Electrical Coiled Tubing N2 Lifting Enabled Successful HT Production Logging and WSO Campaign
Authors: Aklefa Amad , Emhemed Muamer , Sherik Ayoub , Abdalla Mohamed Ben , Sadeg Siraj 
Published: March 2024

Many deep high-temperature (HT) wells of Bu Attifel field, Libya, have died in the past years due to increasing water cut. The lack of artificial lift, issues in the water injection system, and unavailability of workover rigs led to significant oil production loss. Nitrogen (N2) kickoff was attempted several times without success, indicating the need for water shutoff (WSO) procedures. Moreover, production logging (PL) was not feasible at the time as wells were dead, and casedhole saturation logs were proven to be unreliable due to static crossflow between layers (presence of thief zones where reservoir fluid is produced by a high-pressure layer and taken by a lower-pressure layer while the well in shut-in/static condition) and reinvasion phenomena. Due to these challenges, it was suggested to use N2 lifting to assist PL in dynamic condition to determine the source of water for shutoff.

The objective of this project was to kick off the wells with N2 using electrical coiled tubing (e-coil) with PL tools attached to the e-coil bottomhole assembly. Once the well was restored to a predefined water cut, the logging was performed at different N2 injection rates. This was done regardless of well stabilization (i.e., to have downhole and surface parameters stabilized within 0.5% as per the operator's standard) because the presence of N2 along the entire production string affects the flow regime and does not allow the well to fully stabilize as per the operator's standard. Then, the logging was performed again during the transition period when the N2 pump was switched off. This step was important to prove that injecting N2 near PL tools did not affect data quality. A multiphase meter was also available to monitor cleaning up during N2 kickoff. A high-pressure/high-temperature (HP/HT) tool was used because the bottomhole temperature was over 315°F. Special kickoff and logging steps were taken to insure reliability of measurements and safety of tools and operation.

Four wells were logged successfully with HP/HT PL while N2 lifting. Good-quality data were collected, which aided successful water shutoff decisions adding a production gain of more than 4,000 B/D. In the first well, three flowing surveys were performed. The first survey was at high N2 injection rate to accelerate the cleanup and achieve higher flow rates for better PL data. The second was at the minimum N2 injection rate to acquire most reliable data, and the third survey was taken when the N2 pump was switched off to verify that the N2 injection did not affect PL data quality. Comparisons suggest that data were repeating and consistent, especially the downpasses as the tool was moving in the opposite direction to the flow of N2. Special logging procedures, accurate job planning such as for N2 quantities, and logging tool design, among other factors, allowed acquisition of three PL surveys safely in a single run in 9 days after a long cleanup in high-temperature condition.

This project describes the thorough prejob design where all aspects of N2 kickoff, logging tools, and procedures were carefully and intensively studied, enabling 9 days of continuous logging in high-temperature condition. The integrated approach involving multidisciplinary services and utilizing advanced technologies led to a smooth operation and turned a challenging procedure into normal practice in this field.