Effects of alternating stress on casing damage mechanism

Horizontal well staged fracturing technology of horizontal well has been widely used in the exploitation of low-permeability oil and gas reservoirs and coalbed methane, and has achieved good economic results. However, the vertical section of the casing is found to be damaged with serious deformation in the process of multi-stage hydraulic fracturing, which will prevent bridge plugs from being installed at the preferred design depths, thereby resulting in abandonment of all remaining fracturing stages. In this study, a user element is developed to simulate the mechanical behavior at the casing-cement sheath interface under cyclic loads. The developed element is then implemented into an axisymmetric finite element(FE) model of the casing-cement sheath-stratum system through ABAQUS to simulate the mechanical behavior of the casing during horizontal well staged fracturing. The FE results reveal that casing damage with large deformation is induced by the alternation of the resultant stress between injection pressure and geo-stress. The fatigue life of the casing is then estimated through FE-safe based on the obtained results through ABAQUS, thereby finding out that the most vulnerable part of the casing is located at interlaced area between hard and soft strata with the lowest fatigue life. Based on the aforementioned results, increasing the pressure inside the casing at the non-fracturing stage is highly recommended to mitigate casing damage in the interlaced area between hard and soft strata during horizontal well staged fracturing.