Mechanical behavior of calcite vein-bearing shale of the Niutitang Formation in Fenggang area, northern Guizhou based on CT tests

doi:10.13745/j.esf.sf.2024.6.15

Abstract

Abstract:

Core observations of shales from the Niutitang Formation in the northern Qianbei region show that calcite veins often act as natural fracture fillers and largely influence the shale damage patterns. The damage characteristics of calcite vein-bearing shales is important for the prediction of fracture initiation and extension behavior during hydraulic fracturing and for the engineering design. In order to reveal the influence of calcite veins on the mechanical properties and fracture characteristics of shale, uniaxial compression and acoustic emission tests are conducted at seven inclination angles of 0° to 90° in 15° increment. Combined with CT scanning technology and finite element calculations, a three-dimensional (3D) microscopic numerical model is constructed. The effects of calcite vein angle on the fine-scale shale damage process as well as shale mechanical properties are discussed, and the spatiotemporal evolution of shale microcracks are analyzed. The results show that (1) under different calcite vein angles the shale acoustic emission and stress-strain curves show similar curve shape changes, in four stages, namely compression-density, elasticity, yielding, and post-peak damage, with obvious distinctions between stages. The change curve of the characteristic intensity is “U”-shaped with a local minimum at θ of 75°. (2) Calcite veins significantly affect the damage mode: As the vein angle decreases, the damage mode changes from cleavage to cleavage-type shear, to shear-slip, and finally to cleavage-tension. (3) The reconstructed 3D model is largely consistent with physical testing data, providing insights into the process of crack expansion and penetration at shale’s interior and surface. The spatial distribution of acoustic emission reflects the compression, tension, and shear damage types at different stages, revealing the fracture mechanism of calcite-containing shale from the microscopic point of view. (4) Calcite and matrix simulatineously and anisotropically affect the macroscopic mechanical properties of shale: The higher the calcite vein angle, the stronger its slip guidance effect, and the weaker the mechanical properties of the specimens.

Key words: shale, calcite, anisotropy, three-dimensional reconstruction, northern Guizhou

CLC Number:

WU Zhonghu, MENG Xiangrui, LAN Baofeng, LIU Jingshou, GONG Lei, YANG Yuhan. Mechanical behavior of calcite vein-bearing shale of the Niutitang Formation in Fenggang area, northern Guizhou based on CT tests[J]. Earth Science Frontiers, 2024, 31(5): 117-129.

Figures/Tables 19

Fig.1 Core samples from the Niutitang Formation in well FC-1

Fig.2 Core sampling scheme

Fig.3 Calcite vein-bearing shale samples

Fig.4 Microscopic characteristics of shale specimen by SEM

Fig.5 Acoustic emission test setup and sensor arrangement

Fig.6 Microstructural characterization of shale by CT scan imaging

Fig.7 Change of greyscale value at each pixel along the horizontal scanning line

Fig.8 Digital core reconstruction through image superposition/vetorization

Table 1 Material parameters for rock samples

材料	弹性模量/GPa	抗压强度/MPa	泊松比	拉压比	内摩擦角/(°)
页岩	51.6	145	0.22	14	35
方解石	80.5	101	0.30	11	30
石英	96.0	373	0.08	15	60

Fig.9 Digital model of calcite veins with 75° inclination angle

Fig.10 Stress-strain curves of shale with different calcite vein inclinations

Fig.11 Damage patterns of shales with different calcite vein inclinations a—θ=0°;b—θ=15°;c—θ=30°;d—θ=45°;e—θ=60°;f—θ=75°;g—θ=90°。

Fig.12 Plot of AE event count, cumulative event count, and stress strain vs. time for shale specimens with calcite vein angle at 0°

Fig.13 Results of uniaxial compression acoustic emission tests for calcite vein-bearing shales with different vein inclinations a—θ=15°;b—θ=30°;c—θ=45°;d—θ=60°;e—θ=75°;f—θ=90°。

Fig.14 Characteristic of stress variation with vein inclination by physical testing (left) and modeling (right)

Fig.15 3D modeling of fracture evolution in shale specimen with a calcite vein inclination of 60°

Fig.16 2D slice through 3D models showing stress evolution in shale specimen with calcite vein inclination of 45°

Fig.17 Stress-strain diagrams, damage evolution models and AE simulation diagrams under calcite vein inclinations of 0° (a) and 90° (b)

Fig.18 Shale final damage under different vein inclinations by acoustic emission modeling

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