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<p>Directional hydraulic fracturing (DHF) is a favored approach to extract unconventional oil and gas resources. The ambient stress (geological factor) and the pre-crack angle (engineering factor) exert a significant impact on the breakdown pressure and fracture complexity. To gain a comprehensive understanding of the impact of stress magnitude (<i>σ</i>) and pre-crack angle (<i>θ</i>) on the fracturing effectiveness, we conducted DHF experiments on Lushan shale samples with five different axial stresses and five varying pre-crack angles. We monitored the dynamic propagation of hydraulic fractures using an acoustic emission (AE) system and a high-speed camera and subsequently analyzed the fracture pattern using a fluorescent tracer and an optical microscope. The results showed that at low axial stresses (<i>σ</i> &lt; 10&#xa0;MPa), fractures propagated primarily along bedding planes, forming single, bedding-parallel fractures; at moderate stress levels (10–20&#xa0;MPa), fractures preferentially followed pre-crack orientations and developed increasingly complex networks; and at excessively high axial stresses, bedding activation was inhibited, constraining the formation of complex fracture patterns. Higher differential stresses reduced fracture initiation and breakdown pressures owing to stress concentration and preferential propagation along pre-existing cracks, whereas bedding activation at low stresses could lead to abnormally low pressures, consistent with AE evolution. Increasing pre-crack angles generally elevated fracture pressures and promoted shear/mixed-mode cracking, with maximum fracture complexity occurring at <i>θ</i> = 45°. Lower pre-crack angles favored axial, continuous fracture propagation, whereas larger angles triggered bedding activation and fracture arrest, limiting fracture development and complexity. These findings provide mechanistic insights into the interplay of stress, pre-crack orientation, and bedding in controlling shale hydraulic fracture initiation, propagation, and network complexity.</p>

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Directional Hydraulic Fracturing in Lushan Shale: Effects of Stress Magnitude and Pre-crack Angle on Fracture Propagation

  • 期刊论文

  • 发表日期：

    2026年1月12日

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  • Anfa Long,
  • Yongfa Zhang,
  • Yu Zhao,
  • Shengfeng Wu,
  • Chaolin Wang,
  • Jing Bi,
  • Huasen Huang,
  • Kai Wang

摘要

Directional hydraulic fracturing (DHF) is a favored approach to extract unconventional oil and gas resources. The ambient stress (geological factor) and the pre-crack angle (engineering factor) exert a significant impact on the breakdown pressure and fracture complexity. To gain a comprehensive understanding of the impact of stress magnitude (σ) and pre-crack angle (θ) on the fracturing effectiveness, we conducted DHF experiments on Lushan shale samples with five different axial stresses and five varying pre-crack angles. We monitored the dynamic propagation of hydraulic fractures using an acoustic emission (AE) system and a high-speed camera and subsequently analyzed the fracture pattern using a fluorescent tracer and an optical microscope. The results showed that at low axial stresses (σ < 10 MPa), fractures propagated primarily along bedding planes, forming single, bedding-parallel fractures; at moderate stress levels (10–20 MPa), fractures preferentially followed pre-crack orientations and developed increasingly complex networks; and at excessively high axial stresses, bedding activation was inhibited, constraining the formation of complex fracture patterns. Higher differential stresses reduced fracture initiation and breakdown pressures owing to stress concentration and preferential propagation along pre-existing cracks, whereas bedding activation at low stresses could lead to abnormally low pressures, consistent with AE evolution. Increasing pre-crack angles generally elevated fracture pressures and promoted shear/mixed-mode cracking, with maximum fracture complexity occurring at θ = 45°. Lower pre-crack angles favored axial, continuous fracture propagation, whereas larger angles triggered bedding activation and fracture arrest, limiting fracture development and complexity. These findings provide mechanistic insights into the interplay of stress, pre-crack orientation, and bedding in controlling shale hydraulic fracture initiation, propagation, and network complexity.