The Discovery of Fracture Tip-Driven Stress Concentration: A Key Contributor to Casing Deformation in Horizontal Wells

Casing deformation (CD) is generally believed to be caused by the slip of fractures in the strata and its shear effects on horizontal wells. However, the casing deformation mode based on this theory cannot fully match the measurement data, and the differential deformation characteristics and the mechanism behind this phenomenon are not completely clear. To elucidate the mechanisms of CD and enhance prevention and control measures, the CD modes in Shunan Block were identified and deformation mechanisms of these modes were comprehensively investigated. Our research shows the following: (1) Under the mechanism of penetrating fracture shear deformation, CD exhibit obvious shear deformation, and the natural fractures near the intersection point with the wellbore are prone to form a higher risk of deformation. (2) Natural fractures with tips approaching the wellbore experience intense stress concentration (1.6 times higher than shear stress) during activation, resulting in compression and asymmetrical CD. (3) The shear deformation induced by penetrating fractures is 15.52 mm, while the fracture tip-induced compression deformation demonstrates a substantially greater magnitude at 44.17 mm. This compressive deformation exceeds the shear deformation by a factor of approximately 2.85. (4) The stress concentration at the fracture tip is highly sensitive to the injection rate. Hence, adherence to the “avoiding stress concentration” principle is crucial in hydraulic fracturing operations. The conclusion indicates that in addition to penetrating fracture shear deformation, fracture tip compression deformation is another significant mechanism that causes CD. This research finding can offer theoretical guidance for developing effective measures to prevent and control CD in the exploitation of deep shale gas.