Zonal Isolation Challenges and Potential Integrity Solutions in UGS Wells

Abstract Underground Gas Storage (UGS) wells present unique zonal isolation challenges due to cyclic injection and withdrawal operations, aging infrastructure, and the continued use of legacy completions. This technical review identifies the principal causes of integrity loss in UGS wells, including microannulus formation, casing–cement debonding, and long-term cement degradation under pressure cycling. Acoustic log-based cement evaluation methods are frequently inadequate in detecting dry microannuli or quantifying bond quality. Field comparisons show that Cement Bond Logs (CBL) and Variable Density Logs (VDL) often miss behind-casing flow, particularly in gas-contaminated or heterogeneous intervals. These limitations call for complementary diagnostics such as ultrasonic logging, temperature profiling, and noise logging. Drawing on field experience and annual incident reports, this study introduces a revised workflow for precision squeeze cement point detection, integrating injectivity evaluation with risk-informed diagnostics. The influence of fluid type and filtration behavior is emphasized, as standard injectivity tests with brine or freshwater can yield misleading results in certain formations. Operational enhancements, including viscosity-tuned slurries and targeted perforation strategies, are recommended. Failure frequency analysis reveals that Downhole Safety Valves (DHSVs), wellheads, and cement sheaths are the most compromised barriers; however, many of these failures can ultimately be traced back to cement integrity issues. These findings underscore the necessity for standardized integrity protocols tailored to legacy wells, especially in re-entry and abandonment scenarios. A data-driven, adaptive integrity management framework, grounded in regulatory datasets and field diagnostics, is advocated to ensure long-term zonal isolation in gas storage operations.