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An investigation of the evolution of the internal structures and failure modes of Longmaxi shale using novel X-ray microscopy
Journal of Petroleum Science and Engineering 184 (2020) 106479
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An investigation of the evolution of the internal structures and failure modes of Longmaxi shale using novel X-ray microscopy
T
Y.T. Duana,b,c,∗, X. Lia,b,c,∗∗, P.G. Ranjithd, Y.F. Wua,b a
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China c College of Earth Science, University of Chinese Academy of Sciences, Beijing, 100049, China d Deep Earth Energy Laboratory, Department of Civil Engineering, Monash University, Building 60, Melbourne, Victoria, 3800, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: X-ray microscopy Longmaxi shale Void evolution Crack connectivity Meso-scale mineral Intergranular fracture
An accurate description of the evolution of the internal structure of shale during loading and fracturing is important to understand the failure mechanisms that are related to shale-gas migration. The aim of this work was to investigate and characterize the void evolution and failure of Longmaxi shale quantitatively under uniaxial tests, and analyze the relationship between the distribution of the meso-scale (voxel-scale) minerals and the failure mode. Novel X-ray microscopy combined with an in-situ microtest device was used and three groups of shale specimens were tested under uniaxial conditions with different scanning stresses. Some three-dimensional stereograms of different loading forces and an entire force–displacement curve were obtained for each in-situ test. Based on the results, an evolution of the void distribution was characterized and divided into four stages: weakened damage, linear, damage evolution and stable development, and accelerated damage development. The degree of development of the final cracks was distinguished by the crack volume, equivalent aperture width and connectivity rate. The structure evolution and failure mode of shale were described quantitatively by the void changes and the crack characteristics. Three phenomena of intergranular fracture, transgranular fracture and arrest cracks were observed at a meso-scale to explain the effect of mineral distribution on the crack pattern. These quantitative results can provide a guide for the design in shale fracturing engineering and some references for numerical analysis.
1. Introduction Fracturing of reservoir stimulation in natural-gas exploration and the disposal of nuclear waste relate to rock deformation and failure mechanics (Ren et al., 2016; Luo et al., 2018). Rock damage and failure properties are a basic scientific problem in underground space development, water conservancy and hydropower construction, rock tunneling and mineral-resources exploitation (Wang, 2010). The study and detailed understanding of an internal structure evolution during rock failure and its impact on the destruction mode are necessary and significant for science and engineering. Non-destructive X-ray computed tomography (CT) can be used to observe the internal structure of rocks, and makes it possible to explore the progressive failure of rocks by combining a CT device with a specially designed test system. Ge et al. (1999) and Ren (2001) carried out real-time CT scanning tests on coal rocks under triaxial loading.
Bésuelle et al. (2000, 2006) observed the crack evolution of sandstone and clay rocks under triaxial conditions. Liang et al. (2010) studied the micromechanical failure of concrete under uniaxial compression. Higo et al. (2010, 2011; 2013) analyzed the effect of water content on the evolution of the internal structure of sandstone. Mukunoki et al. (2014) developed a bending test apparatus to observe the inner state of clay soil cracking under punching and bending tests. Sun et al. (2016) observed the meso-fracture process in backfill under uniaxial condition. Wang et al. (2014) and Ju et al. (2018a) investigated the damage cracking characteristics of rock and soil mixtures under a uniaxial compressive loading. The damage and failure characteristics of various geological materials have been studied by placing a loading device in the CT scanning chamber. However, because of the type of X-ray CT or the loading system, the resolution of CT image is limited and the strength of the researched materials is low. The possibility to study the structural evolution of high-strength rock and analyze its effect on
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Corresponding author. Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China. ∗∗ Corresponding author. Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China. E-mail addresses: [email protected] (Y.T. Duan), [email protected] (X. Li). https://doi.org/10.1016/j.petrol.2019.106479 Received 10 April 2019; Received in revised form 29 July 2019; Accepted 9 September 2019 Available online 11 September 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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of cracking of shale at some deformation feature points. The third group (S3-1 and S3-2) was tested to capture the crack-propagation characteristics near the peak force. Therefore, these three sets of parallel experimental specimens were tested under in-situ uniaxial conditions but their CT scanning conditions differed.
failure characteristics remains to be developed. In recent years, with the emergence and development of shale gas engineering, researches on shale properties have become a popular topic. In their researches on the internal structure of shale, Li and Diaz (2010), Josh et al. (2012), Boruah and Ganapathi (2015) and Wang et al. (2015) investigated the characteristics and connectivity of threedimensional (3-D) voids in shale using the micro-CT method, and Li (2013), Yang et al. (2013), Lyu et al. (2018), Yasin et al. (2018c) and Zhou et al. (2018) analyzed the mineral types and contents of shale quantitatively. The void and mineral measurements at the micro-scale were in the static state (before or after the test), the structural characteristics at the meso-scale and especially their evolution during loading have rarely been studied. In their researches on the damage and failure of high-strength shale, Li et al. (2017) studied the progressive failure characteristics of Longmaxi shale in uniaxial tests for the first time by using high-resolution in-situ CT equipment, and tension destruction and comprehensive tension-shear destruction modes were found. Some scholars (Duan et al., 2018; Wang et al., 2018; Yang et al., 2018a; Zhou et al., 2018; Liu et al., 2018) have also studied the cracking rule in anisotropic shale samples. However, the quantitative characterization of structural change and failure degree are rare, and the impact of minerals on failure patterns at the CT resolution scale were not analyzed well in these studies. To investigate and characterize the structural evolution and failure mode of Longmaxi shale during the loading process quantitatively, three groups of specimens were tested under uniaxial conditions by using a comprehensive in-situ stage combined with X-ray microscopy (XRM) equipment. Complete force–displacement relationships and 3-D stereograms with a time series were obtained for analysis. The void evolution rule and the failure degree of shale have been described quantitatively and the effect of meso-scale minerals on the final destruction mode has been analyzed further in this study.
3. Experimental procedure and results During the uniaxial compression process, the loading mode was controlled at a minimal constant displacement rate of 0.03 mm/min and the testing force–displacement data were transferred to the computer system. Fig. 3 shows the complete axial force–displacement relationships of the six specimens. S1-1 and S1-2 were tested with no CT scanning and their peak forces were near 1000 N and 1200 N. S2-1 and S2-2 were all scanned at seven different loading forces during testing. S3-1 and S3-2 were scanned at forces as close as possible to the peak value. The numbers marked on the force–displacement curves in the last two groups are the scanning steps. In addition to the deformation curves, 3-D stereograms with a time series are another key result. During loading, a cone-beam X-ray source with a cone angle of 5.11° was used for scanning. At each scanning step, a vertical projection was recorded after the X-ray had passed through the tested specimen, whereafter 1800 projections were obtained after the rotated stage finished a 360° rotation with a same interval angle of 0.2°, and finally, these vertical projections were used for 3-D reconstruction. The voxel size of the reconstructed stereograms was 11.27 μm × 11.27 μm × 11.27 μm. 4. Void distribution and evolution During the loading process, defects in shale were produced continuously and were manifested as damage to the internal structure. When the internal structure changes to a certain extent, it leads to shale fracture. Davudov and Moghanloo (2018) stated that the permeability reduction in shale formations should be corrected to account for microcrack closure at an early stage. Shi et al. (2018) showed that an accurate representation of pore parameters is important in the evaluation of permeability characteristics of reservoir rock. In this study, the use of an in-situ XRM system made it possible to obtain 3-D stereograms at different stress levels, which are basic data to achieve a direct observation of the void-distribution characteristics under loading and a quantitative description of its evolution for Longmaxi shale.
2. Equipment and materials Shale specimens were compressed by using a new type of loading equipment that was manufactured by DEBEN UK and termed the Microtest CT5000 Replacement of Exchangeable Loadcell Assembly, which has a mass less than 6 kg and is suitable for mounting in most common scanning systems. The maximum allowable pressure of this apparatus is 5 kN, the range of compression distance varies from 15 mm to 5 mm, and the tested temperature ranges from −20 °C to 160 °C (Li et al., 2017). By using this compression loading unit with the XRM equipment, which includes the X-ray source, rotating stage and detector, laboratory in-situ compression tests can be achieved and a clear visual interpretation of how the internal structure of shale changes under different loading conditions can be produced. Fig. 1 shows the combined XRM and loading unit and a schematic diagram of its key workflows. Some cylindrical specimens with a diameter of 4 mm and a height–diameter ratio of two were prepared (see Fig. 2(a)). All specimens were cored from a shale block with an angle of 90° between the bedding plane and the direction of drilling (Fig. 2(b)). This shale block was a Longmaxi shale outcrop and originated from a Silurian deposit in Chongqing, China. According to the results of Tong et al. (2017) and Ju et al. (2018b), the micro-scale pores in this shale type include four types of pores: inter-particle, intra-particle, organic matter and fractures. The mineral types with their corresponding mass contents in Longmaxi shale include quartz (32.62%), feldspar (52.75%), carbonates (5.99%), clay minerals (4.75%), pyrite (2.97%), and others (0.92%), according to the latest research of Zhou et al. (2018). Six specimens with an average density of 2.39 g/mm3 were selected for testing and they were divided into three groups with different purposes. The first two specimens (S1-1 and S1-2) were tested to obtain an approximate value of peak force for Longmaxi shale with this core size. The second group (S2-1 and S2-2) was used to observe the degree
4.1. Extraction method and results A CT image can be regarded as a digital image of the grey matrix and the histogram data on the image grey distribution can be obtained from a reconstructed 3-D volume. Therefore, the internal voids can be extracted when a suitable segmentation value is set in this histogram by using the image threshold-segmentation method. First, a surface measurement between the background and material grey distribution (bimodal distribution) was completed. Next, a threshold value was set to select the void boundary, the grey distributions between the surface measurement value and the threshold value that indicates voids (Duan et al., 2019). Because of a correlation between the threshold value and the void ratio, an optimal threshold value should be chosen. In this study, the voxel size of the CT images is 11.27 μm, which means that only a portion of voids greater 11.27 μm can be extracted. Chen et al. (2013) found that the average content of big pores (1 μm ≤ ϕ < 100 μm) is 0.33%, and an optimal threshold value (interpolation = 0.7) is considered suitable to extract the void distribution. This optimal value was kept constant at each scanned step to obtain the void evolution for all specimens. Fig. 4 shows the void distribution during loading for S2-1, S2-2, S3-1 and S3-2. The void ratio at each step was also calculated and labeled in the void-distribution map. 2
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Fig. 1. Combined XRM and loading unit and schematic diagram of key workflows.
are 83.23 MPa, 100.16 MPa, 113.79 MPa and 103.06 MPa, respectively). The negative impact of low-density voids on the rock mechanical response is verified by these quantitative results. In addition to analyzing the initial void differences and their effect, the changes of void distribution during loading can also be observed from Fig. 4. The compaction phenomena of the obvious defects can be found in S2-1 and S2-2 from scanning steps 1 to 2, and some new cracks in other locations appear in these two specimens at step 4. These variation rules are consistent with the conclusion of Liu et al. (2018) that no clear relationship exists between new and pre-existing fractures. In S3-1 and S3-2, voids are distributed in a decentralized manner and no obvious small cracks appeared before the shale fractured. The number of evenly distributed voids increased to varying degrees, and the internal defects developed progressively and formed a final pattern of connected or isolated cracks. Some differences exist in the void-changing rules of the two group specimens, which are related to the different designs of the scanning forces. Fig. 2. Prepared shale specimens (a) and schematic diagram of coring direction (b).
4.3. Quantitative characterization of void evolution To achieve a more accurate quantitative characterization of the void evolution, the changing values of the void surface area are plotted graphically in Fig. 5. The scanning step is selected as the x-axial value in Fig. 5(a) and the stress percentage (ratio of stress value at each step to peak stress) is selected as the x-axial value in Fig. 5(b). For S2-1 and S22, the void values decrease from steps 1 to 2 and are almost unchanged from steps 2 to 3, and they increase gradually from steps 3 to 5. The stress percentages of these scanning steps are distributed evenly throughout the loading process. The void evolution includes three main stages: compaction, elasticity and extension. For S3-1 and S3-2, the void values increase slightly from steps 1 to 4 and then increase rapidly from steps 4 to 7. The stress percentage of scanning step 2 in S3-1/S3-2 is almost 60%, and this point can be considered as the crack-initiation point, according to Xue et al. (2014). S3-1 and S3-2 were scanned almost from the crack initiation to the peak strength to enable a further observation of the void extension. The results show that the extension includes two main stages: stable development and accelerated damage
4.2. Void distribution and changes As shown in Fig. 4, the initial voids are different in the four specimens and their ratios are 1.12%, 1.05%, 0.49% and 0.54%. S2-1/S2-2 have a bigger void ratio than S3-1/S3-2, which can be explained by observing the void maps and the left volume bar, which shows that most initial voids are small and scattered (blue portion) in the four specimens, but three obvious defects exist in S2-1 (red and green portion) and one obvious defect is apparent in S2-2 (green portion). These obvious defects in shale are easier to extract from CT images and result in a larger initial void ratio. Then, the impact of initial void on the uniaxial compression strength (UCS) was analyzed. The void ratio in S2-1 remains at a maximum and in S3-1 it remains at a minimum, which results in a corresponding mechanical response that the UCS of S2-1 is at a minimum and that of S3-1 is at a maximum (the UCS values
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Fig. 3. Axial force–displacement relationships for S1-1, S1-2, S2-1, S2-2, S3-1 and S3-2.
from the CT volumes and the results are shown in Fig. 6, which includes a stereogram of a fractured specimen and the extracted cracks. Based on an overall view of the final cracks, some vertical/oblique cracks exist and their distribution is random, which is related to the uniaxial loading condition. For crack connectivity, it can be observed from the maps that one connective crack exists in each of S2-1 and S3-2, and is termed C2-1 and C3-2, respectively. Two independent cracks termed C2-21 and C2-2-2 were in S2-2 and three independent cracks termed C3-1-1, C31-2 and C3-1-3 were in S3-1. To describe the degree of connectivity quantitatively, cracks C2-1, C2-2-1, C3-1-2 and C3-2 that exist throughout the specimen height were selected as the connected cracks, and the connectivity rate, which is defined as the ratio of connected crack volume to the specimen volume, was calculated. The results are listed in Table 1 and show that S3-2 has the best connectivity. In addition to the description of crack connectivity, the crack volume and surface area were also calculated to characterize the degree of development of these cracks. The quantitative volumes and their corresponding surface area values are listed in Table 1. A good positive correlation exists between the crack volume and the crack surface area, with the exception of C3-1-3. The reason for C3-1-3 having a higher crack volume but a lower surface area than C3-1-2 is related to the bigger width of C3-1-3, which shows that the aperture value of the final crack is important to represent the degree of fracturing. Therefore, the equivalent width of each crack was calculated by defining the ratio of
development. Therefore, the void evolution before the crack-initiation point is concluded from S2-1/S2-2 and goes through two stages, compaction and linear. After the crack-initiation point, void evolution is obtained from S3-1/S3-2 and goes through two stages, stable development and accelerated damage development. By combining the void evolution rule from these two groups of specimens, the internal structure of Longmaxi shale can be considered quantitatively as four stages: weakened damage, linear, stable damage evolution and accelerated damage development. 5. Failure mode characteristics Studying the spatial distribution and presenting the failure degree of the final cracks in Longmaxi shale are very important to evaluate shale cracking under certain conditions and provide guidance on fracturing methods in shale gas engineering (Yang et al., 2018b). In this study, the fractured shale cracks after uniaxial loading were extracted and some quantitative parameters were used to describe the failure degree. The effect of mineral distribution on the crack mode was analyzed at a meso-scale. 5.1. Crack pattern and failure degree The final crack morphologies of the four specimens were extracted 4
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Fig. 4. Distribution of voids during loading for four in-situ specimens of S2-1, S2-2, S3-1 and S3-2.
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Fig. 5. Evolution rule of void surface area during loading for in-situ tested specimens.
Fig. 6. Spatial distribution and connectivity of cracks in fractured shale.
5.2. Meso-scale mineral effect on crack mode
Table 1 Statistical table of quantification parameters of cracks after failure. Sample number
Crack name
Crack volume (mm3)
Crack surface area (mm2)
Crack equivalent width (μm)
Specimen volume (mm3)
Connectivity rate (%)
S2-1 S2-2 / S3-1 / / S3-2
C2-1 C2-2-1 C2-2-2 C3-1-1 C3-1-2 C3-1-3 C3-2
3.23 1.65 0.78 0.11 1.23 1.25 4.75
144.81 112.57 41.44 10.63 84.85 34.80 227.36
22.31 14.66 18.82 10.35 14.50 35.92 20.89
100.06 98.79 / / 91.31 / 105.40
3.23 1.67 / / 1.35 / 4.51
To investigate the effect of meso-scale minerals on the failure characteristics of shale, internal minerals in 3-D stereograms were extracted and the corresponding contents were calculated. Fig. 7 shows the mineral distribution and the content for each specimen. The mineral contents of the four specimens are 2.03%, 2.09%, 2.10% and 2.12% and higher mineral contents within a certain range correspond to higher UCS values (83.23 MPa, 100.16 MPa, 113.79 MPa and 103.06 MPa, respectively), which shows that the positive effect of hard mineral on the rock compressive strength is well quantified. The minerals in S2-1 and S3-2 are discrete and have small size grains. Mineral bands with a small region exist in S2-2 and S3-1 except for the mineral grains. Only one connected crack exists in S2-1/S3-2, two or three independent “Y”shaped cracks exist in S2-2/S3-1 and the meeting point of the “Y” cracks is near the mineral strip. These interesting phenomena indicate that single connective crack develops in specimens with a discrete distribution of mineral particles and complex crack morphologies occur in specimens with mineral bands. Therefore, it can be speculated that a relationship exists between the mineral distribution mode and the final
crack volume to crack surface area and the results are shown in Table 1. Based on the quantitative parameters of crack volume, surface area, equivalent width and connectivity rate, the failure degree of each specimen is well characterized quantitatively and S3-2 has the best degree of failure.
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Fig. 7. Distribution of minerals in four in-situ specimens of S2-1, S2-2, S3-1 and S3-2.
morphologies can be explained by the results of Rigopoulos et al. (2013), who shown that the orientation of most newly formed intragranular and transgranular micro-cracks is nearly parallel to the loading direction.
crack morphology. For a more detailed exploration of the effect of mineral distribution on crack pattern, CT slices of the fractured specimens were studied using the “Rendering setting” mode in VG software. In this mode, materials with different densities, such as hard minerals (pyrite), the shale matrix and cracks can be distinguished and rendered different colors, as shown in Fig. 8, where the minerals are rendered green and cracks are rendered dark blue. After observing the vertical CT slices in four fractured specimens, three main types of mineral effect on crack mode were collected. Some typical phenomena were acquired and shown in Fig. 8. In the first type, the direction of the developing crack is changed by the scattered mineral or mineral band, the final crack bypass the mineral particle or band. In the second type, the mineral band is penetrated by the developing crack and has little effect on crack extension. In the third type, the crack tip is arrested by some minerals and the final crack does not develop to the specimen edge. These phenomena can be termed intergranular fracture, transgranular fracture and crack arrest (Kranz, 1983; Feng et al., 2002; Rigopoulos et al., 2013). The scattered minerals change the crack direction and there are two ways for mineral bands to affect crack development, which can be explained by the principle of minimum-energy dissipation (Zhou, 2001; Yang et al., 2018a). This principle states that during crack propagation, if the energy required to pass around the hard material is less than that required to pass through the material, the crack becomes an intergranular fracture, otherwise, it becomes a transgranular crack. Based on this principle, the reason for the developed vertical or slightly crack
6. Conclusions Six cylindrical specimens were tested under uniaxial compression using a novel X-ray microscopy matched with an in-situ loading stage, and 3-D stereograms with a time series were obtained. The void evolution and final crack morphologies were extracted and some parameters were calculated for quantitative characterization, and the effect of mineral distribution on failure mode was investigated. The following conclusions can be drawn: (1) Three groups of test scheme with different scanning forces were designed, and the internal structure evolution and the failure mode of Longmaxi shale were extracted and described. These structuraldistribution results provide a useful guide and reference for numerical simulation modeling and engineering scheme design of Longmaxi shale. (2) The void evolution rules were expressed well by the void distribution maps and were characterized quantitatively by the changes of void surface area. The gradual evolution of the internal structure in shale can be considered occurring in four stages: weakened damage, linear, stable damage evolution, and accelerated damage 7
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Fig. 8. Typical effect phenomena of the meso-scale minerals on crack mode.
References
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Author contributions Each author contributed to this paper. Xiao Li and Yongting Duan designed and conducted the experiments, Yongting Duan and Yanfang Wu analyzed the test data, Yongting Duan wrote this paper, and Pathegama Gamage Ranjith gave guidance on writing style and language. All authors read and approved the final manuscript. Conflicts of interest The authors declare no conflicts of interest. Acknowledgments The authors would like to thank the Editor and the anonymous reviewers for their helpful and constructive comments. This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB10030301 and XDB10030304), the National Natural Science Foundation of China (Grant No. 41227901) and the China Scholarship Council (CSC). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.106479. 8
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Ju, Y., Gong, W.B., Chang, C., Xie, H.P., Xie, L.Z., Liu, P., 2018b. Three-dimensional characterization of multi-scale structures of the Silurian Longmaxi shale using focused ion beam-scanning electron microscopy and reconstruction technology. J. Nat. Gas Sci. Eng. 46, 26–37. Kranz, R.L., 1983. Microcracks in rocks: a review. Tectonophysics 100, 449–480. Li, G.P., Diaz, E., 2010. Rock physical properties computed from digital core and cutting with application to deep gas exploration and development. In: SPE Deep Gas Conference and Exhibition, Manama, Bahrain, vols. 24–26 January, 2010. Li, J., 2013. Reservoir Characteristics of Longmaxi Shale in the Southeast of Chongqing. Master Dissertation. China University of Geoscience, Beijing. Li, X., Duan, Y.T., Li, S.D., Zhou, R.Q., 2017. Study on the progressive failure characteristics of longmaxi shale under uniaxial compression conditions by X-ray microcomputed tomography. Energies 10, 303–315. Liang, X.Y., Dang, F.N., Tian, W., 2010. CT testing on fracture process of concrete under uniaxial compression. J. China Coal Soc. 35 (Sup), 63–67. Liu, S.X., Wang, Z.X., Zhang, L.Y., 2018. Experimental study on the cracking process of layered shale using X-ray microCT. Energy Explor. Exploit. 36 (2), 297–313. Luo, Y., Xie, H.P., Ren, L., Zhang, R., Li, C.B., Gao, C., 2018. Linear elastic fracture mechanics characterization of an anisotropic shale. Sci. Rep. U.K. 8 (1), 8505–8516. Lyu, Q., Long, X.P., Ranjith, P.G., Tan, J.Q., Zhou, J.P., Wang, Z.H., Luo, W.B., 2018. A laboratory study of geomechanical characteristics of black shales after sub-critical/ super-critical CO2 + brine saturation. Geomech. Geophys. Geo-energy Geo-resour. 4, 141–156. Mukunoki, T., Nakano, T., Otani, J.P., Gourc, J.P., 2014. Study of cracking process of clay cap barrier in landfill using X-ray CT. Appl. Clay Sci. 101, 558–566. Ren, J.X., 2001. Real-time CT monitoring for the meso-damage propagation characteristics of rock under triaxial compression. J. Exp. Mech. 16 (4), 387–395. Ren, L., Xie, L.Z., Xie, H.P., Ai, T., He, B., 2016. Mixed-mode fracture behavior and related surface topography feature of a typical sandstone. Rock Mech. Rock Eng. 49, 3137–3153. Rigopoulos, I., Tsikouras, B., Pomonis, P., Hatzipanagiotou, K., 2013. Petrographic investigation of microcrack initiation in mafic ophiolitic rocks under uniaxial compression. Rock Mech. Rock Eng. 46 (5), 1061–1072. Shi, M., Yu, B.S., Zhang, J.C., Huang, H., Yuan, Y., Li, B., 2018. Microstructural characterization of pores in marine shales of the lower silurian longmaxi formation, southeastern sichuan basin, China. Mar. Pet. Geol. 94, 166–178.
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
An investigation of the evolution of the internal structures and failure modes of Longmaxi shale using novel X-ray microscopy
T
Y.T. Duana,b,c,∗, X. Lia,b,c,∗∗, P.G. Ranjithd, Y.F. Wua,b a
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China c College of Earth Science, University of Chinese Academy of Sciences, Beijing, 100049, China d Deep Earth Energy Laboratory, Department of Civil Engineering, Monash University, Building 60, Melbourne, Victoria, 3800, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: X-ray microscopy Longmaxi shale Void evolution Crack connectivity Meso-scale mineral Intergranular fracture
An accurate description of the evolution of the internal structure of shale during loading and fracturing is important to understand the failure mechanisms that are related to shale-gas migration. The aim of this work was to investigate and characterize the void evolution and failure of Longmaxi shale quantitatively under uniaxial tests, and analyze the relationship between the distribution of the meso-scale (voxel-scale) minerals and the failure mode. Novel X-ray microscopy combined with an in-situ microtest device was used and three groups of shale specimens were tested under uniaxial conditions with different scanning stresses. Some three-dimensional stereograms of different loading forces and an entire force–displacement curve were obtained for each in-situ test. Based on the results, an evolution of the void distribution was characterized and divided into four stages: weakened damage, linear, damage evolution and stable development, and accelerated damage development. The degree of development of the final cracks was distinguished by the crack volume, equivalent aperture width and connectivity rate. The structure evolution and failure mode of shale were described quantitatively by the void changes and the crack characteristics. Three phenomena of intergranular fracture, transgranular fracture and arrest cracks were observed at a meso-scale to explain the effect of mineral distribution on the crack pattern. These quantitative results can provide a guide for the design in shale fracturing engineering and some references for numerical analysis.
1. Introduction Fracturing of reservoir stimulation in natural-gas exploration and the disposal of nuclear waste relate to rock deformation and failure mechanics (Ren et al., 2016; Luo et al., 2018). Rock damage and failure properties are a basic scientific problem in underground space development, water conservancy and hydropower construction, rock tunneling and mineral-resources exploitation (Wang, 2010). The study and detailed understanding of an internal structure evolution during rock failure and its impact on the destruction mode are necessary and significant for science and engineering. Non-destructive X-ray computed tomography (CT) can be used to observe the internal structure of rocks, and makes it possible to explore the progressive failure of rocks by combining a CT device with a specially designed test system. Ge et al. (1999) and Ren (2001) carried out real-time CT scanning tests on coal rocks under triaxial loading.
Bésuelle et al. (2000, 2006) observed the crack evolution of sandstone and clay rocks under triaxial conditions. Liang et al. (2010) studied the micromechanical failure of concrete under uniaxial compression. Higo et al. (2010, 2011; 2013) analyzed the effect of water content on the evolution of the internal structure of sandstone. Mukunoki et al. (2014) developed a bending test apparatus to observe the inner state of clay soil cracking under punching and bending tests. Sun et al. (2016) observed the meso-fracture process in backfill under uniaxial condition. Wang et al. (2014) and Ju et al. (2018a) investigated the damage cracking characteristics of rock and soil mixtures under a uniaxial compressive loading. The damage and failure characteristics of various geological materials have been studied by placing a loading device in the CT scanning chamber. However, because of the type of X-ray CT or the loading system, the resolution of CT image is limited and the strength of the researched materials is low. The possibility to study the structural evolution of high-strength rock and analyze its effect on
∗
Corresponding author. Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China. ∗∗ Corresponding author. Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China. E-mail addresses: [email protected] (Y.T. Duan), [email protected] (X. Li). https://doi.org/10.1016/j.petrol.2019.106479 Received 10 April 2019; Received in revised form 29 July 2019; Accepted 9 September 2019 Available online 11 September 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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of cracking of shale at some deformation feature points. The third group (S3-1 and S3-2) was tested to capture the crack-propagation characteristics near the peak force. Therefore, these three sets of parallel experimental specimens were tested under in-situ uniaxial conditions but their CT scanning conditions differed.
failure characteristics remains to be developed. In recent years, with the emergence and development of shale gas engineering, researches on shale properties have become a popular topic. In their researches on the internal structure of shale, Li and Diaz (2010), Josh et al. (2012), Boruah and Ganapathi (2015) and Wang et al. (2015) investigated the characteristics and connectivity of threedimensional (3-D) voids in shale using the micro-CT method, and Li (2013), Yang et al. (2013), Lyu et al. (2018), Yasin et al. (2018c) and Zhou et al. (2018) analyzed the mineral types and contents of shale quantitatively. The void and mineral measurements at the micro-scale were in the static state (before or after the test), the structural characteristics at the meso-scale and especially their evolution during loading have rarely been studied. In their researches on the damage and failure of high-strength shale, Li et al. (2017) studied the progressive failure characteristics of Longmaxi shale in uniaxial tests for the first time by using high-resolution in-situ CT equipment, and tension destruction and comprehensive tension-shear destruction modes were found. Some scholars (Duan et al., 2018; Wang et al., 2018; Yang et al., 2018a; Zhou et al., 2018; Liu et al., 2018) have also studied the cracking rule in anisotropic shale samples. However, the quantitative characterization of structural change and failure degree are rare, and the impact of minerals on failure patterns at the CT resolution scale were not analyzed well in these studies. To investigate and characterize the structural evolution and failure mode of Longmaxi shale during the loading process quantitatively, three groups of specimens were tested under uniaxial conditions by using a comprehensive in-situ stage combined with X-ray microscopy (XRM) equipment. Complete force–displacement relationships and 3-D stereograms with a time series were obtained for analysis. The void evolution rule and the failure degree of shale have been described quantitatively and the effect of meso-scale minerals on the final destruction mode has been analyzed further in this study.
3. Experimental procedure and results During the uniaxial compression process, the loading mode was controlled at a minimal constant displacement rate of 0.03 mm/min and the testing force–displacement data were transferred to the computer system. Fig. 3 shows the complete axial force–displacement relationships of the six specimens. S1-1 and S1-2 were tested with no CT scanning and their peak forces were near 1000 N and 1200 N. S2-1 and S2-2 were all scanned at seven different loading forces during testing. S3-1 and S3-2 were scanned at forces as close as possible to the peak value. The numbers marked on the force–displacement curves in the last two groups are the scanning steps. In addition to the deformation curves, 3-D stereograms with a time series are another key result. During loading, a cone-beam X-ray source with a cone angle of 5.11° was used for scanning. At each scanning step, a vertical projection was recorded after the X-ray had passed through the tested specimen, whereafter 1800 projections were obtained after the rotated stage finished a 360° rotation with a same interval angle of 0.2°, and finally, these vertical projections were used for 3-D reconstruction. The voxel size of the reconstructed stereograms was 11.27 μm × 11.27 μm × 11.27 μm. 4. Void distribution and evolution During the loading process, defects in shale were produced continuously and were manifested as damage to the internal structure. When the internal structure changes to a certain extent, it leads to shale fracture. Davudov and Moghanloo (2018) stated that the permeability reduction in shale formations should be corrected to account for microcrack closure at an early stage. Shi et al. (2018) showed that an accurate representation of pore parameters is important in the evaluation of permeability characteristics of reservoir rock. In this study, the use of an in-situ XRM system made it possible to obtain 3-D stereograms at different stress levels, which are basic data to achieve a direct observation of the void-distribution characteristics under loading and a quantitative description of its evolution for Longmaxi shale.
2. Equipment and materials Shale specimens were compressed by using a new type of loading equipment that was manufactured by DEBEN UK and termed the Microtest CT5000 Replacement of Exchangeable Loadcell Assembly, which has a mass less than 6 kg and is suitable for mounting in most common scanning systems. The maximum allowable pressure of this apparatus is 5 kN, the range of compression distance varies from 15 mm to 5 mm, and the tested temperature ranges from −20 °C to 160 °C (Li et al., 2017). By using this compression loading unit with the XRM equipment, which includes the X-ray source, rotating stage and detector, laboratory in-situ compression tests can be achieved and a clear visual interpretation of how the internal structure of shale changes under different loading conditions can be produced. Fig. 1 shows the combined XRM and loading unit and a schematic diagram of its key workflows. Some cylindrical specimens with a diameter of 4 mm and a height–diameter ratio of two were prepared (see Fig. 2(a)). All specimens were cored from a shale block with an angle of 90° between the bedding plane and the direction of drilling (Fig. 2(b)). This shale block was a Longmaxi shale outcrop and originated from a Silurian deposit in Chongqing, China. According to the results of Tong et al. (2017) and Ju et al. (2018b), the micro-scale pores in this shale type include four types of pores: inter-particle, intra-particle, organic matter and fractures. The mineral types with their corresponding mass contents in Longmaxi shale include quartz (32.62%), feldspar (52.75%), carbonates (5.99%), clay minerals (4.75%), pyrite (2.97%), and others (0.92%), according to the latest research of Zhou et al. (2018). Six specimens with an average density of 2.39 g/mm3 were selected for testing and they were divided into three groups with different purposes. The first two specimens (S1-1 and S1-2) were tested to obtain an approximate value of peak force for Longmaxi shale with this core size. The second group (S2-1 and S2-2) was used to observe the degree
4.1. Extraction method and results A CT image can be regarded as a digital image of the grey matrix and the histogram data on the image grey distribution can be obtained from a reconstructed 3-D volume. Therefore, the internal voids can be extracted when a suitable segmentation value is set in this histogram by using the image threshold-segmentation method. First, a surface measurement between the background and material grey distribution (bimodal distribution) was completed. Next, a threshold value was set to select the void boundary, the grey distributions between the surface measurement value and the threshold value that indicates voids (Duan et al., 2019). Because of a correlation between the threshold value and the void ratio, an optimal threshold value should be chosen. In this study, the voxel size of the CT images is 11.27 μm, which means that only a portion of voids greater 11.27 μm can be extracted. Chen et al. (2013) found that the average content of big pores (1 μm ≤ ϕ < 100 μm) is 0.33%, and an optimal threshold value (interpolation = 0.7) is considered suitable to extract the void distribution. This optimal value was kept constant at each scanned step to obtain the void evolution for all specimens. Fig. 4 shows the void distribution during loading for S2-1, S2-2, S3-1 and S3-2. The void ratio at each step was also calculated and labeled in the void-distribution map. 2
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Fig. 1. Combined XRM and loading unit and schematic diagram of key workflows.
are 83.23 MPa, 100.16 MPa, 113.79 MPa and 103.06 MPa, respectively). The negative impact of low-density voids on the rock mechanical response is verified by these quantitative results. In addition to analyzing the initial void differences and their effect, the changes of void distribution during loading can also be observed from Fig. 4. The compaction phenomena of the obvious defects can be found in S2-1 and S2-2 from scanning steps 1 to 2, and some new cracks in other locations appear in these two specimens at step 4. These variation rules are consistent with the conclusion of Liu et al. (2018) that no clear relationship exists between new and pre-existing fractures. In S3-1 and S3-2, voids are distributed in a decentralized manner and no obvious small cracks appeared before the shale fractured. The number of evenly distributed voids increased to varying degrees, and the internal defects developed progressively and formed a final pattern of connected or isolated cracks. Some differences exist in the void-changing rules of the two group specimens, which are related to the different designs of the scanning forces. Fig. 2. Prepared shale specimens (a) and schematic diagram of coring direction (b).
4.3. Quantitative characterization of void evolution To achieve a more accurate quantitative characterization of the void evolution, the changing values of the void surface area are plotted graphically in Fig. 5. The scanning step is selected as the x-axial value in Fig. 5(a) and the stress percentage (ratio of stress value at each step to peak stress) is selected as the x-axial value in Fig. 5(b). For S2-1 and S22, the void values decrease from steps 1 to 2 and are almost unchanged from steps 2 to 3, and they increase gradually from steps 3 to 5. The stress percentages of these scanning steps are distributed evenly throughout the loading process. The void evolution includes three main stages: compaction, elasticity and extension. For S3-1 and S3-2, the void values increase slightly from steps 1 to 4 and then increase rapidly from steps 4 to 7. The stress percentage of scanning step 2 in S3-1/S3-2 is almost 60%, and this point can be considered as the crack-initiation point, according to Xue et al. (2014). S3-1 and S3-2 were scanned almost from the crack initiation to the peak strength to enable a further observation of the void extension. The results show that the extension includes two main stages: stable development and accelerated damage
4.2. Void distribution and changes As shown in Fig. 4, the initial voids are different in the four specimens and their ratios are 1.12%, 1.05%, 0.49% and 0.54%. S2-1/S2-2 have a bigger void ratio than S3-1/S3-2, which can be explained by observing the void maps and the left volume bar, which shows that most initial voids are small and scattered (blue portion) in the four specimens, but three obvious defects exist in S2-1 (red and green portion) and one obvious defect is apparent in S2-2 (green portion). These obvious defects in shale are easier to extract from CT images and result in a larger initial void ratio. Then, the impact of initial void on the uniaxial compression strength (UCS) was analyzed. The void ratio in S2-1 remains at a maximum and in S3-1 it remains at a minimum, which results in a corresponding mechanical response that the UCS of S2-1 is at a minimum and that of S3-1 is at a maximum (the UCS values
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Fig. 3. Axial force–displacement relationships for S1-1, S1-2, S2-1, S2-2, S3-1 and S3-2.
from the CT volumes and the results are shown in Fig. 6, which includes a stereogram of a fractured specimen and the extracted cracks. Based on an overall view of the final cracks, some vertical/oblique cracks exist and their distribution is random, which is related to the uniaxial loading condition. For crack connectivity, it can be observed from the maps that one connective crack exists in each of S2-1 and S3-2, and is termed C2-1 and C3-2, respectively. Two independent cracks termed C2-21 and C2-2-2 were in S2-2 and three independent cracks termed C3-1-1, C31-2 and C3-1-3 were in S3-1. To describe the degree of connectivity quantitatively, cracks C2-1, C2-2-1, C3-1-2 and C3-2 that exist throughout the specimen height were selected as the connected cracks, and the connectivity rate, which is defined as the ratio of connected crack volume to the specimen volume, was calculated. The results are listed in Table 1 and show that S3-2 has the best connectivity. In addition to the description of crack connectivity, the crack volume and surface area were also calculated to characterize the degree of development of these cracks. The quantitative volumes and their corresponding surface area values are listed in Table 1. A good positive correlation exists between the crack volume and the crack surface area, with the exception of C3-1-3. The reason for C3-1-3 having a higher crack volume but a lower surface area than C3-1-2 is related to the bigger width of C3-1-3, which shows that the aperture value of the final crack is important to represent the degree of fracturing. Therefore, the equivalent width of each crack was calculated by defining the ratio of
development. Therefore, the void evolution before the crack-initiation point is concluded from S2-1/S2-2 and goes through two stages, compaction and linear. After the crack-initiation point, void evolution is obtained from S3-1/S3-2 and goes through two stages, stable development and accelerated damage development. By combining the void evolution rule from these two groups of specimens, the internal structure of Longmaxi shale can be considered quantitatively as four stages: weakened damage, linear, stable damage evolution and accelerated damage development. 5. Failure mode characteristics Studying the spatial distribution and presenting the failure degree of the final cracks in Longmaxi shale are very important to evaluate shale cracking under certain conditions and provide guidance on fracturing methods in shale gas engineering (Yang et al., 2018b). In this study, the fractured shale cracks after uniaxial loading were extracted and some quantitative parameters were used to describe the failure degree. The effect of mineral distribution on the crack mode was analyzed at a meso-scale. 5.1. Crack pattern and failure degree The final crack morphologies of the four specimens were extracted 4
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Fig. 4. Distribution of voids during loading for four in-situ specimens of S2-1, S2-2, S3-1 and S3-2.
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Fig. 5. Evolution rule of void surface area during loading for in-situ tested specimens.
Fig. 6. Spatial distribution and connectivity of cracks in fractured shale.
5.2. Meso-scale mineral effect on crack mode
Table 1 Statistical table of quantification parameters of cracks after failure. Sample number
Crack name
Crack volume (mm3)
Crack surface area (mm2)
Crack equivalent width (μm)
Specimen volume (mm3)
Connectivity rate (%)
S2-1 S2-2 / S3-1 / / S3-2
C2-1 C2-2-1 C2-2-2 C3-1-1 C3-1-2 C3-1-3 C3-2
3.23 1.65 0.78 0.11 1.23 1.25 4.75
144.81 112.57 41.44 10.63 84.85 34.80 227.36
22.31 14.66 18.82 10.35 14.50 35.92 20.89
100.06 98.79 / / 91.31 / 105.40
3.23 1.67 / / 1.35 / 4.51
To investigate the effect of meso-scale minerals on the failure characteristics of shale, internal minerals in 3-D stereograms were extracted and the corresponding contents were calculated. Fig. 7 shows the mineral distribution and the content for each specimen. The mineral contents of the four specimens are 2.03%, 2.09%, 2.10% and 2.12% and higher mineral contents within a certain range correspond to higher UCS values (83.23 MPa, 100.16 MPa, 113.79 MPa and 103.06 MPa, respectively), which shows that the positive effect of hard mineral on the rock compressive strength is well quantified. The minerals in S2-1 and S3-2 are discrete and have small size grains. Mineral bands with a small region exist in S2-2 and S3-1 except for the mineral grains. Only one connected crack exists in S2-1/S3-2, two or three independent “Y”shaped cracks exist in S2-2/S3-1 and the meeting point of the “Y” cracks is near the mineral strip. These interesting phenomena indicate that single connective crack develops in specimens with a discrete distribution of mineral particles and complex crack morphologies occur in specimens with mineral bands. Therefore, it can be speculated that a relationship exists between the mineral distribution mode and the final
crack volume to crack surface area and the results are shown in Table 1. Based on the quantitative parameters of crack volume, surface area, equivalent width and connectivity rate, the failure degree of each specimen is well characterized quantitatively and S3-2 has the best degree of failure.
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Fig. 7. Distribution of minerals in four in-situ specimens of S2-1, S2-2, S3-1 and S3-2.
morphologies can be explained by the results of Rigopoulos et al. (2013), who shown that the orientation of most newly formed intragranular and transgranular micro-cracks is nearly parallel to the loading direction.
crack morphology. For a more detailed exploration of the effect of mineral distribution on crack pattern, CT slices of the fractured specimens were studied using the “Rendering setting” mode in VG software. In this mode, materials with different densities, such as hard minerals (pyrite), the shale matrix and cracks can be distinguished and rendered different colors, as shown in Fig. 8, where the minerals are rendered green and cracks are rendered dark blue. After observing the vertical CT slices in four fractured specimens, three main types of mineral effect on crack mode were collected. Some typical phenomena were acquired and shown in Fig. 8. In the first type, the direction of the developing crack is changed by the scattered mineral or mineral band, the final crack bypass the mineral particle or band. In the second type, the mineral band is penetrated by the developing crack and has little effect on crack extension. In the third type, the crack tip is arrested by some minerals and the final crack does not develop to the specimen edge. These phenomena can be termed intergranular fracture, transgranular fracture and crack arrest (Kranz, 1983; Feng et al., 2002; Rigopoulos et al., 2013). The scattered minerals change the crack direction and there are two ways for mineral bands to affect crack development, which can be explained by the principle of minimum-energy dissipation (Zhou, 2001; Yang et al., 2018a). This principle states that during crack propagation, if the energy required to pass around the hard material is less than that required to pass through the material, the crack becomes an intergranular fracture, otherwise, it becomes a transgranular crack. Based on this principle, the reason for the developed vertical or slightly crack
6. Conclusions Six cylindrical specimens were tested under uniaxial compression using a novel X-ray microscopy matched with an in-situ loading stage, and 3-D stereograms with a time series were obtained. The void evolution and final crack morphologies were extracted and some parameters were calculated for quantitative characterization, and the effect of mineral distribution on failure mode was investigated. The following conclusions can be drawn: (1) Three groups of test scheme with different scanning forces were designed, and the internal structure evolution and the failure mode of Longmaxi shale were extracted and described. These structuraldistribution results provide a useful guide and reference for numerical simulation modeling and engineering scheme design of Longmaxi shale. (2) The void evolution rules were expressed well by the void distribution maps and were characterized quantitatively by the changes of void surface area. The gradual evolution of the internal structure in shale can be considered occurring in four stages: weakened damage, linear, stable damage evolution, and accelerated damage 7
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Fig. 8. Typical effect phenomena of the meso-scale minerals on crack mode.
References
development. (3) The final crack morphologies and the failure degree were evaluated by the crack volume, equivalent width and connectivity rate. The effect of mineral on crack pattern was explained by their distribution mode at meso-scale and three main crack pattern types were found: intergranular fracture, transgranular fracture and crack arrest.
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Author contributions Each author contributed to this paper. Xiao Li and Yongting Duan designed and conducted the experiments, Yongting Duan and Yanfang Wu analyzed the test data, Yongting Duan wrote this paper, and Pathegama Gamage Ranjith gave guidance on writing style and language. All authors read and approved the final manuscript. Conflicts of interest The authors declare no conflicts of interest. Acknowledgments The authors would like to thank the Editor and the anonymous reviewers for their helpful and constructive comments. This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB10030301 and XDB10030304), the National Natural Science Foundation of China (Grant No. 41227901) and the China Scholarship Council (CSC). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.106479. 8
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