Safety evaluation method for service state of tubing string in CO₂ injection-production well

Aiming at the problem of frequent replacement of the tubing string in CO₂ injection-production well due to corrosion failure, this paper studies a prediction model of CO₂ corrosion rate suitable for the wellbore environment of CO₂ injection-production well and the mechanical analysis method of string, analyzes and discusses the influence of the changes in the condition of CO₂ injection-production well, water content of produced fluid and other factors with time on the corrosion rate and load of tubing string in CO₂ injection-production well, and in combination with the calculation method of the residual strength of the string corrosion, establishes a safety evaluation method for the service state of tubing string in CO₂ injection-production well, as well as conducts example calculation and comparison. The results show that for the corrosion prediction of tubing string in CO₂ injection-production well, the DW-95 model has good applicability. The production stage is the main stage of corrosion in CO₂ injection-production well and the main object of evaluation on safe service state. The safe service time of tubing string is negatively correlated with its corrosion rate and load, and the safe service time is the minimum safe service time for the tubing string, which provides the main basis for the safety evaluation of service state. Applying this as the standard, the replacement cycle of the string can be determined, and the huff and puff period of CO₂ injection-production well can be optimized, thus guiding on-site safety production.