Study of Deformation Law of Casing Local Lateral Collapse Based on the Principle of Virtual Work

In view of the water swelling of mudstone and the creep induction function of formations in the process of oilfield water injection, the casing incurs collapse deformation under local lateral load. In this study, according to the actual collapse deformation characteristics of the casing in the second section of the Qing formation of the Songliao Basin in China, the yield surfaces of the casing collapse deformation are considered as plane plastic areas (half rhombus) with symmetric parabola shaped boundaries, and a mechanical model for the local lateral collapse deformation of casing is presented based on the principle of virtual work. Four types of casing, 4½″J55, 5½″J55, 4½″N80 and 5½″N80, are selected as examples. The relation of the casing intensity, the absolute reduction of intensity and the relative reduction ratio of intensity change with casing wall thickness, yield stress, radial maximum deformation, and deformation length are calculated and analyzed. The results show that the casing intensity of the casing is reduced under local lateral load, which is lower than the design standard value of the American Petroleum Association specification (API SPEC) 5CT. The relative reduction ratio declines linearly with the wall thickness of the casing wall as the yield stress increases, and increases linearly with increasing maximum deformation. In addition, the local lateral bearing capacity of the casing reaches the minimum value when the plastic deformation length reaches the critical value or the deformation quantity is less than the critical value. The conclusions provide scientific guidance for preventing casing failure accidents caused by deformation.