Depleted Reservoirs Gas Storage Casing Mechanical Analysis And Safety Evaluation

The underground gas storage (UGS) on the depleted hydrocarbon reservoirs is anartificial gas reservoir, which processing the injection-production of natural gas to depletedoil and gas stratum through the original gas well or additional wells established in the timewhen gas storage building. Because gas storage accident caused by casing failure accountedfor around31%of total number of gas storage accidents, the casing failure has become aprimary problem to protect the safety of operations of gas storage. The original well casingmust be conducted safety evaluation, when it act as gas reflooded and extractive channel ofnatural gas. This paper processed casing mechanical analysis in the working condition of gasstorage, and systematic researched the factors that affecting the residual strength，then usingreliability analysis method reliability calculation and the remaining life prediction of thecasing in old wells are carried out.first of all by the means of casing mechanical analysis in the working condition of gasstorage, Three-dimensional strength calculation formula of casing under axial force, internalpressure and non-uniform crustal stress is provided, which provide theoretical support to thejudgement of casing damage.After that the influence matching situations of cement sheath properties and formationproperties on the strength of the casing is analysed by using the finite element-method. Inaddition, A three-dimensional finite element model of casing was established, to process thetheoretical analysis and finite element numerical simulation of residual strength after casinguniform corrosion and local corrosion, and the influence of the defect size and the casing loadand stress nonuniformity coefficient on casing residual strength are studied,Finaly the reliability analyse of corroded gas storage casing string under normalcondition and abnormal conditions is proceeded using the stochastic finite element analysismethod. Sensitivity analysis of factors effecting reliability was carried out, and Influence ofcasing internal pressure and gas extraction rate on casing safety of the gas storage is discussed.Then according to the characteristic of the service life of the gas storage is longer, remaininglife prediction of the corroded casing under the action of random factors based on reliabilitytheory is proceeded.