Mechanisms of Fault Activation and Casing Deformation Control During Hydraulic Fracturing in Deep Shale Gas Reservoirs

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Abstract Deep shale gas reservoirs (e.g., the Wufeng–Longmaxi formations) in the southern Sichuan Basin hold vast potential but face severe casing deformation due to the complex geologic conditions, including multi-directional in situ stresses, high brittleness, micro-fracture networks, and high-temperature/high-pressure environments. This study elucidates the fault activation mechanisms during hydraulic fracturing and proposes mitigation strategies to ensure safe and stable shale gas production. Fourteen shale gouge samples from five wells of the Wufeng–Longmaxi formations in the Luzhou Block of the southern Sichuan Basin were collected and analyzed by high-temperature and high-pressure triaxial shear tests (temperature = 120–170°C, confining pressure = 80–120 MPa, and pore pressure = 32–50 MPa), scanning electron microscopy, and X-ray diffraction. The frictional behavior of the shale gouges was systematically analyzed using rate-and-state friction constitutive equations. Dynamic pore pressure simulations and long-term fracturing fluid immersion experiments were performed to evaluate fault stability. A casing deformation calculation model was established based on the stress drop and focal mechanism solutions. Clay mineral content exhibited a significant negative correlation with the friction coefficient (μ = 0.747 − 0.0049φ). The friction coefficient ranged from 0.50 to 0.75 in deep shales. Higher pore pressure (>50 MPa) corresponded to higher frictional strength but lower frictional stability (lower a − b values), resulting in an increased risk of fault activation. Long-term fracturing fluid immersion (pH 5.8→7.0) weakened stability via carbonate dissolution, and the predicted casing deformation aligned with field data (errors <15%). A risk classification method was proposed based on the clay content and frictional stability parameters. The results suggest that fault activation is the primary cause of casing deformation. Optimizing the fracturing fluid composition (e.g., through the addition of KCI stabilizers), controlling the injection pressure gradients (ΔP < 15 MPa/100 m), and monitoring the frictional stability parameters in real time can effectively mitigate risks.

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