Project Background
When the client was conducting tubing leak detection in an empty well at Well XX, they were faced with the challenge of accurate detection and positioning of slow and micro-leakage. The specific challenges are as follows:
- High Construction Difficulty: Optical fibers need to be run into the well along with the casing, involving complex processes such as cementing consolidation and pre-tension control. This requires extremely high standards for wellbore trajectory, well deviation and well diameter, leading to a low deployment success rate in horizontal wells or highly deviated wells.
- Fragile Optical Fibers: The optical fibers themselves are fragile and prone to microcracks or fractures due to stretching, bending or improper clamping during installation, resulting in a high on-site damage rate.
- Difficult Leak Point Identification: Traditional threshold methods struggle to accurately identify multi-point leakage or continuous seepage occurring at the same time, which is likely to cause under-reporting or false positives.
- Insufficient Positioning Accuracy: Distributed Temperature Sensing (DTS) relies on temperature gradients. In the early stage of leakage, the temperature difference is small, leading to a positioning error of ±5–10 meters. Distributed Acoustic Sensing (DAS) is limited by coupling methods and sampling intervals, with errors often exceeding ±20 meters.
Our Solution
To address the challenge of micro-leakage diagnosis, we adopted a systematic engineering approach that integrates the dual-system optical fiber detection technology of DTS and DAS. Through a high-temperature and high-pressure injection test (injecting only 1.5 cubic meters at 40 MPa for 4 hours), we collected temperature and acoustic signals synchronously. Combined with tubing string data, we performed correlation analysis on temperature anomaly points and acoustic energy, achieving accurate identification of leakage positions and channel characteristics.
Final Results
- Accurate Positioning: The DTS system detected 16 distinct temperature rise anomaly points. By comparing with the DAS waterfall plot, the locations of acoustic energy anomalies were highly consistent with the temperature anomaly points, which were confirmed as tubing leak points.
- Cause Analysis: The leak point locations showed a good correspondence with the tubing couplings, indicating that the leaks were caused by thread micro-leakage. For some red anomaly points, the DAS energy was strong, but there was no positive correlation between energy intensity and temperature change, suggesting that the leakage channels were small and complex.
- Technical Advantages: The collaborative verification of the dual-system overcomes the limitations of single technology, realizing high-confidence positioning of leak points under micro-leakage conditions and providing an accurate basis for subsequent plugging operations.
Client Evaluation
The client highly recognized the accuracy and engineering value of this detection. They believed that the solution effectively resolved the industry challenge of difficult positioning of micro-leakage, and demonstrated significant advantages especially in identifying thread micro-leakage. It provided key support for the selection of subsequent plugging materials and construction design, improving the efficiency and reliability of wellbore integrity management.
