Degradation of the internal well equipment steel under continuous service in the corrosive and aggressive environments

Abstract

Purpose is to analyze steel degradation of the internal well equipment during its continuous service while contacting directly the corrosive environments. Methods. A range of research concerning the damaged metal tubes of the internal equipment for oil and gas wells, in particular regarding continuous service tubing, comprised both standard and specific studies involving different variations of X-ray spectral analysis with the use of scanning electron microscope JSM-35CF (JEOL Company, Japan) and SEM-515 with microanalyzer Link by Philips Company. The studied samples have been made of tubing in the period of the unauthorized and emergency well shutdowns; life of the wells is 0 up to 15 years. To analyze both structure and chemical composition of metal inclusive of such gases as oxygen and hydrogen, chippings were produced mechanically from various parts of tube walls. Findings. X-ray structural studies have made it possible to obtain data confirming cementite decay (Fe3С) in the tube metal during continuous operation of the internal well equipment. X-ray structural analysis methods have helped identify the parameters of crystal lattice of a matrix; and a level of elastic distortions of the lattice (i.e. microstresses of the distortions) has been evaluated as well as carbon distribution within ferrite and cementite. The abovementioned offered the possibility to describe both reason and mechanism of the reduced resistance to corrosion in the context of internal well equipment. Originality. New regularities under cementite decay in tube metal have been identified in addition to changes in the parameters of a crystal a lattice; microstresses of the lattice distortions; and carbon distribution within ferrite and cementite. The aforesaid helps explain in a new way both reason and mechanism of the reduced resistance to corrosion in the context of internal well structures operating continuously in aggressive environments. The basic sources and mechanisms of tube steel degradation, resulting from the metal hydrogenation and oxidation, have been defined which becomes the foundation to develop scientifically the substantiated measures mitigating the negative impact on the condition of the internal well facilities operating continuously in the chemically aggressive environments. Practical implications. Degrading hydrogen effect on the crystal lattice of metal has been proved. The effect creates conditions under which tube structures of oil and gas wells experience their failure.