A Correlational Study to Evaluate Minimum Detectable Leak Rate in Gas Wellbores Using Ultrasound Survey

Abstract

Abstract A wide variety of tools and methods are currently available to monitor casing integrity on a case-by-case basis. The use of mechanical integrity test (MIT) methods, to observe pressure communication between casing strings and formation, is often mandated by regulatory agencies when assessing the performance of cemented strings. The pressure monitoring test is the conventional MIT method mostly prescribed for wellbores in gas wells. However, pressure monitoring test require an iterative number of steps to determine the leak source and such determination preclude an estimate of gas leakage rate. However, the ultrasonic or magnetic methods could offer a more direct assessment of casing integrity in gas wells without tubings. In this study, a wellhead test configuration was set-up on a gas cavern well with tubingless completion to simulate different casing leak scenarios whereby flow rates, pressure drops, and ultrasonic noise log responses were measured. A ratio response scale was developed to normalize background noise inherent in ultrasound response data; and thereafter used to develop a minimum detectable leak rate (MDLR) estimate. The degree of association between measured variable (ultrasonic noise log response), and predictor variable (products of flow rates and differential pressures) was statistically established. The result showed that ultrasonic noise log could detect a minimum leak rate of 50 Mcf/d at a differential pressure of 75 psi. The significance of this study was to provide a specific evaluation of the ultrasonic noise log as an alternative to conventional mechanical integrity test (MIT) for cemented strings in gas wells. The study, while contributing to the literature in ultrasonic noise log applications, also provides a framework for quantitative analysis of ultrasound data that may be suitable for future wellbore diagnostic works.