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<p>With the widespread use of the hydraulic fracturing technique, the extensive extraction of shale gas has the potential to mitigate the energy crisis. However, a large amount of fracking fluid that remains in the shale formation after flowback may cause the problem of wellbore instability. Since the shale gas formation orientation is variable, the mechanical behavior of the shale should be considered both the hydration and anisotropic. In addition, little previous studies have focused on a siliceous shale with low clay contents. To better understand the anisotropic mechanical behavior and the failure mechanism of a low-clay siliceous shale after water saturation, triaxial compression tests were carried out on unaltered and saturated shale specimens with different bedding inclinations (0°, 30°, 45°, 60°, 90°). The results showed that water saturation could enhance the anisotropic mechanical characteristics of shale specimens. And the water saturation could make the shale specimens with intermediate and high bedding inclinations (<i>β</i> = 45° ~ 90°) more susceptible to occur shear failure and slippage along the bedding plane and result in a smoother fracture surface. Meanwhile, the microcrack number, displacement field and crack coalescence process of the shale specimen were monitored during triaxial loading by PFC<sup>3D</sup> modeling. The microcracks of shale specimens usually accumulate first on the bedding plane and then mainly on the matrix. Finally, revised nucleation and growth models for the water-saturated shale specimens with different bedding inclinations were proposed. The failure mechanism of the shale matrix is based on the process zone theory induced by tensile microcracks, while the failure mechanism of the bedding plane is based on the slip zone theory induced by shear microcracks. Due to the weak mechanical properties of the hydrated clay mineral layers, the bedding inclination of the shale specimen would determine which failure mechanism is dominant. The results can provide a reference for the research on the shale formation stability evaluation affected by the fluids intrusion.</p>

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Experiment and Numerical Simulation Study of Mechanical Behavior and Failure Mechanism of a Low-Clay Siliceous Shale After Water Saturation

  • 期刊论文

  • 发表日期：

    2025年1月9日

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  • Wang-Xing Hong,
  • Sheng-Qi Yang,
  • Bo-Wen Sun,
  • Wen-Ling Tian,
  • Peng-Fei Yin

摘要

With the widespread use of the hydraulic fracturing technique, the extensive extraction of shale gas has the potential to mitigate the energy crisis. However, a large amount of fracking fluid that remains in the shale formation after flowback may cause the problem of wellbore instability. Since the shale gas formation orientation is variable, the mechanical behavior of the shale should be considered both the hydration and anisotropic. In addition, little previous studies have focused on a siliceous shale with low clay contents. To better understand the anisotropic mechanical behavior and the failure mechanism of a low-clay siliceous shale after water saturation, triaxial compression tests were carried out on unaltered and saturated shale specimens with different bedding inclinations (0°, 30°, 45°, 60°, 90°). The results showed that water saturation could enhance the anisotropic mechanical characteristics of shale specimens. And the water saturation could make the shale specimens with intermediate and high bedding inclinations (β = 45° ~ 90°) more susceptible to occur shear failure and slippage along the bedding plane and result in a smoother fracture surface. Meanwhile, the microcrack number, displacement field and crack coalescence process of the shale specimen were monitored during triaxial loading by PFC3D modeling. The microcracks of shale specimens usually accumulate first on the bedding plane and then mainly on the matrix. Finally, revised nucleation and growth models for the water-saturated shale specimens with different bedding inclinations were proposed. The failure mechanism of the shale matrix is based on the process zone theory induced by tensile microcracks, while the failure mechanism of the bedding plane is based on the slip zone theory induced by shear microcracks. Due to the weak mechanical properties of the hydrated clay mineral layers, the bedding inclination of the shale specimen would determine which failure mechanism is dominant. The results can provide a reference for the research on the shale formation stability evaluation affected by the fluids intrusion.