Effect of normal load and shearing velocity on the interface friction of organic shale – Proppant simulant

Tribology International 144 (2020) 106119

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Effect of normal load and shearing velocity on the interface friction of organic shale – Proppant simulant H. He, L. Luo, K. Senetakis * City University of Hong Kong, Yeung Kin Man Academic Building, Blue Zone 6/F, Kowloon, Hong Kong, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Shale rock Proppant Interface friction Hydraulic fracturing

In the present study, experiments were conducted using an advanced micromechanical apparatus investigating the tribological behavior of interfaces between an organic shale against two different proppant simulants; one composed of Leighton Buzzard sand (LBS) and the other composed of glass beads. A negative correlation was observed between the coefficient of friction and the magnitude of normal load from organic shale-proppant interface shearing tests. At relatively low shearing velocities (0.2–0.4 mm/h) stick-slip shearing behavior was found to be more prominent, and the change of shearing velocity had insignificant influence on the coefficient of friction. Additional discussion is presented comparing these results from the present work with a previously studied inorganic shale.

1. Introduction Benefited from the development and advancement in hydraulic fracturing techniques, the extraction and production of the unconven­ tional hydrocarbon that is trapped in shale gas/oil reservoirs was boosted in recent decades [1–3]. One of the major difficulties of the unconventional hydrocarbon extraction is caused by the low porosity and permeability of the shale rocks; for example, Soeder [4] reported that the permeability of a typical shale was less than 8.34μd. High pressure fluid is pumped into the wellbores to create fractures to the shale rock, and thus making pathways for the gas or oil in the reservoirs to flow to the vertical and horizontal wellbores during hydraulic frac­ turing. To withhold the fracture created and maintain the flow of the gas/oil after the hydraulic pressure is released, proppants are mixed with the hydraulic fracture fluids and transported into the fractures by the pressurized fluids during the fracturing process [5–8]. During the process, the mechanical interaction between proppant and shale directly affects the proppant transport (i.e., whether the proppants can be transported far enough and evenly distribute in the fracture) and proppant settlement (i.e., whether the proppants do not flow-back and withhold the normal stress induced by the fracture closure after the pumping process) [8,9]. Though there is a number of influencing factors on the hydraulic fracturing problem, the interface micromechanical properties of shale against proppant are key parameters to understand fundamentally the mechanics of proppant-shale response and they also

comprise key input parameters to be used in simulations using the discrete element method (DEM), which is a popular micromechanical-based numerical tool used to model hydraulic frac­ turing [e.g., 10–13]. Previous experimental studies focused mainly on the shale-proppant interaction at the macroscopic level with only a few recent studies published in the literature which examined the tribolog­ ical behavior of the shale-proppant interface [14–17], and due to the limited number of experimental works, the micromechanical shale-proppant parameters adopted in DEM models often vary a lot among different numerical studies. For example, the coefficient of fric­ tion between the shale and proppant was taken as 0, 0.1 and 0.5 by Shimizu et al. [10], Zeng et al. [12] and Zhang et al. [13], respectively, in their DEM-computational fluid dynamics (CFD) coupled models. Other than the conventional proppants, including natural quartz sand, resin coated sand and ceramic balls, researchers developed mul­ tiple types of proppants with various shapes for the sake of transporting distance, flow-back resistance and gas/oil conductivity, like bauxite, walnut shells, hollow glass spheres and elongated and rod shaped proppant [18–20]. Shearing two types of quartz sand with different particle shapes against a siliceous (inorganic) shale, He and Senetakis [17] reported that the micromechanical properties of the shale-proppant interface are notably affected by the shape of the proppant. Both Xiao et al. [15] and He and Senetakis [17] reported on the influence of normal load on the coefficient of friction of the shale-proppant com­ posite interface. The friction of the interface or gouge of shale and other

* Corresponding author. E-mail addresses: [email protected] (H. He), [email protected] (L. Luo), [email protected] (K. Senetakis). https://doi.org/10.1016/j.triboint.2019.106119 Received 26 August 2019; Received in revised form 21 November 2019; Accepted 12 December 2019 Available online 13 December 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.

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rock materials was found to be related to the shearing velocity [21–23, among others], and the coefficient of friction of composite interfaces was reported to be dependent to the shearing velocity [24,25]. The fluid injection speed, which affects the velocity of the proppants during the hydraulic fracturing process, was proved to affect markedly the travel distance and distribution of the proppant in the fractures [9]. Kohli and Zoback [23] and Sone and Zoback [26,27] highlighted that the miner­ alogy, especially the amount of clay and organic carbon of the shale dominated many aspects of the mechanical properties of the shale. Based on recent micromechanical experimental studies on geological materials, researchers emphasized that the interparticle contact response is affected mainly by the surface properties, including the microhardness, surface roughness and material type [28–31] and that plowing mechanisms can play an important role on the tribological behavior of geological materials [32,33]. Based on the aforementioned literature studies, the tribological behavior of the shale-proppant con­ tact can be closely related to the shape of the proppant, the magnitude of the normal load, the shearing velocity, the mineralogy of the shale, as well as the material surface characteristics including microhardness and roughness. However, limited studies systematically investigated the ef­ fect of all these factors on the shearing behavior of shale-proppant interfaces. In this study, the micromechanical shearing behavior of a black shale with high organic content against two types of proppant simulants, i.e., Leighton Buzzard sand (a natural quartz sand) and glass beads, was investigated, with a particular focus on the effect of normal load and shearing velocity. The elemental composition as well as the surface characteristics including microhardness and roughness of the materials were quantified in order to be linked to the observed trends from the tribological tests. The results from the present study were also compared with a recent work on the interface behavior of an inorganic shale against proppant simulant by He and Senetakis [17]. 2. Experimental study 2.1. Materials studied A dark-colored shale rock (denoted as black shale, BS) with organic matters was sheared against Leighton Buzzard sand (LBS) with sizes between 1.18 and 2.36 mm (denoted as the “BL” interface) and glass beads (GB) of 2 mm in diameter (denoted as the “BG” interface), to investigate the shale-proppant tribological properties. Through visual comparison with the chart proposed by Krumbein and Sloss [34], the sphericity and roundness of the LBS particles were quantified to be 0.8 and 0.7, respectively, while those of glass bead spheres were equal to 1. He and Senetakis [17] studied the micromechanical behavior of a light-colored siliceous shale rock (denoted as white shale, WS) against Leighton Buzzard sand (denoted as “WL” interface) of sizes between 2 and 4 mm, and the shearing response of the WL interface was used as benchmark in this study. The surfaces that are parallel to the bedding planes of the shale rock pieces were manually slickensided on dry pieces of samples with 600Cw abrasive paper before being characterized and tested. Representative photos together with scanning electron micro­ scope (SEM) images of the BS and WS are given in Fig. 1(a) and Fig. 1(b). It can be observed from the SEM images that the BS specimen is domi­ nated by clay flakes with limited particulate matter, while clear silt sized particles that are bonded and connected by clay minerals could be observed on the WS specimen. A set of energy dispersive spectroscopy (EDS) tests were conducted to quantify the elements of the shale spec­ imens and the results are presented in Fig. 1(c) in the form of bar charts. The predominant elements of both BS and WS were oxygen and silicon (36.49 wt%), with a small amount of aluminum, potassium, as well as some trivial elements like sodium, magnesium and calcium. However, carbon took up to 8.62 wt% in the BS specimen, while only trivial amount of carbon was observed in the WS specimen. The color of the shale pieces [35], as well as the SEM/EDS analysis indicate that the

Fig. 1. Photo and scanning electron microscope (SEM) images of (a) black shale (BS); (b) white shale (WS); (c) the element percentage of the two shale rocks from energy dispersive spectroscopy (EDS) tests.

black shale contains notably more organic matters than the white shale. A set of micro-indentation tests and optical surface profile tests were conducted on the smoothened surfaces of both types of shale rocks to characterize the micro-hardness and the surface roughness (the details of the apparatus and procedures of the surface characterization tests are introduced by He et al. [25]). After the manual slickensiding, the surface roughness (Sq) of the two types of shale rock were close in magnitude, and the roughness can be considered isotropic within the scales of the measurements. The average Sq of the BS and WS were 591.4 nm and 529.2 nm, respectively, based on 10 measurements on each type of shale. From at least 15 measurements on three to four specimens, the average Martens hardness of the BS rock was 0.40 GPa, and that of the WS rock was 0.72 GPa. The difference in hardness and organic contents of the two types of shale rocks is mainly due to their formation origin. For example, Wang et al. [35] reported that the black shale they investigated, which contained 3.9–15.4% of total organic carbon (TOC), was extracted from the bottom of the formation, while the light-colored siliceous shale was extracted from the top. The average surface rough­ ness of the LBS and GB were 223 nm and 145 nm [29], which are both much smaller in magnitude in comparison with the shale surfaces. The micro-hardness of the LBS and GB were measured to be around 4.59 GPa and 3.25 GPa, which are approximately an order of magnitude higher than that of the black shale surface. 2

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2.2. Micromechanical apparatus used

shale against LBS interface was investigated through two brands of tests. Velocity-stepping type of shearing tests, which were adopted by previ­ ous research works on rock interfaces, for example Dieterich and Conrad [36], Dieterich and Kilgore [37] and Kohli and Zoback [23], were per­ formed on specimens BL5, BL6 and BL7 to investigate the effect of shearing velocity on the frictional behavior of the specimens. The FN for tests BL5, BL6 and BL7 was equal to 1 N, 3 N and 5 N, respectively. The range of velocities covered for BL5 and BL6 were 0.4–40 mm/h and 0.2–40 mm/h, while the velocity range for BL7 was extended to 0.2–170 mm/h. For specimens BL8 and BL9, nine cycles of monotonic shearing were performed on individual shearing paths of the shale surface for each pair of specimens under a constant normal load, and the velocity of the shearing cycles varied between 0.1 mm/h to 4096 mm/h. The normal load magnitudes for BL8 and BL9 were 3 N and 1 N, respectively.

A newly developed two-axis dynamic micromechanical testing apparatus constructed and introduced by He et al. [25] at City Univer­ sity of Hong Kong was used to conduct the micromechanical experi­ ments. The schematic illustration of the apparatus is given in Fig. 2, where its key components are illustrated. The vertically positioned loading system was designed to apply the designated normal load, while the horizontal system was utilized to perform shearing on the interface. An image of a representative pair of BL specimen is given in the sub­ figure of Fig. 2, where it can be observed that the proppant simulant is fixed onto the upper specimen mount and the shale is fixed onto the lower mount. The interaction between the proppant and the shale dur­ ing the tests, i.e. the load and displacement in both normal and tangential directions, was monitored by high precision load cells (with a repeatability of 0.01 N) and non-contact eddy current type of displace­ ment sensors (with a repeatability of 0.01 μm), and the sensor signal was collected and transferred to the computer by a data logger with high sampling rate capability (up to 20Hz). More technical details and cali­ brations of the apparatus have been introduced by He et al. [25].

3. Results and discussion 3.1. Effect of normal load on the shearing behavior The shearing response of a representative specimen, BL3, under four different normal loads (0.5 N, 1 N, 3 N and 5 N) is illustrated in Fig. 3(a) in terms of mobilized coefficient of friction against shearing displace­ ment. The four shearing cycles were performed by shearing the same pair of LBS and BS along four variant shearing paths next to one another. The mobilized coefficient of friction reached a steady state value after the initial regime of shearing displacement, where the shearing force increased non-linearly with displacement. The shearing displacement required to reach steady-state sliding is denoted as the microslip displacement [30]. As the normal load increased from 0.5 N to 5 N, the microslip displacement of specimen BL3 increased from around 0.011 mm–0.067 mm and the coefficient of friction decreased by around 12%, from 0.76 to 0.67. The tangential (shearing) stiffness against shearing displacement plots of the four cycles of shearing of BL3 specimen are given in Fig. 3(b), where it can be observed that the stiffness gradually

2.3. Micromechanical testing program Monotonic shearing tests were performed on three pairs of black shale against glass beads specimens (denoted as BG1 to BG3) and four pairs of black shale against Leighton Buzzard sand specimens (denoted as BL1 to BL4) to investigate the effect of the normal load magnitude on the shearing behavior of the specimens. The details of this set of tests are listed in Table 1. For each pair of specimens in this set of tests, multiple cycles of shearing were performed along different shearing paths of the shale surface at a constant shearing velocity of 0.4 mm/h, and the normal load (FN) of the shearing cycles varied from one another (ranging from 0.5 N to 10 N). The effect of shearing velocity on the frictional response of the black

Fig. 2. Schematic illustration of the dynamic apparatus and an image of a representative pair of black shale-Leighton Buzzard sand (BL) sample in contact. 3

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Table 1 Details of monotonic shearing test results under a shearing velocity of 0.4 mm/h. Interface type

Black shaleGlass beads

Test code

Normal load FN (N)

Coefficient of friction μ

Microslip displacement (mm)

Initial Tangential stiffness KTo (N/mm)

BG10.5 N BG11N

0.5

0.42

0.008

25

1

0.38

0.016

31

0.5

0.40

0.009

14

1

0.36

0.016

38

1

0.41

0.018

33

3

0.36

0.025

50

10

0.30

0.058

65

0.5

0.70

0.008

30

1

0.67

0.014

52

3

0.66

0.040

55

5

0.67

0.063

65

0.5

0.68

0.013

26

1

0.74

0.024

40

3

0.72

0.051

49

5

0.71

0.070

50

10

0.69

0.155

56

0.5

0.76

0.011

39

1

0.73

0.018

40

3

0.72

0.049

56

5

0.67

0.067

54

0.5

0.71

0.012

40

1

0.69

0.018

52

5

0.66

0.065

55

10

0.59

0.100

73

BG20.5 N BG21N BG31N BG33N BG310 N BL10.5 N BL11N BL13N BL15N

Black shaleLeighton Buzzard Sand

BL20.5 N BL21N BL23N BL25N BL210 N BL30.5 N BL31N BL33N BL35N BL40.5 N BL41N BL45N BL410 N

Fig. 3. Representative plots of (a) mobilized coefficient of friction against shearing displacement (b) tangential stiffness against shearing displacement of specimen BL3 at different normal loads.

exceptional cases for BL specimens), which confirms the discussion above for specimen BL3. The coefficient of friction of each test is plotted against the corresponding normal load in Fig. 4(a), where the average μ value at each normal load is also given. The coefficient of friction of the BL interface is noticeably higher than that of the BG interface by a magnitude of around 0.30–0.35 at a given normal load. Sandeep and Senetakis [30] reported that the average coefficient of friction of the grain-grain type of contact between two glass beads was around 0.12, while the interparticle friction of the LBS particles was around 60% higher than that of the glass beads equaling to 0.19. Besides the lower surface roughness, the higher roundness of the glass beads is also believed to be part of the reason for the lower coefficient of friction observed for the BG interfaces in comparison with the BL interfaces. He and Senetakis [17] stressed that more angular quartz sand particles, with smaller local contact radius, would result in higher friction values when sheared against the siliceous shale surfaces, which agrees with the observation of the current study on the black shale-proppant interfaces. The average coefficients of friction of the white shale against Leighton Buzzard sand (WL) are plotted in Fig. 4(a) for comparison. The friction of the BL interface is higher than that of the WL interface in the range of 3–10 N of normal load. Since the surface roughness Sq values of the two types of shale rocks are close, the difference in friction could be

dropped to zero as the shearing reached the steady state condition. The tangential stiffness at the initial stage of shearing (initial tangential stiffness, KTo) increased from around 39 N/mm to 54 N/mm (increase of almost 40%) as the normal load increased from 0.5 N to 5 N. The details of all the shearing tests between the black shale-proppant interfaces, including the normal load, coefficient of friction, microslip displacement and initial tangential stiffness, are summarized in Table 1. When comparing the shearing behavior of a given specimen (i.e. given shale sample against given proppant sample) under different normal loads, it is noticed that the μ values decreased, while the microslip displacement and KTo increased as the normal load increased (with a few 4

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N/mm as the normal load increased from 0.5 N to 10 N. In the range of 0.5 N–3 N of normal load, the average KTo of the BL interface was higher than that of the BG interface, while at higher normal loads the two curves converged. The relatively higher initial tangential stiffness observed for the BL interfaces compared with the BG interfaces is speculated to be attributed to the higher roughness and lower local radius of the LBS particle surface compared with the glass beads surface, which could probably resulted in higher level of contact maturing/sur­ face penetration causing adhesion force under relatively low normal loads before starting of the shearing [38,39]. This extra adhesion force caused by the difference in roughness and local contact radius became insignificant when plowing dominated during shearing under higher normal load. The initial tangential stiffness of the BL interface was found to be around 50%–70% lower than that of shearing LBS particles against the siliceous white shale. Based on the SEM and EDS analysis, the black shale consists of clay flakes with organic matter, while the white shale is composed of sili­ ceous silt particles with limited organic content. This compositional and microstructural difference between the two types of shale rocks is believed to be the reason for the different shearing behaviors observed. It was proved by previous researchers that the mineralogy of shale rocks, especially the content of clay and organic matter, notably affects their mechanical properties, such as stiffness, anisotropy, strength, as well as frictional behavior [23,26,27,40]. Compared with the white siliceous shale, the lower hardness of the black shale, which was determined by its mineralogy and microstructure, could result in higher level of plowing during shearing, and the debris (clay dominant for the black shale or siliceous particles dominant for the white shale) and the tribofilm created due to shearing and plowing will, in turn, affect the shearing behavior [15,17,41–43]. 3.2. Effect of shearing velocity on the coefficient of friction The results of the velocity-stepping experiments on specimens BL5 (FN ¼ 1N), BL6 (FN ¼ 3N) and BL7 (FN ¼ 5N) are illustrated in Fig. 5, where the mobilized coefficient of friction, as well as the nominal shearing velocity, are plotted against the shearing displacement. Due to finite apparatus stiffness, a sudden rise of the mobilized coefficient of friction could be observed when the shearing velocity increased, and a dip was observed when the shearing velocity dropped, which is similar to what was reported by previous studies, like Dieterich and Conrad [36] and Dieterich and Kilgore [37]. Due to the morphology of the surface and stick-slip behavior, the curves fluctuated, especially during the shearing at relatively low speeds, as the shearing proceeded, which can be attributed to the possible increased contact area under lower shearing velocities [36]. The frictional stability can be quantified by the following parameter (a-b) to illustrate the shearing velocity effect on the coeffi­ cient of friction:

Fig. 4. Effect of normal load on the (a) coefficient of friction (μ) and (b) initial tangential stiffness (KTo) of the shale-proppant interfaces.

partly attributed to the higher hardness of the white shale compared with the black shale, which could result in less asperity breakage and plowing on the white shale surface. Comparing multiple types of gran­ ular materials, Sandeep and Senetakis [29] observed lower friction from materials with lower hardness. He and Senetakis [17] captured clear plowing tracks with a surface profiler after shearing an angular quartz sand on the white shale surface, and they speculated that due to plowing and shale surface asperity breakage, the coefficient of friction of the WL interface increased as the normal load increased. However, slight negative correlations between coefficient of friction and normal load can be observed from most of the BL and BG specimens tested, based on the results shown in Figs. 3(a), 4(a) and Table 1. The magnitude of the decrease in average coefficient of friction of the BL and BG interfaces were found to be 0.07 and 0.11 as the normal load increased from 0.5 N to 10 N. The initial tangential stiffness of all the tests are plotted against the corresponding normal load in Fig. 4(b), where the average values of initial tangential stiffness of the WL interface is also illustrated. The average KTo of the BG and BL interfaces are given in the dash lines in Fig. 4(b), from which it can be observed that the initial tangential stiffness of the BG interface increased from around 20 N/mm to 65 N/ mm and that of the BL interface increased from around 36 N/mm to 64

ða

bÞ ¼

Δμ lnðViþ1 =Vi Þ

(1)

where Vi and Viþ1 are the shearing velocities under step i and step (iþ1), and Δμ is the change in the steady state coefficient of friction of the two consecutive velocity steps (i.e., from Vi to Viþ1). Positive (a-b) values indicate velocity-strengthening response, i.e. the frictional force in­ creases as the shearing velocity increases, while negative (a-b) values indicate velocity-weakening response. The average friction of the later stage of each velocity step shearing was considered as the steady state coefficient of friction and it was taken into the calculation of Δμ. Mul­ tiple velocity steps were conducted on each of the three tests, and one (ab) value can be calculated each time the velocity change takes place. Among the total 20 (a-b) values derived, 14 of them were not larger than zero, and the maximum and minimum values were equal to 0.0026 and 0.0069. Despite the fluctuation of the curves, for specimens BL7, BL8 and BL9, the average (a-b) values were found to be 0.0035, 0.0007 5

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Fig. 5. Coefficient of friction-displacement curves at varying shearing velocities: (a) specimen BL5 tested under 1 N of normal load; (b) specimen BL6 tested under 3 N of normal load; (c) specimen BL7 tested under 5 N of normal load.

and 0.0033, respectively, which indicate a slight velocity weakening response. Nine cycles of individual shearing cycles were performed on speci­ mens BL8 and BL9 under different shearing velocities (ranging from 0.1 mm/h to 4096 mm/h) to further investigate the effect of shearing ve­ locity on the frictional behavior of the interfaces. Representative mobilized coefficient of friction against shearing displacement curves of

specimen BL8 are plotted in Fig. 6(a). Fluctuation and discrepancies can be observed among the curves; however, the elevated shearing velocity did not systematically alter the shearing response, including the initial tangential stiffness, the microslip displacement and the coefficient of friction. The coefficients of friction of BL8 (FN ¼ 3N) and BL9 (FN ¼ 1N) are plotted against shearing velocity in Fig. 6(b). In general, the co­ efficients of friction of BL8 were lower than those of BL9, however, no 6

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differences in mineralogy, surface microhardness and content of organic matter of the black (organic) and white (inorganic) shale rocks. From the velocity stepping tests, the BL interface tended to exhibit a slight ve­ locity weakening response, however, from a series of shearing tests within a wider range of velocities, the μ values were not systematically affected by the change of shearing velocity. Acknowledgments The work described in this paper was fully supported by the grants from the Research Grants Council of the Hong Kong Special Adminis­ trative Region, China, project no. “CityU 11206617” and project no. “CityU 11214218”. References [1] Boyer C, Clark B, Jochen V, Lewis R, Miller CK. Shale gas: a global resource. Oilfield Rev 2011;23:28–39. [2] DOE, NETL. Shale gas: applying technology to solve America’s energy challenges. Washington, DC: US Departement of Energy (DOE) and National Energy Technology Laboratory (NETL); 2011. [3] Stevens P. The shale gas revolution: developments and changes. Chatham House London; 2012. [4] Soeder DJ. Porosity and permeability of eastern Devonian gas shale. SPE Form Eval 1988;3:116–24. [5] Britt LK, Smith MB, Haddad ZA, Lawrence JP, Chipperfield ST, Hellman TJ. Waterfracs: we do need proppant after all. In: SPE annual technical conference and exhibition. Society of Petroleum Engineers; 2006. [6] Montgomery CT, Smith MB. Hydraulic fracturing: history of an enduring technology. J Pet Technol 2010;62:26–40. [7] Barati R, Liang JT. A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells. J Appl Polym Sci 2014;131. 40735. [8] Wang H, Sharma MM. Modeling of hydraulic fracture closure on proppants with proppant settling. J Pet Sci Eng 2018;171:636–45. [9] Huang H, Babadagli T, Li HA, Develi K, Wei G. Effect of injection parameters on proppant transport in rough vertical fractures: an experimental analysis on visual models. J Pet Sci Eng 2019;180:380–95. [10] Shimizu H, Murata S, Ishida T. The distinct element analysis for hydraulic fracturing in hard rock considering fluid viscosity and particle size distribution. Int J Rock Mech Min Sci 2011;48:712–27. [11] Zhang F, Zhu H, Zhou H, Guo J, Huang B. Discrete-element-method/ computational-fluid-dynamics coupling simulation of proppant embedment and fracture conductivity after hydraulic fracturing. SPE J 2017;22:632–44. [12] Zeng J, Li H, Zhang D. Numerical simulation of proppant transport in hydraulic fracture with the upscaling CFD-DEM method. J Nat Gas Sci Eng 2016;33:264–77. [13] Zhang G, Gutierrez M, Li M. A coupled CFD-DEM approach to model particle-fluid mixture transport between two parallel plates to improve understanding of proppant micromechanics in hydraulic fractures. Powder Technol 2017;308: 235–48. [14] Zhang H, Liu S, Xiao H. Frictional behavior of sliding shale rock-silica contacts under guar gum aqueous solution lubrication in hydraulic fracturing. Tribol Int 2018;120:159–65. [15] Xiao H, Liu S, Wang D. Tribological properties of sliding shale rock–alumina contact in hydraulic fracturing. Tribol Lett 2016;62:20. [16] Zhang H, Liu S, Xiao H. Tribological properties of sliding quartz sand particle and shale rock contact under water and guar gum aqueous solution in hydraulic fracturing. Tribol Int 2019;129:416–26. [17] He H, Senetakis K. A micromechanical study of shale rock-proppant composite interface. J Pet Sci Eng 2020;184:106542. [18] Parker MA, Ramurthy K, Sanchez PW. New proppant for hydraulic fracturing improves well performance and decreases environmental impact of hydraulic fracturing operations. In: SPE eastern regional meeting. Society of Petroleum Engineers; 2012. [19] Alary JA, Parias T. Method of manufacturing and using rod-shaped proppants and anti-flowback additives. United States patent and trademark office 2013;8:562. 900 B2. [20] Zhang C, Zhao L, Yu D, Liu G, Pei Y, Huang F, et al. The evaluation on physical property and fracture conductivity of a new self-generating solid proppant. J Pet Sci Eng 2019;177:841–8. [21] Goldsby DL, Tullis TE. Flash heating leads to low frictional strength of crustal rocks at earthquake slip rates. Science 2011;334:216–8. [22] Zoback MD, Kohli A, Das I, Mcclure MW. The importance of slow slip on faults during hydraulic fracturing stimulation of shale gas reservoirs. In: SPE americas unconventional resources conference. Society of Petroleum Engineers; 2012. [23] Kohli AH, Zoback MD. Frictional properties of shale reservoir rocks. J Geophys Res: solid earth 2013;118:5109–25. [24] Burwell J, Rabinowicz E. The nature of the coefficient of friction. J Appl Phys 1953;24:136–9. [25] He H, Senetakis K, Coop MR. An investigation of the effect of shearing velocity on the inter-particle behavior of granular and composite materials with a new micromechanical dynamic testing apparatus. Tribol Int 2019;134:252–63.

Fig. 6. Illustration of the effect of shearing velocity on the coefficient of friction (a) mobilized coefficient of friction against shearing displacement curves of shearing cycles under various shearing velocities; (b) coefficient of friction against shearing velocity plots of BL8 (FN ¼ 3N) and BL9 (FN ¼ 1N).

systematic effect of shearing velocity can be observed on the steady state coefficient of friction of both specimens. Since each cycle of shearing was performed on a new surface of the shale, the inter-tests variation is believed to be due to surface morphology effects. 4. Summary and conclusions In this study, the frictional behavior of a black shale with organic matter was investigated by conducting micromechanical experiments between two types of interfaces, i.e., black shale against Leighton Buzzard sand (BL) and black shale against glass bead (BG) interface, with a particular focus on the effects of normal load and shearing ve­ locity on friction. Based on shearing tests under normal loads ranging from 0.5 N to 10 N, the coefficient of friction (μ) of the BL interface was found to be around 0.60 to 0.75, while that of the BG interface was found to be lower by a magnitude of around 0.30 in comparison with the BL interface. The average μ values of the BL interface was found to be higher than the friction values of the Leighton Buzzard sand against a white siliceous shale (WL) interface. The coefficient of friction of both types of black shale-proppant contacts decreased as the normal load increased, while the opposite trend was observed for the WL interface. The average initial tangential stiffness of the BL interface increased from around 36 N/mm to 65 N/mm, while that of the BG interfaces increased from 20 N/mm to 65 N/mm as the normal load increased from 0.5 N to 10 N. The initial tangential stiffness of the BL interface was around half to one third of that of the WL interface at a given normal load. The differences in the observed tribological behavior between the BL and WL interfaces were speculated to be predominantly attributed to the 7

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[26] Sone H, Zoback MD. Mechanical properties of shale-gas reservoir rocks—Part 1: static and dynamic elastic properties and anisotropy. Geophysics 2013;78: D381–92. [27] Sone H, Zoback MD. Mechanical properties of shale-gas reservoir rocks—Part 2: ductile creep, brittle strength, and their relation to the elastic modulus. Geophysics 2013;78:D393–402. [28] Nardelli V, Coop MR. The experimental contact behaviour of natural sands: normal and tangential loading. Geotechnique 2019;69:672–86. [29] Sandeep CS, Senetakis K. Effect of Young’s modulus and surface roughness on the inter-particle friction of granular materials. Materials 2018;11:217. [30] Sandeep CS, Senetakis K. An experimental investigation of the microslip displacement of geological materials. Comput Geotech 2019;107:55–67. [31] He H, Senetakis K. An experimental study on the micromechanical behavior of pumice. Acta Geotech 2019;14:1883–904. [32] Yang L, Wang D, Guo Y, Liu S. Tribological behaviors of quartz sand particles for hydraulic fracturing. Tribol Int 2016;102:485–96. [33] Sandeep CS, Senetakis K. Grain-scale mechanics of quartz sand under normal and tangential loading. Tribol Int 2018;117:261–71. [34] Krumbein WC, Sloss LL. Stratigraphy and sedimentation. second ed. W. H Freeman and Company; 1963. [35] Wang Y, Zhu Y, Chen S, Li W. Characteristics of the nanoscale pore structure in Northwestern Hunan shale gas reservoirs using field emission scanning electron

[36] [37] [38] [39] [40] [41] [42] [43]

8

microscopy, high-pressure mercury intrusion, and gas adsorption. Energy Fuels 2014;28:945–55. Dieterich JH, Conrad G. Effect of humidity on time-and velocity-dependent friction in rocks. J Geophys Res: Solid Earth 1984;89:4196–202. Dieterich JH, Kilgore BD. Direct observation of frictional contacts: new insights for state-dependent properties. Pure Appl Geophys 1994;143:283–302. Ando Y, Ishikawa Y, Kitahara T. Friction characteristics and adhesion force under low normal load. J Tribol 1995;117:569–74. Michalowski RL, Wang Z, Nadukuru SS. Maturing of contacts and ageing of silica sand. Geotechnique 2017;68(2):133–45. Yan W, Ge H, Wang J, Wang D, Meng F, Chen J, et al. Experimental study of the friction properties and compressive shear failure behaviors of gas shale under the influence of fluids. J Nat Gas Sci Eng 2016;33:153–61. Chowdhury MA, Helali M. The effect of amplitude of vibration on the coefficient of friction for different materials. Tribol Int 2008;41:307–14. Reches Ze, Lockner DA. Fault weakening and earthquake instability by powder lubrication. Nature 2010;467:452. Sandeep CS, Senetakis K. Exploring the micromechanical sliding behavior of typical quartz grains and completely decomposed volcanic granules subjected to repeating shearing. Energies 2017;10:370.