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<p>The time-delayed initiation of hydraulic fractures plays a critical role in reducing breakdown pressure and suppressing seismic activity. However, its underlying mechanisms and predictive models remain poorly understood, especially in shale reservoirs. In this study, true triaxial fracturing experiments were conducted on Lushan shale samples under different pressure ratios (constant borehole pressure/monotonic breakdown pressure). Acoustic emission (AE) monitoring, microscopic observations, and three-dimensional fracture surface scanning were employed to quantitatively analyze fracture types, morphology, surface roughness, and microseismic intensity. The results show that constant pressure injection reduces the breakdown pressure by 10–15%, compared with monotonic injection. Both the maximum and cumulative AE energy decrease with decreasing pressure ratio, whereas the AE signal quantity increases, indicating that the injected hydraulic energy becomes more dispersed. Dominant frequency analysis reveals that tensile fracturing dominates under all injection schemes. However, decreasing the pressure ratio transitions failure from instantaneous mechanical instability to a time-delayed fracture controlled by fluid–rock coupling, thereby increasing the proportion of shear fractures. The increasing <i>b</i>-value with decreasing pressure ratio further suggests that constant pressure injection replaces large-magnitude microseismic events with more frequent small-magnitude events. A breakdown lifetime prediction model considering fluid infiltration, stress corrosion, and confining pressure was developed and provides reliable predictions when the stress corrosion index equals 48. The time-delayed initiation of hydraulic fractures is governed by the coupled effects of pore-pressure diffusion and stress corrosion. This study provides theoretical support and experimental guidance for optimizing injection schemes.</p>

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Experimental and Mechanistic Investigation on Time-Delayed Initiation of Hydraulic Fractures in Shale Under Constant Pressure Injection

  • 期刊论文

  • 发表日期：

    2026年3月3日

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  • Cong Huang,
  • Yu Zhao,
  • Yongfa Zhang,
  • Dongping Zhou

摘要

The time-delayed initiation of hydraulic fractures plays a critical role in reducing breakdown pressure and suppressing seismic activity. However, its underlying mechanisms and predictive models remain poorly understood, especially in shale reservoirs. In this study, true triaxial fracturing experiments were conducted on Lushan shale samples under different pressure ratios (constant borehole pressure/monotonic breakdown pressure). Acoustic emission (AE) monitoring, microscopic observations, and three-dimensional fracture surface scanning were employed to quantitatively analyze fracture types, morphology, surface roughness, and microseismic intensity. The results show that constant pressure injection reduces the breakdown pressure by 10–15%, compared with monotonic injection. Both the maximum and cumulative AE energy decrease with decreasing pressure ratio, whereas the AE signal quantity increases, indicating that the injected hydraulic energy becomes more dispersed. Dominant frequency analysis reveals that tensile fracturing dominates under all injection schemes. However, decreasing the pressure ratio transitions failure from instantaneous mechanical instability to a time-delayed fracture controlled by fluid–rock coupling, thereby increasing the proportion of shear fractures. The increasing b-value with decreasing pressure ratio further suggests that constant pressure injection replaces large-magnitude microseismic events with more frequent small-magnitude events. A breakdown lifetime prediction model considering fluid infiltration, stress corrosion, and confining pressure was developed and provides reliable predictions when the stress corrosion index equals 48. The time-delayed initiation of hydraulic fractures is governed by the coupled effects of pore-pressure diffusion and stress corrosion. This study provides theoretical support and experimental guidance for optimizing injection schemes.