ABSTRACT:

Casing damages have been found in many stimulated wells worldwide. While other mechanisms such as thermal and chemical can be responsible, this study focuses on geomechanical and mechanical contributions to casing damage during and after hydraulic fracturing. Through 2D numerical models, casing integrity is evaluated with varying pipe eccentricities, cement channels, breakouts, and injection pressures. In addition, near-wellbore 3D geomechanical models are developed to investigate driving mechanics such as axial and lateral compression, distributed and localized shear, and combined compression-shear modes. Coupled reservoir-geomechanics models quantify rock dynamic stress and strain changes induced by stimulation and compare them to the threshold triggering casing damage. For the cases modeled, it is found that the localized shear is most detrimental to casing integrity. Washouts or breakouts can accelerate and aggregate casing risk by as much as 70%. In comparison, cement channels affect little due to stiffness contrast of rock and cement. Even though fluid injection elevates stresses and rock deformation, they are not significant enough to cause casing damage without pipe preloading or slippage of formation discontinuities. The proposed approach, combined with other engineering investigations, can help reduce casing damage risk and optimize well and operation designs.

1. Introduction: Casing Damages and Hydraulic Fracturing

Casing damages during and after injection have been reported in stimulated wells worldwide. For example, a review of over 40,000 wells in Pennsylvania between 2000 and 2012 found a six fold higher incidence of casing and/or cement issues for shale gas wells than conventional wells (Ingraffea et al., 2014); YPF and Chevron experienced casing restrictions due to shear movement in the Vaca Muerta formations (Rimedo et al., 2015); CNPC found casing ovalization and shear in the Sichuan Basin (Chen, 2017; Lin, 2017; Guo 2018); Apache revealed casing failures due to cyclic injection pressure in the Permian Basin (Barreda et al., 2018); Chesapeake reported casing damages in the Eagle Ford and the Powder River Basin (Hoffman et al., 2020). In many cases, operators have had to skip fracturing stages or even abandon wells. These have negative impacts on stimulation efficiency and production.

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