The Value of Downhole Diagnostics to Perform Successful Permanent Isolation of Flow Potential Sources from Surface
Authors: Howley Grace , Rondon Samuel , Volkov Maxim 
Published: October 2023

Managing asset retirement obligations efficiently and effectively is an important objective for operators. Nowadays, Many operators are looking for additional guarantees before and during plug and abandonment (P&A) procedures to properly decommission the wells and permanently seal any potential reservoirs. In order to confirm the integrity of the cement and plugs and the efficacy of the barriers, these procedures may involve the execution of a few surface and downhole tests, such as cement bond logging, the downhole camera runs, and pressure testing. Achieving a P&A with long-term integrity for the lowest cost and best use of limited rig resources requires understanding potential flow paths to place the most effective barriers.

The well may present annulus pressure or flow at the surface in the event of barrier failures. Surface diagnostics, such as flow checks and gas or fluid sampling, can provide some insight into the source of the leak, but surface diagnostics cannot identify the leak path and source with complete certainty in most cases. As a result, standard methods for re-establishing cement barriers, such as cement squeezing, section milling, or casing expansion, may not work if applied at the wrong source, and surface pressure analysis may be misleading and indicate that remediation efforts were ineffective. This set the requirement to locate all potential contributing zone in case of sustained annulus pressure and also recommended to indicate any hidden crossflows in case no surface pressure observed.

Advanced logging techniques are needed to locate the potential flow zones that could contribute to the sustained annulus pressure or could cross-flow amongst one another. They should be able to track and identify unwanted flow caused by barrier failure when they scan behind one or more concentric steel and cement layers. These diagnostics' results are utilized to specify the areas where well barriers should be restored and will result in permanent abandonment. Additionally, the diagnostics should confirm the reliability of the sealing and integrity of previously completed isolation.

This is achieved by integrating high precision temperature and spectral passive acoustics sensors into the diagnostic toolstring and analyzing the results under various well conditions. Both techniques can verify failures, but they consider different leak scenarios and work in concert to create integrated behind-casing scanning. The fluid rate is primarily responsible for the formation of temperature anomalies, including gradient changes and localized heating or cooling peaks. To record a temperature response anomaly or temperature gradient change, the leak rate should be high, particularly if there are several barriers between the sensor and the leak path. Spectral Passive Acoustics are more dependent on delta pressure than leak rate since they are primarily driven by fluid velocity and flow turbulence across restrictions in the leak path.

The integration of temperature and acoustic sensors presents an opportunity to diagnose failures with low leak rates, including those with very slow pressure buildup rates or no identifiable pressure at the surface.

Both High Precision Temperature and Spectrum Passive Acoustics are operated simultaneously: Temperature is logged on the down pass, and Acoustics while stationary on the up pass. Static and dynamic stages are often carried out in pairs. The data collected during both regimes is then compared to show any residual crossflows and to show induced activity during leak activation.

Spectral Passive Acoustics and High Precision Temperature diagnostics were effectively used in a well in the DJ Basin to investigate the source of 50 psi Sustained Annulus Pressure in the B annulus (between 4-1/2″ and 8-5/8″ casings). To analyze the Top of Cement (TOC) behind the production casing, logging was done down to 1833.7 ft inside the production casing. Both regimes, during B-annulus bleed off and during the shut-in condition, were investigated. The information was compared to previously performed Cement Bond Logging (CBL).

High Precision Temperature (HPT) found no significant differences between the two regimes, indicating that the leak rate was extremely low. The gradient was steady, and there was no more than a 0.1 degree Fahrenheit change between shut-in and bleeding off.

Spectral acoustics, in contrast to HPT, clearly indicated a contributing zone across the 8-5/8″ casing shoe. Under static and dynamic regimes, the contribution patterns and traces of annular fluid circulation were both present. The cement barrier behind the 4-½″ production casing below the surface casing shoe appeared to be sealing since no flow below the logging interval was observed.

The diagnostic results allowed the operator to continue with the P&A operations with only one identified contributing zone across 600 ft. Before the plug was set, the production casing was cut and pulled below the identified source. Following the casing pullout, a wellbore camera was run, and it revealed gas bubbling throughout the previously specified zone with no flow below the 600 ft zone identified on the log. This confirmed the accuracy of the source identified by passive spectral acoustics.

No evidence of surface pressure was present after the well was successfully abandoned.

Without proper source identification, deeper barriers would have been established and would have been deemed inadequate or unsuccessful due to the shallow flow. This would have resulted in unnecessary costs to the operator and rig days wasted on unnecessary plugs that could be used to P&A other wells in the basin.

The capacity to find the source of sustained annulus pressure behind the steel and cement barriers was proven by a combination of camera survey, Spectral Acoustics, and high precision temperature logging. This supports the physics and idea behind through barrier diagnostics, which can be utilized to design the P&A prior to the arrival of the rig and tubing retrieval. Additionally, it saves operators time during the P&A and gives them more confidence in creating isolation at the correct zones.