Journal of Shandong University(Engineering Science) ›› 2021, Vol. 51 ›› Issue (5): 91-99, 106.doi: 10.6040/j.issn.1672-3961.0.2020.276

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Influence of calcite veins on shale anisotropy at the microscopic scale based on digital images

Huailei SONG1(),Zhonghu WU1,2,3,*(),Liping LI2,3,Yili LOU1,Wenjibin SUN4,Hao LIU4,Yujun ZUO4   

  1. 1. College of Civil Engineering, Guizhou University, Guiyang 550025, Guizhou, China
    2. School of Qilu Transportation, Shandong University, Jinan 250002, Shandong, China
    3. Research Center of Geotechnical and Structural Engineering, Shandong University, Jinan 250061, Shandong, China
    4. Mining College, Guizhou University, Guiyang 550025, Guizhou, China
  • Received:2020-07-12 Online:2021-10-20 Published:2021-09-29
  • Contact: Zhonghu WU E-mail:songhuaileigzu@163.com;wuzhonghugzu@163.com

Abstract:

The Niutitang Formation shale cores were observed by micro-slice observations and core X-ray whole-rock mineral diffraction analysis, and 7 groups of direct tensile numerical tests under different azimuth angles were performed. The test results showed that the calcite veins had a significant effect on the anisotropy of shale tensile strength. When the azimuth angle increased, the tensile strength gradually decreased. The bedding effect coefficient of tensile strength showed a curve-like growth trend with the increase of azimuth angle, which reached the maximum when α=90°, which was 0.127. The failure modes of shale samples at different angles were very complicated, which could be roughly divided into the following three categories: tree root shape (0°, 15°), step shape (30°, 45°, 60°) and river shape (75°, 90°). Fractures preferentially extended along calcite veins, which might inhibit the formation of complex fracture networks in the shale matrix during hydraulic fracturing. There were also significant differences in the release of dissipated energy under the calcite veins at different angles. The release of dissipated energy under the calcite veins at different angles was also significantly different. When α=0°, 15°, 30°, and 45°, the AE energy was small in the early stage, and increased rapidly to the maximum when it approached the peak stress. When α=60°, 75°, 90°, the AE energy was small in the early stage, and began to increase in the middle stage, and it was the largest when it was close to the peak stress. The cumulative AE energy increased roughly exponentially with increasing strain, and the growth process consisted of three stages: flat period, accelerated period and skyrocketing period. The research results had important reference value for the initiation of hydraulic fractures in shale reservoirs, the prediction of expansion, and the enhancement of oil recovery.

Key words: shale, digital image processing, rock fracture, acoustic emission energy, microstructure

CLC Number: 

  • P618.12

Fig.1

X-ray diffraction analysis of whole rock"

Table 1

Material parameters"

材料 弹性模量/GPa 抗拉强度/MPa 拉压比 泊松比ν 内摩擦角λ/(°)
页岩 51.6 11.67 14 0.22 35
方解石 80.5 9.00 11 0.30 30
石英 96.0 14.00 15 0.08 60

Fig.2

Image acquisition process of shale micro section"

Fig.3

I value change curve on AA' scan line"

Fig.4

Image after threshold segmentation"

Fig.5

Digital image of shale at different azimuths"

Fig.6

Model loading diagram"

Table 2

Uniaxial tensile strength value and bedding effect coefficient of shale"

方位角α/(°) 抗拉强度/MPa S(α)
0 3.562 0.000
15 3.349 0.060
30 3.275 0.081
45 3.235 0.092
60 3.206 0.100
75 3.140 0.118
90 3.110 0.127

Fig.7

Trends of tensile strength, elastic modulus, and lateral and bedding effect coefficients of shale under different azimuth angles"

Fig.8

Correspondence diagrams of damage evolution division process of 45° shale specimen and cumulative acoustic emission"

Fig.9

Uniaxial tensile damage evolution of shale under different azimuth angle"

Fig.10

Trends of stress, AE energy and cumulative AE energy with strain at different azimuth angles"

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