenon expected by the researchers when designing the experiments. Consequently, there is only a low probability for the constituent (e.g., organic matter) of low content to be studied. Moreover, with the observation-after-broken research scheme, what occurred in the crack propagation process is difficult to directly observe or record, especially at micro scale. The description of the interactions between cracks and different constituents in shale is usually unintuitive and indirect. Finally, in the laboratory study of crack initiation and propagation, distinguishing natural cracks from cracks induced from sample preparation and handling always constitutes a problem, which makes the test results mixed and dubious [30]. Considering these challenges and problems, the in situ micromechanical test technique, which has been utilized in recent fracture studies of various materials [31–35], may be a viable choice for the fracture research of shale. Some extant literature [36,37] investigated the fracture characteristics of shale through the application of this technique. Tensile failures of unconventional source rocks were achieved at micro scale, and the fracture behavior and unconfined tensile strength in high and low kerogen regions were compared. However, due to the limitation of the sample preparation method, the distribution of the constituents on the sample surface is not sufficiently clear under the microscope. Moreover, the crack paths were not sufficiently described in these works, which leads to the loss of many details in the crack propagation process. Thus, the understanding of the crack propagation features in shale at micro scale remains incomplete. In this work, based on the in situ micromechanical test technique, we develop a new research scheme to study crack initiation and propagation in shale. Compared with previous works, the sample preparation method is improved. An ion-etched sample is observed under a scanning electron microscope (SEM). Micro cantilever beams are manufactured in the selected test areas using a gallium focused ion beam (GaFIB). Combining SEM observation and the micro fabrication method, we are free to choose any area of interest on the etched surface, and investigate the influence from determined constituents. After fabrication, the sample is put into SEM again, and the micro beams are loaded by a flat diamond indenter. Video of the deforming and fracturing process, and the corresponding load and displacement data, are recorded simultaneously. Thus, the interactions between micro cracks and different constituents can be visible, and the issue of discerning natural cracks from man-induced cracks no longer presents a problem. Considering the mechanical heterogeneity of shale, three test areas dominated by different representative constituents are selected. The crack propagation process in shale at micrometer scale is clearly characterized for the first time. Through analysis of the key frames and the loaddisplacement curves, the fracture mechanisms are discussed and summarized. 2. Materials and methods 2.1. Sample information The shale core used in this study was from the Triassic Yanchang Formation [38], a typical terrestrial shale formation in the Ordos Basin. X-ray diffraction (XRD) was performed to determine the mass percentage of minerals. The sample has high clay content that is approximately 70% by weight, and quartz, feldspar, and pyrite comprise the remaining 30%. The results from mercury intrusion porosimetry (MIP) and total organic content (TOC) test show 7.3% pore volume and 4.7% organic content by weight. In accordance with XRD results, we can find that there are complicated constituents in the shale sample by using backscattered electron (BSE) mode (Fig. 1a) and the advanced mineral identification and characterization system (AMICS) (Fig. 1b). The stiff minerals and organic matter randomly distribute in the clay matrix, and clay minerals, which are mainly illite, are generally parallel to the bedding plane of the shale sample. 2.2. Sample preparation Sample preparation is crucial for micromechanical tests. For conventional micromechanical test techniques, for example, nanoindentation, the sample surface usually experiences mechanical polishing and ion polishing. The roughness of the surface is below 100 nm, so that the indentation test can generate reliable results. For the in situ micromechanical test, FIB is commonly utilized to produce microstructures, such as micro pillars and nanowires. Here, ion etching (EM TIC 3X, Leica) and FIB milling (Orion NanoFab, Zeiss) were combined to manufacture the micro cantilever beams for the test. There are three steps for sample preparation. 1) Cutting and mechanical polishing. A block sample, which measures 5 mm × 5 mm × 2 mm, was cut from a shale core. The 5 mm × 5 mm face is parallel to the bedding plane. Then, the sample block was mechanically polished to decrease surface roughness and keep the corresponding faces parallel to each other (Fig. 2a). 2) Ion etching and test area selection. One 5 mm × 2 mm side was selected to perform ion etching (Fig. 2b). The ion-etched face was observed with an environmental scanning electron microscope (ESEM, Quanta 2
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Fig. 1. SEM observation of shale rock from the Yanchang Formation. (a) BSE image and (b) mineral identification results of the shale sample.
Fig. 2. Sample preparation for test area selection and micro fabrication. (a) Shale block which measures 5 mm × 5 mm × 2 mm. (b) Ion-etched region in the shale sample.
200F, FEI). Three representative areas, which were dominated by a stiff mineral, clay layers and a blob of organic matter, respectively, were selected to manufacture the micro cantilever beams. 3) FIB milling and refining. A gallium ion beam was used to prepare the microstructures. With the understanding of the contrast difference between the SEM image and gallium ion image, we located the selected test areas under the gallium ion microscope (Appendix A Fig. A1). The part above the test area was removed with a 15 nA beam current. The overall shape of the cantilever beam was also produced by a 15 nA beam current. All of the faces of the cantilever beam were then refined using a 7 nA beam current, except for the face that experienced ion etching. The geometry of the cantilever beams was designed with lengths of 30 μm, widths of 10 μm, and depths of 15 μm. The blank spaces were sufficiently reserved for the conical indenter to avoid unexpected collisions. For additional details about the sample 3
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Fig. 3. SEM observation and EDS characterization of the representative test areas. (a) SEM image of the test area dominated by inclusion mineral. (b) The manufactured micro cantilever beam in the first test area. (c) EDS result of the inclusion mineral. The mineral was determined to be feldspar. (d) SEM image of the test area dominated by organic matter. (e) The manufactured micro cantilever beam in the second test area. (f) EDS result of the organic matter. High carbon content was detected. (g) SEM image of the test area dominated by clay layers. (h) The manufactured micro cantilever beam in the third test area. (i) EDS result of the clay layers. Characteristic elements of clay mineral were detected.
preparation, refer to Appendix B Fig. B1. Fig. 3 exhibits the micro cantilever beams in the representative test areas. Energy dispersive spectrometer (EDS, PV7760/68 ME, EDAX) was performed to prove the chemical composition of the dominant element in each test area. 2.3. In situ fracture test The sample with micro cantilever beams was placed on the micro mechanical test stage (Fig. 4a and b), and the whole in situ mechanical test system (PI 88, Hysitron) was put into the ESEM (Fig. 4c and e). A conical diamond indenter with a 10 μm flat tip was used for the test. The sample and the indenter were observed under the microscope in a low vacuum condition using an accelerating voltage of 15 kV. At first, the test system was rotated 15° counterclockwise in the ESEM (Fig. 4c and d), so that the indenter could be moved to the test areas. Then, the test system was rotated 15° clockwise to keep the ion-etched face to be perpendicular to the electron beam (Fig. 4e and f). The flat indenter was placed at the free end of the cantilever beam. In displacement control mode, the indenter moved downwards at a set velocity of 15 nm/s. The loading direction is normal to the length direction of the cantilever beam. The contact force and displacement were recorded by the mechanical test system, and the deforming and fracturing process of the cantilever beam was captured by the ESEM simultaneously. Finally, the punch indenter moved back when the displacement reached the set maximum value.
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Fig. 4. Experimental settings for in situ fracture test. (a) SEM in situ mechanical test instrument. (b) Upper surface of the shale sample was perpendicular to the indenter. (c) Chamber scope and (d) SEM image of the micro cantilever beam when the test system was rotated 15° counterclockwise in the ESEM. (e) Chamber scope and (f) SEM image of the micro cantilever beam when the rotation angle moved to 0°.
3. Results 3.1. Crack propagation in the test area dominated by inclusion mineral The area containing a large inclusion mineral was selected as the first test area (Fig. 3a and b). The size of the inclusion is approximately 40 μm. Through EDS analysis, the inclusion was determined to be feldspar (Fig. 3c). By comparing the SEM image and the gallium ion image, the selected test areas were located under an ion microscope. Due to the advanced fabrication method, we were able to precisely place the dominant constituent at the fixed end of the cantilever beam. During the mechanical test, the flat indenter tip was placed at the free end of the cantilever beam (Fig. 5a). When the indenter moved downwards, the stress was concentrated in the fixed end of the beam. Thus, the micro crack was certain to be initiated and propagated in the region next to the fixed end. The region indicated by a red rectangle in Fig. 5a is the failure region. For better visualization, the failure region is magnified when revealing the test results (Fig. 5b). The relatively bright parts in the SEM images of the failure region, such as the large inclusion and the small mineral chips (black arrows), usually indicate stiff minerals. In contrast, the matrix with lower modulus and strength is the relatively dark part around the small chips. In addition, some micro pores are seen on the surface (white arrows). The edge of the inclusion is distinct in the failure region (red dotted line). Linear response of deformation was observed at the beginning of the test (Fig. 5c). At 69 s, a micro crack was initiated in the matrix (Fig. 5d and e), and a sudden load drop occurred in the load-displacement curve (Fig. 5f). The cantilever beam was then loaded continuously. When the crack tip moved to the edge of the inclusion (Fig. 5g), a branch crack was generated and trapped in 5
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the pore next to the inclusion (red arrow in Fig. 5h). A load turn can be found in the load-displacement curve (black circle in Fig. 5i). Then, the main crack moved along the edge of the inclusion (Fig. 5j and k), and the load decreased gradually with the crack propagation (Fig. 5l). At 289 s, the cantilever beam was almost broken (Fig. 5m and n). The total displacement of the indenter tip is 6
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Fig. 5. Micro crack propagation in the test area dominated by inclusion mineral and corresponding load-displacement curves. (a–c) Linear response of deformation at the beginning of the mechanical test. The red rectangle indicates the failure region. Black and white arrows indicate small mineral chips and pores on the surface, respectively. The red dotted line indicates the edge of the inclusion mineral. The load linearly increased with the displacement. (d–f) At 69 s, a micro crack (red arrow) was initiated in the matrix. The load suddenly dropped due to the crack initiation. (g–i) At 76 s, the crack tip reached the edge of the inclusion. A branch crack (red arrow) was generated and trapped in the pore. A load turn (black circle) can be found in the load-displacement curve. (j–l) The micro crack continued moving downwards along the edge of the inclusion. The load gradually decreased with the displacement. (m–o) At 289 s, the cantilever beam was almost broken. The load on the beam decreased to 0. The total displacement of the indenter was approximately 4 μm (see Supplementary material Movie S1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
approximately 4 μm. 3.2. Crack propagation in the test area dominated by organic matter The area containing a large blob of organic matter was selected as the second test area (Fig. 3d and e). The size of the organic matter blob is approximately 40 μm. High carbon content was detected in the organic matter (Fig. 3f). Similarly, the dominant element was placed at the fixed end of the cantilever beam, while the indenter tip was placed at the free end (Fig. 6a). In the failure region (red rectangle in Fig. 6a), the constituents are basically organic matter (dark part) and the matrix (bright part). The red dotted line indicates the edge of the organic matter blob (Fig. 6b). The cantilever beam experienced the linear response of deformation at first (Fig. 6c). The micro crack was initiated at 95 s (Fig. 6d and e), and the first load drop occurred in the load-displacement curve (Fig. 6f). Then, the flat indenter kept moving downwards. When the crack tip reached the boundary between the organic matter and the matrix (Fig. 6g), the crack penetrated into the organic matter (Fig. 6h). The second load drop then occurred (Fig. 6i). Subsequently, the crack propagated in the organic matter (Fig. 6j and k), and the load increased with the crack propagation (Fig. 6l). At the conclusion of the test, the cantilever beam was still sustained by the organic matter (Fig. 6m). The direction of the crack in organic matter (white double arrow in Fig. 6n) is generally parallel to the loading direction. The final load of the test was approximately 8 mN (Fig. 6o). 3.3. Crack propagation in the test area dominated by clay layers The area containing a high-percentage of clay layers was selected as the third test area (Fig. 3g and h). Characteristic elements for clay minerals were detected through EDS (Fig. 3i). Clay layers are the dominant constituent in this case (Fig. 7a). Different from the other two cases, there is no large particle in the test area. Most of the stiff minerals are smaller than 5 μm in the failure region (Fig. 7b). Moreover, the distribution of the fine clay layers is not uniform. In the upper part of the failure region, there are more small mineral chips, and the clay layers are disordered (black arrows). The small mineral chips restrict the distribution of the clay layers (red dotted lines). In contrast, the lower part of the failure region contains fewer small mineral chips, and the clay layers (white arrows) are generally along the length direction of the cantilever beam (red dotted double arrow). During the mechanical test, the load increased with the displacement linearly (Fig. 7c), until a micro crack was initiated at the fixed end of the cantilever beam (Fig. 7d). In the failure region, the initial crack is along the layering direction of the clay layer at the top left (Fig. 7e). The load drop occurred with the crack initiation (Fig. 7f). At 101 s, the crack tip moved to the middle of the failure region (Fig. 7g). In the upper part of the failure region, the micro crack is along the clay layers, and the crack flanks appear smooth (Fig. 7h). The load decreased with the crack propagation (Fig. 7i). Then, the crack propagated in the lower part of the failure region (Fig. 7j). The crack path is normal to the layering direction of the ordered clay layers, and the crack flanks appear distorted (Fig. 7k). We can also find that the load reduction slows from line part ① to line part ② in the load-displacement curve (Fig. 7l). At 455 s, the cantilever beam was almost broken (Fig. 7m). The smooth and distorted crack flanks are clearer in the failure region (Fig. 7n). The total displacement of the indenter tip is approximately 6.5 μm (Fig. 7o). 4. Discussion 4.1. Load-displacement curves and fracture characteristics Through the in situ micro mechanical test, we observed micro crack propagation in selected test areas with different dominant elements, and recorded load and displacement data during the deforming and fracturing process. The load-displacement curves for the three test areas are summarized in Fig. 8. For all of the three cases, the pre-failure curves indicate the linear response of deformation of the micro cantilever beams. For the test areas dominated by the inclusion mineral and clay layers, the load for the micro crack initiation is approximately 2.6 mN and 2.3 mN, respectively. The crack initiation load is the peak load of the curve. The load-displacement curves exhibit the features of brittle failure. However, the load for the crack initiation in the test area dominated by organic matter is approximately 4.7 mN, which is much higher than the other two cases. There are two load drops in the loaddisplacement curve, which indicates crack initiation in the matrix and penetration into the organic matter, respectively. In addition, after the load drop, the load did not decrease like the other two cases. We failed to completely break the cantilever beam dominated by organic matter with the experimental settings. Prior to the retreat of the indenter tip, a high load still exists, which is approximately 8 mN, on the cantilever beam. The load-displacement curve of this case exhibits the features of ductile failure. 7
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4.2. Crack paths and fracture mechanisms The paths of the micro cracks reveal the interactions between the cracks and the constituents in shale. The micro cracks were all generated in the matrix in the three cases. For the test area dominated by clay layers, the crack was initiated at the fixed end of the
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Fig. 6. Micro crack propagation in the test area dominated by organic matter and corresponding load-displacement curves. (a–c) Linear response of deformation when the displacement was smaller than 1 μm. The red rectangle indicates the failure region. The red dotted line indicates the edge of the organic matter. (d–f) At 95 s, a micro crack (red arrow) was initiated. The first load drop (number ①) occurred in the load-displacement curve. (g–i) At 139 s, the micro crack penetrated into the organic matter, and the second load drop (number ②) occurred in the load-displacement curve. (j–l) After the penetration, the load was increased to keep the crack propagation in the organic matter. (m–o) At 489 s, the displacement of the indenter reached the maximum value set before the test. The cantilever beam was still sustained by the organic matter after the indenter moved back. The direction of the crack in organic matter (white double arrow) is generally parallel to the loading direction. The final load on the cantilever beam was approximately 8 mN (see Supplementary material Movie S2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cantilever beam (Fig. 7e). However, for the other two cases, the initial crack is not at the end of the cantilever beam (Figs. 5e and 6e), due to support from the inclusion mineral and organic matter blob, respectively. Before the crack tip touched the edge of the mineral inclusion, micro cracks tended to avoid the stiff mineral chips, and propagated in clay matrix with lower modulus and strength (Fig. 5h and 7h,n). Such a crack deflection is obvious in the failure region. The mechanical contrast of the main constituents in shale significantly influences the propagation paths. The interactions between the micro crack and the large inclusions in shale are quite different. For the first case, when the crack tip touched the edge of the inclusion mineral, the crack did not penetrate into the mineral. Crack branches were generated and moved along the edge of the inclusion mineral (Fig. 5h and k). The load decrease indicated by the black circle after the load drop in Fig. 5i suggests energy release due to the branch crack initiation. For the second case, the crack tip penetrated the boundary between the organic matter and the matrix, and moved in the organic matter in the depth direction (Fig. 6h and k). The behavior of the organic matter in the fracture problem is intriguing. With the lowest modulus and strength in shale [15], organic matter cannot prevent invasion from the micro crack. However, after penetration, increased energy was demanded to sustain the propagation of the micro crack (Fig. 6k and n). Previous works have also discussed this behavior of organic matter [39,40]. Many researches attempted to define a brittleness index based on the empirical identification of brittle and ductile constituents [1,41]. These two cases intuitively exhibit the interaction between the micro crack and the brittle and ductile constituents in shale, and demonstrate the significant influence of organic matter on the brittleness [6,42]. For formations with high organic content, extra energy is needed to induce fractures. However, thermal maturity and organic type may influence the way that organic matters deform and fracture [43,44], which needs to be studied in future work. Besides organic matter, the mechanical properties of clay have also long been studied [19,45,46]. As a kind of phyllosilicate mineral, clay is usually treated as the main source of the anisotropy of shale [18,47,48]. With less compressibility, the modulus and strength of clay in the direction parallel to the layer structure are higher than those in the normal direction. From the third case, we can find that the relationship between the loading direction and the layering direction also influences the mechanical behavior of clay in the fracturing problem. From the upper part to the lower part of the failure region, the propagation direction of the micro crack is from along the clay layer to perpendicular to the layer structure, and the crack paths change from smooth to zig-zag (Fig. 7h and k). The change of the crack flank appearance indicates a transition from brittle fracture to ductile fracture. Interestingly, a similar phenomenon has been identified in some biological materials containing fiber structures, such as bone [49] and wood [50]. The difference is that fracture mechanism investigations for most biological materials aim to prevent the generation of micro cracks. However, for the development of shale oil/gas, the generation of complex cracks is requisite. From the discussion above, we can find that due to the mechanical heterogeneity and the anisotropic structure, a variety of fracture mechanisms in shale exist at micro scale. Crack deflection and crack branching owing to mechanical contrast, toughening in organic matter, and crack bridging of clay layers together influence crack propagation in shale (Fig. 9). Rock is assumed to be homogeneous at micro scale in most hydraulic fracturing models. However, it is difficult to directly observe the microscopic phenomena via macroscopic fracture experiments. This work provides a detailed description of the micro crack initiation and propagation process in typical cases by focusing on the influence of single constituents. Such information may give the mechanical parameters and lead to modification of the fracture criteria used at a greater scale. In addition, microscopic phenomena can also yield information useful for engineering practice. For example, crack paths (penetrating into or moving around the minerals) may influence the fracture geometry and the selection of proppants. Finally, recent simulations attempted to investigate the interactions between micro crack and different constituents in shale with mesoscale modeling approaches [21,51]. Detailed characterization of crack propagation processes and crack mechanisms for microscale constituents are required in order to render such simulations realistic. This microscale crack characterization can serve as a basis or building blocks for mesoscale modeling of fracturing in shale, provide a valuable dataset for validating the mesoscale modeling approaches, and assist in understanding the complexity in fracture surfaces and fracture patterns at macroscopic scale. 5. Conclusion This paper experimentally investigates micro crack propagation in shale. Three representative test areas in the shale block from the Yanchang Formation are selected to manufacture the micro cantilever beams and perform in situ fracture tests. Based on the loaddisplacement curves, the fracture characteristics in the three cases are compared. For the test areas dominated by the inclusion mineral and clay layers, the crack initiation load is the maximum load of the curve, which is approximately 2.6 mN and 2.3 mN, respectively. The load-displacement curves exhibit the features of brittle failure. For the test area dominated by organic matter, the crack initiation load is approximately 4.7 mN. After the crack penetration into the organic matter blob, the load increases with the 9
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Fig. 7. Micro crack propagation in the test area dominated by clay layers and corresponding load-displacement curves. (a–c) At 16 s, there was no obvious change in the failure region (indicated by the red rectangle). Red dotted lines indicate the layering direction of the disordered clay layers (black arrows). The red dotted double arrow indicates the layering direction of the ordered clay layers (white arrow). (d–f) At 58 s, a micro crack (red arrow) was initiated at the fixed end of the cantilever beam. The load dropped from over 2 mN to lower than 1 mN. (g–i) After initiation, the micro crack propagated along clay layers (red arrow). The load decreased gradually with the propagation. (j–l) In the lower part of the failure region, the micro crack propagated perpendicular to the layering direction (red arrow). The reduction rate of load in this part (number ②) was smaller than in the previous part (number ①). (m–o) At 455 s, the cantilever beam was almost broken. The total displacement of the indenter was approximately 6.5 μm (see Supplementary material Movie S3). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Load-displacement curves in three test areas.
Fig. 9. The fracture mechanisms in shale at microscale. (a) Crack deflection. The magenta ellipse indicates the stiff mineral chips in shale. (b) Crack branching. The red dotted line indicates the edge of the inclusion mineral. (c) Toughening in organic matter. The blue ellipse indicates the organic blobs in shale. White arrows indicate resistance to the crack propagation in organic matter. (d) Crack bridging. The green lines indicate the clay layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
crack propagation. The load-displacement curve for this case exhibits the features of ductile failure. Furthermore, through the analysis of the crack paths, four micro fracture mechanisms are summarized. Crack deflection, crack branching, toughening in organic matter, and crack bridging together influence the crack propagation in shale at microscale. The characterization results may serve as a basis or building blocks for mesoscale fracturing modeling, and the observed cracking behaviors may be used for (in) validating such modeling approaches. Different from previous experimental studies on crack propagation in shale rock, this work chooses three typical cases to simplify complex fracturing problems. Moreover, the test areas for the fracture test are determined beforehand. Based on the micro fabrication and in situ mechanical test method, a new framework is provided for the investigation of the fracture characteristics of shale at the
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mineral level. The experimental settings are given in detail in this paper, and further studies and improvements can be made based on this work. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is partially funded by the National Natural Science Foundation of China (Grant No. U1663208 and 51520105005) and the National Science and Technology Major Project of China (Grant No. 2017ZX05009-005 and 2017ZX05049-003). Appendix A. . Location of selected test areas In Section 2, three representative test areas were selected to manufacture the micro cantilever beams through SEM observation. For micro fabrication, the selected test areas of tens of micrometers need to be located under the ion microscope. The imaging principles of the electron microscope and the ion microscope are different. For the electron microscope, the electron beam scans the sample surface, and the secondary electron (SE) or BSE signal generated by the incident electrons is collected. When observing a composite material by using the SE signal, the electrons accumulate on the surface of the nonconductive parts in the material, which makes the nonconductive parts relatively bright. However, for the ion microscope, the ion-generated secondary electron (iSE) signal contains the information of the sample surface [52]. When observing a composite material by using the iSE signal, the ions tend to accumulate on the surface of the nonconductive parts and prevent the collection of the ion-generated secondary electrons, which makes the nonconductive parts relatively dark. In Fig. A1, we can find that the image contrasts are generally opposite under different microscopes for both inclusion minerals and organic matters (red dashed circles). The image contrast features can assist to locate the selected test areas.
Fig. A1. Image contrast difference between SEM image and gallium ion image. (a) Under the electron microscope, organic matters are usually black, while the inclusion minerals, such as quartz, feldspar, and carbonate are white and bright. (b) Under the gallium ion microscope, due to the poor electro conductivity, the ions tend to accumulate on the surface of the inclusion minerals, and the secondary electrons generated by ion injection cannot be detected by the ET detector. Thus, the inclusion minerals become black, while organic matters are much brighter. Red dashed circles indicate the same organic matter region under different microscopes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 12
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Appendix B. . Fabrication of micro cantilever beam There are various factors that influence the fabrication of the micro cantilever beam. The dose of the gallium ions determines the milling depth. The magnitude of the beam current influences the milling efficiency. A higher beam current results in a shorter fabrication time, but a higher risk of beam damage on the sample surface. Moreover, the scanning direction is also important. The red arrows in Fig. B1 indicate the scanning directions of the ion beams. The smooth faces are usually generated in the front sides of the
(caption on next page) 13
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Fig. B1. Experimental procedures for the fabrication of micro cantilever beam. (a–h) Gallium ion images of the sample in the fabrication process. Red rectangles indicate the milling region. Red arrows indicate the scanning directions of the gallium ion beams. The accelerating voltage is 30 kV. Blue dashed boxes indicate deposition and pollution on the sample after FIB milling. (a) Rotate and tilt the sample stage to make the ion-etched face perpendicular to the gallium ion beam. Scan the red region with a beam current of 15 nA and a dose of 150 nC/μm2 to obtain the upper surface of the cantilever beam and to reserve sufficient blank space for the indenter tip. (b) Scan the red region with a beam current of 15 nA and a dose of 100 nC/μm2 to prepare the right side of the cantilever beam. (c) Scan the red region with a beam current of 15 nA and a dose of 100 nC/μm2 to obtain the lower face of the cantilever beam. (d) The fabrication result of the ion-etched face. (e) Rotate and tilt the sample stage to make the 5 mm × 5 mm face perpendicular to the gallium ion beam. Scan the red region with a beam current of 15 nA and a dose of 100 nC/μm2 to obtain the inner face of the cantilever beam. (f) The fabrication result of the 5 mm × 5 mm face. (g) Refine the faces of the cantilever beam with a beam current of 7 nA and a dose of 60 nC/μm2. (h) The gallium ion image of the manufactured cantilever beam. (i) SEM image of the manufactured micro cantilever beam. (j) Chamber scope for step a-d, g, and h. The ion-etched face is perpendicular to the gallium ion beam. (k) Chamber scope for step e and f. The upper surface of the sample is perpendicular to the gallium ion beam. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
arrows, while deposition and pollution may occur in the other sides (blue dashed boxes). Fig. B1 illustrates the detailed experimental settings of the micro fabrication on shale samples. The settings may change with the sample material and fabrication purpose. Appendix C. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.engfracmech.2020.106906.
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Dynamic microscale crack propagation in shale Junliang Zhaoa, Dongxiao Zhangb, a b
⁎
T
BIC-ESAT, ERE and SKLTCS, College of Engineering, Peking University, Beijing 100871, PR China School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, PR China
ARTICLE INFO
ABSTRACT
Keywords: Mechanical heterogeneity Crack propagation Micro fabrication In situ fracture experiments Fracture mechanisms
Hydraulic fracturing and horizontal well are the two key techniques for the development of shale oil/gas. Mechanical heterogeneity and anisotropy of shale strongly influence the effects of fracturing. While numerous studies have focused on the fracture behavior of shale, the microscale fracture mechanisms of shale remain poorly understood. In this study, three representative test areas, which were dominated by a stiff mineral, a blob of organic matter and clay layers, respectively, were selected in a terrestrial shale sample from the Yanchang Formation. Micro cantilever beams were manufactured using the micro fabrication method in each test area, and in situ fracture experiments were performed to investigate crack propagation under a scanning electron microscope. The crack propagation process was characterized at micrometer scale in shale. The crack paths and the load-displacement curves revealed various fracture mechanisms. Crack deflection and crack branching owing to mechanical contrast, toughening in organic matter, and crack bridging of clay layers were observed and discussed. This study provided a novel insight into micro crack behavior in shale and presents a new framework for the investigation of the fracture characteristics of shale. The microscale shale characterization and crack mechanisms can serve as a basis or building blocks for mesoscale modeling of fracturing in shale, and the load-displacement data and fracture behaviors can provide a valuable dataset for validating modeling approaches.
1. Introduction Shale oil/gas reservoirs are characterized by their extremely low permeability [1,2]. To increase the permeability, horizontal well and hydraulic fracturing have been widely utilized in the development of unconventional shale oil/gas. During fracturing, waterinjection induced cracks propagate into the rock formation and create a path for the migration of hydrocarbons. The mechanical property of shale constitutes an important factor influencing the fracturing effect and the design of the fracturing plan [3–5]. As a kind of natural composite material, shale is a clay-based mixture of various inorganic and organic constituents [6,7]. In this multiphase system, clay minerals usually play the role of the matrix, and non-clay minerals and organic matter are the inclusions. Like other materials possessing hierarchical structures, such as bone [8,9] and wood [10], the macroscopic mechanical behavior of shale depends on the microscopic properties of different constituents [11,12]. Taking advantage of micromechanical test methods, numerous works have investigated the elastic properties of the main constituents in shale [13–16]. Quasi-static nanoindentation, modulus mapping by nanoindenter, and quantitative nanomechanical mapping by atomic force microscope (AFM) are commonly used micromechanical techniques. These previous works found strong mechanical heterogeneity in shale. The reported Young’s moduli of inclusion minerals, such as quartz, feldspar and carbonate, are ⁎
Corresponding author. E-mail address: [email protected] (D. Zhang).
https://doi.org/10.1016/j.engfracmech.2020.106906 Received 29 September 2019; Received in revised form 22 January 2020; Accepted 23 January 2020 Available online 03 February 2020 0013-7944/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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generally above 50 GPa, while the Young’s modulus of organic matter is approximately 10 GPa [14,15,17]. In addition, compared with the inclusion minerals and organic matter, the mechanical properties of clay minerals exhibit significant anisotropy [18–20]. Besides elastic behavior, mechanical variation within the micro constituents may also affect fracture behavior during hydraulic fracturing. Field-scale and mineral-scale simulations, as well as core-scale experiments, suggested that besides the macroscopic stress field, the existence of different minerals with microscale mechanical heterogeneity and the microstructure in shale may also influence the development of the hydraulic fractures, resulting in more tortuous fracture surfaces and more complex fracture patterns [21–23]. Therefore, the study of the microscale fracture behavior of shale is important. Conventional investigations of fracture propagation in shale focused on macroscopic mechanical tests, such as uniaxial/triaxial compression [24–26] and laboratory fracturing simulation experiments [22,27–29]. In these studies, the researchers cannot predict or determine what kind of constituents will interact with the crack before the test. The summary of the crack phenomenon and the analysis of the fracture mechanism are usually based on the image of the broken sample after the test. However, since the distributions of the constituents are random, the crack propagation paths can also be random, which means that it is difficult to make the selected constituents fail, and the test results may be unable to provide the phenomenon expected by the researchers when designing the experiments. Consequently, there is only a low probability for the constituent (e.g., organic matter) of low content to be studied. Moreover, with the observation-after-broken research scheme, what occurred in the crack propagation process is difficult to directly observe or record, especially at micro scale. The description of the interactions between cracks and different constituents in shale is usually unintuitive and indirect. Finally, in the laboratory study of crack initiation and propagation, distinguishing natural cracks from cracks induced from sample preparation and handling always constitutes a problem, which makes the test results mixed and dubious [30]. Considering these challenges and problems, the in situ micromechanical test technique, which has been utilized in recent fracture studies of various materials [31–35], may be a viable choice for the fracture research of shale. Some extant literature [36,37] investigated the fracture characteristics of shale through the application of this technique. Tensile failures of unconventional source rocks were achieved at micro scale, and the fracture behavior and unconfined tensile strength in high and low kerogen regions were compared. However, due to the limitation of the sample preparation method, the distribution of the constituents on the sample surface is not sufficiently clear under the microscope. Moreover, the crack paths were not sufficiently described in these works, which leads to the loss of many details in the crack propagation process. Thus, the understanding of the crack propagation features in shale at micro scale remains incomplete. In this work, based on the in situ micromechanical test technique, we develop a new research scheme to study crack initiation and propagation in shale. Compared with previous works, the sample preparation method is improved. An ion-etched sample is observed under a scanning electron microscope (SEM). Micro cantilever beams are manufactured in the selected test areas using a gallium focused ion beam (GaFIB). Combining SEM observation and the micro fabrication method, we are free to choose any area of interest on the etched surface, and investigate the influence from determined constituents. After fabrication, the sample is put into SEM again, and the micro beams are loaded by a flat diamond indenter. Video of the deforming and fracturing process, and the corresponding load and displacement data, are recorded simultaneously. Thus, the interactions between micro cracks and different constituents can be visible, and the issue of discerning natural cracks from man-induced cracks no longer presents a problem. Considering the mechanical heterogeneity of shale, three test areas dominated by different representative constituents are selected. The crack propagation process in shale at micrometer scale is clearly characterized for the first time. Through analysis of the key frames and the loaddisplacement curves, the fracture mechanisms are discussed and summarized. 2. Materials and methods 2.1. Sample information The shale core used in this study was from the Triassic Yanchang Formation [38], a typical terrestrial shale formation in the Ordos Basin. X-ray diffraction (XRD) was performed to determine the mass percentage of minerals. The sample has high clay content that is approximately 70% by weight, and quartz, feldspar, and pyrite comprise the remaining 30%. The results from mercury intrusion porosimetry (MIP) and total organic content (TOC) test show 7.3% pore volume and 4.7% organic content by weight. In accordance with XRD results, we can find that there are complicated constituents in the shale sample by using backscattered electron (BSE) mode (Fig. 1a) and the advanced mineral identification and characterization system (AMICS) (Fig. 1b). The stiff minerals and organic matter randomly distribute in the clay matrix, and clay minerals, which are mainly illite, are generally parallel to the bedding plane of the shale sample. 2.2. Sample preparation Sample preparation is crucial for micromechanical tests. For conventional micromechanical test techniques, for example, nanoindentation, the sample surface usually experiences mechanical polishing and ion polishing. The roughness of the surface is below 100 nm, so that the indentation test can generate reliable results. For the in situ micromechanical test, FIB is commonly utilized to produce microstructures, such as micro pillars and nanowires. Here, ion etching (EM TIC 3X, Leica) and FIB milling (Orion NanoFab, Zeiss) were combined to manufacture the micro cantilever beams for the test. There are three steps for sample preparation. 1) Cutting and mechanical polishing. A block sample, which measures 5 mm × 5 mm × 2 mm, was cut from a shale core. The 5 mm × 5 mm face is parallel to the bedding plane. Then, the sample block was mechanically polished to decrease surface roughness and keep the corresponding faces parallel to each other (Fig. 2a). 2) Ion etching and test area selection. One 5 mm × 2 mm side was selected to perform ion etching (Fig. 2b). The ion-etched face was observed with an environmental scanning electron microscope (ESEM, Quanta 2
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Fig. 1. SEM observation of shale rock from the Yanchang Formation. (a) BSE image and (b) mineral identification results of the shale sample.
Fig. 2. Sample preparation for test area selection and micro fabrication. (a) Shale block which measures 5 mm × 5 mm × 2 mm. (b) Ion-etched region in the shale sample.
200F, FEI). Three representative areas, which were dominated by a stiff mineral, clay layers and a blob of organic matter, respectively, were selected to manufacture the micro cantilever beams. 3) FIB milling and refining. A gallium ion beam was used to prepare the microstructures. With the understanding of the contrast difference between the SEM image and gallium ion image, we located the selected test areas under the gallium ion microscope (Appendix A Fig. A1). The part above the test area was removed with a 15 nA beam current. The overall shape of the cantilever beam was also produced by a 15 nA beam current. All of the faces of the cantilever beam were then refined using a 7 nA beam current, except for the face that experienced ion etching. The geometry of the cantilever beams was designed with lengths of 30 μm, widths of 10 μm, and depths of 15 μm. The blank spaces were sufficiently reserved for the conical indenter to avoid unexpected collisions. For additional details about the sample 3
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Fig. 3. SEM observation and EDS characterization of the representative test areas. (a) SEM image of the test area dominated by inclusion mineral. (b) The manufactured micro cantilever beam in the first test area. (c) EDS result of the inclusion mineral. The mineral was determined to be feldspar. (d) SEM image of the test area dominated by organic matter. (e) The manufactured micro cantilever beam in the second test area. (f) EDS result of the organic matter. High carbon content was detected. (g) SEM image of the test area dominated by clay layers. (h) The manufactured micro cantilever beam in the third test area. (i) EDS result of the clay layers. Characteristic elements of clay mineral were detected.
preparation, refer to Appendix B Fig. B1. Fig. 3 exhibits the micro cantilever beams in the representative test areas. Energy dispersive spectrometer (EDS, PV7760/68 ME, EDAX) was performed to prove the chemical composition of the dominant element in each test area. 2.3. In situ fracture test The sample with micro cantilever beams was placed on the micro mechanical test stage (Fig. 4a and b), and the whole in situ mechanical test system (PI 88, Hysitron) was put into the ESEM (Fig. 4c and e). A conical diamond indenter with a 10 μm flat tip was used for the test. The sample and the indenter were observed under the microscope in a low vacuum condition using an accelerating voltage of 15 kV. At first, the test system was rotated 15° counterclockwise in the ESEM (Fig. 4c and d), so that the indenter could be moved to the test areas. Then, the test system was rotated 15° clockwise to keep the ion-etched face to be perpendicular to the electron beam (Fig. 4e and f). The flat indenter was placed at the free end of the cantilever beam. In displacement control mode, the indenter moved downwards at a set velocity of 15 nm/s. The loading direction is normal to the length direction of the cantilever beam. The contact force and displacement were recorded by the mechanical test system, and the deforming and fracturing process of the cantilever beam was captured by the ESEM simultaneously. Finally, the punch indenter moved back when the displacement reached the set maximum value.
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Fig. 4. Experimental settings for in situ fracture test. (a) SEM in situ mechanical test instrument. (b) Upper surface of the shale sample was perpendicular to the indenter. (c) Chamber scope and (d) SEM image of the micro cantilever beam when the test system was rotated 15° counterclockwise in the ESEM. (e) Chamber scope and (f) SEM image of the micro cantilever beam when the rotation angle moved to 0°.
3. Results 3.1. Crack propagation in the test area dominated by inclusion mineral The area containing a large inclusion mineral was selected as the first test area (Fig. 3a and b). The size of the inclusion is approximately 40 μm. Through EDS analysis, the inclusion was determined to be feldspar (Fig. 3c). By comparing the SEM image and the gallium ion image, the selected test areas were located under an ion microscope. Due to the advanced fabrication method, we were able to precisely place the dominant constituent at the fixed end of the cantilever beam. During the mechanical test, the flat indenter tip was placed at the free end of the cantilever beam (Fig. 5a). When the indenter moved downwards, the stress was concentrated in the fixed end of the beam. Thus, the micro crack was certain to be initiated and propagated in the region next to the fixed end. The region indicated by a red rectangle in Fig. 5a is the failure region. For better visualization, the failure region is magnified when revealing the test results (Fig. 5b). The relatively bright parts in the SEM images of the failure region, such as the large inclusion and the small mineral chips (black arrows), usually indicate stiff minerals. In contrast, the matrix with lower modulus and strength is the relatively dark part around the small chips. In addition, some micro pores are seen on the surface (white arrows). The edge of the inclusion is distinct in the failure region (red dotted line). Linear response of deformation was observed at the beginning of the test (Fig. 5c). At 69 s, a micro crack was initiated in the matrix (Fig. 5d and e), and a sudden load drop occurred in the load-displacement curve (Fig. 5f). The cantilever beam was then loaded continuously. When the crack tip moved to the edge of the inclusion (Fig. 5g), a branch crack was generated and trapped in 5
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the pore next to the inclusion (red arrow in Fig. 5h). A load turn can be found in the load-displacement curve (black circle in Fig. 5i). Then, the main crack moved along the edge of the inclusion (Fig. 5j and k), and the load decreased gradually with the crack propagation (Fig. 5l). At 289 s, the cantilever beam was almost broken (Fig. 5m and n). The total displacement of the indenter tip is 6
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Fig. 5. Micro crack propagation in the test area dominated by inclusion mineral and corresponding load-displacement curves. (a–c) Linear response of deformation at the beginning of the mechanical test. The red rectangle indicates the failure region. Black and white arrows indicate small mineral chips and pores on the surface, respectively. The red dotted line indicates the edge of the inclusion mineral. The load linearly increased with the displacement. (d–f) At 69 s, a micro crack (red arrow) was initiated in the matrix. The load suddenly dropped due to the crack initiation. (g–i) At 76 s, the crack tip reached the edge of the inclusion. A branch crack (red arrow) was generated and trapped in the pore. A load turn (black circle) can be found in the load-displacement curve. (j–l) The micro crack continued moving downwards along the edge of the inclusion. The load gradually decreased with the displacement. (m–o) At 289 s, the cantilever beam was almost broken. The load on the beam decreased to 0. The total displacement of the indenter was approximately 4 μm (see Supplementary material Movie S1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
approximately 4 μm. 3.2. Crack propagation in the test area dominated by organic matter The area containing a large blob of organic matter was selected as the second test area (Fig. 3d and e). The size of the organic matter blob is approximately 40 μm. High carbon content was detected in the organic matter (Fig. 3f). Similarly, the dominant element was placed at the fixed end of the cantilever beam, while the indenter tip was placed at the free end (Fig. 6a). In the failure region (red rectangle in Fig. 6a), the constituents are basically organic matter (dark part) and the matrix (bright part). The red dotted line indicates the edge of the organic matter blob (Fig. 6b). The cantilever beam experienced the linear response of deformation at first (Fig. 6c). The micro crack was initiated at 95 s (Fig. 6d and e), and the first load drop occurred in the load-displacement curve (Fig. 6f). Then, the flat indenter kept moving downwards. When the crack tip reached the boundary between the organic matter and the matrix (Fig. 6g), the crack penetrated into the organic matter (Fig. 6h). The second load drop then occurred (Fig. 6i). Subsequently, the crack propagated in the organic matter (Fig. 6j and k), and the load increased with the crack propagation (Fig. 6l). At the conclusion of the test, the cantilever beam was still sustained by the organic matter (Fig. 6m). The direction of the crack in organic matter (white double arrow in Fig. 6n) is generally parallel to the loading direction. The final load of the test was approximately 8 mN (Fig. 6o). 3.3. Crack propagation in the test area dominated by clay layers The area containing a high-percentage of clay layers was selected as the third test area (Fig. 3g and h). Characteristic elements for clay minerals were detected through EDS (Fig. 3i). Clay layers are the dominant constituent in this case (Fig. 7a). Different from the other two cases, there is no large particle in the test area. Most of the stiff minerals are smaller than 5 μm in the failure region (Fig. 7b). Moreover, the distribution of the fine clay layers is not uniform. In the upper part of the failure region, there are more small mineral chips, and the clay layers are disordered (black arrows). The small mineral chips restrict the distribution of the clay layers (red dotted lines). In contrast, the lower part of the failure region contains fewer small mineral chips, and the clay layers (white arrows) are generally along the length direction of the cantilever beam (red dotted double arrow). During the mechanical test, the load increased with the displacement linearly (Fig. 7c), until a micro crack was initiated at the fixed end of the cantilever beam (Fig. 7d). In the failure region, the initial crack is along the layering direction of the clay layer at the top left (Fig. 7e). The load drop occurred with the crack initiation (Fig. 7f). At 101 s, the crack tip moved to the middle of the failure region (Fig. 7g). In the upper part of the failure region, the micro crack is along the clay layers, and the crack flanks appear smooth (Fig. 7h). The load decreased with the crack propagation (Fig. 7i). Then, the crack propagated in the lower part of the failure region (Fig. 7j). The crack path is normal to the layering direction of the ordered clay layers, and the crack flanks appear distorted (Fig. 7k). We can also find that the load reduction slows from line part ① to line part ② in the load-displacement curve (Fig. 7l). At 455 s, the cantilever beam was almost broken (Fig. 7m). The smooth and distorted crack flanks are clearer in the failure region (Fig. 7n). The total displacement of the indenter tip is approximately 6.5 μm (Fig. 7o). 4. Discussion 4.1. Load-displacement curves and fracture characteristics Through the in situ micro mechanical test, we observed micro crack propagation in selected test areas with different dominant elements, and recorded load and displacement data during the deforming and fracturing process. The load-displacement curves for the three test areas are summarized in Fig. 8. For all of the three cases, the pre-failure curves indicate the linear response of deformation of the micro cantilever beams. For the test areas dominated by the inclusion mineral and clay layers, the load for the micro crack initiation is approximately 2.6 mN and 2.3 mN, respectively. The crack initiation load is the peak load of the curve. The load-displacement curves exhibit the features of brittle failure. However, the load for the crack initiation in the test area dominated by organic matter is approximately 4.7 mN, which is much higher than the other two cases. There are two load drops in the loaddisplacement curve, which indicates crack initiation in the matrix and penetration into the organic matter, respectively. In addition, after the load drop, the load did not decrease like the other two cases. We failed to completely break the cantilever beam dominated by organic matter with the experimental settings. Prior to the retreat of the indenter tip, a high load still exists, which is approximately 8 mN, on the cantilever beam. The load-displacement curve of this case exhibits the features of ductile failure. 7
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4.2. Crack paths and fracture mechanisms The paths of the micro cracks reveal the interactions between the cracks and the constituents in shale. The micro cracks were all generated in the matrix in the three cases. For the test area dominated by clay layers, the crack was initiated at the fixed end of the
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Fig. 6. Micro crack propagation in the test area dominated by organic matter and corresponding load-displacement curves. (a–c) Linear response of deformation when the displacement was smaller than 1 μm. The red rectangle indicates the failure region. The red dotted line indicates the edge of the organic matter. (d–f) At 95 s, a micro crack (red arrow) was initiated. The first load drop (number ①) occurred in the load-displacement curve. (g–i) At 139 s, the micro crack penetrated into the organic matter, and the second load drop (number ②) occurred in the load-displacement curve. (j–l) After the penetration, the load was increased to keep the crack propagation in the organic matter. (m–o) At 489 s, the displacement of the indenter reached the maximum value set before the test. The cantilever beam was still sustained by the organic matter after the indenter moved back. The direction of the crack in organic matter (white double arrow) is generally parallel to the loading direction. The final load on the cantilever beam was approximately 8 mN (see Supplementary material Movie S2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cantilever beam (Fig. 7e). However, for the other two cases, the initial crack is not at the end of the cantilever beam (Figs. 5e and 6e), due to support from the inclusion mineral and organic matter blob, respectively. Before the crack tip touched the edge of the mineral inclusion, micro cracks tended to avoid the stiff mineral chips, and propagated in clay matrix with lower modulus and strength (Fig. 5h and 7h,n). Such a crack deflection is obvious in the failure region. The mechanical contrast of the main constituents in shale significantly influences the propagation paths. The interactions between the micro crack and the large inclusions in shale are quite different. For the first case, when the crack tip touched the edge of the inclusion mineral, the crack did not penetrate into the mineral. Crack branches were generated and moved along the edge of the inclusion mineral (Fig. 5h and k). The load decrease indicated by the black circle after the load drop in Fig. 5i suggests energy release due to the branch crack initiation. For the second case, the crack tip penetrated the boundary between the organic matter and the matrix, and moved in the organic matter in the depth direction (Fig. 6h and k). The behavior of the organic matter in the fracture problem is intriguing. With the lowest modulus and strength in shale [15], organic matter cannot prevent invasion from the micro crack. However, after penetration, increased energy was demanded to sustain the propagation of the micro crack (Fig. 6k and n). Previous works have also discussed this behavior of organic matter [39,40]. Many researches attempted to define a brittleness index based on the empirical identification of brittle and ductile constituents [1,41]. These two cases intuitively exhibit the interaction between the micro crack and the brittle and ductile constituents in shale, and demonstrate the significant influence of organic matter on the brittleness [6,42]. For formations with high organic content, extra energy is needed to induce fractures. However, thermal maturity and organic type may influence the way that organic matters deform and fracture [43,44], which needs to be studied in future work. Besides organic matter, the mechanical properties of clay have also long been studied [19,45,46]. As a kind of phyllosilicate mineral, clay is usually treated as the main source of the anisotropy of shale [18,47,48]. With less compressibility, the modulus and strength of clay in the direction parallel to the layer structure are higher than those in the normal direction. From the third case, we can find that the relationship between the loading direction and the layering direction also influences the mechanical behavior of clay in the fracturing problem. From the upper part to the lower part of the failure region, the propagation direction of the micro crack is from along the clay layer to perpendicular to the layer structure, and the crack paths change from smooth to zig-zag (Fig. 7h and k). The change of the crack flank appearance indicates a transition from brittle fracture to ductile fracture. Interestingly, a similar phenomenon has been identified in some biological materials containing fiber structures, such as bone [49] and wood [50]. The difference is that fracture mechanism investigations for most biological materials aim to prevent the generation of micro cracks. However, for the development of shale oil/gas, the generation of complex cracks is requisite. From the discussion above, we can find that due to the mechanical heterogeneity and the anisotropic structure, a variety of fracture mechanisms in shale exist at micro scale. Crack deflection and crack branching owing to mechanical contrast, toughening in organic matter, and crack bridging of clay layers together influence crack propagation in shale (Fig. 9). Rock is assumed to be homogeneous at micro scale in most hydraulic fracturing models. However, it is difficult to directly observe the microscopic phenomena via macroscopic fracture experiments. This work provides a detailed description of the micro crack initiation and propagation process in typical cases by focusing on the influence of single constituents. Such information may give the mechanical parameters and lead to modification of the fracture criteria used at a greater scale. In addition, microscopic phenomena can also yield information useful for engineering practice. For example, crack paths (penetrating into or moving around the minerals) may influence the fracture geometry and the selection of proppants. Finally, recent simulations attempted to investigate the interactions between micro crack and different constituents in shale with mesoscale modeling approaches [21,51]. Detailed characterization of crack propagation processes and crack mechanisms for microscale constituents are required in order to render such simulations realistic. This microscale crack characterization can serve as a basis or building blocks for mesoscale modeling of fracturing in shale, provide a valuable dataset for validating the mesoscale modeling approaches, and assist in understanding the complexity in fracture surfaces and fracture patterns at macroscopic scale. 5. Conclusion This paper experimentally investigates micro crack propagation in shale. Three representative test areas in the shale block from the Yanchang Formation are selected to manufacture the micro cantilever beams and perform in situ fracture tests. Based on the loaddisplacement curves, the fracture characteristics in the three cases are compared. For the test areas dominated by the inclusion mineral and clay layers, the crack initiation load is the maximum load of the curve, which is approximately 2.6 mN and 2.3 mN, respectively. The load-displacement curves exhibit the features of brittle failure. For the test area dominated by organic matter, the crack initiation load is approximately 4.7 mN. After the crack penetration into the organic matter blob, the load increases with the 9
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Fig. 7. Micro crack propagation in the test area dominated by clay layers and corresponding load-displacement curves. (a–c) At 16 s, there was no obvious change in the failure region (indicated by the red rectangle). Red dotted lines indicate the layering direction of the disordered clay layers (black arrows). The red dotted double arrow indicates the layering direction of the ordered clay layers (white arrow). (d–f) At 58 s, a micro crack (red arrow) was initiated at the fixed end of the cantilever beam. The load dropped from over 2 mN to lower than 1 mN. (g–i) After initiation, the micro crack propagated along clay layers (red arrow). The load decreased gradually with the propagation. (j–l) In the lower part of the failure region, the micro crack propagated perpendicular to the layering direction (red arrow). The reduction rate of load in this part (number ②) was smaller than in the previous part (number ①). (m–o) At 455 s, the cantilever beam was almost broken. The total displacement of the indenter was approximately 6.5 μm (see Supplementary material Movie S3). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Load-displacement curves in three test areas.
Fig. 9. The fracture mechanisms in shale at microscale. (a) Crack deflection. The magenta ellipse indicates the stiff mineral chips in shale. (b) Crack branching. The red dotted line indicates the edge of the inclusion mineral. (c) Toughening in organic matter. The blue ellipse indicates the organic blobs in shale. White arrows indicate resistance to the crack propagation in organic matter. (d) Crack bridging. The green lines indicate the clay layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
crack propagation. The load-displacement curve for this case exhibits the features of ductile failure. Furthermore, through the analysis of the crack paths, four micro fracture mechanisms are summarized. Crack deflection, crack branching, toughening in organic matter, and crack bridging together influence the crack propagation in shale at microscale. The characterization results may serve as a basis or building blocks for mesoscale fracturing modeling, and the observed cracking behaviors may be used for (in) validating such modeling approaches. Different from previous experimental studies on crack propagation in shale rock, this work chooses three typical cases to simplify complex fracturing problems. Moreover, the test areas for the fracture test are determined beforehand. Based on the micro fabrication and in situ mechanical test method, a new framework is provided for the investigation of the fracture characteristics of shale at the
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mineral level. The experimental settings are given in detail in this paper, and further studies and improvements can be made based on this work. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is partially funded by the National Natural Science Foundation of China (Grant No. U1663208 and 51520105005) and the National Science and Technology Major Project of China (Grant No. 2017ZX05009-005 and 2017ZX05049-003). Appendix A. . Location of selected test areas In Section 2, three representative test areas were selected to manufacture the micro cantilever beams through SEM observation. For micro fabrication, the selected test areas of tens of micrometers need to be located under the ion microscope. The imaging principles of the electron microscope and the ion microscope are different. For the electron microscope, the electron beam scans the sample surface, and the secondary electron (SE) or BSE signal generated by the incident electrons is collected. When observing a composite material by using the SE signal, the electrons accumulate on the surface of the nonconductive parts in the material, which makes the nonconductive parts relatively bright. However, for the ion microscope, the ion-generated secondary electron (iSE) signal contains the information of the sample surface [52]. When observing a composite material by using the iSE signal, the ions tend to accumulate on the surface of the nonconductive parts and prevent the collection of the ion-generated secondary electrons, which makes the nonconductive parts relatively dark. In Fig. A1, we can find that the image contrasts are generally opposite under different microscopes for both inclusion minerals and organic matters (red dashed circles). The image contrast features can assist to locate the selected test areas.
Fig. A1. Image contrast difference between SEM image and gallium ion image. (a) Under the electron microscope, organic matters are usually black, while the inclusion minerals, such as quartz, feldspar, and carbonate are white and bright. (b) Under the gallium ion microscope, due to the poor electro conductivity, the ions tend to accumulate on the surface of the inclusion minerals, and the secondary electrons generated by ion injection cannot be detected by the ET detector. Thus, the inclusion minerals become black, while organic matters are much brighter. Red dashed circles indicate the same organic matter region under different microscopes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 12
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Appendix B. . Fabrication of micro cantilever beam There are various factors that influence the fabrication of the micro cantilever beam. The dose of the gallium ions determines the milling depth. The magnitude of the beam current influences the milling efficiency. A higher beam current results in a shorter fabrication time, but a higher risk of beam damage on the sample surface. Moreover, the scanning direction is also important. The red arrows in Fig. B1 indicate the scanning directions of the ion beams. The smooth faces are usually generated in the front sides of the
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Fig. B1. Experimental procedures for the fabrication of micro cantilever beam. (a–h) Gallium ion images of the sample in the fabrication process. Red rectangles indicate the milling region. Red arrows indicate the scanning directions of the gallium ion beams. The accelerating voltage is 30 kV. Blue dashed boxes indicate deposition and pollution on the sample after FIB milling. (a) Rotate and tilt the sample stage to make the ion-etched face perpendicular to the gallium ion beam. Scan the red region with a beam current of 15 nA and a dose of 150 nC/μm2 to obtain the upper surface of the cantilever beam and to reserve sufficient blank space for the indenter tip. (b) Scan the red region with a beam current of 15 nA and a dose of 100 nC/μm2 to prepare the right side of the cantilever beam. (c) Scan the red region with a beam current of 15 nA and a dose of 100 nC/μm2 to obtain the lower face of the cantilever beam. (d) The fabrication result of the ion-etched face. (e) Rotate and tilt the sample stage to make the 5 mm × 5 mm face perpendicular to the gallium ion beam. Scan the red region with a beam current of 15 nA and a dose of 100 nC/μm2 to obtain the inner face of the cantilever beam. (f) The fabrication result of the 5 mm × 5 mm face. (g) Refine the faces of the cantilever beam with a beam current of 7 nA and a dose of 60 nC/μm2. (h) The gallium ion image of the manufactured cantilever beam. (i) SEM image of the manufactured micro cantilever beam. (j) Chamber scope for step a-d, g, and h. The ion-etched face is perpendicular to the gallium ion beam. (k) Chamber scope for step e and f. The upper surface of the sample is perpendicular to the gallium ion beam. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
arrows, while deposition and pollution may occur in the other sides (blue dashed boxes). Fig. B1 illustrates the detailed experimental settings of the micro fabrication on shale samples. The settings may change with the sample material and fabrication purpose. Appendix C. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.engfracmech.2020.106906.
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