| Summary: | Compared with traditional injection tubular strings, the stresses on CO<sub>2</sub> injection tubular strings are more complex. The results from field applications show that the phase transition of CO<sub>2</sub> fluid in CO<sub>2</sub> injection strings is an important factor in the calculation of temperature distribution and analysis of string mechanics. Therefore, we propose a strength analysis method for CO<sub>2</sub> injection tubular strings that considers the CO<sub>2</sub> phase transition. We selected four CO<sub>2</sub> injection strings in an oil field in China as examples to evaluate their strength and safety. First, we established coupled differential equations for the temperature, pressure, and physical parameters of CO<sub>2</sub> injection strings according to the theory of fluid flow and heat transfer. Then, we used an adaptive fuzzy neural network to construct the model for calculating the CO<sub>2</sub> convection heat transfer coefficient and used this to obtain the high-precision convection heat transfer coefficients of tubular strings under conditions of CO<sub>2</sub> flooding. We analyzed the injection-string deformations that resulted from the piston, spiral bending, expansion, friction and temperature effects according to the stress characteristics of the CO<sub>2</sub> injection strings with packers under different working conditions. Finally, we performed mechanical analyses on the collapse resistance, internal pressure resistance, and tensile and triaxial stresses of the CO<sub>2</sub> injection strings, and the results of these analyses provide a theoretical basis for the strength analysis of CO<sub>2</sub> injection strings.
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