The Quality Safety Evaluation and Life Prediction of Casing String in Underground Gas Storage

Underground natural gas storage can carry out seasonal peak regulation and provide emergency gas supply to industry; however, its completion casing string is subjected to many undesirable influences by high-pressure gas injection and corrosion environment. Therefore, the bearing capacity and service life of casing string will be reduced. By establishing the reliability life prediction model of casing string, the expected life of casing can be computed quantificationally. The collapsing strengths are calculated based on the API 5C3 standard and the ISO10400-2007 standard, respectively. The results are compared with that of a casing collapse experiment; the application ranges of the two standards are obtained under different casing thicknesses. In this paper, the authors calculated the casing collapsing strength and the collapsing safety factor of the Su-11 well according to the casing collapsing strength theory. The results indicate that collapsing strength satisfies the strength requirement of casing in the whole range of well depth. According to the Norsok corrosion model, casing corrosion rates are predicted in an acid environment considering the impact of corrosion protection film. The actual casing corrosion rates and the life span of the Su-11well’s casing string are calculated by the casing string residual life comprehensive evaluation theory, which gives the well a remaining casing life of more than 30 years for underground gas storage.