Analysis and investigation of the leakage failure of a casing used in a shale gas well

Abstract

A casing leaked in a shale gas well during pressure testing. A series of experiments including macroscopic observation, non-destructive inspection, physical and chemical inspection, optical microscope, scanning electron microscope (SEM) and energy dispersive spectrometry techniques (EDS) were carried out to analyze the failure accident. The finite simulation analysis (FEA) method was used to analyze the effect of internal pressure on the defective parts of the casing. Based on the experiment result and manufacturing process investigation, the root cause was obtained. During the casing pipe rolling process, the bolts of size M12 × 60 mm on the balance cylinder of the continuous pipe mill were loosened and dropped onto the pipe. Under the dual action of the roller and the mandrel, the bolts are 'inlaid' inside the casing as a dissimilar metal substance. The bolt head almost occupies the entire wall thickness of the casing, resulting in the remaining wall thickness of the casing being less than 1 mm. The dissimilar metal inside the casing destroys the continuity of the pipe and severely reduces the pressure bearing capacity of the casing. The FEA calculation results show that the equivalent stress at the defect is lower than the yield strength of the pipe body during the factory hydraulic test. While, in the field pressure test, the equivalent stress at the defect is far greater than the yield strength of the casing under the action of axial tension and internal pressure. Therefore, the crack at the defect quickly propagated and penetrated the entire wall thickness of the casing, resulting in leakage. In order to avoid similar accidents, suggestions to prevent such failures are proposed.