Evaluation of High-Resolution Downhole Tubular Inspection Tools by Full-Scale Field Testing

Abstract Downhole tubular corrosion poses a significant threat to underground gas storage (UGS) well integrity. Therefore, integrity management of UGS wells requires periodic inspection of tubular conditions using various inspection tools to properly identify and size corrosion anomalies. The corrosion inspection data are then used to estimate corrosion growth rates and the remaining strength of the tubulars through various methods. However, the calculated remaining life and remaining strength can have a significant uncertainty due to the variations of the input parameters, including the corrosion growth rate, accuracy of reported corrosion feature sizes from the inspection tools, tubular material property variations, uncertain in-situ load conditions, and prediction model uncertainties. In order to improve the tubular corrosion management for UGS wells, this study focused on a quantitative performance evaluation of a few selected state-of-the-art downhole tubular corrosion inspection tools based on a full-scale field test program. This paper presents the process and methodology of the field test and performance evaluation of four high-resolution tools based on principles of ultrasonic testing and magnetic flux leakage, which can provide relatively accurate sizes (including depth, length and width) of corrosion features. Statistical approaches were employed to determine the quantitative tool performance metrics, including the probability of detection, probability of false call and sizing accuracy. Moreover, the statistical performance results not only allow the determination of the feature sizing tolerances that can be used in deterministic integrity assessment methods but also provide valuable inputs for further development of a probability-based tubular integrity assessment workflow.