Case: Slow/Minor Leak Localization

1. Core Issue: Identification of Micro-Leakage

• Observation: Only 1.5 m³ of fluid was injected over 4 hours under a high pressure of 40 MPa.

• Interpretation: The extremely low injection efficiency observed under high pressure confirms the presence of micro-leaks in the wellbore. The fluid is being lost through minor leaks in the tubing string rather than entering the target zone.

2. Detection Methodology and Findings

• Observation: Analysis of fiber-optic leak detection logging data identified 16 distinct temperature anomaly points via DTS. DAS waterfall plot analysis showed that acoustic energy anomalies corresponded closely with these temperature anomalies.

• Interpretation:

• Method: Combined Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) technologies were deployed.

• DTS Finding: The 16 localized temperature increases are caused by the Joule-Thomson effect and friction as fluid escapes under high pressure, pinpointing the leak locations.

• DAS Corroboration: The DAS waterfall plot showed anomalies at the same depths. This cross-validation from a separate physical domain (acoustics) strongly confirms the leak points.

3. Diagnosis of Leak Cause

• Observation: Comparison with tubing string data revealed a strong correlation between the DTS temperature anomalies and tubing collar depths.

• Interpretation: By mapping the 16 anomaly depths against known tubing collar (connection) depths, a near-perfect match was found. This diagnoses the root cause as seal failure at the threaded ("pin & box") connections, allowing micro-leakage under pressure.

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4. Supplementary Analysis and Key Notes

• Observation: Points highlighted in red in the table showed relatively stronger DAS energy. However, under micro-leak conditions, this energy intensity is influenced by multiple factors like fracture geometry and does not show a direct positive correlation with the magnitude of temperature change.

• Interpretation: The report notes that a stronger DAS signal does not necessarily indicate a larger leak or greater temperature change in micro-leak scenarios. The leak paths (e.g., thread gaps) are tiny and intricate. DAS signal strength is affected by various flow dynamics (turbulence, impingement), explaining why acoustic energy (DAS) and temperature change (DTS) are not proportionally related. This complexity further supports the diagnosis of subtle thread leaks.

5. Summary and Conclusions

Recommended Action: Based on this report, the operator should repair or replace the specific tubing collars corresponding to the identified anomalies to eliminate the micro-leaks and restore injection efficiency.