Fatigue Life Of Wellbore In CNG Underground Gas Storage Well

With the rapid development of natural gas industry, the progress of CNG automobile industry was promoted greatly. CNG stations as places for CNG car fuel, its gas storage facilities is an important part of CNG stations. Underground gas storage well is the rapid expansion of gas storage facilities in last decade, as a kind of gas efficient, convenient operation and maintenance, economy mode of gas storage, is gradually widely adopted by replacing the high pressure gas cylinder and high pressure air tank. However, due to the gas storage well buried underground, it is not easy to discover the metallurgical defects of material itself or processing, manufacturing, assembly and defects occurring in the process of using injury, this wellbore casing with defects under the action of alternating load, fatigue failure may occur. Underground gas storage well as a high pressure device, often been built in a crowded place, the consequences will be serious once the accident occur. Therefore, the study on fatigue life of CNG underground gas storage well borehole is critical.This paper based on the gas storage well which belongs to the CNG stations in the eastern district of Erdos dongsheng as the research object, conduct a study on the fatigue life of gas storage well wellbore casing, and for the first time, analyzed the casing which contain volume defects, then reach a conclusion about the influence law of casing with different internal and external surface defect form to fatigue life.First, according to the structure and working characteristics of gas storage well, established proper mathematical model. Then according to three different conditions included wellbore casting without defects, with inner surface defects and outer surface defects, used finite element software ANSYS Workbench to analysis them, then obtained the relationship with pits size and the maximum equivalent stress, and provided data analysis for fatigue simulation; By investigating the practical operation of gas storage well in CNG station, and recording day cycles of gas storage well with mathematical statistic method, it is concluded that average number in12day cycle, and determined the cycle of casing load spectrum; By combining material S-N curve and fatigue analysis theory, using nCode Designlife fatigue analysis software for fatigue life estimation, it is concluded that when indentation depth is constant, the fatigue life decreases with increasing indentation length ratio of the shaft; When the pit depth and length of axis ratio are constant, the fatigue life is inversely proportional to the long axis of the pit; When indentation axle ratio is the same, for the same degree of sharp (equal to2x/Z) pits, depth and the ratio of wall thickness (Z/B) older the greater its fatigue failure; By comparing the fatigue life of the pits of inside and outside suz’ face, it is concluded that under the same size, the inner surface pits were more dangerous; Finally, analysised the load frequency effect on the fatigue life of gas storage well casing.Through this article research results, make the maintenance more targeted for CNG underground gas storage well, provided the reference data for judgeing for store the wellbore casing safety and residual life evaluation.