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<p>Shale reservoirs that have undergone intense tectonic transformation typically display significant heterogeneity and anisotropy in fracture structures and seepage properties, which severely hinder the development of shale gas resources in tectonically complex areas. This paper focused on the micro-fracture structures of tectonically deformed shale (TDS), of which the 3-D structural characteristics were investigated qualitatively and quantitatively. The size effects of fracture void percentage, spatial attitude angles, and permeability coefficients (determined by seepage numerical simulations) were then analyzed to estimate the permeability REV (representative elementary volume) sizes of various TDSs. The results indicate that structural deformation (especially brittle deformation) promotes fracture development. The fracture intensity and connectivity of TDSs generally follow the evolution pattern of “brittle TDS series ≈ brittle-ductile transitional TDS series &gt; ductile TDS series”. The variations of fracture void percentage and spatial orientation angles effectively characterize the size effects of fracture seepage properties and are reliable structural characteristic parameters for determining the permeability REV size of intensely fractured shale. Based on the variation criterion of permeability coefficients (i.e., the relative standard deviation (<i>RSD</i>) of 0.5), the fracture sub-regions that are representative of the entire samples should exhibit a spatial orientation distribution pattern similar to the parent fracture structures and a fracture void percentage <i>RSD</i> value of less than 15%. The REV sizes of seepage properties of TDSs were estimated accordingly, showing apparent correlations with the development degree and distribution uniformity of fractures. Flaky and scaly shales were expected to have the smallest REV size (6–8&#xa0;mm). The REV size of porphyritic and platy shales was expected to be slightly larger than 8&#xa0;mm. Cataclastic, fractured-crumpled, crumpled, and mylonitized shales were expected to exhibit the largest REV size of seepage properties (around or slightly greater than 12&#xa0;mm).</p>

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Characteristics and Size Effects of 3-D Micro-fracture Structures of Tectonically Deformed Shale Based on X-ray Tomography and Digital Image Processing Technology: Implications for Estimating Permeability REV Size of Intensely Fractured Shale

  • 期刊论文

  • 发表日期：

    2026年1月29日

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  • Cheng Guoxi,
  • Li Fengli,
  • Jiang Bo,
  • Wu Caifang,
  • Li Ming,
  • Song Yu

摘要

Shale reservoirs that have undergone intense tectonic transformation typically display significant heterogeneity and anisotropy in fracture structures and seepage properties, which severely hinder the development of shale gas resources in tectonically complex areas. This paper focused on the micro-fracture structures of tectonically deformed shale (TDS), of which the 3-D structural characteristics were investigated qualitatively and quantitatively. The size effects of fracture void percentage, spatial attitude angles, and permeability coefficients (determined by seepage numerical simulations) were then analyzed to estimate the permeability REV (representative elementary volume) sizes of various TDSs. The results indicate that structural deformation (especially brittle deformation) promotes fracture development. The fracture intensity and connectivity of TDSs generally follow the evolution pattern of “brittle TDS series ≈ brittle-ductile transitional TDS series > ductile TDS series”. The variations of fracture void percentage and spatial orientation angles effectively characterize the size effects of fracture seepage properties and are reliable structural characteristic parameters for determining the permeability REV size of intensely fractured shale. Based on the variation criterion of permeability coefficients (i.e., the relative standard deviation (RSD) of 0.5), the fracture sub-regions that are representative of the entire samples should exhibit a spatial orientation distribution pattern similar to the parent fracture structures and a fracture void percentage RSD value of less than 15%. The REV sizes of seepage properties of TDSs were estimated accordingly, showing apparent correlations with the development degree and distribution uniformity of fractures. Flaky and scaly shales were expected to have the smallest REV size (6–8 mm). The REV size of porphyritic and platy shales was expected to be slightly larger than 8 mm. Cataclastic, fractured-crumpled, crumpled, and mylonitized shales were expected to exhibit the largest REV size of seepage properties (around or slightly greater than 12 mm).